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2015-01-20T18:35:59.493 | <p>Is it possible to "pump" a powder the same way liquids can be pumped?</p>
<p>If so, what are the challenges? If not, what are some alternatives?</p>
| |pumps|liquid|fluid-mechanics| | <p>Yes you can pump powders using a diaphragm pump. The above comments all focus on alternate ways to transport powder they are not talking about pumping. Check out powder pumping with a diaphragm pump on youtube.</p>
| 1 | Is it possible to "pump" a powder? |
2015-01-20T18:41:51.527 | <p>What is the <a href="https://en.wikipedia.org/wiki/Barometric_formula" rel="nofollow noreferrer">barometric height formula</a> for a mixture of gases?</p>
<p>The main problem I can see, the curves describing the minimal energy (kinetic + gravitational/centrifugal) won't be any more a simple derivative, but a functional equation.</p>
<p>I think, this or similar equations could be useful in very separate areas (isotope separation, estimating high atmosphere components of planets, etc.) Despite I couldn't find anything with google about this.</p>
<p>So, the question is: we have a high gas mixture in constant g gravity and T temperature. We know the <span class="math-container">$p_1$</span>, <span class="math-container">$p_2$</span>, ... partial pressures of the gases on the <span class="math-container">$h=0$</span>. Of course we know every relevant constants of the gases, too. What is their minimal-energy, isothermic, height-dependent partial pressure function <span class="math-container">$p_n(h)$</span>?</p>
| |pressure|gas|nuclear-technology| | <p>The barometric height formula is defined as:</p>
<p>$$ p(z) = p(0)\exp(-Mgz/(RT))$$</p>
<p>For a centrifuge the upper expression is transformed to:
$$ p(r) = p(0)\exp(M\omega^2r^2/(2RT))$$</p>
<p>As you see the $g$ is transformed to the radial acceleration $a = \omega^2 r$. The factor $2$ stems from the integration which you have to do during the derivation of the barometric height formula (see <a href="https://en.wikipedia.org/wiki/Barometric_formula#Derivation" rel="noreferrer">barometric height formula</a>, derivation). And of course, the minus sign disappeared because the force points outward.</p>
<p>Both formulas hold also for a mixture of gases, I cite "<a href="https://scholar.google.de/scholar?cluster=8946199746943967206&hl=de&as_sdt=0,5&sciodt=0,5" rel="noreferrer">Kemp, R. Scott. "Gas Centrifuge Theory and Development: A Review of US Programs." Science and Global Security 17 (2009): 1-19.</a>":</p>
<blockquote>
<p>When the rotor contains a mixture of gases, the distribution holds independently for each species.</p>
</blockquote>
<p>So, as a first approximation, it is as I suggested in the comment section. The cited source gives the following equations for a two-isotope gas:</p>
<p>$$ p_A(r) = p_A(0)\exp(M_A\omega^2r^2/(2RT))$$
$$ p_B(r) = p_B(0)\exp(M_B\omega^2r^2/(2RT))$$</p>
<p>A separation factor can be calculated:</p>
<p>$$ \alpha_0 = \frac{p_A(0)}{p_B(0)}/\frac{p_A(r)}{p_B(r)} =\exp((M_B- M_A)\omega^2r^2/(2RT))$$</p>
<p>Keep in mind that all formulas here don't include convection of any kind. In earth's atmosphere, we have too much convection and temperature differences for those formulas to work. Furthermore, in centrifuges, to improve their performance, there is a current-flow introduced (see the <a href="https://scholar.google.de/scholar?cluster=8946199746943967206&hl=de&as_sdt=0,5&sciodt=0,5" rel="noreferrer">cited source</a>).</p>
| 4 | Multi-gas barometric height formula in isotope separation devices? |
2015-01-20T19:49:53.683 | <p>The United States have different rules about exactly how one obtains an engineering license, but the general process is the FE exam, a few years of work, and the PE exam. Suppose one then wishes to practice engineering in Germany. What are the legal requirements for doing so?</p>
| |licensure|international| | <p>Take all this with a grain of salt. I work in Germany as an engineer and don't have any special license sides my degree, but I'm employed and your situation may be different.</p>
<p>Generally, engineer is a free profession in Germany. This means, if you are an engineer (have an engineering degree from a university or a university of applied science), you can freelance as an engineer. Each state has an <em>Ingenieurgesetz</em> that states how to handle foreign degrees. Mostly, if the degree is equivalent (Master/BA of engineering) you can request the right to call yourself an engineer (But I don't know yet where).</p>
<p>Additionally, I <em>think</em> for certain tasks in civil engineering you have to be part of an engineering association.</p>
<p>Lastly, if you work with a company, you generally don't need a special license (for example to sign of plans)</p>
| 6 | What is the process of transferring an engineering license from the United States to Germany? |
2015-01-20T22:03:47.020 | <p>I think, here is the main problem the difference between the internal and the external temperature.</p>
<p>For example, in Saudi Arabia, in 50 C, a passive house needed probably much sophisticated planning as in Paris.</p>
<p>Compared to the traditional cooling systems, in the second case is enough only to get a cooling system with bigger power. I think, they are much more scalable.</p>
<p>Is it anyways possible?</p>
| |civil-engineering|architecture|cooling| | <p>In order to build a passive house you have to limit the final energy use for cooling to 15 kWh/m² per year and your primary energy use for HVAC and lighting has to be less than 90 kWh/m² per year minus the deduction for compactness, so let's say around 70 kWh/m².</p>
<p>I guess getting to 15 kWh/m² final energy use for cooling (i.e. the heat extracted from spaces including all the insulation losses) in a climate like Saudi Arabia is almost impossible. In a classic building technique it would mean very thick insulation and small windows in order to limit solar heat loads through fenestrations.</p>
<p>The other criterion however, the primary energy use, would not be that difficult to meet because it represents the balance between the primary energy used and generated on-site by renewables (e.g. PV panels). So if your vapour compression cooling system uses 100 kWh/m² per year and your PV panels produce 40 kWh/m² per year (per m² of building net surface, not per surface of panels) you get to 60 kWh/m² in total which is quite good. If your PV panels create 100 kWh/m² then your balance is 0 and you have a net zero energy building.</p>
<p>So in conclusion, it does not make really sense to apply the 15 kWh/m² rule in Saudi Arabia. This rule (like the whole passive house concept) was made up in Germany and is a completely political choice which does not have any meaning (it could also be 10 or 25 kWh/m²). What is more important is the net primary energy use and this one you can keep in the passive limits. In fact, to build a building that has a low net primary energy balance Saudi Arabia is a good place because you have plenty of sun and you can generate a lot of green electricity on site with PV panels, something that is not possible in cold climates where the solar energy is not so abundant.</p>
| 14 | How are passive houses made in very hot regions (like Saudi Arabia)? |
2015-01-20T22:16:03.560 | <p>Liquid hydrogen cooling could be some intermediate solution between helium and liquid nitrogen based cooling. For example, some type of superconductors could be cooled by liquid H<sub>2</sub>, and this could be much cheaper than the liquid He.</p>
<p>What are their most characteristic applications in the industry?</p>
| |superconduction|cryogenics| | <p>The combustion chamber and nozzle of the Space Shuttle main engine were cooled with liquid hydrogen. Liquid hydrogen was also the fuel. It was used as a coolant before it was burned (<a href="https://en.wikipedia.org/wiki/Regenerative_cooling_%28rocket%29" rel="nofollow noreferrer">regenerative cooling</a>).</p>
<p><img src="https://upload.wikimedia.org/wikipedia/commons/b/bc/Ssme_schematic.svg"> </p>
<p>Another example is the <a href="http://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29" rel="nofollow noreferrer">air breathing rocket engine SABRE</a>. Here too liquid hydrogen is used as fuel. It's also used for liquefying the atmospheric oxygen.</p>
<p><img src="https://i.stack.imgur.com/PqrXj.jpg" alt="enter image description here"></p>
<p>In the industry, one advantage that liquid helium and nitrogen have over hydrogen is that helium and nitrogen are inert and non-combustible. Probably less corrosive too.</p>
<p><em>p.s.</em> The question has a <a href="/questions/tagged/superconduction" class="post-tag" title="show questions tagged 'superconduction'" rel="tag">superconduction</a> tag. So, I don't know if this is the sort of application of liquid hydrogen cooling that the O.P. is looking for. </p>
| 16 | Do liquid hydrogen-based cooling devices exist and what are they used for? |
2015-01-20T22:41:57.420 | <p>I'm making a robot for the Sparkfun AVC. I was curious if I could use only the knowledge about</p>
<ol>
<li><p>How the car is being steered at every time interval,</p></li>
<li><p>How many times the wheels have rotated,</p></li>
</ol>
<p>to get a general sense of where the car is. I would use computer vision to avoid immediate dangers. </p>
<p>The biggest problem is any slipping causing false counts.</p>
| |control-engineering|robotics| | <p>The correct way to do this is by using what is known as a <a href="http://en.wikipedia.org/wiki/Particle_filter" rel="nofollow">particle filter</a>.</p>
<p>The maths for estimating your next position is quite simple and other answers have already provided that, but this is how you deal with the uncertainty. <a href="https://www.youtube.com/watch?v=aUkBa1zMKv4" rel="nofollow">This video</a> explains the basic principle rather well. You will notice that you need to take a measurement of some aspects relating to where you are, for example distance to known objects, compass reading, position of visible markings, etc. (Use your computer vision for that, plus anything else you can such as sonic range finders). It doesn't have to be perfect, the particle filter deals really well with 'noisy' measurements and 'noisy' (error-prone) predictions of where you are.</p>
<p>Also do a search for "slam particle filter" to get some further insight.</p>
| 20 | Knowing how many times the rear wheels have rotated, and the angle of the front wheels at every time interval, how do I find the location of a car? |
2015-01-20T22:53:44.837 | <p>I recently had a project where I made an aluminum linkage (similar to a scissor lift) operated by linear actuators. Aluminum pieces were fabricated from 6061 "standard" aluminum stock, 6mm (0.25 inch) thick. The bars are at most 750x75mm (30x3"). The application was mostly cosmetic (for a special effect) rather than lifting or exerting much force.</p>
<p>I used shoulder bolts with a nylon washer between the aluminum pieces to reduce friction, and standard zinc-plated steel washers on the exterior sides.</p>
<p>The problem I encountered was tightening the bolt too much would result in too much friction, whereas too loose would result in one of the aluminum pieces coming slightly off the shoulder of the bolt, skewing, and causing similar problems.</p>
<p>What would be a better way to create a reliable pivot point that doesn't require constant adjustment?</p>
<p>I put together this illustration as an example:</p>
<p><img src="https://i.stack.imgur.com/m1Psy.png" alt="Linkage Description"></p>
| |bolting|aluminum|linkage| | <p>Steel and aluminum don't like each other! that's why you are getting the binding. Only use Stainless steel with aluminum. And of course never use aluminum fasteners for an application like this due to strength. And use all nylon washer if you can or between the SS washer and where is touches the moving part.</p>
| 22 | What method can I use to make a pivot point in an aluminum linkage? |
2015-01-20T23:14:48.790 | <p>As far I know, the depleted fuel cells are crushed, and solved in nitric acid.</p>
<p>What is coming after that? This nitric acid should probably contain a very wide spectrum of different salts (practically, all of the elements between 35-65, and yet a lot of transurans, and a lot of uranium (both of 235 ad 238), and plutonium).</p>
<p>To reach an efficient reprocessing, the uranium (or, at least, the actinid elements) should be somehow separated from this solution. But, AFAIK, they have very different chemical properties. How is it possible to separate only the transuranic materials?</p>
| |nuclear-technology|nuclear-reprocessing| | <p><strong>Fission Products Extraction</strong></p>
<p>The seperation of plutionium and uranium from the other fission products is done with the organic molecule <a href="http://en.wikipedia.org/wiki/Tributyl_phosphate" rel="noreferrer">tributyl phosphate</a> by <a href="http://en.wikipedia.org/wiki/Liquid%E2%80%93liquid_extraction" rel="noreferrer">liquid-liquid-extraction</a>. You have two phases, one organic and one aqueous. The fission products will solve in the aqueous phase and uranium/plutonium will solve in the organic phase with the tributyl phosphate.</p>
<p><strong>Uranium/Plutonium Separation</strong></p>
<p>To seperate uranium from plutonium you have to reduce the plutonium with uranium nitrate. Again, you have two streams: One with the uranium/plutonium from before and one aqueous stream with uranium nitrate (U4+). Plutonium will be reduced and solve in the aqueous solution. </p>
<p>To accomplish the aforementioned chemical processes, the liquid-liquid extraction you can use the following desgins:</p>
<p><strong>Mixer-Settler</strong></p>
<p><img src="https://i.stack.imgur.com/lOtdo.png" alt="Mixer-Settler"> (<a href="https://scholar.google.de/scholar?cluster=16705612692502180121&hl=de&as_sdt=0,5&sciodt=0,5" rel="noreferrer">source</a>)</p>
<p><strong>Pulse Column</strong></p>
<p><img src="https://i.stack.imgur.com/AbjZf.png" alt="Column"> (<a href="https://scholar.google.de/scholar?cluster=16705612692502180121&hl=de&as_sdt=0,5&sciodt=0,5" rel="noreferrer">source</a>)</p>
<p>The principle is always: Organic phase is lighter than the aqueous phase. Both phases are first separated, than mixed, and then again separated. During the mixing the chemical reactions occur.</p>
<p>Please see the mentioned <a href="https://scholar.google.de/scholar?cluster=16705612692502180121&hl=de&as_sdt=0,5&sciodt=0,5" rel="noreferrer">source</a> for more information. </p>
| 26 | How does nuclear fuel reprocessing work? |
2015-01-20T23:41:29.103 | <p>I've found myself tapping and threading a lot lately, and doing so with hand taps and dies. What would be the next step up from using a drill-press to cut holes for tapping, and a lathe for turning parts to diameter for threading?</p>
<p>What are options for making this easier and more precise short of a 6-figure screw or CNC machine?</p>
| |machining| | <p>For tapping threads, there are a few steps between free hand tapping and fully automatic machines. Which one is right for you will depend on the material you're tapping, the size of the threads you're tapping, and how large your production run is.</p>
<p>The first step would be to buy a guided tap wrench, which is a regular hand tap wrench with a bushing you can put on the top and chuck into a drill press (or mill) or lathe. This helps keep your tap square to the workpiece and also lets you apply a little pressure with the quill.</p>
<p>If you want to get into power tapping (And need more precision than a cordless drill with the clutch turned down,) there are reversing tapping heads you can attach to a drill which reverses the tap out when threading is complete. These are available for drill presses or hand-held drills. They run around \$500-$1,000. here's another style of tap chuck called a tension compression chuck, which allows a little vertical give but I believe that is only of much use if your spindle is computer controlled.</p>
<p>If you want a more robust solution and have some money, there are tapping presses that start at around $3,000. It's not quite 6 figures, but they aren't cheap.</p>
<p>Depending on what you are making, self threading screws may also be a valid option. There are a wide variety of types that work in different situations.</p>
<p>As for external threads, after die cutting, the only cheaper method I'm aware of is roll threading. This requires a dedicated machine that is relatively expensive unless you're working on very small threads.</p>
| 32 | What machinery should I invest in for tapping and threading? |
2015-01-20T23:46:47.343 | <p>It's true that most bridges are "two directional."</p>
<p>But <a href="http://en.wikipedia.org/wiki/Three-way_bridge">three way bridges</a> are pretty rare, globally. I can understand why there wouldn't be many for rivers, but if bridges are designed based on the lie of the surrounding ground, why wouldn't there be a large number of non-river sites that would support such bridges.</p>
<p>On the other hand, three out of the world's bridges exist in Michigan (and only ten or so elsewhere in the United States). What is it about the land, topography, or other features of Michigan that cause it to have a disproportionate number of the country's and world's three way bridges.</p>
| |structures|design|civil-engineering| | <p>Multi-way bridges are rare for river crossings for the reasons you describe; however, they are seen as a component of highway interchanges, usually when left entrances/exits are present or access ramps have been "braided" into an existing system interchange, or where a SPUI has been built over an existing divided highway. Viaducts over railroad tracks or highways are also built in this configuration on occasion.</p>
<p>An example of this is the <a href="https://goo.gl/maps/uC5kx">intersection of Russel Blvd. and Gravois Ave. in St. Louis, MO</a>, where a highway intersection is partially undercut by Interstate 55; another good example (in SPUI form), is at <a href="https://goo.gl/maps/d2a3C">Hampton Ave and I-64, also in St. Louis</a>.</p>
<p>Even more extreme forms of this can be found in Atlanta, GA, where the Downtown Connector freeway was tunneled under two intersections: <a href="https://goo.gl/maps/0vJNa">Baker St NE and Piedmont Ave NE</a> and <a href="https://goo.gl/maps/rH46k">Memorial Dr SE and Capitol Ave SE</a>.</p>
<p>Finally, New York calls itself home to a true four-way bridge, the little-known <a href="https://goo.gl/maps/K3wlb">Macomb's Dam Bridge</a> in the Bronx (the four-way part comes on the east approach, where it intersects ramps from the Major Deegan Expressway that are elevated in order to pass over trackwork to the south).</p>
| 33 | Why are three way bridges rare? |
2015-01-20T23:50:02.410 | <p>My design uses a round steel bar inserted through a hole in an aluminium component with a tight fit (no movement). The operating environment is dry, 20-40°C.</p>
<p>Do I need to worry about galvanic corrosion, and if so how long will this last before starting to corrode? Also, what storage conditions should I avoid that would accelerate the corrosion?</p>
| |steel|corrosion|materials| | <p>Aluminum and steel are very near to each other in the <a href="http://en.wikipedia.org/wiki/Galvanic_series" rel="noreferrer">Galvanic Series</a> for seawater. So even if your apparatus is exposed to a corrosive environment along those lines, it should not corrode very quickly.</p>
<p>The aluminum will corrode first, if corrosion occurs. If the exposed surface area of the steel is small compared to that of the aluminum, the corrosion that occurs will be very slow. This is part of the principle behind galvanized steel pipe: even if the zinc is scratched through, the remainder of it will corrode slowly and galvanically protect the steel. Because the surface area of the exposed steel is so much less than that of the zinc, the corrosion of the zinc is negligible.</p>
| 34 | Do I need to worry about galvanic corrosion when inserting a steel bar through aluminium? |
2015-01-20T23:51:19.933 | <p>If I have structural or tool steel that has been treated to some standard (ASTM, SAE, ISO -- e.g., for hardness) but I don't know the details of the treatment, is there a "safe" temperature below which I can work the steel without affecting its performance characteristics?</p>
| |steel|machining|metallurgy| | <p>There are three key temperature which may affect the properties of steel. As mentioned in the accepted answer the recrystallization temperature is the most significant as it can potentially affect any steel, especially ones which have been cold worked to improve their properties or have a high alloy content eg stainless, chrome-moly steels and some castings. </p>
<p>The second consideration is the tempering temperature. This only applies to steels which have been heat treated, generally cutting tools, dies, springs and certain other very high strength/hardness parts. The tempering range can vary between 180 and 300 C or up to 600C for high speed steels. Heating above the tempering temperature will remember the steel and consequently soften it. this is usually only a concern for finished components although some types of stock are supplied hardened and tempered, typically high alloy tool steels. </p>
<p>The final concern is that very high temperatures, approaching the melting point of the steel may cause the growth of very large crystals of even deep oxidation of the surface. This is a concern for all grades but particularly those containing chromium. </p>
<p>Finally, while many hot rolled steel grades can be hot worked without any loss of mechanical properties some have a narrow working window (red short) and the manufacturers data should be consulted in all cases. </p>
| 35 | At what temperature do I risk altering the structure of steel? |
2015-01-20T23:59:10.837 | <p>I am starting to use steel in some of my designs, and while trying to learn more about this material and its properties I have realized that there is so much more to it than I thought.</p>
<p>How does the grading system used for steel work? For instance, what does "Grade 11SMn30" mean, and how does this affect the properties of the material?</p>
<p>Answers/comments indicate there are multiple standards, which I did not know, but I've found out that this is an EN grading.</p>
| |steel|materials| | <p>Based on it being an EN steel grade:</p>
<p>The first number is 100x the carbon content percentage (so 0.11%), the letters are added elements (sulphur and maganese), and the last number is the sulfur content (0.30%). <a href="http://www.steelnumber.com/en/steel_composition_eu.php?name_id=155">You can see the full details here.</a></p>
<p>The full format seems to be:</p>
<pre><code>[X][% carbon][added elements][% of added elements, hyphenated]
</code></pre>
<p>Note that the X is only present for stainless steels. <a href="http://mobile.euro-inox.org/map/EN_name/EN_en_name.php">Here is a good example</a>.</p>
<p>Note also that this scheme is somewhat ambiguous. The percentages are only an approximation, and the example you gave is interesting because it lists Sulfur before Maganese, despite the naming convention stating that they should be listed in order of content.</p>
<p>That's alright for getting quick basic info about the steel, but for anything else you may want to use the EN number, 1.0715, rather than the name. <a href="http://en.wikipedia.org/wiki/Steel_grades#European_standard_steel_number">Wikipedia has details on the format</a>. Given this classification you can find out much more about the steel's properties and see what general category it fits into. The site I linked first says this:</p>
<blockquote>
<p>EN 10277-3: 2008 Bright steel products<br>
Technical delivery conditions. Free-cutting steels </p>
<p>EN 10087: 1999 Free cutting steels<br>
Technical delivery conditions for semi-finished products, hot rolled bars and rods</p>
</blockquote>
| 37 | What does the steel grade "11SMn30" mean? |
2015-01-21T00:03:53.533 | <p>The LTE specification specifies SC-FDMA as the encoding scheme for the reverse link (mobile to base station) when the forward link uses OFDMA.</p>
<p>What is the reason for the difference?</p>
| |electrical-engineering|telecommunication|lte|modulation| | <p>SC-FDMA is encoded and transmitted from the handset where there are power tradeoffs. This encoding is better than OFDMA for the handset for the following related reasons:</p>
<ul>
<li>Reduces peak to average power ratio</li>
<li>Increases efficiency of the power amplifier</li>
<li>Increases battery life</li>
</ul>
<p>So at the base station they can employ transmitters that can handle a higher peak transmit power, with lower efficiency as it has a significant amount of power available. But on a mobile handset, this requires more expensive components, more space, and, probably most importantly, more power since peak transmissions are not as efficient as transmitting at a lower level but for a longer period of time (ie, higher peak to average ratio means lower energy efficiency, more heat, more power). The tradeoff is a little lower spectrum efficiency, but it's worth it for increased battery life, and particularly since most high speed data is going to the handset, not coming from it.</p>
<p><a href="https://www.youtube.com/watch?v=dr4YQAfifKA" rel="nofollow">This short video</a> explains further.</p>
| 38 | Why does the LTE Specification define SC-FDMA as the modulation scheme for the reverse link? |
2015-01-21T00:46:44.707 | <p>Recently, both Zigbee and Z-wave tend to be the most commonly used protocol to communicate between Internet of Things (IoT) devices. Most of these IoT devices tend to have a combination of mechanical or optical related transducer or actuator device component coupled with a micro-controller and respective software stack. What are the major differences between Zigbee 3.0 and Z-Wave in relation to communication between IoT devices?</p>
| |electrical-engineering|telecommunication|iot| | <p>I will leave the main part of the previous answer below, as it contains more information about the differences between several radio protocols.</p>
<p>As for the differences between Zigbee 3.0 and Z-Wave, let's do a side by side comparison:</p>
<ul>
<li>Frequency: Zigbee uses the 2.4GHz band, while Z-wave uses the 868MHz band in Europe and the 900MHz ISM band in the US (see the frequency coverage <a href="http://z-wave.sigmadesigns.com/docs/Z-Wave_Frequency_Coverage.pdf" rel="nofollow">here</a>)</li>
<li>OSI Layers: Zigbee is based on the IEEE 802.15.4 spec for the PHY/MAC Layers, where Z-Wave is based on ITU-T G.9959 rPHY/MAC. The Z-Wave protocol stack was developed by Sigma Designs (and the standard is maintained by the Z-Wave Alliance), whereas Zigbee is developed by the Zigbee Alliance. Both networks have mesh capabilities</li>
<li>Ecosystem: Z-Wave claims to have "more than 1200 products" on the market, Zigbee claims a similar figure.</li>
<li>Documentation: Zigbee 3.0 spec is (for now) only available to Alliance members (the other Zigbee standards can be downloaded after entering some information (which can be bogus if you somehow want to stay anonymous)). Z-Wave requires you to buy the development kit or join the Alliance to access the spec.</li>
<li>Certification: Both protocols will require a certification to be fully certified and to be able to use the protocol logo.</li>
</ul>
<p>More generally, here is what you need to take into account when choosing a protocol:</p>
<ul>
<li>Frequency: The protocols you named all uses the 2.4GHz band, but a lot of other protocols uses the 868MHz, the 915MHz, the 433MHz band. The idea is the lower the frequency, the lower the energy use and greater the range. At least in theory. One also have to consider the physical size of the antenna (the lower the frequency, the bigger the antenna).</li>
<li>OSI Layers used: Some of the "protocols" out there only implements one layer of the OSI model, and you will need to have another layer implemented to be able to communicate</li>
<li>Ease of use of protocol: some protocols are very easy to use (for example EnOcean), some are more complicated (Zigbee, BLE). They differ in their capabilities and the freedom you have to send the information</li>
<li>Bitrate, frame size, useful payload: How much information you can send in a given timeframe. Depending on your use, you may need to send a lot of data, where different protocols may or may not behave well with this.</li>
<li>Collision and packet collision avoidance: how does the protocol manages the band use and prevent packet collision (two objects sending at the same time).</li>
<li>Ecosystem: Are there a lot of other devices out there using this protocol? What do you want to connect your object to (smartphone directly? Internet directly? Something else)? Do you need a local gateway to access internet?</li>
<li>Documentation availability: Is all the information necessary freely available? Is it locked (does it require membership to an association to be read)? Is it behind a paywall?</li>
<li>Last but not least: Cost of use. To be implemented and sold in your products, some protocols will require you to join the Alliance so you can certify your product compliance with the protocol. This often cost a lot of money (depends on the protocol).</li>
</ul>
<p>I thinks this pretty much sums it all, and should give good pointers to the right direction to look for more information.</p>
| 50 | What are the pros and cons of between Zigbee and Z-Wave communication protocols used in IoT device space? |
2015-01-21T01:19:55.740 | <p>I am thinking mainly in $\frac{kWh}{m^3}$ and $\frac{\$}{m^3}$.</p>
<p>In the past some decades a wide range of surprisingly efficient water desalination plants were built, mainly in desert regions (Middle East). These plants use reverse osmosis via a system of multiple pressered membranes. This solution seems to be very effective in the sense of energy usage.</p>
<p>But it is not enough. Comparing the desalination prices (coming mainly from energy costs) to the alternatives, a further 60-90% reduction is yet needed. Comparing them, what development potential is there in water desalination?</p>
<p>I think water desalination has probably a theoretical energy limit, which could be maybe calculated from entropy and binding energy formulas. How near are we to this theoretical limit?</p>
| |water-resources| | <p><strong>For reverse Osmosis</strong></p>
<p><a href="http://urila.tripod.com/desalination.htm" rel="nofollow noreferrer">This site gives the minimal required energy for sea water desalination by RO as 2.78 kJ/l (fresh water)</a>, this is if you consider only the reversible process. According to <a href="https://en.wikipedia.org/wiki/Reverse_osmosis#Desalination" rel="nofollow noreferrer">wikipedia, the best RO desalination plants operate at 3kWh/m³</a> which trasnlates to 10.8 kJ/l.</p>
<p>AFAIK, energy losses are pressure loss through the membrane (in addition to the osmotic pressure, a membrane introduces irreversible pressure losses), water pretreatment and energy (in the form of pressure) in the brine. Also a lot of water simply needs to be moved about, there's pre treatment steps etc.</p>
<p><a href="http://www.iwapublishing.com/sites/default/files/ebooks/IWA_GlobalTrendReport2016.pdf" rel="nofollow noreferrer">According to this IWA trend report</a>, two areas within the wide field of membranes where more research is done are better membranes in terms of pressure loss and fouling resistance (fouling directly influences pressure losses). Recentish developments in RO desalination like <a href="https://en.wikipedia.org/wiki/Forward_osmosis#Desalination" rel="nofollow noreferrer">forward osmosis</a> mostly benefit from better fouling resistance compared to RO.</p>
<p><strong>For thermal desalination</strong><br>
<em>crickets</em><br>
(will update when I find more info)</p>
| 54 | What development possibilities could yet exist in water desalination? |
2015-01-21T01:23:13.033 | <p>I have been investigating recapturing energy from house lighting using solar panels.</p>
<p>Are there solar panels specifically designed to capture indoor lighting with a higher efficiency?</p>
<p>If so, what are the differences between the outdoor and indoor solar panels designed for this purpose?</p>
<hr>
<p><strong>References</strong>: </p>
<ul>
<li><a href="http://solarcellcentral.com/limits_page.html" rel="nofollow noreferrer">Solar Efficiency Limits</a></li>
<li><a href="http://classroom.synonym.com/can-solar-batteries-charge-using-incandescent-light-7189.html" rel="nofollow noreferrer">Can Solar Batteries Charge Using Incandescent Light?</a></li>
<li><a href="https://www.linkedin.com/groups/Can-solar-panels-charge-from-96493.S.209515615" rel="nofollow noreferrer">Can solar panels charge from street lights, during the night? As it does charge from the sun during the day!</a></li>
</ul>
| |electrical-engineering|photovoltaics| | <p>I have this 150 Watt panel, 12 volt(said to be). In testing it with two 100 Watt (equal too regular 100 watt old kind) bulbs used for lighting the room. This one panel can light up two other LED bulbs from the two on the ceiling. WOW, I thought that was impossible. That you only get 10% from the panel. Not so...</p>
<p><a href="https://i.stack.imgur.com/0YCfX.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0YCfXm.jpg" alt="enter image description here"></a></p>
<p><a href="https://i.stack.imgur.com/unYNP.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/unYNPm.jpg" alt="enter image description here"></a></p>
<p>Two of these to power solar panel.
The LEDs look a little dimmer then normal. But the panel is only getting 11.61 Volts (about).</p>
<p>Plugged LEDs in a 12 volt power supply and there's a big difference in lighting strength.</p>
| 55 | Are there different solar panels for recapturing energy from household lighting? |
2015-01-21T02:29:22.187 | <p>I would like be able to measure LED brightness using a cost effective instrument.</p>
<p>What are the available instruments to measure LED brightness and respective cost effective techniques?</p>
<p>Background: I'm interested in converting what the customer thinks is acceptable brightness to actual LED brightness specification?.</p>
<hr>
<p><strong>Reference:</strong></p>
<ul>
<li><a href="https://electronics.stackexchange.com/questions/150511/how-can-i-measure-the-brightness-of-a-led">How can I measure the brightness of a LED?</a></li>
</ul>
| |electrical-engineering|optics| | <p>There are two aspects that you need to consider here: One is physical measurement of luminance, luminous flux, or illuminance (I'll come on to the differences in a moment), and the other is the much less clear-cut issue of human perception.</p>
<p>Let's deal with the physics first. There are three concepts to explain, which are all related:</p>
<p><strong>Luminous flux</strong> is the total amount of light emitted by a light source, in all directions. Its SI unit is lumens. This is difficult to measure without expensive equipment (an <a href="https://en.wikipedia.org/wiki/Integrating_sphere" rel="nofollow">integrating sphere</a>). Luminous flux is relevant because it gives a measure of the total amount of light that a source is putting into a room, and will typically be given in the specifications of a light source.</p>
<p><strong>Illuminance</strong> is the amount of light that falls on a surface of unit area. Its SI unit is lux, where 1 lux = 1 lm/m<sup>2</sup>. It is easy to measure with commonly available "light meters" and photodiodes. Illuminance is the most important value when asking, for example, "will enough light fall on this desk for somebody to work by". Building codes or standards often specify the illuminance required on floors, walls, or work surfaces, for specific tasks. This is a function of the overall design of lighting in the space rather than just of the light source - type of fixture, quantity and spacing, colour of the walls, etc., are all important - but marketing materials for some LED light sources in particular will claim "equivalent to xxx wattage incandescent" by comparing the illuminance on a small area of work surface from an omnidirectional incandescent lamp vs a narrow-beam LED, and imply that they are equivalent - despite them having radically different effects everywhere else in the room.</p>
<p><strong>Luminance</strong> is a measure of the amount of light being <em>given off</em> by a surface <em>in a specific direction</em>, per unit of area. If that surface is the surface of a light source - e.g. if you look at a light bulb - then this is related to how bright the source appears. This measurement is relevant because if a customer looks directly at a range of light sources to compare "how bright they are", it is differences in luminance that they will see. Luminance meters are much more expensive than illuminance meters.</p>
<p>So in summary,</p>
<ul>
<li>Luminous flux is the total amount of light given off by a source</li>
<li>Illuminance is how brightly a surface is lit</li>
<li>Luminance is (part of) how bright the light source appears to the eye</li>
</ul>
<p>Which one(s) you need to measure will depend on exactly what you are trying to achieve.</p>
<p><strong>Human perception</strong> is the tricky part here. There is more to a feeling of "brightness" than either of the measures above. For example, people tend to perceive light of a higher colour temperature as being brighter than light of a lower colour temperature - a common issue when comparing LEDs to other sources. People are also strongly influenced by contrast; we mostly see relative rather than absolute brightness, so a patch of light on a small area of workspace in an otherwise dark room may seem brighter than the same illuminance throughout the space, especially once the viewer's eyes have adapted to the dark. So <em>be very careful</em> if giving practical demonstrations, or you may find that the customer's perception does not match your calculations.
(it's also worth noting, of course, that for some purposes it's the perception that matters...)</p>
| 62 | How can I measure the brightness of a LED? |
2015-01-21T02:42:04.263 | <p>The debate of traffic circles (also called roundabouts or rotaries) versus traffic light intersections has been in progress for a while. Those in favor of traffic circles <a href="http://www.wsdot.wa.gov/Safety/roundabouts/benefits.htm" rel="noreferrer">say that</a>, among other things, that they are safer than traffic light intersections. This claim has been scientifically proven. On the other hand, traffic light intersections are more space-inefficient.</p>
<p>Even <a href="http://www.treehugger.com/cars/mythbusters-roundabouts-vs-4-way-stop-intersection-which-more-efficient-video.html" rel="noreferrer">Mythbusters</a> has joined the fun, testing the efficiency (which is one of the main arguments both sides seem to concern themselves with) of each method.</p>
<p>For comparison, here's a quick picture of a traffic circle:</p>
<p><img src="https://i.stack.imgur.com/n5wI9.jpg" alt="Traffic circle"></p>
<p>And of a four-way traffic light intersection:</p>
<p><img src="https://i.stack.imgur.com/QL3y9.jpg" alt="Intersection"></p>
<p>So, <strong>what are the pros and cons of a traffic circle versus a traffic light intersection?</strong></p>
| |traffic-light|highway-engineering| | <p>Roundabouts let traffic flow more smoothly and there is also less reliance on technology that can malfunction due to a series of factors (i.e. power outage, etc).</p>
| 63 | What are the pros and cons of a traffic circle versus a traffic light intersection? |
2015-01-21T03:15:17.020 | <p>I have a 2 mm thick steel plate which is 300 mm long and 30 mm wide, supported at either end. It supports a weight-bearing wheel that can roll along the plate. It currently supports the maximum weight that I expect it to support when the wheel is in the middle, but it flexes a little bit too much. Would making it wider help to support the weight and increase its stiffness, or do I need to make it thicker?</p>
<p>Also is there a way to calculate how the stiffness will change with the thickness (or width if that would affect it)?</p>
| |mechanical-engineering|steel|stiffness| | <p>As mentioned in other answers, what controls the deflection is the second moment of area, and the easiest way to increase it is increase the thickness. Doubling the thickness would increase the 16 fold the second moment of area and it would increase the weight by 100%</p>
<p>However below I am outlining, another way to improve the second moment of area is to change the cross-section, which is more efficient. I am presenting the base line example (30 mm X 2 mm), then I am giving two additional configurations which increase the weight by 2/3 (or 66%) and I am also presenting the increase in <strong>second moment of area which is significantly greater</strong>)</p>
<p>More specifically</p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th style="text-align: center;">Cross-section</th>
<th style="text-align: center;">Description</th>
<th style="text-align: center;"><span class="math-container">$I_{xx}$</span></th>
<th style="text-align: center;">Increase</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align: center;"><a href="https://i.stack.imgur.com/npwTym.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/npwTym.png" alt="enter image description here" /></a></td>
<td style="text-align: center;">Flat sheet</td>
<td style="text-align: center;"><span class="math-container">$$20 mm^4$$</span></td>
<td style="text-align: center;">-</td>
</tr>
<tr>
<td style="text-align: center;"><a href="https://i.stack.imgur.com/xMgfMm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xMgfMm.png" alt="enter image description here" /></a></td>
<td style="text-align: center;">H- section (adding 5mm flanges at either side)</td>
<td style="text-align: center;"><span class="math-container">$$593 mm^4$$</span></td>
<td style="text-align: center;">~ x30</td>
</tr>
<tr>
<td style="text-align: center;"><a href="https://i.stack.imgur.com/xboSWm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xboSWm.png" alt="enter image description here" /></a></td>
<td style="text-align: center;">C- section (adding 10mm flanges)</td>
<td style="text-align: center;"><span class="math-container">$$1217 mm^4$$</span></td>
<td style="text-align: center;">~60 times</td>
</tr>
</tbody>
</table>
</div>
<p>From the above three configurations you can see the vast difference the relatively small addition of flanges can make on the stiffness of the beam ( I won't go through the calculation because its in most textbooks - and the question has already an accepted answer).</p>
<p>As a final point, it is noteworthy that the configurations H and C have the weight, yet the C section will have half the deflection. (So its really a matter of <strong>intelligently using the cross-section</strong>).</p>
| 68 | How does width and thickness affect the stiffness of steel plate? |
2015-01-21T04:05:04.740 | <p>One of the valves on my 3 bar line just failed. Without it, our production has ground to a stop and we can't operate until we get a replacement (which we have already ordered). In order to make a deadline, I am trying to find a way to use a (properly rated for pressure) DN 16 flanged valve to fit with the existing DN 10 piping.</p>
<p>Is there any way to (safely) connect these two flanges together temporarily?</p>
| |steam|piping| | <p>If you have access to a lathe you should be able to quickly machine a flanged reducer out of <a href="http://en.wikipedia.org/wiki/Nylon_6" rel="nofollow">Nylon 6</a>. Not sure what temperature you're operating at but make it a bit thick and it will easily be able to handle 3 bar.</p>
| 74 | How can I fit a DN16 flange to a DN10 flanged pipe? |
2015-01-21T09:16:09.513 | <p>For one-off, bespoke electronic products, or ventures on low budgets, what is the cheapest way of ensuring the product conforms to EMI/EMC requirements of CE marking? This can be at design time and / or throughout the manufacturing process.</p>
<p>Examples:</p>
<ul>
<li><p>A small-run, electronic microprocessor based controller (using a CE-Marked zero-cross solid state relay) intended to switch on or off a 2 kW load such as a consumer grade 240 V, 2 kW blower heater. </p></li>
<li><p>A one-off CNC machine that utilizes CE-marked, low voltage motors and servos, and is controlled by a microcontroller and miscellaneous circuitry</p></li>
<li><p>A new venture into producing LED lighting with controllers - typically switching 10 W loads; using PWM to control brightness; controlled by a micro-controller; using SMD LEDs, and a CE-marked switched-mode 24 V, 3 A PSU.</p></li>
</ul>
<p>Obviously there must be logical principles which one can apply to allow one to bypass other expensive conforming methods. For example, connecting two CE-marked appliances to each other where they are intended to do so would most likely not need any further testing.</p>
<p>The problem is that for more complicated scenarios getting products approved by external companies that provide CE-marking assessments can be incredibly expensive. Also, I have not found any particularly affordable equipment/methods to carry out EMI/EMC testing - and not being an expert in EMI/EMC means I would not be confident about using such equipment effectively. </p>
| |electrical-engineering|emc| | <p>First of all, the main documents you need to know:</p>
<ul>
<li>DIRECTIVE 2004/108/EC relating to electromagnetic compatibility:
<a href="http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2004:390:0024:0037:en:PDF" rel="nofollow">http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2004:390:0024:0037:en:PDF</a> </li>
<li>DIRECTIVE 2006/95/EC on low voltage devices: <a href="http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2006:374:0010:0019:en:PDF" rel="nofollow">http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2006:374:0010:0019:en:PDF</a> (and the version entering into force in April 2016: <a href="http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32014L0035&from=EN" rel="nofollow">http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32014L0035&from=EN</a> )</li>
</ul>
<p>Those documents form the basis of your work. Also, they define if your device is subject to the directive or not. Depending on the case, you could also be subject to the R&TTE Directive and RoHs Directive).</p>
<p>The Annex I of both documents informs us on the essentials requirements to meet. Mainly, it says that your product must be safe for humans, pets and property and that it should not be impacted by the electromagnetic emissions of other nearby devices nor it should impact them with its own emissions.</p>
<p>What's really interesting lies in the Annex II of the first document and in Annex IV of the second document. These parts defines the conformity assessment procedure you'll have to follow.</p>
<p>There are 8 procedures (called modules) of conformity assessment: </p>
<ul>
<li>internal production control (module A);</li>
<li>CE type-examination (module B);</li>
<li>conformity to type (module C);</li>
<li>production quality assurance (module D);</li>
<li>product quality assurance (module E);</li>
<li>product verification (module F);</li>
<li>unit verification (module G);</li>
<li>full quality assurance (module H).</li>
</ul>
<p>In our case, the one to follow is called Internal Production Control. Basically, what you have to do is write the technical documentation. This document should be made of those parts:</p>
<blockquote>
<ul>
<li>a general description of the electrical equipment,</li>
<li>conceptual design and manufacturing drawings and schemes of components, sub-assemblies, circuits, etc.,</li>
<li>descriptions and explanations necessary for the understanding of said drawings and schemes and the operation of the electrical equipment,</li>
<li>a list of the standards applied in full or in part, and descriptions of the solutions adopted to satisfy the safety aspects of this Directive where standards have not been applied,</li>
<li>results of design calculations made, examinations carried out, etc.,</li>
<li>test reports.</li>
</ul>
</blockquote>
<p>We learn what we have to do in order to respect the directives. The main constraint here will be showing that you have indeed accessed the harmonised standards. This can cost quite a hefty sum of money. However Afnor (the French certification organism) provides subscription to their database, which can be cost effective compared to buying individual standards). Your local certification organism should have a similar offer.</p>
<p>The harmonised standards list to be followed is available here:</p>
<ul>
<li>for the EMC directive: <a href="http://ec.europa.eu/enterprise/policies/european-standards/harmonised-standards/electromagnetic-compatibility/index_en.htm" rel="nofollow">http://ec.europa.eu/enterprise/policies/european-standards/harmonised-standards/electromagnetic-compatibility/index_en.htm</a></li>
<li>for the LVD directive: <a href="http://ec.europa.eu/enterprise/policies/european-standards/harmonised-standards/low-voltage/index_en.htm" rel="nofollow">http://ec.europa.eu/enterprise/policies/european-standards/harmonised-standards/low-voltage/index_en.htm</a></li>
</ul>
<p>This document will then have to be kept for up to ten years after the last product manufactured was sold.</p>
<p>Doing this will allow you to affix the CE marking and provide a certificate of conformity to your customer.</p>
<p>Even if this is a lighter method for CE certification, it still involves a bit of work. However, this is something that should be done anyway (if just to make sure your product will perform well and as expected in all conditions).</p>
<p><strong>EDIT :</strong>
There is a European Union website that shows all the currents harmonized standards and all the different Directives your product may be subject to:
<a href="http://www.newapproach.org/Directives/" rel="nofollow">http://www.newapproach.org/Directives/</a></p>
<p><strong>TL;DR:</strong> At least two directives applies, one related to EMC, the other to Low Voltage Devices. The procedure to follow is called Internal Production Control. To comply, one has to draw up a technical documentation whose main part is the harmonised standard list followed is added.</p>
| 86 | CE Marking on low budgets: conforming to EMI / EMC requirements in ELV and mains circuits |
2015-01-21T10:09:38.163 | <p>There has recently been a series of <a href="http://www.theguardian.com/business/2015/jan/14/cheesegrater-leadenhall-building-bolt-city-london" rel="nofollow">high profile bolt failures</a> on <a href="https://en.wikipedia.org/wiki/122_Leadenhall_Street" rel="nofollow">'The Cheesegrater'</a> in London, which have been attributed to <a href="https://en.wikipedia.org/wiki/Hydrogen_embrittlement" rel="nofollow">hydrogen embrittlement</a>.</p>
<p>What can engineers do to ensure that hydrogen embrittlement doesn't effect their structures?</p>
| |mechanical-engineering|civil-engineering|structures|metallurgy| | <p>I know it's an old thread, but I'm putting my solution on here because I've just come across this issue.</p>
<p>Understanding the causes could help you deal better with hydrogen embrittlement.</p>
<p><strong>Overstressing:</strong>
Primarily, extra stress put on material results in the form of blisters, cracks, hydride formation on the structures. In this case, the atomic hydrogen present in the atmosphere reacts with the material. This could lead to hydrogen embrittlement.</p>
<p><strong>Electroplating Steel:</strong>
Hydrogen embrittlement also occurs while plating and pickling due to the exposure of the parts to hydrogen. The only solution to prevent this baking the part immediately after plating. A typical temperature is to bake at 375ºF for 4 hours within 1 hour after plating.</p>
<p><strong>Tensile strength > 145 KSI:</strong>
High-strength steels, titanium alloys and aluminium alloys are most vulnerable to hydrogen embrittlement.</p>
<p>The following resources may be useful </p>
<ul>
<li><a href="https://www.imetllc.com/training-article/hydrogen-embrittlement-steel/" rel="nofollow noreferrer">Hydrogen Embrittlement of Steel</a></li>
<li><a href="https://itafasteners.com/hydrogen-embrittlement-types-causes-and-solutions.php" rel="nofollow noreferrer">Hydrogen Embrittlement - Causes and Solutions</a></li>
</ul>
| 87 | How can we prevent/detect hydrogen embrittlement? |
2015-01-21T10:24:26.413 | <p>When designing a steel beam, the resistance to buckling is related to M<sub>cr</sub>; the elastic critical moment for lateral-torsional buckling.</p>
<p>However the Eurocodes give no advice about how to calculate this parameter.</p>
<p>How would you calculate it?</p>
| |civil-engineering|steel|beam|eurocodes| | <p>The answer by Frédéric Bourgeon is great. You may also find useful this <a href="https://www.eurocodeapplied.com/design/en1993/elastic-critical-moment" rel="nofollow noreferrer">free online calculation tool for elastic critical moment</a> in eurocodeapplied.com. It is based on the aforementioned NCCI but it is much simpler to apply as compared to LTBeam, at least for the typical cases encountered in design practice.</p>
<p>It covers typical I- or H- profiles (IPE, HEA, HEB, HEM), three end support conditions (both ends simply supported, both ends fixed, one end simply supported and one end fixed), and three loading arrangements (linear bending moment, parabolic bending moment, triangular bending moment).</p>
<p>Hope this helps.</p>
| 88 | How do you calculate Mcr (critical buckling moment) |
2015-01-21T10:34:49.407 | <p>It is common to lift bridges to replace bearings, etc.</p>
<p>In an ideal world the lifting capacity required of the jacks would be the self-weight of the bridge divided by the number of jacks (+ allowances for wind/snow, etc.).</p>
<p>From my (limited) experience, however, bridges begin to 'stick' to their bearings, and an additional allowance for over-coming this has to be provided.</p>
<p>Does anyone have any guidance about how to determine this figure?</p>
| |civil-engineering|structures|bridges|maintenance|friction| | <p>So there's an incorrect assumption underlying your question.</p>
<blockquote>
<p>In an ideal world the lifting capacity required of the jacks would be the self-weight of the bridge divided by the number of jacks (+ allowances for wind/snow, etc.).</p>
</blockquote>
<p>And the assumption there is that the lifting capacity is equivalent only to the weight of the bridge. The problem is that if anything goes wrong, you're likely to see a catastrophic failure of some sort which could lead to irreparable damage.</p>
<p>Real world lifts don't operate in that "ideal" manner, and instead rely upon a <a href="http://en.wikipedia.org/wiki/Factor_of_safety">safety factor</a> in order to make sure that the lifted weight is well within the limitations of the equipment. And in some cases, the safe working limit (SWL) may be derated further from the working lift limit (WLL) if there are any extenuating circumstances such as worn equipment or hazardous weather.</p>
<p>So the ideal lift capacity is one that is significantly larger than load to be lifted. The actual lift capacity used is tempered by the fact that you generally pay for that lift capacity whether you need it or not.</p>
<p>According to the Wikipedia article on <a href="http://en.wikipedia.org/wiki/Factor_of_safety">safety factors</a>, a factor of 2 is common with building materials and 3 is common for automobiles. You need to weigh the risk to human health or safety within the lift you're considering and use an appropriate safety factor. A conservative approach would be to use a higher safety factor of 3, so you need at least 3 times the bridge weight for the lifting capacity.</p>
<hr>
<p>Assuming you're staying within the SWL and WLL of the lift equipment you're using, that still doesn't necessarily account for the binding forces caused by corrosion between the bridge and the bearings supporting it. Static friction can also come into play if the bridge itself has to be slid out of the supporting structure.</p>
<p>Unfortunately, it's hard to determine what that binding force is going to add up to without a lot more detail. At a minimum, you would need to know the materials involved and the cross sectional area of contact between them. You would also want to approximate how long they've been exposed to the elements and what sort of conditions the elements have brought - such as salt water exposure vs mountain air.<br>
<sub>This is where I'm going to wave my hands in an airy fashion and not attempt to swag the binding force created by corrosion.</sub> </p>
<p><a href="http://en.wikipedia.org/wiki/Friction">Static frictional force</a> can be guessed at a little more easily than the corrosion binding. $\mu_{static}$ ranges from 0.6 to 0.8 for various materials in contact with steel. And while the general formula for calculating the <code>Force</code> from friction is $F_{static} = \mu_{static} * F_{normal}$, that equation also assumes horizontal movement. As you're likely lifting vertically, not sliding horizontally, the static frictional forces will be less since the equivalent $F_{normal}$ for vertical motion will be less.</p>
<p>Based upon that, I'd use a conservative guess of $\frac 23$ or $\frac 34$ of the weight of the bridge to estimate the static friction forces involved in the lift. Experience and the particulars of the lift will guide you in adjusting that guess up or down.</p>
<p>So depending upon your safety factor, it could very well be that the SWL of the equipment will provide sufficient lift to overcome any static friction or binding caused by corrosion. Or it could be that you need to increase the lift capacity requirements to overcome that effect. And it's worth pointing out that equipment can exceed those limits, so a lower safety factor may be "good enough" to get past the effects of static friction at the start of the lift.</p>
| 89 | Additional jacking force to overcome stiction/dry friction |
2015-01-21T22:09:23.393 | <p>I've looked around for different types of <a href="http://en.wikipedia.org/wiki/Concrete_pump" rel="nofollow noreferrer">concrete pump</a>s and simply don't know if there are many different types, what their mechanisms are and which ones are the most predominant ones.</p>
<p>What I've found:</p>
<ul>
<li>This <a href="https://www.youtube.com/watch?v=dgmRN3BDE4E" rel="nofollow noreferrer">video</a> explains how a pump with an S-shaped tube operates.</li>
<li>The <a href="http://en.wikipedia.org/wiki/Concrete_pump" rel="nofollow noreferrer">relevant wikipedia page</a> explains another type, which does differ in the way how the concrete is released</li>
</ul>
<p><img src="https://i.stack.imgur.com/l9sDNm.jpg" alt="An example of a concrete pump"> (<em>a concrete pump</em>)</p>
| |mechanical-engineering|pumps| | <p>I've found <a href="http://www.ritchiewiki.com/wiki/index.php/Concrete_Pump" rel="nofollow">a fairly comprehensive source</a>. There are five main types:</p>
<ul>
<li><strong>Mechanical pumps:</strong> A piston and a rotary valve are used the push the concrete through.</li>
<li><strong>Hydraulic pumps:</strong> Hydraulics are used to pump the concrete.</li>
<li><strong>Schwing pumps:</strong> Two cylinders are used: One to receive concrete from the input hopper and one to release it into the output pipe. The two cylinders are connected by a flat gate valve. Apparently, Fredrich Schwing started <a href="http://schwing.com/about-us/" rel="nofollow">a company</a> to manufacture concrete pumps. Unfortunately, their website does not have much information.</li>
<li><strong>Thomsen pumps:</strong> Similarly to the Schwing pump, two cylinders are used with internal pistons. The difference is that a flapper valve (a type of <a href="https://en.wikipedia.org/wiki/Check_valve" rel="nofollow">check valve</a>) is used.</li>
<li><strong>Squeeze-crete pumps:</strong> A rotating cylinder is used to squeeze the concrete through.</li>
</ul>
| 120 | What are the predominant types of concrete pumps and what are their working mechanisms? |
2015-01-21T22:18:05.123 | <p>How are spindle speeds calculated for a drill bit?</p>
<p>I've seen dozens of charts that highlight the rpm that should be used for specific drill bit types, bit diameter, and material. However, what if my chart doesn't have the particular type of material or bit that I am using? I'd also like to have some intuition to know if the chart looks right or wrong.</p>
<p>Upon some quick research, it appears that the "cutting speed" is what is ultimately needed for a particular material. I assume the cutting speed for each material must be looked up? Is there a standard or "go-to" place to find these? Then, information about the drill bit can be used to determine spindle speed. Again, what if I'm using a big hole saw or circle cutter, and it's not listed? How do I model the bit to use the cutting speed to determine rpm (for a given material of course)?</p>
<p>I'd also like to know to calculate feed speeds for a drill or mill, but there are presumably more variables. It is probably better answered in another question.</p>
| |machining|mechanical-engineering| | <p>Rotational speed is a by-product of the desired result and is generally a compromise to optimise other factors.</p>
<p>To give a simple example of why this is, consider that a twist drill cuts across its entire frontal surface. While the rotational speed of the bit is constant across the face, the linear speed varies from 0 (at the centre) through angular rotation multiplied by circumference (at the periphery). This continuum of speeds cannot all be correct, and some other variable must determine the selection.</p>
<p>The values provided by tables and formulae are necessarily approximations of the true state and this is why so often guidance is given to try a setting and then adjust it to improve the sound, for example, of the cut.</p>
<p>In 1906, F. W. Taylor presented <a href="https://ir.library.oregonstate.edu/concern/technical_reports/3x816s35t" rel="nofollow noreferrer">On the art of cutting metals</a> as the first rigorous explanation for setting up machine tools. This description of the process used to determine the principles for selecting cutting speeds remains an empirical marvel in engineering.</p>
<p>So far, this has ignored your second important variable, that of how fast to feed the drill into the material to complement the rotational cutting motion.</p>
<p>In many cases, rather than calculating the speed, select the value the tool manufacturer recommends for the application closest to what you require. This takes advantage of the century of experimentation and design embodied in the humble twist drill.</p>
| 121 | Determining Drill Speed |
2015-01-21T23:55:26.270 | <p>What is an effective method to re-engineer a current product to reduce engineering bill of material (EBOM) cost?</p>
<p>This is a continues improvement project for a current product in electronics space. Attached is an example of a PCB board. The product consists of many similar boards. The BOM includes active and passive electronic components such as capacitors, resistor, microprocessors, transistors, PCB boards, connectors etc.</p>
<p>What is an effective method to tackle this engineering cost reduction problem? </p>
<p><img src="https://i.stack.imgur.com/UCzai.jpg" alt="Beaglebone black"></p>
| |electrical-engineering|cost-engineering| | <p>In addition to the above,</p>
<ul>
<li>Consider arrayed components instead of discretes. i.e., a 4-unit 1k SMT 1206 for an IC's pull-ups instead of 4pcs 1k 0805. Saves board space and assembly time also.</li>
<li>Weigh the expense of going double-sided with any possible reduction in PCB footprint.</li>
<li>If out-sourcing PCB fab, component assembly, final assembly... call around. Discuss larger orders.</li>
<li>If in-sourcing the previous, look at tools, jigs, fixtures, supplies, binning, lighting, proper seating and work area... anything that can help improve speed and efficiency.</li>
</ul>
| 126 | How to reduce Engineering Bill of Material Cost for electronics products? |
2015-01-22T10:12:39.987 | <p>Given a tolerance within which your workpiece should be manufactured, say some length should be $10\pm1$mm. If you determine that your uncertainty in measuring this length is $0.2$mm (at 95%). How should a measurement of $9.1$mm be treated?</p>
<p>Clearly there is a significant probability that this value will actually be outside of tolerance. Do you need to decrease you tolerance range based on the uncertainty in your measurement?</p>
| |tolerance|measurements|statistics| | <p>Measurement variations are very common and should be taken in to consideration when engineering systems. In most cases high precision equipment is available but might be cost prohibitive to justify purchasing for the project. Therefore, the goal of the engineer is to design the system to account for measurement variation. In this case the min and max limits are 9mm and 11mm with 10mm being nominal. There are few strategies that can be used. They are</p>
<ul>
<li><p>Define an upper and lower control measurement limit to account for maximum 0.2 mm variation. Therefore the LCL and UCL would be 9.2mm and 10.8mm. This will guarantee the work pieces are always $10\pm1$mm</p></li>
<li><p>Another would be to perform a gauge R&R study to understand true measurement variation and include this data in the design. Make sure calibration is included in the preventive maintenance schedule.</p></li>
<li><p>Using a Design for Six Sigma (DFSS) might be a better approach. Hopefully, the design is 6 sigma capable, after accounting for 0.2mm worst-case measurement variation. If so measurement variation might be insignificant.</p></li>
</ul>
<p>In most case a combination of the above plus other strategies will be required to achieve a good design</p>
<hr>
<p><strong>References:</strong> </p>
<ul>
<li><a href="http://www.qualitydigest.com/inside/metrology-article/basics-gauge-rr.html" rel="nofollow">Basics of Gauge R&R</a></li>
<li><a href="http://en.wikipedia.org/wiki/Design_for_Six_Sigma" rel="nofollow">Design for Six Sigma</a></li>
</ul>
| 137 | How does measurement uncertainty combine with tolerances? |
2015-01-22T17:56:18.063 | <p>As I heard, the life expectancy of photovoltaic cells is normally some decades, as so. This is very long, regarding their high cost, it has significant (negative) effect on the total costs.</p>
<p>Why do they age? I can see the only possibility which could damage their atomic structure, and this is the very high energetic ultraviolet (or even soft röntgen) photon spectrum of the sun (which is only a small part of its power).</p>
<p>And, I think, these photons could be maybe easily filtered out by a transparent plastic layer over the silicon.</p>
<p>Thus, why do the solar cells age, and what happens in them on the atomic level?</p>
| |photovoltaics| | <p>For a different angle, my experience with solar cells is space-based, rather than terrestrial. The biggest drivers for degradation of space based solar cells are particle radiation and orbital debris impacts. The particle radiation degrades by wearing down the junction, and the orbital debris degrades primarily by increasing the internal series resistance of the cells. Because there is no air to cool the cells, reverse bias heating can also be a serious problem if a solar array gets partially shadowed by other structure.</p>
| 150 | Do solar cells age? Why, and how? |
2015-01-22T18:25:15.817 | <p>I think, it could be a reactor utilizing californium-242 (or, at least, weapon-grade U-235) cooled and moderated by heavy water.</p>
<p>Essentially, it were similar to an atomic bomb, but - of course - it would be optimized for stay around the equilibrial state.</p>
<p>The result were probably a very strong neutron source.</p>
<p>I think, it could be used for various things, mainly in the space applications.</p>
<p>Does any cost/size estimations about this ever created?</p>
| |nuclear-technology| | <p>An almost-critical sphere of fissionable material would do. Add some neutron reflecting material that can be rotated to increase or decrease the neutron reflection. Or you could have two halves of a critical sphere that are moved closer together or further apart to control the reactivity. </p>
<p>A moderator is not necessary if the reactor operates on <a href="https://en.wikipedia.org/wiki/Fast-neutron_reactor" rel="nofollow noreferrer">fast neutrons</a>, thermal (slow) neutron reactors are just easier/cheaper to operate and more proliferation resistant.</p>
<p>If you only operate it at low power levels you would not need a lot of cooling (though the reactor would probably not be very useful). </p>
<p>The largest amount of mass would be the shielding. So as long as you don't care about that, you're fine (and in your question you don't mention it). There has been research into nuclear powered <a href="https://en.wikipedia.org/wiki/Nuclear_propulsion#Cars" rel="nofollow noreferrer">cars</a>, <a href="https://en.wikipedia.org/wiki/Nuclear-powered_aircraft" rel="nofollow noreferrer">aircraft</a>, and trains, but the size and weight of the shielding was the main technical problem for such applications.</p>
<p>The output of nuclear reactions is heat, if you want to convert it to electricity or another more useful form of energy, you will need some kind of heat engine, which (depending on the temperature) needs to dump 75% of the heat input as low temperature waste heat. So you will need some kind of heat sink for that. But again that is not part of the question.</p>
<p><em>edit:</em> To be more specific, the output of a nuclear reactor is some heat in the reaction products, and most of the energy in the neutron radiation. Reactors usually also capture those neutrons in order to convert their energy to heat, so that is anoher reason why you might want to use neutron reflectors. The most difficult to shield radiation are the gamma rays, which typically require several meters of concrete or a thick slab of lead to block.</p>
| 151 | Smallest possible controlled chain reaction-based nuclear fission reactor? |
2015-01-22T20:40:52.330 | <p>I punch metals before I start a bit in them, but with small-diameter bits I am finding that they wander inside the material. E.g., drilling a 1/8" hole in 1/2" of aluminum the (titanium-coated HSS) bit will come out the other end a measurable distance to a side (orthogonal to the drill feed direction) from where it entered. </p>
<p>To be clear, the bit starts where I want it to since I punched the metal. But the bit appears to be flexing within the material, causing the resulting hole to be angled. </p>
<p>How can I prevent this? Is there something about the material that I need to understand in order to prevent the bit from wandering and causing the hole to be skewed? I'd like to know the mechanics behind the materials that is causing these skewed holes.</p>
| |materials|machining| | <p>It's common to start with a shorter, stiffer tool such as a center drill or a spotting drill. In addition, using the shortest drill bit that drills the hole you need will increase stiffness. Because of the flutes in a drill, the stiffness goes down geometrically as the length increases. The other variable you have control over is how you are holding the tool. A collet will keep the tool in better alignment than a chuck.</p>
<p>If you're doing this often, it's worth making sure you have a good grasp of feeds and speeds, and that you're using the best type of drill for the job. Both of those could have some affect on hole perpendicularity.</p>
| 160 | How do I keep my bit from wandering on a drill press? |
2015-01-22T20:59:09.643 | <p>I want to design a small wind turbine that can be easily carried out of the classroom into the sports field in relatively low-wind conditions (5-15km/h) that can power a small ultra-bright 5V LED - just enough to show that it works.</p>
<p>How do I calculate the power I can get from different blade diameters in these wind conditions, and the power needed to drive a small DC motor/generator enough to light the LED?</p>
| |electrical-engineering|renewable-energy|power-engineering|wind-power| | <p>There is a useful calculator at <a href="https://rechneronline.de/wind-power/" rel="nofollow noreferrer">https://rechneronline.de/wind-power/</a></p>
| 162 | How to correctly size the blades for a small educational wind turbine? |
2015-01-22T23:16:05.397 | <p><a href="https://en.wikipedia.org/wiki/Pontoon_bridge">Pontoon bridges</a> differ from traditional bridges in that they are supported not by structures anchored to the floor of the body being spanned but by floating pontoons that are connected by a more rigid structure that supports a roadway. They're often used by militaries to provide a temporary crossing point, but they're also <a href="https://en.wikipedia.org/wiki/List_of_pontoon_bridges">used for permanent civilian crossings</a>.</p>
<p>I would assume that they make it easier to cross larger bodies of water because there is less structure to be secured below the surface. In areas of deep water, support structures can become unfeasibly large. These larger spans could, though, make the pontoon bridges susceptible to damage from strong winds and currents. Are there plans to use pontoon bridges to cross long distances?</p>
| |civil-engineering|bridges| | <p>Yes, and they have been used successfully in some major applications. One of the example in your list is <a href="https://en.wikipedia.org/wiki/Evergreen_Point_Floating_Bridge">Evergreen Point Floating Bridge</a> "Its 2,310 meters (7,580 ft) floating section is the longest floating bridge in the world.". It carries 4 lanes of traffic on SR 520.</p>
<p>Having lived in the area and used the bridge, I can recall a few times that bridge has been closed, or had restricted traffic, I currently live in the Pittsburgh PA area where tunnels on major interstates are routinely closed. There is not a significantly different traffic impact when compare a pontoon bridge to a tunnel. Different sources for the impact, but overall driver impact is similar. In my experience pontoon bridges are a viable solution in some cases. I think probably the major limiter is tidal. A pontoon bridge subject to tidal (ocean) impacts or seasonal variation (river), are going to have significantly higher stress issues. </p>
| 165 | Are pontoon bridges being considered to extend bridge span? |
2015-01-22T23:16:44.680 | <p>By "modern computers," I mean electronic programmable computers such as those that were developed about the middle of the 20th century.</p>
<p>My understanding is that early computers such as Charles Babbage's "calculating machines" worked mainly on mechanical principles, like a modern "abacus". Apparently later machines were more electronically based. I'm talking about computers that evolved from using vacuum tubes, to transistors to integrated circuits, and ultimately silicon chips.</p>
<p>What engineering advances enabled the above transition to occur from physical means (e.g. vacuum tubes) to electronic means (e.g. transistors) when it did (middle 20th century)?</p>
| |computer|electrical-engineering|engineering-history| | <p>HDE 226868 already mentioned the transistor in detail, so I will add many, many technological advances and theoretical backgrounds for the evolution of the computer.</p>
<ul>
<li><p><strong>Metallurgy</strong><br>
For the mass-market you need cheap and reliable methods to produce the desired electronic elements. For mass production you need integrated circuits and that means, especially if you go down to small dimensions, you need homogenous high-quality base material.
Only with high-grade silicon wafers this was possible and while the <a href="https://en.wikipedia.org/wiki/Czochralski_process" rel="noreferrer">Czochralski process</a> from 1916 is cheap it did not give such good results for silicon like <a href="https://en.wikipedia.org/wiki/Zone_melting" rel="noreferrer">zone melting</a> which was invented 1950/51.</p></li>
<li><p><strong>Boolsche Algebra und Cybernetics</strong>.
<a href="https://en.wikipedia.org/wiki/Boolean_algebra" rel="noreferrer">The logical algebra developed by Boole</a> allowed to build computers on binary states. If you ever had the pleasure to try using decimal systems on an electronic machine (BCD), you see how unbelievably easier it is to implement logical and numerical functions. With progressing complexity, it was also necessary to invent the necessary background for developing and controlling complex electronic system, so cybernetics as a new study began to develop.
It really cannot be underestimated what Babbage and later von Neumann did: Instead of building specialized models for tasks they invented the idea of a programmable machine: Provide simple building blocks of commands and express your solution with this simple commands instead of rewiring the machine (Perhaps there are still some people here who literally "reprogrammed" computers by rewiring parts !). Allow conditional execution. Treat data and program as unit. The general-purpose computer was born.</p></li>
<li><p><strong>Photography and Chemistry</strong>
Only photography was able to provide people to allow scaling down developed solutions cheaply, robustly and efficiently. Without photo lithography and the development of chemicals allowing the etching of microscopic structures, computers could be small, but they would be extremely expensive.</p></li>
</ul>
| 166 | What engineering advances paved the way for "modern" computers? |
2015-01-23T01:27:14.347 | <p>I understand that you can have a device with angular measurements for rotation and elevation, and use trigonometry to calculate the distances... but only if you have some distances to start with. How did they accurately measure the first straight-line distance over any distance significantly long enough to give usable angles? And wouldn't the error in other distances calculated from this very quickly accumulate out of control?</p>
| |engineering-history|surveying| | <p>The device for taking horizontal and vertical angles that you mention is called a theodolite. Theodolites only started being phased out as the main surveying tool in the 1980s when total stations where introduced. Below is a Soviet theodolite from 1958, (ex Wikipedia).</p>
<p><img src="https://i.stack.imgur.com/V1KXM.jpg" alt="enter image description here"></p>
<p>Theodolites were analogue devices and the angles measured had to be written in a notebook. Total stations were electronic devices, essentially electronic theodolites, with electronic distance measuring devices, based on infrared signals. These devices could be connected to a portable electronic memory unit with a keypad to store the measurements. The surveyor still had to manually enter a point identifier for each reading, but didn't have to enter the measured angles.</p>
<p>When starting a survey, a reference marker from the nation system of surveying markers closest to the surveying region was chosen as this had a known/established northing, easting and elevation. A picture of US survey marker follows (from Wikipedia).</p>
<p><img src="https://i.stack.imgur.com/8c4xc.jpg" alt="enter image description here"></p>
<p>The theodolite would be set up and the first reading would be to the known marker peg to establish the baseline for the survey.</p>
<p>For very accurate surveys a surveying target, on tripod, was placed over the survey marker; either a plate with a cross on it or a short pointed rod with the point upwards. A similar target would then be placed on a temporary marker and the horizontal angle between the two targets measured. The vertical angle from theodolite's horizontal plane (in the eye piece) to the first target would be measured as would the vertical angle to the second target.</p>
<p>Each theodolite has specifically marker dot on it, at eye piece (telescope) height. This is the reference marker for the theodolite from which lateral distances are measured. A measuring tape was place against the dot on the theodolite and the other end of the tape was place at the centre of each target cross or the tips of each pointed target rod, to measure the slope distances. The measuring tape had to have a certain tension applied and the readings would be recorded. Later, in the office, the measured slope distances would be corrected for tape sag. Additionally, the heights of the theodolite and the two targets, above the ground, would be measured with a tape measure.</p>
<p>Having done all of that, another temporary marker would be established, the theodolite moved between the last two pegs and the process repeated.</p>
<p>For each set up, the heights of the theodolite and targets was needed as were the slope distances, vertical angles and horizontal angle. Using trigonometry on all this data one could determine the co-ordinates and elevation of each peg.</p>
<p>Another method used of measuring was called stadia. This used a theodolite but instead of a cross targets or pointed rod targets being used to sight to at each of the survey pegs, surveying rods were used. See the picture below from <a href="http://www.tigersupplies.com" rel="noreferrer">http://www.tigersupplies.com</a></p>
<p><img src="https://i.stack.imgur.com/ohAUd.jpg" alt="enter image description here"></p>
<p>The surveying rod would be placed on each peg and three height reading were taken from the surveying rod: the top cross hair, the central (main) cross hairs and the bottom cross. See the picture below.</p>
<p><img src="https://i.stack.imgur.com/1PAlf.png" alt="enter image description here"></p>
<p>The reading from the central cross hairs gives the height for the elevation. The difference between the upper and lower cross hair readings multiplied by an optical constant for the optics of the theodolite gave the distance between the surveying rod and the theodolite. Except for some Japanese theodolites, the optical constant was 100.</p>
<p>In the picture above, the cross hair readings are 1.500, 1.422 and 1.344.</p>
<p>Irrespective of which method was used. To make adjustments for surveying errors, a closed traverse was done whereby after everything that needed to be surveyed was measured, the last reading was back to the first peg surveyed. If the co-ordinates in 3D matched there were no errors. If they didn't each of the readings would need to be adjusted to close the traverse with "no errors"</p>
<p>To minimize errors, the shorter the lateral distances the better, as there was less tape sag. For measurements that required high levels of accuracy, such as when assembling large equipment in hot climates the work would be done during the early mornings to minimize or eliminate heat shimmer. </p>
| 170 | How was surveying for maps done before lasers? |
2015-01-23T13:38:05.777 | <p>The Eurocodes gives the following equation for estimating a "simply supported bridge subject to bending only"*:</p>
<p>$$n_0 = \frac{17.75}{\sqrt{\delta_0}}$$</p>
<p>Where</p>
<ul>
<li>$n_0$ is the natural frequency <em>in hertz</em></li>
<li>$\delta_0$ is the deflection at mid-span under permanent actions <em>in mm</em></li>
</ul>
<p>The equation is seemly plucked from thin air, and there is no explanation as to where the constant 17.75 comes from. As an engineer I'm loath to use a formula I don't understand, but more than that it would be helpful to learn the fundamentals behind it so that I can see if it can be altered to work with other support conditions.</p>
<p>Can anyone provide a derivation / fundamental origin to this relationship?</p>
<p><em>*Full reference is: EN 1991-2:2003 6.4.4 [Note 8] (Equation 6.3), if that helps.</em></p>
| |bridges|dynamics|eurocodes| | <p>There is some more information on this in Ladislav Fryba's book "Dynamics of Railway Bridges" (1996). If you read chapter 4, you will see formula 4.53 on page 92:</p>
<p>$$f_1 = 17.753 v_{st}^{-1/2}$$</p>
<p>With $f_1$ being the first natural frequency in Hertz and $v_{st}$ the midspan deflection in mm. This is exacly the formula you are asking about.</p>
<p>This equation follows from the formula for the midspan deflection of a simply supported beam loaded by a uniformly distributed load <em>μg</em></p>
<p>$$v_{st} = {5 \above 1pt 384} {\mu gl^4 \above 1pt EI} $$</p>
<p>which is substituted in</p>
<p>$$ f_j = {\lambda_j^4 \above 1pt l^4} ({EI \above 1pt \mu})^{1/2} $$</p>
<p>It yields $$ \lambda_1 = \pi $$</p>
<p>Substituting those equations into each other using g = 9.81 m/s^2 gives</p>
<p>$$ f_1 = {\pi \above 1pt 2} ({5 \above 1pt 384} g)^{1/2} v_{st}^{-1/2}$$</p>
<p>The numerical evaluation of this equation yields the desired equation.</p>
| 179 | Derivation for bridge natural frequency estimate in Eurocodes |
2015-01-23T17:34:03.773 | <p>There are numerous steel alloys, containing mostly iron, carbon, and some other metals. Generally speaking, we can think of them as if they were some type of steel.</p>
<p>My question is: do "non-ferrous steels" exist? I am thinking of pure, non-iron metals, containing a little carbon, just as is added to iron to turn it into steel. Or asked another way, are there other metals besides iron that are doped with carbon in order to form an alloy like steel?</p>
<p>In general, how does the addition of carbon affect the properties of these metals?</p>
| |steel|alloys|metals| | <p><strong>Summary: The Fe-C system, and thus steel, is unique due to a eutectoid transformation from a high-solubility phase to a low solubility phase that allows for a wide variety of microstructures and properties which are highly and relatively easily tunable. Other first-row transition metals have different, and less exploitable, behavior when alloyed with carbon.</strong></p>
<p>Fe-C is the only first-row transition metal-carbon system that has a eutectoid transformation in its phase diagram.The eutectoid transformation changes austenite to ferrite and cementite on cooling. Austenite has high carbon solubility, and ferrite has low carbon solubility. I am picking on first-row transition metals as they tend to have chemical behavior "close" to that of steel, with similar cost, density, and other "obvious" properties (with the exception of scandium, which is extremely rare and expensive), and examining all 70+ metals is a fair amount of work for this answer.</p>
<p>The nature of the eutectoid transformation allows for many microstructures and thus a high degree of tunable properties. Consider a eutectoid steel austenitized and cooled at varying rates:</p>
<ul>
<li>If cooled slowly, a moderately ductile, moderately strong pearlite microstructure forms. Pearlite results from a cooperative nulceation and growth process as carbon leaves austenite during its transformation to ferrite, forming alternating lamellae of ferrite and cementite.</li>
<li>If cooled moderately rapidly and then held isothermally for a period of time, a much harder bainite microstructure forms. The kinetics of bainite formation are not well understood, but the microstructure is a less-organized arrangement of cementite and ferrite, again resulting from carbon coming out of solution as austenite transforms into ferrite.</li>
<li>If cooled extremely rapidly, an extremely strong and hard martensite microstructure forms. Martensite formation is a diffusionless process in which carbon is trapped in austenite while it transforms to a BCC structure, distorting the lattice into a strained BCT structure which is difficult to strain further, hence its high strength. By altering the quantity of carbon and being creative with heat treatment schedules, a wide array of microstructural combinations are available.</li>
</ul>
<p>With appropriate alloying and heat treatment, it is possible to have a steel with retained austenite, ferrite, pearlite, bainite and martensite all in the same material. Such complex microstructures are impossible in other first-row transition metal-carbon systems.</p>
<p>All of the wide heat-treatability and wide array of microstructures and properties are entirely due to the presence of a eutectoid transformation which takes a high-solubility phase to a low-solubility phase. The eutectoid transformation itself is due to a phase change from austenite (FCC) to ferrite (BCC) and the resulting significant loss of carbon solubility. <strong>The answer to your question is effectively no</strong>, there are no other alloys (of which I am aware) that behave like steel during processing. The answer to your alternate question is that carbon has less useful and less exploitable effects on other first-row transition metals.</p>
<p>Below are the Fe-C, Ni-C and Mn-C phase diagrams for comparison. Note that the Fe-C phase diagram stops at 0.2 a/a C while the others go to 1.0 a/a C. Ni-C has no eutectoid, only a eutectic transformation, and thus can only be precipitation hardened. Any other microstructure formation would have to occur during solidification. Mn-C phase diagram has a eutectoid, but it goes from a high-solubility phase to another high-solubility phase, which means that extremely large amounts of carbon would be present in the lower temperature phase (nearly 10% a/a C compared with less than 1% a/a C in steel), which would result in extreme brittleness.</p>
<p><a href="https://i.stack.imgur.com/bXw6p.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bXw6p.png" alt="Fe-C Phase Diagram"></a>
<a href="https://i.stack.imgur.com/ZuNb8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZuNb8.png" alt="Ni-C Phase Diagram"></a>
<a href="https://i.stack.imgur.com/LpYYv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LpYYv.png" alt="Mn-C Phase Diagram"></a></p>
| 186 | Do "non-iron steels" exist? |
2015-01-23T18:03:09.120 | <p>As I know, a very considerable part of the currently processed steels (around half of it) is coming from recycling.</p>
<p>But during the steels coming into the recycling process are coming normally from various sources, and thus they are containing very different alloying materials.</p>
<p>But the output of the reprocessed steel must be steel containing alloys exactly in the specified ratios.</p>
<p>Do some type of "separation", or "removal" of the previous alloys of the recycled steel happen? And if yes, how does it work?</p>
| |steel|metallurgy|alloys|recycling| | <p>First the scrap is separated at the source ; for example cast iron generally only contains Si and Mn. High vapor pressure elements boil off or collected in the flux/slag : eg, Zn, Pb, Sn, Bi, An ,,,,Aluminum oxidizes and goes into the slag. Steels do pick up Cr, Ni, Mo, and Cu residuals , generally these are advantageous ; they all add to hardenability except Cu. ( Cu is important in atmospheric corrosion resistance). V and Nb and W are present in very small amounts so insignificant. , And Co , expensive and has specialized applications so it is also separated at the scrape source ; Co scrape is easy to identify; medical prosthesis and jet engine hot section blades and vanes ,also in some high speed tools -again separated at the scrape source. Ni alloys and austenitic stainless are separated at the source as they are not ferromagnetic. Magnetic Martensitic and ferritic stainless ( typically 13 % Cr) can be separated at the scrape source . The separation of steels at the sources is done because all the alloy elements are worth more than carbon steel . There must be books available on this ; it is a major factor in the steel industry. An example of what happens in the real world ; grade A 516 carbon steel plate is the workhorse of industry but when a thick section with high strength is ordered , "somehow" the Cr, Mo, Ni residuals are high enabling acceptable heat-treatment results. </p>
| 191 | How can the alloying materials of recycled steels be separated? |
2015-01-23T18:05:35.530 | <p>Consider a large auditorium, a church, or some other very large, essentially one roomed building with a high ceiling. Suppose that the building has many entrances which enable cold air in/hot air out and traffic in and out of the building is unavoidably large. </p>
<p>I would imagine that any attempts to control the temperature of such a large building would be very inefficient in terms of energy and cost, particularly because warm air rises.</p>
<p>Assuming that it's very cold outside and we're interested primarily in keeping the building warm at the ground level so that it is comfortable for humans to work and interact, what is the best method to keep such a large, essentially one roomed building with a high ceiling warm when outdoor cold air exposure is frequent and unavoidable? </p>
<p>When I say "best," I'm interested in balancing energy, maintenance, and monetary costs over the life of the building. </p>
| |civil-engineering|building-physics|energy|hvac| | <p>Another option to the radiant solution above would be under floor air distribution systems (UFAD) using displacement ventilation. These systems require a raised floor plenum which works well in theatre style auditoriums, where you often have this anyway. The plenum essentially becomes your supply duct.</p>
<p>The systems are sized to condition the occupied zone only, say the first 2m over the outlet. So no matter how high you ceiling is, you'll end up heating only a 2m high volume.</p>
<p>Unlike in radiant systems, which heat people directly and do not change the air temperature, the outside air component is treated. Whether this is an advantage or not depends on the required outside air volumes, where these volumes are introduced relative to the occupants (i.e. are you blowing cold air on someone?) as well as expected comfort levels.</p>
<p><strong>Opportunities for energy savings in UFAD systems</strong></p>
<ul>
<li>Lower supply temperatures when heating, higher supply temperatures when cooling</li>
<li>The conditioned volume is limited to the occupied zone no matter what the ceiling height is</li>
<li>Supply velocity is lower, however supply volumes are increased; there might be some value in optimisation here and thinking about perimeters and internal zoning</li>
<li>Lower pressure drop in supply</li>
<li>Throw in some heat exchanger to capture the heat from the return air</li>
</ul>
<p><strong>A few other things to note:</strong></p>
<ul>
<li>Lower velocity and higher volumes taken together means that the number of outlets might increase and that these systems cannot handle the same peak loads as conventional systems; this will be important for perimeter zones. The building fabric must be optimised to limit peak loads (losses/gains through glazing)</li>
<li>Lower velocities also means less problems with draft (in theory at least)</li>
<li>If the acoustic zoning doesn't match the mechanical zoning, things will get a bit tricky; acoustic separation might require walls being taken down through to the plenum</li>
<li>If you have a tiled flooring system, the outlet diffusers can be swapped around which has multiple benefits: for one, it provides a very flexible room layout that can be changed at any time; in addition, the systems introduce some flexibility for occupants to make themselves comfortable by relocating diffuser tiles</li>
<li>Wikipedia has an article which provides a general overview of <a href="https://en.wikipedia.org/wiki/Underfloor_air_distribution" rel="noreferrer">UFAD systems</a> and one for <a href="https://en.wikipedia.org/wiki/Displacement_ventilation" rel="noreferrer">displacement ventilation</a></li>
</ul>
<p>What is "most efficient" depends on many other things besides space heights. UFADs can be a good solution in certain situations but same is true for e.g. radiant systems. What you'd definitely like to avoid is ending up conditioning a 4m high volume of air.</p>
<p>A few UFAD images from the wiki page to save you a click:
<img src="https://i.stack.imgur.com/gdkz7.png" alt="UFAD concept section"></p>
<p><img src="https://i.stack.imgur.com/iOFq0.jpg" alt="UFAD stratification"></p>
| 193 | What is the most efficient means of warming a building with a high ceiling? |
2015-01-23T20:21:06.483 | <p>For a <a href="http://en.wikipedia.org/wiki/Piston_pump" rel="nofollow noreferrer">piston pump</a> you can calculate/measure things like,</p>
<ul>
<li>Average Volume flow rate (m³/s)</li>
<li>Geometric Values (Surface)</li>
<li>Pressure over time</li>
<li>closing times of the valves</li>
<li>energy conversion efficiency $\eta$</li>
</ul>
<p>besides those, what charateristic values are there?</p>
<p><img src="https://i.stack.imgur.com/c6X9H.gif" alt="enter image description here"> (<em>example of a type of piston pump</em> <a href="https://nationalvetcontent.edu.au/alfresco/d/d/workspace/SpacesStore/dc6a7f1f-e3d3-44d8-9bf1-984ea8cb3c01/204/pmaops201b/images/proc201_010100_g04.gif" rel="nofollow noreferrer">source</a>)</p>
| |mechanical-engineering|pumps| | <p>• Piston pump has a wide range of pressure
• It can manage force without the moving flow rate.
• The Pressure and flow rate variations have a small effect on the pump performance.
• It has the capability to move high viscosity fluids such as abrasives and slurries with an excellent design.
• Low speed.
• The fluid discharge doesn’t depend on the head. Due to this feature, these pumps can be used as metering pumps.
• This <a href="https://mechanicalboost.com/what-is-a-pump-types-of-pumps-and-applications/" rel="nofollow noreferrer">type of pump</a> has high efficiency</p>
| 200 | What are characteristic values for a piston pump? |
2015-01-24T05:36:09.180 | <p><strong><em>Disclaimer</em></strong>
I'm an applied mathematician by training, not an engineer. My work research primarily focuses on creating new "methods" to solve different PDE's related to solid deformation (elasticity) and fluid mechanics. In this sense, i know how to solve a pde problem computationally. From my perspective, engineers use my work as "tools" to accomplish their work.</p>
<p>However, due to my lack of education/experience in engineering, i admit i'm actually rather clueless on how numerical solutions to pde's are really used in an engineers actual practice. <strong><em>The primary source of my confusion is the following:</em></strong></p>
<p>I've been told that engineers never (or should never) conduct numerical simulations (e.g. finite element analysis, CFD, etc...) without knowing or having a good idea ahead of time what the simulation "should" look like. This helps engineers discriminate realistic results from questionable ones.</p>
<p>However, i argue that <strong><em>if the engineer already knows what is supposed to happen in the simulation, then what's the point of simulation in the first place???</em></strong> I've always assumed that simulations are needed for predictive purposes, which assumes ignorance of what is to come. That is, <strong><em>I think of a simulation as a stand-alone tool to predict the future when you don't know what to expect</em></strong>.</p>
<p>What i'm looking for is a broader perspective into how/when/why engineers use numerical simulations like CFD and Finite Element Analysis, especially if good engineering practice dictates that you should already know what to expect when you're simulating?</p>
| |modeling| | <p>There isn't much left to say but that knowing result before runing simulation isn't knowing exact numerical value but to have certain expectations regarding solution based on understanding the physics of the problem. Usually engineers set problem and choose general method and when we finally formulate problem as set of equations and boundaries we seek help from mathematicians to help us solve it in most effective way. Usually engineers are those who define equations, mathematicians solve them.
If you have no understanding of bending than, though you can solve biharmonic equation, your solution will Probably not be set of right deflections.
When mathematician learnes to use tools for solwing pde he can solve most pde problems but eg. mechanical engineer though will understand basics of solwer will not try to use it to solve radar imaging or electric flow.</p>
| 207 | How do engineers really use numerical simulation? |
2015-01-24T13:13:19.357 | <p>These days most of modern electronics use rechargeable batteries as a power source. Also, these days most modern rechargeable batteries are <strong>Lithium Ion</strong> or <strong>Lithium Polymer</strong> based. Like any other devices, over time these rechargeable batteries lose the ability to recharge, retain and discharge energy thus users have to replace the devices or rechargeable batteries. </p>
<p>It is my understanding that the rise in battery internal resistance is the primary cause the rechargeable battery aging. Is this accurate? If so what can be done to lower or eliminate the internal resistance in rechargeable batteries. </p>
<p>If my understanding is inaccurate what is the cause for rechargeable battery aging?</p>
<p>If the causes for battery aging are understood, how can electronic engineers design charging and discharging circuits to extend the rechargeable battery life?</p>
<hr>
<p><strong>References:</strong></p>
<ol>
<li><a href="http://batteryuniversity.com/" rel="nofollow">Battery University</a></li>
<li><a href="http://www.eetimes.com/author.asp?section_id=36&doc_id=1320292" rel="nofollow">All About Batteries, Part 1: Introduction</a></li>
<li><a href="http://All%20About%20Batteries,%20Part%202:%20Specifications%20&%20Terminology">All About Batteries, Part 2: Specifications & Terminology</a></li>
<li><a href="http://www.eetimes.com/author.asp?doc_id=1322276" rel="nofollow">All About Batteries, Part 7: Lithium Thionyl Chloride</a></li>
</ol>
| |energy|electrical-engineering|energy-storage|chemical-engineering|battery| | <p>One of the problems that plagued older rechargeable batteries (e.g. Nickel Cadmium ($\text{NiCad}$) and Nickel Metal Hydride ($\text{NiMH}$)) was the <a href="https://en.wikipedia.org/wiki/Memory_effect">memory effect</a>. The memory effect occurs when a rechargeable battery is not fully discharged. It then "forgets" that it has a greater capacity than it thinks it has, and so in the future it discharges less.</p>
<p>A good example is a water bottle. Initially, water bottles have a certain capacity for water. Let's say that I drink most of the water in a water bottle during one usage. If the memory effect affected water bottles, I would not be able to drink any water in the future occupying the space that had held the water that had not been drunken the last time. That extra space would be forever lost. Over time, this can wear down a rechargeable battery. Fortunately, <a href="http://copquest.com/battery_care.htm">this generally only affects $\text{NiCad}$ and $\text{NiMH}$ rechargeable batteries</a>.</p>
<p>I haven't been able to find much about effects that influence only lithium ion batteries, but there are a lot of <a href="http://www.mpoweruk.com/life.htm">across-the-board factors</a>. Here's a short list:</p>
<p><ul>
<li>Chemicals breaking down</li>
<li>Passivation (which affects lithium ion batteries), which is when a layer of unwanted chemicals form on the battery cell. <a href="http://pubs.acs.org/doi/pdf/10.1021/cr020730k">This</a> discusses a related phenomenon on page 4258:</p>
<blockquote>
<p>Unfortunately, on recharge, the lithium has a strong tendency to form mossy deposits and dendrites in the usual liquid organic solvents (cf. Figure 15B). This limits the cycle life to 100-150 cycles (considerably lower that the 300 cycles required for a commercial cell), as well as increasing the risk of a safety incident.</li>
<li>Mechanical stresses and leaking. Batteries can be damaged in a variety of ways, causing internal components to break and causing chemicals to leak out. This can be very dangerous to humans.</li>
</ul>
There are other long-term factors that increase battery aging. The page I linked for the above list seems to be fond of the <a href="https://en.wikipedia.org/wiki/Arrhenius_equation">Arrhenius equation</a>:
$$k=Ae^{-E_a/RT}$$
which shows that the rate of chemical reations changes as temperature changes. High temperatures mean faster reactions but also possibly a shorter life; this can affect non-rechargeable batteries significantly.</p>
</blockquote>
<p>Finally, there's the phenomenon of <a href="http://www.corrosion-doctors.org/Batteries/self-compare.htm">self-discharge</a>, which is when unwanted reactions in the battery "eat away," so to speak, at the battery's capacity. <a href="https://en.wikipedia.org/wiki/Self-discharge">The process can differ</a> based on the type of battery. <a href="http://batteryuniversity.com/learn/article/elevating_self_discharge">Battery University has a page on it</a>, which you may have already seen. It reiterates that temperature can speed up this process. Scarily enough, lithium ion batteries may discharge as much as 5% within the first 24 hours, slowing down to 1-2% per month after that.</p>
| 211 | What causes rechargeable batteries to age? What can be done to extend life of these batteries? |
2015-01-24T15:27:54.717 | <p>EMI/EMC issues are common in most electronic designs. Electronic and Hardware Engineers use many different strategies to mitigate these issues: These include (But not limited too)</p>
<ul>
<li>Designing filter circuits </li>
<li>Strategic selection of micro-controller clocking frequency </li>
<li>PCB layout consideration including good grounding practices</li>
<li>Mechanical Shielding of critical electrical circuits for immunity or suppression</li>
</ul>
<p><strong>Question:</strong> This particular design in question, has a plastic enclosure, LCD display and USB type to connector for recharge a battery. What are some of the mechanical engineering best practices to help design product meet EMC/EMI requirement?</p>
<p><strong>References:</strong></p>
<ol>
<li><a href="http://www.ieee.li/pdf/viewgraphs/emc_design_fundamentals.pdf" rel="nofollow">EMC Design Fundamentals</a></li>
<li><a href="http://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf" rel="nofollow">EMI Design Guideline for USB components</a></li>
<li><a href="http://cache.freescale.com/files/microcontrollers/doc/app_note/AN4438.pdf" rel="nofollow">EMC Design Considerations</a></li>
</ol>
| |mechanical-engineering|electrical-engineering|emc| | <p>Generally speaking meeting the EMC/EMI requirements is the EE job. The biggest part of radiated EM waves usually comes from a poor PCB, and of course there is little the ME can do about that. Providing more space, if possible, can relax some constraints for the EE that will have more space to properly route his/hers tracks.</p>
<p>In your case it appears to me that no EM problem should be present: the fastest signal lying around would pass through the USB connection, but you say it's used only for power. Your device is also battery power and that gives a huge help since there's no risk to inject some unwanted frequencies in the mains through the power supply.</p>
<p>If there's some high speed clock inside, and I guess there is, proper routing should be enough. A great help would come from a metallic enclosure, as you probably know a grounded metallic enclouser greatly helps to keep unwanted EM from escaping your device. Bonus points: it also keeps unwanted EM from messing with your circuit, that might be a bigger issue.</p>
<p>To help with the routing, relaxing the position of the usb connector and/or the lcd can help, but really in this case it seems quite trivial to me to make a circuit that has no problems.</p>
<p>Without additional informations on the specific device in question I'd say just relax, make a prototype and test it, you're probably getting away with it without any problem.</p>
| 212 | Can Mechanical Design help meet EMC/EMI requirements for electronic products? |
2015-01-24T16:22:45.257 | <p>Austenite is <strong>non magnetic</strong> while $\alpha$-ferrite and pearlite <strong>are magnetic</strong>. (<a href="http://www.sciencedirect.com/science/article/pii/030488539390454A">Magnetic properties of pearlite vary as a function of carbon content</a>) </p>
<p>If a strong magnetic field is applied in a particular direction while the steel is being quenched (rather, austenite is being quenched!), would the grain structure change? Is it possible to get superior grain structure and hence tougher steel by application of cyclic magnetic field?</p>
<p>My speculation is that, at the <a href="http://www-g.eng.cam.ac.uk/mmg/teaching/typd/addenda/eutectoidreaction1.html">eutectoid point</a> while quenching the steel, as low carbon zone of pearlite has more permeability, that zone should align itself to the strong magnetic field by pushing the carbon in orthogonal direction, so grain boundaries should take a different shape.</p>
<p>Will it actually happen? </p>
| |steel|process-engineering|materials|metallurgy|magnets| | <p>This will at least depend on the:</p>
<ul>
<li>Rate of Cooling</li>
<li>Magnetic field strength</li>
<li>Exact composition</li>
</ul>
<p>The magnetic field will alter the microstructure as you can read in, for example,</p>
<ul>
<li>Yudong Zhang, Nathalie Gey, Changshu He, Xiang Zhao, Liang Zuo, Claude Esling, High temperature tempering behaviors in a structural steel under high magnetic field, Acta Materialia, Volume 52, Issue 12, 12 July 2004, Pages 3467-3474, ISSN 1359-6454, <a href="http://dx.doi.org/10.1016/j.actamat.2004.03.044">http://dx.doi.org/10.1016/j.actamat.2004.03.044</a>.</li>
<li>G.M. Ludtka, R.A. Jaramillo, R.A. Kisner, D.M. Nicholson, J.B. Wilgen, G. Mackiewicz-Ludtka, P.N. Kalu, In situ evidence of enhanced transformation kinetics in a medium carbon steel due to a high magnetic field, Scripta Materialia, Volume 51, Issue 2, July 2004, Pages 171-174, ISSN 1359-6462, <a href="http://dx.doi.org/10.1016/j.scriptamat.2004.03.029">http://dx.doi.org/10.1016/j.scriptamat.2004.03.029</a>.
(<a href="http://www.sciencedirect.com/science/article/pii/S1359646204001770">http://www.sciencedirect.com/science/article/pii/S1359646204001770</a>)</li>
</ul>
<p>For me, it is behind a paywall. But as you can read in the abstract, 30 Tesla will result in, for example, more ferrite. The other paper reveals that for a hyper eutectoid steel, you will have particle like cementite.</p>
<p>I am not aware of any models with which you can make predictions about tensile strength and so on. But, for the question <strong>Can we change steel properties by application of magnetic field while quenching?</strong>, it is a clear <strong>yes</strong>. A more complete lookup in the literature would be the next step for models and experiments for a more specific case.</p>
| 214 | Can we change steel properties by application of magnetic field while quenching? |
2015-01-24T16:41:54.090 | <p>Localities in the US have different road layout and setups.</p>
<p>For example the <a href="http://en.wikipedia.org/wiki/Michigan_left">Michigan Left</a>, <a href="http://en.wikipedia.org/wiki/Jughandle">Jersey Left/Jug Handle</a>, vs a standard 4 way stop with left turns at the intersection.</p>
<p>Have any of them come out as clear winners? </p>
| |civil-engineering|highway-engineering| | <p>Yes.</p>
<p><strong>Michigan left</strong></p>
<p><a href="http://web.archive.org/web/20090510121808/http://www.tfhrc.gov/safety/pubs/04091/10.htm#1022">This page</a> (pdf <a href="http://web.archive.org/web/20090510122331/http://www.tfhrc.gov/safety/pubs/04091/04091.pdf">here</a>) is very informative, though you really have to dig to get what you want. In a sub-section of 10.2.2 Median U-Turn Crossover, I found this (A "Michigan left" is referred to as a "median U-turn crossover"):</p>
<blockquote>
<p>A study on a Michigan corridor used simulation to compare median U-turn crossovers with two-way left-turn lanes (TWLTL). The study showed that during peak hours, the corridor with median U-turn crossovers had a lower travel time by 17 percent and a 25 percent higher average speed than the same corridor with a TWLTL. However, vehicles made more stops on the arterial with median U-turn crossovers. In nonpeak hours, the median U-turn crossovers had the same efficiency as the TWLTL, even though a higher delay for left-turning vehicles had been expected due to the higher travel distance a vehicle must cover to turn left using a median crossover.</p>
</blockquote>
<p>So that's a yes for stopping congestion during peak hours. More information on that specific simulation can be found under found under footnote 149, which isn't too easy to find. Other simulations reportedly found similar results:</p>
<blockquote>
<p>Simulation studies using a range of intersection configurations (number of through lanes on the major and minor street) and volumes from intersections in Virginia and North Carolina suggest a reduction in overall travel time for all movements through the intersection when compared to a conventional intersection: -21 to -2 percent during off-peak conditions, and -21 to +6 percent during peak conditions. The studies also show a general increase in the overall percent of stops when compared to a conventional intersection: -20 to +76 percent during off-peak conditions, and -2 to +30 percent during peak conditions.</p>
</blockquote>
<p>The rest of 10.2.2 has some more safety information:</p>
<ul>
<li>The collision rate is lowered slightly</li>
<li>There are less "conflict points" (i.e locations where collisions are likely to happen)</li>
</ul>
<p><strong>Jersey jughandle</strong></p>
<p>The Jersey jughandle (referred to as simply a "Jughandle") does reduce conflict points, though not as much as a Michigan left. It, too, appears to increase efficiency:</p>
<blockquote>
<p>Simulation studies using a range of intersection configurations (number of through lanes on the major and minor street) and volumes from intersections in Virginia and North Carolina suggest a reduction in overall travel time through the intersection when compared to a conventional intersection: -6 to +51 percent during off-peak conditions, and +4 to +45 percent during peak conditions. The studies also show a large increase in the overall percent of stops when compared to a conventional intersection: +15 to +193 percent during off-peak conditions, and +19 to +108 percent during peak conditions.</p>
</blockquote>
<hr>
<p>Is there a clear winner? Both clearly reduce travel time and congestion, so the answer to your question is a definite yes. The Michigan left has many less conflict points (16) than the Jersey Jughandle (26), which I consider quite the advantage (the standard four-way intersection has 32). It also has a lesser increase in stops. I'd give the edge here to the Michigan left, though both are probably improvements over your standard four-way intersection.</p>
| 215 | Has a specific type of alternative intersection been shown to reduce congestion & accidents? |
2015-01-24T19:17:31.397 | <p>I was looking at high pressure pipes and their pressure ratings. I'd like to know how these pressure ratings are determined.</p>
<p>I assume that pipes are tested until failure, and the failure pressure is multiplied by some safety factor to determine its 'rated' pressure, but is there a formula to calculate what the failure pressure should be before testing it (based on the material, wall thickness, diameter, or other measurements of the pipe)?</p>
| |mechanical-engineering|pressure|mechanical-failure| | <p>In the world of plastic piping, the formula is different, because the material doesn't <a href="https://en.wikipedia.org/wiki/Yield_(engineering)" rel="nofollow">yield</a>. For <a href="https://en.wikipedia.org/wiki/Isotropy#Materials_science" rel="nofollow">isotropic</a> plastics, B31.3 shows piping as:</p>
<p>$$ p = \frac{2St}{D-t} $$ </p>
<p>Where D, t and S remain the same as above. However, the allowable strength (S) is given by an applicable ASTM specification, which functions the same as the yield stress - but is not always based on the materials ultimate strength. </p>
<p>Composite piping, being composed of <a href="https://en.wikipedia.org/wiki/Transverse_isotropy" rel="nofollow">Orthotropic</a> laminates, doesn't have a well defined strength - the material is designed with the pipe. In these cases, the original assumption that pipes are <strong><em>tested until failure</em></strong> is absolutely correct. B31.3 states again:</p>
<p>$$ p = \frac{2SFt}{D-t} $$ </p>
<p>Where a new factor, F is introduced. S is obtained from the <a href="http://www.astm.org/Standards/D2992.htm" rel="nofollow">Hydrostatic Design Basis</a> - and it is essentially an <a href="https://en.wikipedia.org/wiki/Fatigue_(material)#High-cycle_fatigue" rel="nofollow">S-N Curve</a> for that particular sequence of lamination. F allows conversion between the two tests - 0.5 for the static test, 1.0 for the dynamic test.</p>
<p>ASME is currently reviewing this method - and this is an exciting new area of development for them as they are generating a <a href="http://gasapps.gastechnology.org/webroot/app/gtiishome/files/ASMEStandardsCommittee.pdf" rel="nofollow">new piping standard</a> to relieve the industry of the expensive and extensive HDB testing requirement.</p>
<p>Quality testing mandated by ASTM D2996 / ASTM D2992 ensures the piping is made the same way - any change in the formula requires a re-test. Using this method, composite piping is typically designed for a 50 year life-cycle.</p>
| 218 | How to calculate the pressure a pipe can withstand? |
2015-01-24T20:39:22.510 | <p>Weight lifters will use the term "one rep maximum" in order to refer to the maximum amount of weight that they can lift one time. The one rep max is often used as a proxy for how strong someone is (or isn't).</p>
<p>Recreational lifters, other types of athletes, and the general population rarely bother with testing themselves in order to determine their maximum strength.</p>
<p>From a human factors and usability point of view, it can be useful to know what the average maximum strength of a population is so that design can take those limitations into account. Please see the footnotes <sup>1,2,3</sup> for some practical applications of this question.</p>
<p>My question is if it's possible to estimate a person's maximum strength for a given activity by seeing how many repetitions they can complete at a lower weight. For example, if someone can bench-press 100 lbs 10 times in a row, can that be used to estimate the maximum amount they could bench-press once?</p>
<p>If so, does that estimation model:</p>
<ul>
<li>hold consistently across activity types?</li>
<li>hold consistently across increasing amounts of weight?</li>
<li>hold consistently across age and gender distributions?</li>
</ul>
<hr>
<p><sup>1</sup><sub>An entertaining example: amusement park / carnival attractions can be designed differently in order to accommodate the various distributions of strength.</sub><br>
<sup>2</sup><sub>A mobility example: aids for the disabled can be enhanced if it's found that their maximum strength is below the threshold for a device.</sub><br>
<sup>3</sup><sub>A diagnostics example: restraints can be designed just to the maximum of a particular demographic's abilities in order to provide a more comforting or friendly appearance. This provides a humanizing touch while maintaining the accuracy of the diagnostic test.</sub> </p>
| |statistics|biomechanics| | <p>This actually isn't as much of an engineering question as it is a physiology question. There are actually a number of widely used estimates to predict your one-rep maximum, aka "1RM", if you know how many repetitions you can do at a lower weight. <a href="http://www.weightrainer.net/training/coefficients.html" rel="nofollow">See here for more info</a>.</p>
<p>All of the methods are based on empirical studies, and are basically look-up tables of coefficients.</p>
<ul>
<li><a href="https://en.wikipedia.org/wiki/One-repetition_maximum#Brzycki" rel="nofollow">Brzycki</a></li>
<li><a href="https://en.wikipedia.org/wiki/One-repetition_maximum#Epley_Formula" rel="nofollow">Epley</a></li>
<li><a href="https://en.wikipedia.org/wiki/One-repetition_maximum#Lander" rel="nofollow">Lander</a></li>
<li><a href="https://en.wikipedia.org/wiki/One-repetition_maximum#Lombardi" rel="nofollow">Lombardi</a></li>
<li><a href="https://en.wikipedia.org/wiki/One-repetition_maximum#Mayhew_et_al." rel="nofollow">Mayhew et al.</a></li>
<li><a href="https://en.wikipedia.org/wiki/One-repetition_maximum#O.27Conner_et_al." rel="nofollow">Connor et al.</a></li>
<li><a href="https://en.wikipedia.org/wiki/One-repetition_maximum#Wathen" rel="nofollow">Wathen</a></li>
<li>National Strength And Conditioning Association (NSCA) Coefficients</li>
</ul>
| 223 | Estimating maximum lift or strength |
2015-01-24T20:46:33.150 | <p><a href="https://www.refrigerationbasics.com/RBIII/controls1.htm" rel="nofollow">Cut-in, cut-out</a> <a href="http://en.wikipedia.org/wiki/Thermostat" rel="nofollow">thermostats</a> are commonly used with refrigeration devices in order to hold a consistent temperature range. </p>
<p>Their general operations is such that when the temperature by the probe reaches a particular temperature set as the upper limit, the thermostat cuts-in and the compressor is turned on providing cold air.<br>
And when the temperature by the probe reaches the lower temperature set point, the thermostat cuts-out and turns off the compressor.</p>
<p>What materials are used in within an analog or mechanical cut-in, cut-out thermostat? Likewise, how is the internal circuit designed such that one probe feeds both cut circuits and what control is put in place for the cut-out to override the cut-in signal?</p>
<p>To help scope this question: based upon the materials used in an analog cut-in, cut-out thermostat, would it be reasonable to attempt to test and adjust the set points of a thermostat by using jars of water at known temperatures and measuring the output off of the relay within the thermostat?</p>
| |electrical-engineering|circuit-design|refrigeration| | <p>A <a href="http://en.wikipedia.org/wiki/Bimetallic_strip" rel="nofollow noreferrer">bimetallic strip</a> is used. Two different metals like steel and copper expand at different rate and a strip of the two bound together as result bends with temperature changes. A contact placed at the end of the strip will close when the temperature is right - and the temperature can be tuned by turning a screw that pushes the strip closer or farther from the contact.</p>
<p>In simpler systems the natural inertia of the system is used to create the hysteresis - as the refrigerator compressor works, it takes time for the temperature drop to reach the bimetallic strip, and switch it off, so the temperature is brought below the cut-off point by some hard to control factor.</p>
<p>In more complex systems, a two-level switch (either through two strips or just mechanism that shorts two switches at two different temperature levels), in connection with an analog RS switch (usually based on a relay) creates a better-controllable hysteresis.
<img src="https://i.stack.imgur.com/x8eOO.png" alt="enter image description here"></p>
<p>As temperature is too high, both switches are engaged and so the cooling compressor starts. Temperature drops.</p>
<p>At cut-in temperature the "On" switch disengages, but power to the relay is still supplied through the "Off" switch.</p>
<p>At cut-out temperature the "Off" switch disengages, and the cooling process stops.</p>
<p>With temperature rising, the "Off" switch engages, but since the relay is open, it doesn't supply power to the coil.</p>
<p>Further rise causes the "On" switch to power up the coil and the relay switches both the compressor and the "Off" switch circuit on. With temperature drop, the "on" switch will disconnect but the coil powered through the relay will "remember" the state until "Off" opens.</p>
<p>(and I'm sorry, but I don't know about calibrating thermostats with jars of water. My vote would be "against" as they are rarely submersible.)</p>
| 224 | Thermostat design properties |
2015-01-24T21:51:11.753 | <p>Some dc motors can be used as generators as well by applying mechanical torque to the output shaft to induce a current. However, even if a dc motor <em>can</em> do this, I imagine they were not designed for this purpose and thus perform less efficiently when used as a generator rather than as a motor.</p>
<p>In my admittedly naive understanding, dc generators and dc motors are essentially the same machinery, but with inputs and outputs reversed. This leads me to believe that some other design considerations are used to make one direction more efficient than the other.</p>
<p><strong><em>How differently are dc generators and dc motors designed to make one direction of input/output more efficient than the other?</em></strong> What can one do electrically or mechanically to improve the efficiency in either direction?</p>
<p>In particular, I'm interested in converting a dc motor into a generator and want to know how I can improve its efficiency in converting mechanical energy into electrical energy.</p>
| |mechanical-engineering|electrical-engineering| | <p>In Ye olden days DC generators were brushed commutated devices. They had a one or more stator windings and an armature winding. Field wound DC generators as well as motors were commonly connected in one of three methods: Series, Shunt and Compound. Without getting into details, each had its own set of strengths and weaknesses. But you only have to remember these two things: the voltage of a DC motor is dependent on its input shaft speed. Current is a function of torque. More voltage means more RPM's and more amps means more newton-meters (or foot-pounds).</p>
<p>So with all that, you need a constant speed source to get a constant voltage. And you need to ensure you have enough torque to satisfy the current demand of your load otherwise voltage drops off. Old automobiles had commutated generators. They couldn't regulate the voltage so they used a range of around 10-14 volts and used a relay that simply closed when the engines speed was within the voltage range. If the voltage went too low or too high, the relay opened. Primitive by today's standards. The Alternator in today's automobile uses a voltage regulation circuit that varies the armature current which changes the field strength based on the stators output voltage. Lower speed means more current to the armature and less current at higher speeds.</p>
<p>So how different were DC generators from motors? Not very different at all. If anything they mostly differed in mechanical design as they were to be coupled to a prime mover (steam, ICE, electric etc.). Though, in much larger dynamos they had adjustable commutator brushes to compensate for the shift in the commutation plane as a result of heavy load characteristics. A hand wheel would turn a worm gear which would advance or retard the commutation plane to bring the generator back into its normal operating parameters. You don't need to worry about this as I am sure you motor isn't megawatt capable.</p>
<p>I am guessing your motor is a permanent magnet type motor. Its nameplate RPM is what you need to spin the motor at to get the nameplate voltage. This means if you have a 12V motor that spins at 6000 RPM, you need 6000 RPM to get 12V. If you don't have a constant speed source you have no way to regulate the voltage. You would need a buck-boost switching regulator to get a constant voltage from your motor.</p>
<p>If you are using this for a renewable energy project like wind or hydro, a charge controller is usually designed for a wide input voltage swing via a buck/boost regulator. Solar panels are a close analogy to a permanent magnet DC generator, no internal voltage regulation and a varying amount if input energy. Sun might be shining bright one minute and a minute late, be blocked by a cloud. So the charge controller does its best to make a useful steady-voltage from its varying input. From there, use storage batteries to capture that power for later use and to act as a buffer for low input events.</p>
<p>And just for reference, an AC motor can also generate power if you spin it faster than its nameplate RPM, usually at synchronous speed. But again, no voltage regulation and a constant speed is needed. More trouble than its worth. Also of note: jet planes use a very elaborate mechanical speed regulator to produce constant shaft speeds which ensures a constant 60 or 400Hz AC frequency as the throttle is varied.</p>
| 226 | How different are dc motor designs from dc generator designs? |
2015-01-25T12:13:28.800 | <p>Since the units are the same ($\frac{N}{m^2}$), what's the difference between pressure and stress?</p>
| |pressure|stresses| | <p>Pressure is force applied per unit area. It arises due to external forces on the surface of an object.</p>
<p>When external forces are applied, in order to avoid deformation internal forces are generated which are called Stresses. Both pressure and stress have the same unit. </p>
| 229 | What is the difference between pressure and stress? |
2015-01-25T16:00:44.127 | <p>I want to do some very simplified drag calculations on a vessel. My hope was that calculating the skin-friction resistance would be enough to get a good estimate of the surge resistance.</p>
<p>Because wave-making resistance is very speed dependent, I assume you can neglect it when the vessel is below a certain speed. I also assume that I have to work with <a href="https://en.wikipedia.org/wiki/Froude_number">Froude numbers</a> instead of velocities, to take vessel size into account.</p>
<p>I have seen Froude numbers below Fn = 0.1 and Fn = 0.2 mentioned in books and on the Internet, but if you calculate the velocity for a vessel with a 100 m long waterline you get:</p>
<p>$$V = 0.1 \cdot \sqrt{9.81\ \text{m/s}^2 \cdot 100\ \text{m}} \approx 3.13\ \text{m/s} \approx 6.08\ \text{knots}$$</p>
<p>$$V = 0.2 \cdot \sqrt{9.81\ \text{m/s}^2 \cdot 100\ \text{m}} \approx 6.26\ \text{m/s} \approx 12.16\ \text{knots}$$</p>
<p>This values seem way too high in my opinion. 12.16 knots is almost service speed for some vessels and 6 knots is also quite high.</p>
<p>Are Fn = 0.1 and Fn = 0.2 reasonable numbers, and if not, below what Froude numbers should I stay to be able to neglect wave-making resistance? </p>
| |drag|marine-engineering| | <p>"Hull speed" is actually the ratio of speed to the square root of length.
To make things even more confusing, length is in feet, and speed is in
knots. That's how the constant 1.34 arises. (ProTip: Let's never speak of
it again!)</p>
<p>Wave resistance ($R_w$) begins its rapid rise at a Froude number (Fr) of
about 0.35. Below that Fr, $R_w$ is usually small compared to the skin-friction
and other hydrodynamic drag components.</p>
<p>Now, for sake of example, let the wave resistance coefficient be defined as
$C_w = R_w/(0.5 \rho U^2 S)$, where $\rho$ is water density, $U$ is ship speed,
and $S$ is the (static) wetted surface area of the hull. </p>
<p>In deep water, $C_w$ increases roughly like Fr to the 6th power.</p>
<p>The depth-based Froude number is $F_h = U/\sqrt{g h}$, where $g$ is
gravitational acceleration, and $h$ is water depth. </p>
<p>For finite depth water, $C_w$ can increase almost like Fh to the 10th power
as $F_h \rightarrow 1$. Once through (the critical value) $F_h =1$, wave resistance
begins to decrease, and it can be lower than in deep water for the same
length-based Froude number (Fr). </p>
<p>$F_h < 1$ is usually referred to as sub-critical; $F_h > 1$ is super-critical,
and (roughly) $0.9 < F_h < 1.1$ is trans-critical.</p>
<p>In the trans-critical regime, the hull also experiences forces and moments
that significantly change its attitude with respect to the undisturbed free-surface of water.
The trim and heave of a hull is known as "squat". This phenomenon is difficult
to predict accurately. It can have some effects on resistance but, more importantly, in
shallow water there is also a danger of the ship grounding against the sea-bed.
This can cause large losses of income, and there have also been fatalities attributed
to the phenomenon.</p>
<p>Wave patterns for finite depth are quite interesting...</p>
<p><a href="http://www.cyberiad.net/gallery/shipwakes/hw8fd/fh080.gif" rel="nofollow">fh080.gif</a></p>
<p><a href="http://www.cyberiad.net/gallery/shipwakes/hw8fd/fh090.gif" rel="nofollow">fh090.gif</a></p>
<p><a href="http://www.cyberiad.net/gallery/shipwakes/hw8fd/fh099.gif" rel="nofollow">fh099.gif</a></p>
<p><a href="http://www.cyberiad.net/gallery/shipwakes/hw8fd/fh101.gif" rel="nofollow">fh101.gif</a></p>
<p><a href="http://www.cyberiad.net/gallery/shipwakes/hw8fd/fh110.gif" rel="nofollow">fh110.gif</a></p>
<p><a href="http://www.cyberiad.net/gallery/shipwakes/hw8fd/fh120.gif" rel="nofollow">fh120.gif</a></p>
<p>As $F_h$ enters the trans-critical regime, wave patterns change dramatically.
The angle of the V-shape opens out and becomes 90 degrees at $F_h = 1$.</p>
<p>For sub-critical speeds, transverse waves (those perpendicular to the ship's
track) are apparent.
In super-critical flow, transverse waves disappear. (In short, they cannot keep
up with the ship).</p>
<p>DISCLOSURE: These patterns were made using my (free) program Flotilla.</p>
<p>More patterns can be found at:</p>
<p><a href="http://www.cyberiad.net/wakeimages.htm" rel="nofollow">www.cyberiad.net/wakeimages.htm</a></p>
| 231 | Below what Froude number can I neglect wave-making resistance on a vessel? |
2015-01-26T11:41:47.390 | <p>As a hydraulics layman thinking about hydraulic systems, it seems that the important factor is to have a liquid that doesn't compress much or at all. Doesn't water meet this requirement, and what other properties should the liquid have (if any) that water doesn't?</p>
| |mechanical-engineering|hydraulics| | <p>In addition to the other answers, water is commonly used as a primary working fluid in industrial hydraulic systems <em>with</em> additives to prevent the issues mentioned above. Typical applications include furnaces, foundries, plastic extrusion, etc. Anywhere that standard hydraulic oil represents a major fire or contamination hazard, yet operating costs do not justify purchasing synthetic fluids for $1100+/barrel. </p>
<p>Factory hydraulics are ideal for water-based fluids because the environment is controlled. Atmospheric temperature is typically maintained within 15C-30C year-round. So boiling and freezing are non-issues. Oil properties can be continuously monitored for changes in pH, composition, or particulate levels. New water can be regularly added to the system to replace losses from evaporation.</p>
<p>The percentage of water in a typical mixture may be between 30 - 80%. As the fraction of water increases, the viscosity of the mixture decreases. Low viscosity is the primary challenge of water-based hydraulics. Modern hydraulic pumps (axial-piston, radial, vane, etc) are designed with internal leakage passages for cooling. Somewhere between 2-6% of total flow is "leaked" or intentionally siphoned away from the flow outlet for cooling of the pump's sliding surfaces. The flow rate through these passages is obviously dependent on viscosity. The problem with running 100% water is that viscosity falls below 1.0 cSt at 20C. Compare that to 46 cSt viscosity of standard hydraulic oil. The machining tolerances of pump surfaces made for pure water would be extremely fine (<1 micron), or leakage could siphon away the majority of total flow <em>and</em> surfaces could frequently experience contact due to the thin film of water separating them. </p>
<p>Current manufacturing tolerances <em>in series production</em> are simply not that good yet. There are specialized pump and valve manufacturers like Oilgear and Dynex-Rivett with more expensive product lines targeted at the extreme end of the water-based fluid market (80-95% water). There are also products like the Danfoss PAH pumps which can operate at reduced pressures with pure tap water. However a PAH costs approx 25x more than an equivalent-displacement pump for standard fluids. </p>
<p>Source - my former boss spent years trying to redesign standard axial-piston pumps for 100% water</p>
| 270 | Why do hydraulic systems use special fluid - what's wrong with water? |
2015-01-26T12:03:23.730 | <p>As this will depend on Pressure drop $\Delta p$, assume that it does not leave the range from 0 to 100 bar. The <a href="http://en.wikipedia.org/wiki/Hagen%E2%80%93Poiseuille_equation" rel="noreferrer">Hagen-Poiseuille equation</a> for an incompressible fluid is defined as:</p>
<p>$$ \dot{V} = \frac{\pi R^4 \Delta p}{8\eta L}$$</p>
<p>I realize that it wont be applicable for very small (nm) diameters, so this question is in the context of microfluidics. Fluids of interest in this case have a kinematic viscosity of 1 cSt to 10000 cSt.</p>
| |fluid-mechanics|microfluidics| | <p>Same answer as above, "Yes BUT..."</p>
<p>This is from a practical point of view:</p>
<p>Other than HPLC, nearly all "microfluidics" applications are far into the laminar region. Poiseuille's law works very well most of the time.</p>
<p>When going below 100<span class="math-container">$\mu$</span>m dimensions, the problems come.</p>
<ul>
<li>lack of stiffness -> violates assumption that fluid into a network segment equals flow out of it. (easy to model)</li>
<li>thermal effects -> varying viscosity and thermal expansion of fluid and solids. (easy to model)</li>
<li>violation of the assumption of a single homogeneous phase (the difficult problem, in my opinion).</li>
</ul>
<p>To expand on the final point, it is mainly from presence of gas or bubbles, but also from "old" fluid not having fully cleared out, if multiple fluids are involved. Focusing on gas/bubbles, it has two effects. First of all, "bulk modulus" again. More significantly, surface tension. It can dominate everything else at low dimension scales and low flows, and can often be intermittent (such as if resulting from dissolved gases vs temperature variation etc), leading to much confusion.</p>
| 271 | Can you use the Hagen-Poiseuille equation for a pipe which radius is in the sub-millimeter region? |
2015-01-26T17:56:37.703 | <p>For a project I had built a <a href="http://en.wikipedia.org/wiki/De_Laval_nozzle" rel="nofollow noreferrer">convergent divergent nozzle</a> designed for Mach number = 3. In that project, I could know the flow has gone supersonic by seeing the manometer fixed between the throat and the divergent section (drop in pressure, as the divergent section acts like a nozzle for the supersonic flow). </p>
<p>However, this got me thinking, If I am to build a nozzle for the propulsion purpose (or any practical purpose), it is not desirable to have holes in it for the manometer in order to maintain the uniform strength. My theoretical calculations tell me that the flow should go supersonic and no shock in the nozzle, but while building, the surface finish, geometric tolerances and supply pressure might not be what I expect. In that case, <strong>how do I know if the flow has gone supersonic?</strong></p>
<p>I thought about following ways. So far I haven't tried any of them. </p>
<ol>
<li><p>Using a Pitot tube might not be useful since there will be a bow shock in front of the tube if in case the flow is indeed supersonic (as shown in the figure), <img src="https://i.stack.imgur.com/oc1oR.jpg" alt="enter image description here"> which will increase the total pressure. We can use <a href="http://nptel.ac.in/courses/101103004/module7/lec6/3.html" rel="nofollow noreferrer">Reyleigh pitot tube formula</a>, but how to compute static free stream pressure without affecting the flow / nozzle? </p></li>
<li><p><a href="https://en.wikipedia.org/wiki/Schlieren_photography" rel="nofollow noreferrer">Schlieren Photography</a>: If we see oblique shocks / shock diamonds, then the inference will be: 'flow is supersonic'. This will work only when the shock features are super clear. </p></li>
</ol>
| |fluid-mechanics|gas|pipelines|measurements| | <p>If you still looking for answer, </p>
<p>You could keep a well-designed wedge, with static holes on the wedge surface, eighther 1. the surface of the wedge is aligned with the flow axis or 2. aligned symmetric line with the flow axis. you will have pitot pressure from Raleigh pitot-tube.</p>
<p>Now you could measure the static pressure directly in case 1. or 2. with Oblique shock relations having known values of wedge angle($\theta$), $P_0$ & $P_{\infty}$ you will get oblique shock angle($\beta$) and Mach number($M$).</p>
| 280 | How to know whether the flow is supersonic in a nozzle? |
2015-01-26T20:33:45.380 | <p>A friend and I wrote a paper for a fluids class discussing the details (as they relate to fluid dynamics) of constructing a cannon that could shoot a steak fast enough to cook it.</p>
<p>We quickly discovered (but not quickly enough to change our topic) that our paper was a bit too ambitious for two twenty year old undergrads taking an introductory course in fluid mechanics. Nonetheless, we still busted out a ballistics simulator, a cookbook, and a compressive heating calculator and did our best.</p>
<p>One of the issues that stumped us was compressing the gas we used to launch the steak. We chose Helium because it was the least dense gas that probably won't burst into flames (like Hydrogen).</p>
<p>Using a compressive heating calculator, we found the velocity we needed to shoot the steak, and were using Bernoulli's equation to find the pressure we needed to launch at our chosen velocity.</p>
<p>The issue we ran into was density is dependent on pressure, but we needed the density to calculate the pressure needed. </p>
<p>How does one determine the pressure given the issue above? Is it simply several rounds of iteration until an acceptable answer is found? </p>
| |fluid-mechanics|gas| | <p>I know this is an old post, however there is a fairly simple way to determine the gas pressure required for launching things from a tube. </p>
<p>Assume isentropic expansion, so that the expression $\frac{p}{p_0}=\left(\frac{V_0}{V} \right)^\gamma$ holds. Here, $p_0$ and $V_0$ are the initial pressure and volume behind the projectile, $p$ and $V$ are the pressure and volume after the projectile has moved some amount, and $\gamma$ is the specific heat capacity ratio of the driver gas (air = 1.4).</p>
<p>Now integrate the pressure on the projectile over the volume it covers in the tube to get its kinetic energy at exit (i.e. "PV" work):</p>
<p>$$
KE=\frac{1}{2}mv^2 = \int_{V_0}^{V_e}p\cdot dV
$$</p>
<p>where $m$ is projectile mass, $v$ is exit velocity, and $V_e$ is the volume of gas behind the projectile just as it's exiting. Now just substitute the isentropic equation into the integral and solve it:</p>
<p>$$
\frac{1}{2}mv^2 = \int_{V_0}^{V_e} p_0 \cdot \left(\frac{V_0}{V} \right)^\gamma dV
$$
$$ = p_0 V_0^\gamma \int_{V_0}^{V_e} \frac{dV}{V^{\gamma}}
$$
$$ = \frac{p_0 V_0^\gamma}{1-\gamma} \left( V_{e}^{1-\gamma} - V_{0}^{1-\gamma} \right)
$$</p>
<p>Then you can just plug in values for everything else and solve for $p_0$.</p>
| 283 | Determining pressures of a compressible gas |
2015-01-27T14:31:24.163 | <p>This is inspired by a project I undertook for my Physics class last year; I'd like to apply it in the future.</p>
<p><strong>Background:</strong> For the project, I and a few classmates were required to build a small car powered by any of a number of sources; we opted for a mousetrap that turned two rear wheels on a four-wheel car as it was released. Our group's goal was to make the car go as far as possible.</p>
<p>One problem was that the power source was not constant. The lever arm of the trap lifted in an arc for about five seconds, then returned to its resting position. During this time, it propelled the car. The car then coasted for the rest of the way.</p>
<p>In the powered stage, we wanted to increase traction. The wheels were CDs (because we had a $5.00 budget), which have a tendency to spin out. So we attached pieces of cloth to them to get better traction. On the second stage, however, we found that this cloth slowed down the car quite a bit because it dramatically increased friction (as we found out after several hours of testing different combinations).</p>
<p>Other groups used duct tape to cover the wheels, and some used records, which seemed to do slightly better than CDs (though that violated the size restrictions). Cloth seemed to have the best traction, though - we didn't have many spinouts. The tests were done on a typical classroom floor (I'm not sure what it's made of - linoleum? - but it's the same as in almost every school, at least in America).</p>
<p>In a wheeled vehicle in general - obviously not just a small car powered by a mousetrap - how can I substantially improve traction on the wheels while the power source is on yet reduce friction while it coasts? Is it as simple as choosing certain tires, or is there a bigger and better engineering solution?</p>
<hr>
<p>As a final wrap-up: My thought would be to have a shifting center of mass for the vehicle, where the powered wheels have a lesser tendency to slip and the front wheels are nearly frictionless. During the powered stage, the center of mass would be near the back, while in the coasting stage, it would be near the front. This could help reduce the normal force on certain wheels and thereby produce or avoid extra friction.</p>
| |mechanical-engineering|friction|applied-mechanics| | <p>The friction you are talking about when referencing the cloth would more correctly be refered to as rolling resistance. In a true sense, you need to increase (the coeficient of) friction to increase traction or increase the force acting normal to the ground.</p>
<p>You can increase the coeficient of friction by changing the wheel material, as you did with the cloth wrap but you found this increased rolling resistance once the vehicle was moving, thus reducing efficiency. An alternative would be to increase mass, thus increasing the force available to generate traction using the formula for traction GlenH7 provided $F_t = \mu_tmg$ where $m$ is mass and $g$ is gravitational acceleration. Increasing mass will negatively affect your acceleration so you could adjust your design to shift as much of the existing mass to the drive wheels as possible (see the dragster analogy, cars are very long, all the weight is at the back), just enough that the front wheel(s) only just maintains contact with the ground on acceleration (see dragster again).</p>
| 298 | How can I improve traction on a wheeled vehicle yet reduce friction? |
2015-01-27T15:52:56.227 | <p>It's been a long time since I went over heat exchangers and even then it was all theoretical. I'm looking for anyone with real world experience with different types of heat exchangers.</p>
<p>I'm looking at building a external wood burning stove. <a href="http://www.crownroyalstoves.com/Websites/crownroyalstoves/images/crs/waterjacket.jpg" rel="nofollow">Most of the designs are pretty much just a firebox surrounded by water</a> which is a step up from just putting a pot of water over a stove. They a bit overpriced/simple for what they are and most are 'dumb' devices. Temp too cold, turn on fan, temp too hot, turn off fan.</p>
<p>Most of my combustion experience is with events that occur multiple times/second rather than in hours. I think there is a lot of room for improvement starting with a proper heat exchanger.</p>
<p>Rough initial estimates:</p>
<ul>
<li>Air Temp: ~1500F | ~800C</li>
<li>Water Temp: <212F | <100C (Not making a boiler due to safety).</li>
</ul>
| |mechanical-engineering|thermodynamics|heat-exchanger| | <p>I can't speak from real (=hands on with my own hands) experience, but in industrial applications I see mostly <strong><a href="http://en.wikipedia.org/wiki/Shell_and_tube_heat_exchanger" rel="nofollow">shell and tube</a></strong> type HX. Exhaust is in the tubes, the shell has the water.</p>
<p>Pros:</p>
<ul>
<li>lots of surface</li>
<li>when you add flanges opposite the tube openings, cleaning is doable</li>
</ul>
<p>Cons: </p>
<ul>
<li>Pressure loss in the small tubes</li>
</ul>
<p>An alternative would be to <strong>jacket the (existing) chimney</strong>, with water between chimney and jacket. Insulation would be addded on the outside.</p>
<p>Pros:</p>
<ul>
<li>easier to retrofit</li>
<li>little change to convection etc. characteristics of the stove</li>
</ul>
<p>Cons:</p>
<ul>
<li>Less surface, so less heat transfer</li>
</ul>
| 302 | Most efficient heat exchanger for dirty air/water medium? |
2015-01-27T21:37:38.480 | <p>With the huge metal shredders that can shred an entire car or a bus, they can shred parts like the axle and engine which are large solid chunks of metal, just like the massive spinning shredder blades.</p>
<p>So why does the car get shredded and not the shredder? Are the blades made from harder/stronger metal, or is there something about their shape that makes them stronger (they just look like large plates with notches on)?</p>
| |crushing|shredding|waste-disposal| | <p>The blades are indeed made of, or tipped with a hard steel/carbide, but they also don't contact each other. The shredding action is created by the shear forces of the two corotating drums of "teeth", and this non-contact between the hard points, high shear strength, and gearing advantages allow it to power through whatever you throw at it. </p>
<p><a href="https://www.youtube.com/watch?v=aVkTj9VrH4o" rel="noreferrer">https://www.youtube.com/watch?v=aVkTj9VrH4o</a></p>
| 309 | Why don't massive industrial shredders shred themselves? |
2015-01-27T22:28:43.523 | <p>I live in Louisiana these days, in an area that is known for its numerous antebellum plantation homes (circa early 1800s). While touring one of these homes it was clear that almost everything about the house was designed around keeping cool in the summer. Some examples:</p>
<ul>
<li>4-meter-high ceilings to allow hot air to rise to the ceiling.</li>
<li>Floor-to-ceiling windows to allow hot air at the top to escape and cool air to be drawn in at the bottom. </li>
<li>Porches on the sunny sides of the house to prevent sunlight from entering the windows.</li>
<li>Large central staircases to allow hot air to rise to the second floor, drawing cool air in on the bottom floor.</li>
<li>Some have a cupola, a central observation room at the top of the house, again to allow hot air to escape at the top of the house and draw air in from the bottom.</li>
</ul>
<hr>
<p>My question is: Given our modern understanding of thermodynamics, how could one design a home today to be cooled passively? Could we do any better than the plantation owners of the 1800s?</p>
<p>Let's define cooling as making the house more comfortable for humans. This means that it is not only important to reduce the temperature, but also to block sunlight and maintain airflow. Also, if possible, it would be very beneficial to extract moisture from the air. </p>
| |civil-engineering|energy-efficiency|heat-exchange|cooling| | <p>In dry/arid areas a <a href="http://en.wikipedia.org/wiki/Windcatcher" rel="noreferrer">wind catcher</a> tower in conjunction with a <a href="http://en.wikipedia.org/wiki/Qanat" rel="noreferrer">qanat</a> is a great way to keep buildings cool. The underground water stays cool and cools the air passing over it that is drawn in through the wind catcher.</p>
<p>However, in a humid climate you would want to isolate the incoming warm/moist air from the cool underground water. You could pass pipes through a large underground water tank and draw the air through those, this may also condense some of the moisture out of the air and you'd need drainage outlets from the air-pipe that come outside of the water tank.</p>
<p><img src="https://i.stack.imgur.com/tPl5D.png" alt="Passive house cooling system" /></p>
<p>Obviously you'd use a much better heat exchange method than I drew in this rather crude picture, but hopefully it explains the basic idea.</p>
| 313 | How to design a house to be cooled passively? |
2015-01-28T08:12:50.767 | <p>To size a heat exchanger, I need to know (among other things) the Reynolds number (Re) as an indication of the flow conditions. The Re number depends on the viscosity. In a <a href="http://en.wikipedia.org/wiki/Shear_thinning" rel="nofollow noreferrer">shear thinning fluid</a>, I can't assume a constant viscosity, instead it will depend on the shear rate that is not constant throughout the pipe. <a href="https://physics.stackexchange.com/questions/86178/what-is-the-apparent-viscosity-in-shear-thinning-turbulent-flow-through-a-pipe">I've asked on Physics SE about the shear rate in turbulent shear thinning fluid flow in a pipe</a>, but received no helpful answer.</p>
<p>Here at Engineering.SE, I am not looking for an in-depth examination of the beauty of the Navier-Stokes equation, but for a practical approach to sizing a heat exchange for such a fluid. How to go about it?</p>
| |mechanical-engineering|heat-exchanger|fluid-dynamics| | <p>First, as Arthur notes, even the best Nusselt number correlations are often as much as 20% off, so don't expect any analytical method to give results that are much better than approximate.</p>
<p>With that said, there are ways to compute the Reynolds number for shear thinning fluids. <a href="http://www.sciencedirect.com/science/article/pii/S0377025704000473" rel="nofollow">Rudman, Blackburn, et al</a> suggest using the effective viscosity for the mean wall shear stress. If you have a fluid that can be modeled as a <a href="https://en.wikipedia.org/wiki/Herschel%E2%80%93Bulkley_fluid" rel="nofollow">Herschel-Bulkley Fluid</a>, then the Reynolds number equation takes the following form.
$$ Re = \frac{\rho \bar{U} D}{\eta_w} $$
$$ \eta_w = K^{\frac{1}{n}} \frac{\tau_w}{(\tau_w - \tau_y)^{1/n}} $$
$$ \tau_w \approx \frac{4}{D} \frac{\Delta p}{L} $$</p>
<p>The authors note that other formulations of the Reynolds number exist for shear thinning flows, and there is no "perfect" Reynolds number for shear thinning flows, but state that this formulation takes the dynamics near the walls of the pipe into account well.</p>
| 329 | How to size a heat exchanger for a shear thinning fluid? |
2015-01-28T15:00:33.337 | <p>A <a href="http://en.wikipedia.org/wiki/Helicopter_rotor#Flybar_.28stabilizer_bar.29" rel="nofollow noreferrer">helicopter flybar</a> is a kind of gyroscopic stabilizer which acts in feedback to the pitch of the blades. The effect, in words, is that as the helicopter starts to pitch or roll in a particular direction more lift is applied on that side, keeping the helicopter level. These devices are commonly found on the inexpensive RC helicopters which are on sale at all major department stores. An example of the flybar from one of these RC helicopters is shown below.</p>
<p><a href="http://www.modifyrcheli.com/what-is-flybar-shorter-or-longer-better" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tGFwG.jpg" alt="enter image description here"></a> </p>
<p>Although it isn't obvious from the picture, there is an angle between the flybar and the rotors. On my RC helicpoter at home the angle is roughly $35^\circ$. I believe that this angle has the effect of introducing extra phase in the feedback loop.</p>
<p>My questions are:</p>
<ul>
<li>What is the effect of changing the angle between the flybar and the rotor?</li>
<li>How can I mathematically describe the effect of this angle?</li>
<li>What would happen if the angle was changed to $90^\circ$? What about $0^\circ$?</li>
</ul>
| |aerospace-engineering|feedback-loop| | <p>This angle is determined by the lift characteristics of the rotor and the rotational inertia of the whole helicopter. You are exactly correct that the angle changes the phase of the feedback.</p>
<p>For the purpose of discussion let's assume the helicopter has pitched forwards slightly and needs to be corrected backwards, also let's assume a single rotor that spins anti-clockwise when viewed from above.</p>
<p>If we adjust the rotor when it is exactly at the front to give some more lift, the time it takes for the additional lift to overcome the rotational inertia of the whole helicopter would mean that the net effect of the lift was applied at some point to the front-left of the helicopter instead of exactly at the front. The next correction would then be slightly further around, and the next further round again. This would cause the helicopter to oscillate in the same manner that a coin does when you knock it over on the table.</p>
<p>So you pretty much answered it yourself, the angle controls the phase offset between the detected error and where the lift <em>begins</em> to be applied. Remembering that the net effect of the lift will be as if applied between where the rotor is adjusted and where the rotor is reset (after some time the helicopter has moved to correct the error).</p>
<p>Changing the offset angle of the flybar will change the angle at which the correcting force is applied relative to the error. The effect of this will be an oscillation in one direction or the other. Up to 90 degrees from where it should be the flybar will give negative feedback out of phase, so should (at least mathematically) remain in a stable oscillation. Beyond 90 degrees from where it should be the flybar will start to give positive feedback instead of negative feedback, again out of phase until 180 degrees, causing the thing to exponentially spin out of control.</p>
| 332 | What is the effect of changing the angle of a helicopter flybar? |
2015-01-28T19:27:50.297 | <p>Many commercial and open-source CFD codes implement several closure methods for the non-linear convective acceleration term of the Reynolds-averaged Navier-Stokes (RANS) equations. Common methods (also known as <em>turbulence models</em>) include</p>
<ul>
<li><a href="http://en.wikipedia.org/wiki/Spalart%E2%80%93Allmaras_turbulence_model">Spalart–Allmaras</a> (S–A)</li>
<li><a href="http://en.wikipedia.org/wiki/K-epsilon_turbulence_model">k–ε</a> (k–epsilon)</li>
<li><a href="http://en.wikipedia.org/wiki/K-omega_turbulence_model">k–ω</a> (k–omega)</li>
<li><a href="http://en.wikipedia.org/wiki/SST_(Menter%E2%80%99s_Shear_Stress_Transport)">SST</a> (Menter’s Shear Stress Transport)</li>
<li><a href="http://en.wikipedia.org/wiki/Reynolds_stress_equation_model">Reynolds stress equation model</a></li>
</ul>
<p>Which of these are suitable for CFD simulation of a streamlined vehicle body? The purpose of the simulations is to guide the refinement of the body shape to minimize aerodynamic drag forces. An exemplary answer would briefly outline the advantages and disadvantages of each method for this simulation application.</p>
<hr>
<p>Potentially useful details:</p>
<p>The vehicle is a small one-person vehicle with approximate dimensions</p>
<ul>
<li>L = 2.5 m,</li>
<li>W = 0.7 m, and</li>
<li>H = 0.5 m.</li>
</ul>
<p>It will be travelling at speeds ranging from 0 m/s to approximately 12 m/s. All three wheels are enclosed by the body envelope, and the vehicle has an approximate ground clearance of 15 cm except near the wheels, where the body shell extends down to within 1 cm of the road surface.</p>
<p>Normally aerodynamic forces at these speeds are very nearly negligible, but assume that this vehicle is being designed to compete in a "Super Mileage" competition on a smooth track, is very light-weight, and uses low friction drivetrain components throughout, so the aerodynamic forces have a significant effect on the achievable fuel consumption.</p>
| |mechanical-engineering|fluid-mechanics|fluid-dynamics|simulation|modeling| | <p>In case you only have the resources to perform one simulation only I would agree with @Subodh and use $ k-\omega\:SST$.</p>
<p>In case you can afford multiple simulations I would use different models and compare. This way you can identify the influence of the turbulence model in your particular application.</p>
<p>Could you clarify if you are looking for an optimal velocity distribution or if you are more interested in separations?</p>
| 336 | Which turbulence models are suitable for CFD analysis on a streamlined vehicle body? |
2015-01-28T22:00:19.550 | <p>The specific example I have in mind is a car tire with a small leak in it. As the pressure increases, does the outflow of air increase linearly, i.e. $v\propto P$, or does it have some more interesting behavior?</p>
| |fluid-mechanics|pressure|fluid-dynamics|valves| | <p>Yep, the answer is a bit more interesting.</p>
<p>The mass flow rate ($\dot m$) will vary as $C_d A \sqrt{2 \Delta P}$ (discharge coefficient, cross sectional area, and change in pressure across the valve). Barring compression of the fluid, velocity will behave the same way.</p>
<p>The devil of the thing is in $C_d$. This is pretty much something that you have, measure experimentally or (try to) model numerically with computational fluid dynamics. That little number captures all of the non-ideal aspects of the flow involved (viscosity, turbulence). It's always less than one (nothing is ever ideal), so a simple prediction from Bernoulli will always over-predict (just a question of how much).</p>
<p>In practice, $C_d$ will change as you adjust the valve (as will area). So the manufacturer will usually just give you a few values or a curve for flow as a function of valve position and pressure or a total flow coefficient $C_V$ as a function of valve position ($\sqrt{\Delta P }$ dependence assumed).</p>
<p>As mentioned in the other answer, this all goes funny if the flow becomes compressible.</p>
| 337 | For a small valve in a fluid system, what is the relationship between pressure and flow rate? |
2015-01-29T08:59:38.777 | <p>Suppose I have a concrete tank, round, vertical, diameter in the range 16m-24m. One half the floor is filled with gravel or concrete to a height of maybe 2 meters. When the tank is empty, that will mean I have about 5 tons per m² more weight on this half, when the tank is full (with water or a slurry that's mostly water) it's still 3 tons (assuming 2.5 t/m³ density which is exact enough for my ballpark). Most of the times (>90%) it will be full. The tank will be above ground, 8-10 m high.</p>
<p>I want to know if the tank will tilt during its lifetime, say 20 years. I'm not a civil engineer and I have no feeling for the numbers involved. My gut feeling is that my tank will tilt visibly in a matter of a few years and that my idea is not feasible as is. Can someone weigh in and comment on ...</p>
<ul>
<li>Will I have tilt/uneven settling problems? At what magnitude over the tanks lifetime?</li>
<li>What's the easiest (=cheapest) remedy, leaving the tank interior alone?</li>
</ul>
<hr>
<p><em>Clarifying points</em>: The tank is not yet built or even planned. It's just an idea I'm thinking about that calls for half filling the tank to create a sort of funnel. I wonder if this idea is worth pursuing, and uneven loading/settling is one issue want to consider. I'm not in the "call a structural engineer and let him calculate the static" phase, I'm in the tossing around harebrained ideas in my skull phase. I'm sure such a tank can be built to last for 20 or 200 years, but at what price? </p>
| |civil-engineering|soil|geotechnical-engineering| | <p>Here's a really (and I mean really!) quick and dirty set of calculations that might give you an idea of the magnitudes of settlement you could be dealing with. </p>
<p>The settlement potential of the tank location can be determined a number of ways, but probably the best thing to do would be a plate load bearing test. The test can be run to simulate the range (though not the duration) of loads you are expecting. A test like this will give you a spring constant $k$ that represents the "modulus of subgrade reaction" of the bearing soil (for the tested loading range). However, it's a short term test that doesn't take into account creep, so the long-term $k$ value will be lower. </p>
<p>In general, a short-term $k$ will run from something like 80pci for a very soft clay to something like 250pci for a very dense sand (caveat: this is just from the top of my head without looking anything up). </p>
<p>So let's use the worst case scenario here, and to take into account creep, let's do what geotech engineers do best and slap a 2.5 safety factor on it. So we have about a 30pci modulus of subgrade reaction. </p>
<p>Let's also assume that most of the differential settlement will occur as a result of the uneven loading of the empty tank, and that the emptying/filling of the tank is going to have a negligible contribution to the differential settlement. This isn't too terrible of an assumption, since the difference in applied surface pressure (which determines differential settlement) is much greater in the empty state, and it's also conservative because it will only be empty 10% of the time anyway. </p>
<p>So here we go (I'm American, so we're doing everything other than what you gave for dimensions in imperial-scum units first and then converting - sorry!): </p>
<p>$k = 30 \frac{lbf}{in^3}$, $\gamma_{concrete}=150\frac{lbf}{ft^3}$, $H_{concrete}=2m$</p>
<p>Applied pressure under half the tank: $q_c=H_c\times\gamma_c=0.98ksf=5.3 \frac{tonf}{m^2}$</p>
<p>Settlement under loaded half of tank: $S=\frac{q_c}{k} = 0.23in = 5.8mm$</p>
<p>If we assume the other side of the tank does not settle at all, our differential settlement comes out to about 6mm. </p>
<p>Now, this number assumes the loaded side of the tank is free to settle while the unloaded side remains static. This is not the case. Assuming the tank is nice and stiff, some of the applied pressure on the loaded side will be transferred to the unloaded side (which will reduce the settlement of the loaded side). </p>
<p>I don't know what the application is for this tank, but the above is probably a pretty conservative analysis of the situation you described. I would be surprised if differential settlement potential turns out to be a problem for you. </p>
<p>EDIT: One thing to note is that the tank will "wiggle" when it is being filled/drained. What I mean is, the entire thing will settle more when it is filled, but it will settle <em>more</em> in the unloaded side (thereby undoing some of the differential settlement in the empty condition). Then when drained, the soil will rebound and the tank will return to the more-tilted empty condition when the unloaded side rebounds more than the loaded side (though it is likely neither side would rebound fully). </p>
<p>Assuming the 6mm of settlement from above, the deflection angle for the 24m diameter tank comes out to be $\arctan\frac{6mm}{24m}=0.014^{\circ}$. Pretty tiny. </p>
| 346 | Settling of unevenly loaded storage tank |
2015-01-29T21:18:31.507 | <p>This is related to the question <a href="https://diy.stackexchange.com/questions/58612/will-dark-gutters-stay-ice-free-better-than-light-colored-gutters">Will dark gutters stay ice free better than light colored gutters?</a> The current consensus seems to be that the mass of the gutters and limited light may make simply painting them black insufficient.</p>
<p>In my particular application the two major gutter runs are facing East and West so get decent amount of sun light. Physical location is Pittsburgh PA. </p>
<p>From an engineering perspective, these are some of the questions that would need to be addressed: </p>
<ul>
<li>How much would I need to raise the temperature of the gutters to prevent ice build-up? </li>
<li>How can I calculate the energy required to raise the temperature of the gutters?</li>
<li>How much energy can I theoretically obtain from sunlight throughout the day for a given area during the winter at my latitude?</li>
<li>How would the analysis change if I also wanted to melt ice that had built up during the night?</li>
<li>What other factors should one consider when designing such a system?</li>
</ul>
| |solar-energy| | <p>And so, as promised, I'll do it.</p>
<p>First assumption:
Let's work with 1m long strips of gutter. It'll be easier to calculate everything starting from here.</p>
<p>Let's say the gutter is already full of ice. (We'll work on the ice filling problem later on)</p>
<p>The standard gutter size (according to <a href="http://stormmaster.com/gutter-sizing/" rel="nofollow">this site</a>) is 5-inch K-Style, or 6-inch half round. If we use the half round version, we can <a href="http://www.wolframalpha.com/input/?i=6%20inch%20diameter%201m%20long%20cylinder%20volume%20in%20Liters" rel="nofollow">learn</a> it holds around 9L of ice. The <a href="http://www.wolframalpha.com/input/?i=9Liter%20of%20ice" rel="nofollow">latent heat of fusion</a> of 9L of ice is 3000kJ, or 833Wh. This means that if you wanted to melt this ice in an hour, you'll need 833 Watts of power. For each meter of gutter.</p>
<p>But what's the available solar energy?</p>
<p>Second assumption: Let's say the sun shines all day, with the perfect orientation regarding the gutter, and the gutter absorb all the energy it receives from the sun. Let's assume it's the shortest day of the year, at the US mean latitude (around 38°N).</p>
<p>According to <a href="http://pveducation.org/pvcdrom/properties-of-sunlight/calculation-of-solar-insolation" rel="nofollow">the third chart</a>, the length of day on the shortest day of the year is around 9h30m. Let's round this up to 10h (I like round and easy numbers for my ballpark calculations).</p>
<p>The cross section of our gutter <a href="http://www.wolframalpha.com/input/?i=6%20inch%20by%201m%20long%20rectangle%20area%20in%20m%C2%B2" rel="nofollow">is around</a> 0.15m². The sun irradiance reaching the ground stands at around 1000W/m². This means around 150W reaches our gutter. On a 10h day course, that would be 1500Wh. Hey, that would be enough! Well, yes, if you take your gutter and put in the perfect orientation regarding the sun. Which it's not true.</p>
<p>In this case, the energy received will be lower. Moreover, one also has to take into account the efficiency of the energy conversion. High quality solar thermal collector (which collect solar radiation and convert it to heat) typically have efficiency at around 60%. This means, in our case (where the efficiency will be lower, we'll receive at best 900Wh.</p>
<p>If we take into account the fact that our gutter has a fixed orientation, the energy received will be even lower than that. Thus, we won't have enough energy to melt the ice.</p>
<p>Given this data, I'd say it's not possible.</p>
<p>As for the ice filling the gutter. The problem is still the same. Making sure that the water flowing from the roof stays hot enough will still means providing enough energy to keep it above freezing. Also, usually, water melts on the roof, but doesn't flow alone, i.e. it brings down some snow and ice in the gutter, which you'll have to melt of you want to prevent ice buildup in the gutter.</p>
| 356 | Is it possible to modify a home gutter to absorb sufficient solar energy to melt ice? |
2015-01-30T10:54:13.783 | <p>I recall learning about the bronze age in school. Why was bronze so important to the technological development of humans? Why not some other metal? </p>
| |materials|metallurgy|engineering-history| | <p>I'd like to add to what @Fred said.</p>
<p>Bronze wasn't the first. Before the Bronze Age, there was a comparatively brief <a href="http://en.wikipedia.org/wiki/Copper_Age" rel="nofollow">Copper Age</a> [also <a href="http://en.wikipedia.org/wiki/Metallurgy_during_the_Copper_Age_in_Europe" rel="nofollow">this</a>]. Copper is comparatively abundant, and it sometimes naturally occurs in pure state (nuggets), as well as ores.</p>
<p>In some places, polymetallic ores were used for producing copper. Early metalworkers noticed that the resulting "copper" had different properties. I've used quotes, because it wasn't copper anymore: accidentally, it was bronze. Subsequently, alloying materials were added on purpose.</p>
| 371 | What made bronze the first widely used (non-ornamental) metal in human civilization? |
2015-01-30T16:30:13.347 | <p><a href="http://en.wikipedia.org/wiki/Otto_Lilienthal">Otto Lilienthal</a>, sometimes hailed as the first aviator, became famous for his many gliders, which pioneered new experiments in aeronautics. However, powered aircraft didn't take off - pun intended - until the Wright brothers successfully flew their powered airplane. Did Lilienthal ever experiment with non-human-powered flight? Wikipedia says he built a small engine, but doesn't say whether or not it was ever used in an aircraft.</p>
| |engineering-history|aerospace-engineering|aircraft-design| | <p>I'm going to answer my own question because just before posting it I found an answer.</p>
<p><a href="http://www.lilienthal-museum.de/olma/e213.htm" rel="nofollow noreferrer">The Otto Lilienthal Museum</a> has a comprehensive list of Lilienthal's designs. One is listed as the "small wing-flapping machine." It didn't use a propeller (or jet engine, of course!) but instead used a small engine weighing about 22 pounds when fully fueled. Its wingspan was 22 feet, which was quite impressive. <a href="http://en.wikipedia.org/wiki/Otto_Lilienthal" rel="nofollow noreferrer">Wikipedia</a> lists it as "Small Ornithopter." <a href="http://en.wikipedia.org/wiki/Ornithopter" rel="nofollow noreferrer">Another page</a> calls it the <em>kleiner Schlagflügelapparat</em>, which Google Translate translates to "small flapping wing apparatus."</p>
<p><a href="https://upload.wikimedia.org/wikipedia/commons/d/d9/Otto_is_going_to_fly.jpg" rel="nofollow noreferrer">Here's</a> a picture of Lilienthal with it, pre-flight.</p>
<p><img src="https://upload.wikimedia.org/wikipedia/commons/d/d9/Otto_is_going_to_fly.jpg" alt="Small ornithopter pre-flight"></p>
| 380 | Did Otto Lilienthal ever experiment with non-human-powered aircraft? |
2015-01-30T18:28:07.380 | <p>The cabin noise of vehicles is <a href="http://elevatingsound.com/the-top-30-quietest-cars-a-cabin-noise-test-by-auto-bild/" rel="nofollow">a large concern in the luxury market</a>. Why haven't any automakers used the same techniques as are employed in <a href="http://en.wikipedia.org/wiki/Noise-cancelling_headphones" rel="nofollow">active noise cancelling headphones</a>. In these devices a microphone is used as an in-loop sensor to suppress low frequency ($f\lt500\ \text{Hz}$) pressure fluctuations in the enclosed space near the ear. </p>
<p>Why isn't similar technology used in vehicles? What technical improvements are required over the standard vehicle sound system to implement it? Given the increased volume of the system, what differences can be expected between vehicles and headphones? </p>
| |electrical-engineering|feedback-loop|audio-engineering|automotive-engineering| | <p>Some reasons why noise reduction in vehicle cabins is not a standard feature, yet:</p>
<ul>
<li>As @Trevor Archibald states, safety is a very good reason. There is still a need to hear
some noises from outside the vehicle such as the sirens of emergency
vehicles: police, ambulance, fire fighters truck</li>
<li>Hearing car horns from other drivers is still needed</li>
<li>The sound of the engine lets people know if the engine is performing as
it should</li>
<li>It's an added cost some people may not want to pay</li>
<li>People haven't asked for it</li>
<li>Most people don't object to hearing some noise, as long as it's not intrusive</li>
<li>Insulating vehicle cabins against noise by using sound proofing
materials has suited most peoples needs until now</li>
</ul>
<p>It has been introduced in a small number of cars: <a href="http://www.usatoday.com/story/money/cars/driveon/2012/09/26/cars-noise-cancellation-technology/1590703/" rel="nofollow">Auto Makers Shush Cars</a>, but these are a bit more expensive than the average car. See also: <a href="http://www.nydailynews.com/autos/ford-honda-noise-canceling-tech-new-cars-bmw-porsche-bring-noise-boosting-features-article-1.1168670" rel="nofollow">Cars Go Quiet</a>, <a href="http://www.autoblog.com/2013/12/22/bose-noise-cancelling-in-new-cars/" rel="nofollow">Bose Noise Cancelling in Cars</a></p>
<p>However, introducing electronic noise reduction technology in cars, could reduce the weight of cars by reducing the amount of sound proofing materials used, <a href="http://news.harman.com/Press-Releases/Industry-first-HARMAN-Quiets-the-Car-by-Minimizing-Road-Noise-1ef.aspx" rel="nofollow">Harman Quiets the Car</a></p>
<p>On a different angle, in the 1980s electronic noise cancelling had been used to cancel the engine noise made by heavy vehicles used in inner urban development site to reduce the amount of noise heard by nearby residents.</p>
| 382 | Is active noise cancellation useful in vehicles? |
2015-01-31T19:04:29.037 | <p><strong>Background:</strong><br>
In an automobile, only <sup>1</sup>/<sub>3</sub> of the potential energy in the fuel is converted into mechanical energy and significant portion of the energy is lost as heat. </p>
<p>There have been previous attempts to recuperate this lost energy. In the early 1990's, Porsche developed automotive thermoelectric generators (ATEG) which didn't go past prototyping stage. Currently, Porsche Motorsports is testing a thermal energy harvesting system in their LeMans series Race car.</p>
<p><img src="https://i.stack.imgur.com/KbiMM.jpg" alt="enter image description here"></p>
<p>In addition to Porsche's research, GM is in collaboration with Future Tech, LLC. to explore the idea of using themoelectric technology to harvest energy from internal combustion engines. Other automotive manufactures, such as BMW, are also exploring this technology. </p>
<p>Currently the power usage in a</p>
<ul>
<li>Small car is approximately 150 W</li>
<li>Full size truck is approximately 500 W</li>
</ul>
<p>If this technology can successfully be implemented, then components such as the radiator, water pump, and alternator could effectively have reduced workload or removed from the system, thus reducing the load to the internal combustion engine.</p>
<hr>
<p><strong>Question:</strong><br>
With the growing interest in green technology, are there technology barriers <strong>beside efficiency</strong> that are preventing the implementation of energy harvesting from internal combustion engines using thermoelectric technology?</p>
<hr>
<p><strong>References:</strong> </p>
<ul>
<li><a href="https://electronics.stackexchange.com/questions/197038/which-one-must-be-used-matched-output-voltage-or-open-circuit-voltage">Which one must be used matched output voltage or open circuit voltage?</a></li>
<li><a href="http://energy.gov/sites/prod/files/2014/03/f13/stabler.pdf" rel="noreferrer">Benefits of Thermoelectric Technology for the Automobile</a> </li>
<li><a href="http://www.o-flexx.com/fileadmin/media/documents/chip_silver_edition_-_The_promise_and_problems_of_thermoelectric_generators.pdf" rel="noreferrer">The Promise and Problems of Thermoelectric Generators</a> </li>
<li><a href="http://www.academia.edu/3648232/Modeling_of_an_Automotive_ThermoElectric_Generator_ATEG_" rel="noreferrer">Modeling of an Automotive ThermoElectricGenerator (ATEG)</a> </li>
<li><a href="http://www.gizmag.com/thermoelectric-cars-improve-mpg/10928/" rel="noreferrer">Thermoelectrics to replace car alternators and improve MPG</a> </li>
<li><a href="http://www.ijiset.com/v1s3/IJISET_V1_I3_47.pdf" rel="noreferrer">Thermo-Electric Generator in Turbocharged Diesel Engine</a> </li>
<li><a href="http://www.kettering.edu/news/recovering-wasted-energy" rel="noreferrer">Kettering University researchers are working with General Dynamics to convert the unused heat energy of their propulsion systems to useful and clean energy</a> </li>
<li><a href="http://www.thetruthaboutcars.com/2014/03/porsche-919-hybrid-lemans-racer-goes-after-the-two-thirds-of-gasolines-energy-thats-wasted-as-heat/" rel="noreferrer">Porsche 919 Hybrid LeMans Racer Goes After The Two Thirds of Gasoline’s Energy That’s Wasted As Heat</a> </li>
<li><a href="http://ecomodder.com/forum/showthread.php/germans-trying-replace-alternator-thermoelectric-generators-tegs-3124.html" rel="noreferrer">Germans trying to replace Alternator with Thermoelectric Generators or TEGs</a> </li>
<li><a href="http://www.greencarcongress.com/2014/03/20140305-919.html" rel="noreferrer">Porsche 919 Hybrid LMP1 Le Mans prototype...</a></li>
</ul>
<hr>
<p><strong>Footnote</strong></p>
<p>The <a href="https://engineering.stackexchange.com/questions/372/energy-harvesting-using-thermoelectric-technology">suggested duplicate</a> is related, but still distinctly different. The order of magnitude of energy available to recover from an internal combustion engine is significantly greater than within the GPU of a video card. As such, the economies of scale are different and different solutions are therefore possible. </p>
| |mechanical-engineering|electrical-engineering|thermodynamics| | <p>The use of any idea/technology is dependent on its cost effectiveness. Efficiency is just one part of the analysis. If the modules were free and never went bad, they would certainly be in use. However, high temperature modules are expensive for their power produced; making them currently not cost effective.</p>
<p>EnergyNumbers does a good job of explaining the thermodynamics involved. Because of Carnot you cant harvest significant amount of power in this fashion. This is also why money for peltier modules is better spent on increased engine efficiency/performance instead. However, high temperature waste heat will be there regardless and when thermoelectric becomes cost effective, you will see it in use.</p>
<p>Despite what people may advertise, no idea/technology is "green" until it can pull its own weight in a cost benefit analysis. You have to look at the whole picture; how many dollars and raw materials did it consume to provide this "green" energy.</p>
<p>If you would like to discover it for yourself, here are some high temperature modules you can price and evaluate. Remember the watts listed is the consumed power not the generated power. <a href="http://tetech.com/peltier-thermoelectric-cooler-modules/high-temperature/" rel="nofollow">tetech.com</a></p>
| 389 | Thermoelectric Technology to Harvest Energy from Internal Combustions Engines |
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