Talk:Prandtl–Glauert singularity

message: The wording in this article makes it seem as though the vapor starts behind the shock, i.e. the first vertical rise of the N.  Pressure, and temperature, increase across a shock wave.  I am pretty sure that this phenomena is created at super-critical Mach numbers (M=~0.8 - 0.9) where the flow is in a compressible regime, but the craft is not traveling at supersonic speeds.  As the flow accelerates around the bulging features it accelerates to Mach one, and then beyond as it expands supersonically over the diverging features.  (Think Laval nozzle, or the flow over a supercritical airfoil.)  Thus there is no N wave, there is notionally half an N wave, a region of supersonic expansion (M > 1) in some cone/wedge like regions which are terminated with a shock to decelerate the flow back to the free stream velocity.  It is in this sonic region that the flow is expanding, thereby dropping the temperature and pressure causing the visible condensation.  The shock that terminates the sonic region then increases the temperature causes the condensation to vaporize and disappear.  Also see the last two subjects "Big Mistake " and "Prandtl-Glauert Singularity"
:You are right. In addition, condensation clouds can occur whenever sufficiently wet air is moving into a low-pressure region: On wing-tip vortices, at the edges of flaps or rudders, in the intakes of civil jet engines (standing aircraft, engines near full-power: The air accelerates sharply into the intake).  I've personally seen them several times as passenger in an A320 (both above the wing and at the edge of high-lift flaps), during landing approach on rainy days, just after descending through low-hanging clouds; a good opportunity is also on cloudy, humid days in the approach lane of a big airport. www.airliners.net has lots of pictures of this.
: The cloud is completely independent of the Mach number or presence of a supersonic area, and is just wrongly attributed to those because the most famous examples include supersonic jets close to water.
: The volume that has visible vapour in it marks the volume with sufficiently low pressure to cause condensation, which may or may not require locally supersonic conditions.138.250.80.173 (talk) 14:17, 13 October 2014 (UTC)
thread_title:  Clarification Request 

message: I really think these pictures are interesting.  Keep posting more.  What kind of camera can catch the Prandtl-Glauert singularity? - Fat Tony
:any normal camera or camcorder will do, though the effect can be quite short so the photographer has to be on his/her toes.  See some of the videos of this effect.  There are also some photographers' details of how some of these shots were taken over at my site, Wilk4: Breaking the Sound Barrier jwilkinson 00:20, 7 January 2007 (UTC)
thread_title:  Pictures 

message: How about a simple graph to accompany (or supplant) the paragraph of text explaining the N curve?  The text requires a couple of readings to understand.
72.14.224.1 (talk) 18:45, 20 July 2009 (UTC)
replies: text: see the links to Mark Cramer's explanations.  jwilkinson 00:20, 7 January 2007 (UTC)
thread_title: expand request

message: I tagged this for merger with Prandtl-Glauert method, which doesn't contain enough information to indicate whether it should really be a separate topic. From what I can tell, the "method" is simply one way of deriving the equation of the singularity. If I'm wrong here, then that article certainly needs expansion. --Dhartung | Talk 07:19, 15 June 2007 (UTC)
replies: text: I don't know much about the topic, but from what I ascertained I agree.  23:28, 5 August 2007 (UTC)
thread_title:  Merge 

message: Gentlemen, I think there is a big mistake here.
What you call the Prandtl-Glauert singularity,is not what is really happening to the airplane.
Prandtl and Glauert did not work together. There is a rule that have their names, but they work on it separatedly. It is based on potential flow, small disturbances, and it is used to correct the perfect flow around a body due to compressibility.
But, instead of that, Prandtl worked with Theodor Meyer 8one of his students) to modelize shock and expansion waves.
What we are viewing in this photographs is that. expansion flow, with Mach waves (sound waves).
The aircraft is flying subsonic, but it has a Mach number greater that the critical one. Then there is supersonic flow over the airplane. For those parts were an expansion (cockpit, tail before the local shock)occurs the humidity of the air is condensed. You can find an introduction on this subject in "Foundations of aerodynamics" Kuethe-Chow.
Hope this helps. [Carlos Gayer]190.48.115.60 06:35, 6 October 2007 (UTC)
replies: text: Thank you, I believe that the "Prandtl-Glauert-singularity-clowd" is more like an urban myth. So far nobody could explain, where the singularity is. Neither a shock wave nor a steady pressure drop into a condensation condition describes a singularity. There must be an error here. Is there anybody with a good aerodynmics textbook who can explain? 88.217.24.161 (talk) 19:44, 18 December 2007 (UTC)

text: Carlos is right here. The correct explanation for the visible phenomenon is this: In transonic flight (inflow Mach Number slightly < 1) there are some supersonic regions, mainly on the wing or behind the cockpit of the aircraft. When flying low in humid air (the F-18 in the picture was next to an aircraft carrier, I believe), Water condensates in the supersonic region. At the shock, temperature, pressure and density increase, thus the rear end of the cloud nicely marks the shock location. Sometimes you can see a similar effect in landing passenger aircraft, where water condensates in the core of flap- or wing-tip vortices (where pressure is also very low). "Prandtl-Glauert Transformation" is a mathematical method that was developed and taught by Ludwig Prandtl but first published by Glauert. This method enables one to extend potential flow methods (which are only for strictly incompressible flow) to compressible flows. It becomes physically incorrect above M = 0.7! There is a nice article on it in the German Wikipedia (  -- anyone care to translate it?). The Prandtl-Glauert Singularity is not an observable phenomenon described by the theory but the reason why it is invalid at M = 1. Following the formula, pressure would go to Infinity at the speed of sound and then switch signs. Which is obviously not physical, thus ... sorry, you'll need to change this article :( --129.247.247.239 (talk) 17:41, 2 August 2008 (UTC)

text: I translated the German article, see Prandtl-Glauert transformation. Besides this I fully agree that the "Prandtl-Glauert singularity" is a purely mathematical problem, and it is not related to the condensation clouds seen in the pictures. Maybe "Prandtl-Glauert singularity" just sounds very eloquent so people tend to use it too often... ;) NickFr (talk) 17:27, 9 August 2008 (UTC)

text: I have done some editing to try and clairfy this situation. Key points: there is no actual singularity - the tranformation is not valid for the speeds. Still, extreme pressures exist and this can result in condensation. --Kvng (talk) 17:16, 13 March 2011 (UTC)
thread_title:  Big mistake 

message: In a science article like this, in a politically neutral encyclopedia, the U.S. flag shouldn't be on the foreground starring the picture. Please change or modify the picture. Thank you very much. --158.42.244.159 (talk) 18:58, 13 May 2008 (UTC)
thread_title:  U.S. Flag Picture 

message: Operation Crossroads#Sequence of blast events mentions the formation of a "Wilson cloud" following the detonation of an atom bomb at Bikini Atoll. The same photo is shown in this article. Is a Wilson Clud the same thing as a Prandtl–Glauert singularity? Can we coordinate our terms and make sure we're being consistent? See also Talk:Operation Crossroads#Wilson cloud.   Will Beback  talk   09:55, 28 May 2009 (UTC)
thread_title:  Wilson cloud 

message: The PGS can also make rainbows.
http://scienceblogs.com/startswithabang/2010/09/the_physics_of_a_fighter_jet_r.php
Hcobb (talk) 04:18, 9 September 2010 (UTC)
thread_title:  Prandtl–Glauert singularity rainbow 

message: I don't get this...I think....possibly...I'm not sure if I understand it....particularly the second sentence of the introduction, and I took AP Physics in the mid-80s, getting a 4.  I should know if I get this or not.  I am just too confused by all jargon.  I am not sure if I have understood the jargon correctly.  I think that I can figure it out but it should be clearer and in less technical language.  Perhaps "singularity," "transformation" and "apply" should be defined or other wording used.  What is being transformed?  And what is it being transformed into?  As for "apply", what is being applied to what?  One applies nail polish and paint.  Or applies oneself to a task. How does one apply a transformation?  A transformation may occur after something is applied, but how can that be invalid. I hope you see my point.  "Applying the transformation" might be a phrase that engineers and scientist understand; I don't see too many others understanding it. 
The whole "N wave" discussion should be moved down if not junked altogether.
I came to this article because I saw the photography of the F/A-18 with the cloud and wanted to see if it was real or somehow digitally enhanced.  I now think that it is real and is a cloud caused by condensation, condensation in turned created by the sound wave, I think. The Bikini Atoll picture is interesting; I always though that cloud was caused by ocean water being raised into the air by the explosion.  Now I am not too sure.  Maybe it is water that was in the air and is being formed into a cloud be the explosion's shockwave.  I think that this article should basically answer that question in layman's terms: Why is there this odd looking cloud attached to the F-18?  is it a cloud? What made it? Since I can't be sure if I understand the article, I cannot edit it for clarity.  --Bruce Hall (talk) 10:38, 1 November 2011 (UTC)
replies: text: P.S. Over at the Sonic Boom article it says this "Rapid condensation of water vapor due to a sonic shock produced at sub-sonic speed creates a vapor cone (known as a Prandtl–Glauert singularity), which can be seen with the naked eye"  That makes sense to me, though it still could be tweaked.  Having "sonic shock" and "sub-sonic" in the same sentence is probably confusing to some.  Perhaps better: "Sound waves produced at sub-sonic speed causes water in the air to condense creating a vapor cone (known as a Prandtl–Glauert singularity), which can be seen by the naked eye." But then again I don't know if that vapor cone is the singularity or not. --Bruce Hall (talk) 10:38, 1 November 2011 (UTC)
thread_title: Too technical? Too much jargon

message: This article handles at least three relatively unrelated subjects, and not all of them that accurately...
The Prandtl-Glauert singularity is not really about the pressure of a shock wave, it's about the forces experienced by an aircraft traveling at high-subsonic or supersonic speeds.  The subsonic speeds, the Prandtl-Glauert transformation predicts that the lift and drag forces on an aircraft are proportional to the inverse of the square root of one minus the Mach number squared.  At Mach 1, this would indicate that the force on the body is infinite.  This is the Prandtl-Glauert singularity (and the reason why some engineers of the early 20th Century thought of the "sound barrier" as impenetrable).
The section on "Pressure Profile" all appears to be accurate enough, but it has nothing whatsoever to do with the Prandtl-Glauert singularity.  This would belong much more in an article on the N-Wave.  My one complaint is that the region of expansion behind a shock wave where the pressure might be reduced below the dew point is not "part of the shock wave."  In an inviscid flow, shock waves are discrete, discontinuous phenomena of infinitesimal thickness.  In a real, viscous flow, they are only slightly more thick than that.  This region of rarefaction is more properly referred to at the "expansion region."
The images of vapor cones again have nothing whatsoever to do with the Prandtl-Glauert singularity.  They're not even really all that connected to supersonic flow, though they do frequently occur at high-subsonic and low-supersonic speeds.  Vapor cones are the result of regions of extremely low pressures around an aircraft (such as the lower peak of an N-Wave).  This can potentially happen at any flight condition.  The region right behind a shock wave is a region of extremely high pressure.
--VallWoodshadow (talk) 13:57, 19 November 2011 (UTC)
replies: text: I absolutely agree with this. The vapor cone images should be removed entirely, and we should make an attempt to remove references to the Prandtl–Glauert singularity from other articles relating to vapor cones. Dziban303 (talk) 20:28, 8 February 2012 (UTC)
thread_title:  Accuracy and Relevance of This Article 

message: It would be nice to see an explanation in the difference between the Prandtl-Glauert Singularity and the vapor condensation that is formed over a wing that is at a high angle of attack. I see many people confusing the two, and I believe that the picture of the F-16 in the "transonic" Wiki article is actually demonstrating vapor condensation and not a Prandtl-Glauert Singularity. (75.65.220.204 (talk) 16:06, 18 August 2013 (UTC)
thread_title:  Prandtl-Glauert Singularity vs. vapor condensation from maneuvering?