Talk:Prandtl–Glauert singularity

Question
Question: should this article have a 'see also' link over to sound barrier or is that misleading since it is an effect that is not necessarily dependent on being near the speed of sound? (though it's often mistaken for 'breaking the sound barrier'. Thoughts? jwilkinson 00:20, 7 January 2007 (UTC)

I can't tell -- do these clouds follow the plane around while Mach >=1? Or is it a phenomenon that occurs just at the moment of Mach = 1? Chrisvls 22:59, 30 Sep 2004 (UTC)


 * It occures at transonic speeds, not only at Mach-1. The cloud follows the aircraft; it is not stationary. Rsduhamel 00:52, 21 December 2005 (UTC)


 * As far as I know, it should occur at speeds >= mach 1, as this should follow from the underlying principles. What would change with additional speed would be the angle of the cone, becoming more acute, and if (can we presume?) the strength of the shockwave is stronger at higher speed then so should be its diameter, and if the sweep of the wing is less than this angle then the cone should have an irregular shape in the wing area. The diameter of the cone will of course be dependent upon also the local relative humidity.


 * An anecdote that may be of interest. Note that the shock wave can be reflected from an underlying surface. I did not personally see this, but a fellow naval airman described a low flying supersonic overwater demonstration. He said it appeared that the aircraft was "flying inside an ice cube". -- Leonard G. 02:35, 1 Oct 2004 (UTC)

I personally observed this effect around an F-4 Phantom II that was certainly not exceeding the speed of sound. It was at the Miramar Air Show sometime in the late 1970s. It was a cool very humid day and all of the military aircraft had visible clouds in the shock waves that were forming over the wings. It seemed that the pilots were making a point of doing fast flybys to produce these clouds. The Blue Angels announcer made a point of talking about the clouds and their cause. As an F-4 flew by, with the clouds showing over his wings, for about a half second at most, a huge doughnut-shaped cloud formed around the middle of the aircraft. I cursed myself for not having my 8mm movie camera rolling. I find it very hard to believe the plane had exceeded the speed of sound since there was no sonic boom. (Actually, as I remember back, this was more than 25 years ago and the aircraft may have been an F-14 but I'm pretty sure it was an F-4.) Rsduhamel 19:19, 29 Oct 2004 (UTC)


 * You all already have a link to my page where I gather information about these photos. On there is a B-2 with the effect. B-2's are subsonic aircraft.  --jwilkinson 21:16, 14 May 2006 (UTC)

Clarification Request
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)

Pictures
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)

This is a general effect. Rapid condensation occurs due to the air pressure falling lower than the local dew point. Transonic weak shocks are only one means of achieving this around aircraft. Manoeuvering aircraft can induce this effect without entering the compressible regime. In an highly accelerated subsonic turn the pressure can drop significantly around the top of the leading edge and induce the effect there. Nothing to do with the P-G rule. In fact the SU-35 picture is a case in point. This aircraft is manoeuvering at subsonic speeds. The condensation is forming in the low pressure cores of the vortices that are formed by the strakes at high angles of attack. Once the cores rupture down stream the effect is lost. What ever about the other photos relationship to the P-G rule this one has nothing to do with it. The source of the low pressure there is not due to compressibility effects. — Preceding unsigned comment added by Simonicusfacilis (talk • contribs) 11:33, 8 September 2011 (UTC)

Reliable technical information and photographic sources for the stunning Prandtl-Glauert condensation clouds
Collection of "Sound Barrier" tutorials by Dr. Mark S. Cramer. Covers the sonic boom, the sound barrier, and the Prandtl-Glauert condensation clouds. An authoritative site. Site URL: http://FluidMech.net
 * http://FluidMech.net/tutorials/sonic/sonicboom-intro.htm

"Prandtl-Glauert Condensation Clouds" tutorial by Dr. Mark S. Cramer. Part of the "Sound Barrier" collection of tutorials. The site is authoritative. Site URL: http://FluidMech.net
 * http://FluidMech.net/tutorials/sonic/prandtl-glauert-clouds.htm

"Gallery of Fluid Mechanics - Condensation due to the Prandtl-Glauert Singularity" by Dr. Mark S. Cramer. One of the best authenticated and documented photo collections of Prandtl-Glauert condensation clouds (alternatively, Prandtl-Glauert Singularity or Prandtl-Glauert clouds). The site really is an authority on the subject. Site URL: http://GalleryOfFluidMechanics.com
 * http://GalleryOfFluidMechanics.com/conden/pg_sing.htm

ChamorroBible.org, Tenjos (Agosto) 17, 2004, "Manguaeyayon na Palabran Si Yuus - God's Precious Words, with the Photograph of the Day". One of the best authenticated and documented photo collections of Prandtl-Glauert condensation clouds (alternatively, Prandtl-Glauert Singularity clouds or Prandtl-Glauert clouds). Eight photographs are included in Part 1 of 2 (August 17, 2004). All public domain. Site URL: http://ChamorroBible.org (generally referenced as "ChamorroBible.org" or the "Chamorro Bible" WWW site).
 * http://ChamorroBible.org/gpw/gpw-20040817.htm

ChamorroBible.org, Tenjos (Agosto) 18, 2004, "Manguaeyayon na Palabran Si Yuus - God's Precious Words, with The Photograph of the Day". One of the best authenticated and documented photo collections of Prandtl-Glauert condensation clouds (alternatively, Prandtl-Glauert Singularity or Prandtl-Glauert clouds). Eight additional photos in Part 2 of 2 (August 18, 2004). Included in this collection is a remarkable NASA photo of a Prandtl-Glauert cloud around the Apollo 11 Saturn V rocket at launchtime. All public domain. Site URL: http://ChamorroBible.org (generally referenced as "ChamorroBible.org" or the "Chamorro Bible" WWW site).
 * http://ChamorroBible.org/gpw/gpw-20040818.htm

--TheStudent 21:48, Dec 6, 2004 (UTC)

expand request
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)

If anyone could give an expansive discussion on this effect and it role in fluid dynamics - compressible fluids and newtonian fluids, etc; with some weighting in the mathematics, that would be wonderful. If not, des someone have a reference they could put out. Something heavier on the math/fluid dynmaics than what is currently there. Thanks much. Ya'll are the best! 128.62.97.227 20:35, 1 September 2005 (UTC)


 * see the links to Mark Cramer's explanations. jwilkinson 00:20, 7 January 2007 (UTC)

Merge
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)
 * I don't know much about the topic, but from what I ascertained I agree. 23:28, 5 August 2007 (UTC)

Prandtl-Glauert singularity
The phenomenon is not necessarily airspeed related; it occurs when there is a change in air pressure. According to Bernoulli's principle, the airflow across the top of the wing is faster than that across the bottom because the wings are cambered, and the airflow over the top has further to travel in the same amount of time. This creates a high pressure beneath the wing, and low pressure on top. The two pressure areas attempt to swap, which is what creates wingtip vortices. Increasing the airflow (with airspeed, for example) will increase the difference between the high and low pressure areas, which creates lift. As the Prandtl-Glauert Singularity suggests, the drop in air pressure can drop the air temperature below the dew point, which causes visible moisture.

While Prandtl-Glauert Singularity seems to be specific to airspeed related visible moisture, there there is another way to create this effect -- an effect fighter pilots refer to as "vapes." It usually occurs in humid air during high-G maneuvering. We've all seen this airshows before; it forms on the wings and leading edge extensions of aircraft like the F-16 and F-18.

Big mistake
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)
 * 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)

"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)
 * 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).


 * 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)

So why does the article say otherwise? It is an example of a mathematical singularity in aerodynamics. The German article about the cloud phenomenon is separate from the article about the singularity, and it says that it's incorrect to conflate the phenomenon with the mathematical singularity: Eine Singularität gibt es in Wirklichkeit nicht.--87.162.40.71 (talk) 15:22, 18 August 2009 (UTC)


 * 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)

U.S. Flag Picture
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)

As German I don't feel offended by the picture mentioned above. It's from the 60s (nearly historic, space race age) and I don't see any political tendencies in using it. However if someone else does, this one here will do the job as well. Regards NickFr (talk) 17:59, 9 August 2008 (UTC)

Apollo 11 at Max Q?
I've marked the reference to Max Q as dubious because, though I know nothing about fluid dynamics, the altitude pictured looks too low for Max Q. According to the sources in the Max Q article, the Apollo launches reached max Q at around 8-9 miles up. That's what it sounds like from the NASA's recording of the launch in the Apollo 11 flight journal. Given that the Saturn V was 101.6 metres tall, either the U.S. flag in the image was almost microscopically small, or the vapour cloud happened at a far lower altitude. Or have I missed something? - Pointillist (talk) 17:53, 22 April 2009 (UTC)
 * Isn't the flag just simply a lot closer to the camera than the Apollo is? It looks legit to me. Ehque (talk) 04:00, 8 May 2009 (UTC)
 * I don't think angular diameter calculations allow the spacecraft to be so high. Here's how I see it:
 * Assume the flag is four feet tall (1.22 metres). Given that the spacecraft is 101.6m tall and the flag is about 18% taller than the spacecraft in the original image, the spacecraft must be about 100 times further from the camera than the top of the flagpole.
 * If the condensation happened at Max Q, by then the spacecraft was 13.6km high (table 4-1 in Apollo 11 launch vehicle flight evaluation report) so the flagpole would have to be roughly 136m tall. But that's impossible, because in 2001 the world's tallest unsupported flagpole was only 86m tall (per this BBC story).  Even if the condensation happened at Mach 1, the spacecraft was already 7.8km high by then which would make the flagpole about 78 metres tall (256 ft).  That's pretty unlikely.
 * Another factor is the thin cloud in the background, which (per para A.3 of the evaluation report) is either 1/10 Cumulus at 700m or 2/10 Altocumulus at 4.6km - far lower than the altitude at Mach 1, let alone Max Q.
 * So the Max Q suggestion is probably rubbish.
 * Personally (this is just speculation) I think this picture probably shows the flag pole at the KSC Press Site (which is maybe 12-15m tall) and was taken much earlier in the flight. - 18:26, 8 May 2009 (UTC)

Definitely a composite photo, light sources between the Flag pole and the Apollo are completely different. I submit it is a Max Q event photographed through a telescopic lens. Then post composited likely (given its age) through dye transfer retouching (very old school) Ice Czar (talk) comment added 17:01, 31 May 2009 (UTC).

I agree with Ice Czar, it's a composite photo. It could be a real photo, but that would have taken being in a really lucky position, and being really lucky snapping the shot, but I doubt it.

Looking at the Apollo 9 launch journal (http://history.nasa.gov/ap09fj/001_day01_launch.htm) and assuming them to be pretty similar, Mach 1 would have been achieved at +00:01:08, and Max Q at +00:01:25. Myself, being that I don't work for NASA, and this happened 4 years before I was born, I am not an expert in it, but I would place the photo closer to +00:01:08.

JW Smythe (talk) 18:06, 3 June 2009 (UTC)

If you look at the image close up, it looks like a painting.

alex-thong (talk) 22:40, 8 June 2009 (UTC) —Preceding unsigned comment added by 86.171.20.205 (talk)

Max Q is ruled out by the fact that Prandtl-Glauert singularities are transsonic phenomena, i.e. Mach 1; as JW Smythe points out, Mach 1 came 17 seconds earlier in the flight than max Q. As to the composite photo issue, Ice Czar is correct, the sunlight on Apollo 11 is from the right, while that on the flagpole (note the ball on top) is from the left. This is unambiguously a composite shot. The anonymous speculations about the height of the flagpole are flat wrong; possible variations in shooting angle and focal length make any attempt to deduce the flagpole's height completely fruitless. In deriving and applying the factor-of-100 height ratio, Mr. Anonymous is making major assumptions that just aren't valid. I could shoot just such a shot using a 1-foot-high flag on a 10-foot pole. It wouldn't even be terribly lucky, provided I set the camera up in advance; at a height of 7.8 km, the angular speed of the rocket is relatively slow (I was 7 at the time, and watched this and later Apollo launches on TV), so split-second timing isn't necessary. As to the cloudy "background", the yellow-orange glow around the exhaust plume pretty clearly shows that the clouds are below the spacecraft, not above. SeanWillard (talk) 17:50, 16 June 2009 (UTC)

Wilson cloud


Operation Crossroads 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.  Will Beback   talk    09:55, 28 May 2009 (UTC)

Prandtl–Glauert singularity rainbow
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)

Too technical? Too much jargon
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)
 * 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)

Accuracy and Relevance of This Article
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)

In the process of making some changes, but I think all of the vapor cone material should be moved into a separate page, as it is really not the same phenomenon. It may be a common perception that vapor cones are related to the Prandtl-Glauert singularity, but that's really not true.

--VallWoodshadow (talk) 22:07, 19 November 2011 (UTC)


 * 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)

Almost 10 years has passed since your comment, but the article is still in the same misleading state. The current state of affairs (correct me if I am wrong) is that there are the following 3 things:

-There is PG singularity, which a prediction of an incorrect theory and is not an observable phenomenon;

-there is condensation happening in the regions of low pressure, both in low-speed regime (above the wing) and in supersonic regime (in a cone). None of that has anything to do with PG singularity;

-there is a "PG effect", sometimes also called "PG singularity", which is apparently just an urban myth, coming from the confusion of the above two things, and to my understanding "PG effect" is not mentioned in books on fluid dynamics. Yet, this term seem to live its own life on the internet, usually accompanying blog posts, pictures, and videos with planes and vapor.

Because of this confusion, it could make sense to make a section, or even a separate article, about this urban myth with explanation.24.17.214.179 (talk) 20:37, 2 September 2020 (UTC)anon

Prandtl-Glauert Singularity vs. vapor condensation from maneuvering?
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))