Talk:Shock wave/Archive 1

message: When the Shuttle takes of, why don't people on the ground hear a sonic boom?  Is it the sound wave is radiated away from the ground toward space?  If so, I would think some of the sound waves would be reflected back towards earth and something would be heard.  Or is a boom heard but the Shuttle engines are too loud and the boom is drowned out?
When a vehicle goes supersonic, do the people in the vehicle hear the boom?  Or is it now that they are traveling faster then the sped of sound the sound wave never catches up to them so nothing is heard?
When a vehicle goes subsonic, does it create a sonic boom?  If so, is the wave radiated backward or forward?
When the shuttle returns to earth, is the reason the sound is different, “the crack sound”, because it’s not really a sonic boom it’s a thunder clap?  Is this because the air is so thin an air pressure wave is not created?
—The preceding unsigned comment was added by 70.90.163.246 (talk) 17:06, 11 December 2006 (UTC)
thread_title: Questions about Sonic Booms

message: i'm surprised the most common type of shockwave, the mach wave (sound wave), is not mentioned in the article.  it is an infintely weak shock. 141.211.174.250 23:00, 8 Nov 2004 (UTC)
thread_title:  mach wave 

message: I must say, I found this article really quite misleading and inaccurate.
The equations of fluid mechanics are non-linear, with wave-like solutions. In an ideal fluid, compression waves tend to collate and form discontinuities that are known as "shock waves", while expansion waves tend to disperse. Thus, even the first sentence of the article needs to be more specific in its definition; a shock wave is specifically a compression wave, as a non-linear expansion wave will not form a shock.
It can also be when the actual molecular or particle speed is moving faster than the wave propagation speed
This doesn't really describe a shock wave, more a possible cause of a shock wave. Even then, I'm not sure it makes sense.
In compressible fluids such as air, disturbances such as the pressure changes caused by a solid object moving through the medium will propagate through the fluid as pressure waves traveling at the speed of sound.
In general, the speed of a shock is a function of its strength and the compression waves caused by a subsonically moving disturbance will collate and form shocks just like any other compression waves. The speed of sound is the speed of infinitessimally small waves, but there is nothing magical about it. There is no strong distinction between linear and non-linear waves; all fluid waves are non-linear in their behaviour, but for very small ones, the non-linearity is often insignificant in the length- and time-scales of interest.
When the cause of the disturbance is moving slowly relative to the speed of sound, the pressure wave takes the form of conventional sound waves.
And yet, in the case of explosive shock waves, the disturbance is stationary. These waves move at a speed depending on their size. Again, there isn't anything different about sound waves, they are simply the limiting form of a shock or expansion wave as its strength approaches zero.
The pressure waves enable the fluid to redistribute itself to accommodate the disturbance, and the fluid behaves similarly to an incompressible fluid
This in general is not true, the fluid doesn't "behave like" an incompressible fluid. Low mach number flows can and do behave compressibly. Take acoustics, for example; Imagine a loadspeaker generating sound at 1KHz, with its diaphragm moving 1mm. The maximum disturbance speed is therefore 6.3m/s, which is way below the sound speed in air of 340m/s. Yet, it is still the compressible, wave-like behaviour of the fluid that is of interest. Note that 6.3m/s will also be a typical fluid velocity of the flow. 
If the flow is steady and low mach number, then the steady-state solution will look incompressible, but transient behaviour will still be wave-like. It's important to remember this when considering things like explosive flows, where the disturbance is stationary, and the flow can be relatively low mach number, but it's the fast transient that dominates the flow picture.
However, when a disturbance moves faster than the pressure waves it causes, fluid near the disturbance cannot react to it or "get out of the way" before it arrives. The properties of the fluid (density, pressure, temperature, velocity, etc.) thus change almost instantaneously as they adjust to the disturbance, creating thin disturbance waves called shock waves and shock heating.
Again, I really don't like this! A supersonic disturbance creates supersonic waves. If these are compression waves, then they collate to form discontinuous shocks, if they are expansion waves then they spread out (e.g. a Prandtl-Meyer expansion fan).
Shock waves are not sound waves...Over time a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy
I find this strong distinction between shock waves and sound waves very misleading. Also it should be mentioned that the shock strength tends to be reduced mostly by spreading.
I think this page could use some heavy editing!
Steve Thomas 16:48, 21 September 2005 (UTC)
thread_title:  Bad article 

message: Since user Robinh insists on reinstating material, and I have no particular interest in a reversion war, this section is a discussion of what material belongs in this article and what doesn't.
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However, when a disturbance moves faster than the pressure waves it causes, fluid near the disturbance cannot react to it or "get out of the way" before it arrives. The properties of the fluid (density, pressure, temperature, velocity, etc.) thus change almost instantaneously as they adjust to the disturbance, creating thin disturbance waves called shock waves and shock heating.
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My problem with this paragraph is that there is no such thing as a disturbance wave, and that there is certainly no "thin disturbance wave called shock heating". Additionally, since a shock wave is a particular kind of pressure wave, it can't be caused by a disturbance moving faster than the pressure wave it causes. I know what you're trying to say, but as formulated it is either impossible to understand or wrong. Take your pick.
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Shock waves are not sound waves; a shock wave takes the form of a very thin membrane (sheet of energy) on the order of micro-meters in thickness. The pressure excursion within the shock wave is so extreme that it causes the speed of sound within the wave to change. Shock waves in air are heard as a loud "crack" or "snap" noise. Over time a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom, commonly created by the supersonic flight of aircraft.	
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My problem with this paragraph is that shock waves are not membranes, and are not sheets of energy in any literal sense, and I think that this explanation is just confusing. One speaks of pressure change, not "excursion", and although some models use a speed of sound change within the shock wave to model it, I am aware of no measurements of this phenomenon (if you have a reference, maybe I'll agree with this). The speed of sound changes behind the wave.
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There are two types of shock waves: normal shocks and oblique shocks. A normal shock extends perpendicular to the flow of fluid, and the flow goes from supersonic upstream of the shock wave to subsonic downstream. An oblique shock is formed at an angle to the flow, and although the component of flow perpendicular to the oblique shock goes from supersonic to subsonic in crossing the wave, the tangent component of flow is not affected, so the net flow may remain supersonic downstream of an oblique shock wave.	 
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My problem with this explaination is mainly that normal and oblique waves are not different. A normal shock is merely a special case where the oblique angle is 90 degrees. The explanation here is impossible to understand, and I would suggest either expanding it significantly or deleting it.
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frame|A cage around the engine reflects any shock waves.	 
	A spike behind the engine converts them into trust.	 
frame|To generate lift a supersonic airplane has to produce at least two shock waves: One over-pressure downwards wave, and one under-pressure upwards wave.	 
Whitcomb area rule states, we can reuse air displacement without generating additional shock waves.	 
	In this case the fuselage reuses some displacement of the wings.	 
	
(Images linked here)	
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These images make no sense here. They are not referenced or explained in any way. They appear to be copied from supersonic, where they are at least somewhat at home. It's my opinion that the quality of the first picture is so poor as to be useless, especially since it's just explaining Busemann's shockless internal flow concept. The area rule picture is taken from Whitcomb_area_rule and should be referenced as such rather than duplicating data.
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Analogous phenomena are known outside fluid mechanics. For example, particles accelerated beyond the speed of light in a refractive medium (where the speed of light is less than that in a vacuum, such as water) create shock effects, a phenomenon known as Cerenkov radiation.	 
		 
There are two basic types of shock waves: blast waves and driven waves. A blast wave is produced by explosive phenomena. Blast waves can travel out from their source at supersonic speeds. A driven wave is produced by a source that constantly ejects matter (for example, the solar wind). A driven wave can reach a static state where it bounds the wind.	 
		 
When meteors enter the earth's atmosphere, this phenomenon causes them to heat up and disintegrate; this is sometimes erroneously attributed to friction.	 
		 
Another example of a shock wave is the boundary of a magnetosphere. At the shock wave, particles from the solar wind will abruptly slow to subsonic speeds.
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I suppose that the Cerenkov radiation example is relevant, although it's drawing a long bow. The description of the different kinds of shock waves is basically wrong, and I don't see that it's close enough to being a good description to be kept. The meteor example is wrong, you're thinking of the combination of stress by differential heating and vaporisation of gaseous internal components. The third example is ok.
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I've linked this on your talk page, and I hope we can come to some resolution AKAF 13:37, 8 December 2005 (UTC)
thread_title:  Reinstated material 

message: Physically I'd expect this was possible (just view a normal shock with small sideways translation) and it's apparently easy to find such solutions on the nasa 'wedge' applet.
Here (at a  NACA/NASA webpage)
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Could you tell me what data you've put in?, I don't see M2<1 for an oblique shock, but if you could tell me what upstream Mach number and turning angle you used?AKAF 06:55, 26 April 2006 (UTC)
replies: text: 'For the Mach number change across an oblique shock there are two possible solutions; one supersonic and one subsonic. In nature, the supersonic ("weak shock") solution occurs most often. However, under some conditions the "strong shock", subsonic solution is possible'.
and, at
essentially develops the point made earlier (only the normal component of flow goes from supersonic to subsonic)
Linuxlad 07:58, 26 April 2006 (UTC)

text: I don't necessarily see the applicability of the strong shock approximation to converging pipe flow (I've noted it in the detached shock section-thanks). In this case wouldn't the effect of any wall convergence be equal to the farfield effect of a detached shock, which is the weak shock solution? I think (correct me if I'm wrong) that in the case where the strong shock is applicable to the general duct flow, that the duct must choke and push a normal shock upstream. I'm not sure whether the Mach number < 1 area can extend to be a significant area of the pipe? I'm not saying it's wrong (and I'll leave the comment there), but I'm not managing to get my head around it.AKAF 11:02, 26 April 2006 (UTC)
thread_title: M<1 behind an oblique shock?

message: In my opinion shock waves can be created by supersonic (but not only) objects, but the created waves themselves travel with speed of sound. Take evergreen example of a jet plane. The plane creates shock waves, but at the time the shock wave arrives to your ears, the plane is gone far far away and you need some amount of experience to get a sight of it.
Marcel
:That's not a shockwave that's just ordinary soundwaves.
:To get a shockwave something has to be travelling faster than sound. In that case some parts of the air around the aircraft are travelling at exactly the speed of sound *towards* the aircraft, so that the soundwaves leaving the aircraft pile up on each other, sort of like a tailback on a road, and a shockwave forms, the pressure goes up and up and up there, and then spreads out sideways. Because of this amplification effect, a shockwave is very intense, more like an explosion when you hear it (not coincidentally, since explosions create shockwaves.)WolfKeeper 13:18, 11 May 2006 (UTC)
replies: text: I really like this clarification, so I copied it to the main page. Hope that's OK. AKAF 14:12, 11 May 2006 (UTC)

text: This is totally wrong! You have to understand that get shock wave you do not need to move above the speed of sound. In fact, any movement creates shock wave. The faster the movement the larger  the shock.  In every explanation of the speed of sound is originated by a very small movement. Check any compressible flow book that you like, you can use my  here www.potto.org or you can use shapiro or Saad or any thermo book. No matter what book you choose the results should be the same.  Now take for example piston moving with Mach number 0.1 what will be the velocity of shock ahead and according to you it will be below the speed of sound. Of course you are wrong! if you where right than
you never hear car approaching you. The shock will move in Mach=1.062    for =1.4. Think differently, if you were right than zero movement (almost zero creates sound wave) yet larger velocity creates smaller velocity. You were right it will violate the second law of thermo. --potto 04:10, 2 February 2007 (UTC)
thread_title:  Supersonic? 

message: The article seems to be written primarily from an aviation perspective, but many shock waves exist that are not due to fast objects moving through air.  Examples should include blast waves, termination shocks, standing shock waves in flows (such as entrance to a conventional ramjet that is traveling supersonically), rate limiting of Petchek-style magnetic reconnection (in an MHD context),  Cerenkov radiation and other wake effects like shocked wakes from power boats and the Earth's Alfven bow shock in the solar wind, and (as others have mentioned above) detonation waves such as knocking in an internal-combustion engine. zowie 19:49, 14 July 2006 (UTC)
thread_title: Overhaul needed

message: The entire first part of the entry uses "shock wave".  Then in the second half of the entry the word "shockwave" appears.  Personally I think the second form doesn't belong here, but I don't care about that as much as I'd rather see one or the other used used throughout.
: Agree, I have replaced shockwave by shock wave. myth 03:28, 15 September 2006 (UTC)
thread_title: Shock Wave vs. Shockwave

message: What are the fundamntal diffs between ordinary logitudinal compression/rearefaction waves in , say, air or water, and real shock waves?--Light current 07:39, 21 October 2006 (UTC)
replies: text: The classical difference is that standard waves follow a sine wave in pressure (or height, whatever), and that shock waves follow a step function. AKAF 15:52, 23 October 2006 (UTC)
replies: text:  Also shock waves move faster than the speed of sound in the medium. This can be a bit confusing if there are several frames of reference, since it means that the speed difference between the fluid directly before the shock, and the shock is greater than the local speed of sound in the fluid directly before the shock. This means one property of a shock is that it is independant of the properties of the gas behind the shock. AKAF 15:57, 23 October 2006 (UTC)
thread_title:  Fundamental properties 

message: I don't see how a shock wave can be electromagnetic, or otherwise propagate in free space.  I think the defining difference between a sound wave and a shock wave is that non-linear effects are important in shock waves.  David R. Ingham (talk) 10:03, 11 July 2012 (UTC)
replies: text: Yes, the article is not didactic about it. Must be corrected/improved. About "electromagnetic shock waves" see Shock waves in astrophysics... Well, suggestions for re-writing fragments of the article?   --Krauss (talk) 12:01, 18 February 2013 (UTC)
thread_title:  electromagnetic 

message: Is “shockwave” really an alternate spelling for “shock”? I have half a dozen fluid mechanics and aerodynamics books at hand, and none of them use that spelling. Every time, it is introduced as “shock wave” and from there on just designated as “shock”. Ariadacapo (talk) 05:22, 24 April 2016 (UTC)
replies: text: Well, there are quite a few hits in Google Scholar.  It's true that initially they seem to be dominated by therapeutic uses rather than articles on mechanics.  I admit to being surprised by this; it may not be as common a spelling as I thought. --Trovatore (talk) 08:31, 24 April 2016 (UTC)
replies: text:  You are correct! I found 1 and 2 which use that spelling in the domain of fluid mechanics. I think that is sufficient to keep it in the lead sentence without having to add a citation. Thank you Trovatore. -- Ariadacapo (talk) 09:03, 24 April 2016 (UTC)
thread_title:  Alternative spelling "shockwave"