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## The Beauty of Fluid Mechanics In Video and Photos31

An anonymous reader writes "F/A-18 Hornet jet fighters just having some fun — and making science interesting at the same time: Video #1, #2, #3, and a photo gallery. The formal name for the cool 'vapor cone' is Prandtl-Glauert Condensation Cloud, which is due the Prandtl-Glauert singularity."
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## The Beauty of Fluid Mechanics In Video and Photos

• #### How does it follow the jet? (Score:3, Interesting)

on Monday November 20, 2006 @12:28PM (#16916076) Homepage
How/why does the condensation follow the jet, instead of leaving a trail like you would expect? (Or at least like I would expect).
• #### Re:How does it follow the jet? (Score:4, Informative)

on Monday November 20, 2006 @12:35PM (#16916174)
Quick answer is the condensation isn't following the jet. It occurs in air at the edge of the shockwave. As the shockwave passes, the air returns to the same state.
• #### Re: (Score:2)

From the article I understood that the condensation was due to the cooling of the air due to the low pressure zones created. How can the air so quickly return to the state that it was in so as to immediately absorb (I don't know the proper term) the condensation?
• #### Re:How does it follow the jet? (Score:4, Informative)

<.endergt2k. .at. .verizon.net.> on Monday November 20, 2006 @12:54PM (#16916506)
The first link about prandtl-glauert clouds [fluidmech.net] explains it better than I can, but the summary answer is that the shock wave actually follows the cloud, not precedes it, and it is the shock wave that rapidly reheats the air.
• #### Re: (Score:2)

it is the shock wave that rapidly reheats the air.

Yeah, I get lost about half-way down that page, but this makes sense.
Or at least as much sense as I need it to make, not being a physicist :-)

Thanks.

• #### Re: (Score:2, Informative)

The shock wave doesn't have anything to do with it. The effect occurs because at speeds close to the speed of sound (both above AND below) there is an extreme amplification of the temperature gradient in the air around the wing. At subsonic speeds there is no shock wave, and yet these clouds form at subsonic as well as supersonic speeds. So it cannot be that the shock wave causes the evaporation of the cloud.

The cloud evaporates simply because when the disturbance passes, the air returns to its equilibriu

• #### Re:How does it follow the jet? (Score:4, Informative)

<.endergt2k. .at. .verizon.net.> on Monday November 20, 2006 @02:05PM (#16917656)
You are correct that the vehicle does not have to be travelling at supersonic speeds to cause the cloud to form. You are also correct that it is the return of the air to its previous pressure and temperature that causes the cloud to evaporate. You are incorrect in that the shock wave has nothing to do with it - the shock wave greatly accelerates this state change, giving us the clean sharp termination of the cloud.

If you read the page [fluidmech.net], you will see that even at subsonic speeds variations in the surface can result in transonic flow, which in turn results in a terminating shock wave.

I don't want to quote the entire discussion here, but the page clearly states that subsonic clouds may be amplified by the prandtl-glauert singularity, but will not be cleanly terminated (see this image [galleryoff...hanics.com] for reference). The sharply defined cone (which is, I believe, what the original question was about) only appears when transonic flow is in play. In this case, it is the shock wave that causes the clean termination of the cloud.

• #### Re: (Score:2, Interesting)

The temperature is directly related to pressure. The pressure changes very quickly, so the temperature changes just as quickly. The condensation of water into a cloud is directly related to the temperature and the pressure. If you have the right conditions, it's going to be a cloud. The time it takes for the water vapor to condense into a cloud is related to the speed of the molecules of water vapor. They're moving pretty fast. So quickly that the 700MPH jet is practically standing still by comparison.

So, u
• #### Re:How does it follow the jet? (Score:5, Interesting)

on Monday November 20, 2006 @01:33PM (#16917124) Homepage Journal
How can the air so quickly return to the state that it was in so as to immediately absorb (I don't know the proper term) the condensation?

Thermodynamics... Pressure, Temperature and Volume are all related. Sound is nothing more than pressure waves, and the aircraft moving through the air at close to Mach speeds will create very interesting pressure waves around it, that can extend quite far from the craft itself. Since the source of the pressure wave is the aircraft itself, and moving at close to the speed of sound (speed that pressure waves propagate), the pressure wave will become a standing wave with respect to the aircraft. As substances compress (increase pressure in the same volume), thermodynamics regulates that temperature will rise, and as they expand (pressure drops), temperature will drop. These same principals are what cause your Air conditioner and refrigerator to work. The atmosphere also has what is known as the dew point, which simply stated, is the temperature that a cloud of condensation will form. When the ground temperature is at the dew point, you get fog. It is a temperature based on pressure and humidity. Since the aircraft causes radical changes in temperature and pressure, if you get the right combination of conditions, the aircraft's movement through the air can cause a local zone of air that will hit the dew point and create the cloud. It is when it hits this point that the cloud will magically appear. The thermal "momentum" for the change is dictated by the specific heat of the air, and the heat of vaporization for the water in the air at the location (amount of energy needed to change its temp, and energy required to change phases of the water). Since the energy is being provided by the aircraft, there is no shortage, and it is powerful enough to drive these changes very quickly, which is why it appears and disappears so fast.

Besides my contorted description here, the referenced articles do a decent job going into more detail. There are also some nice java applets that can visualized the pressure waves created at near-mach speeds to demonstrate why the shape of the cone is as large as it is (mainly due to the amplification predicted by the Prandtl-Glauert singularity equations). I tried looking for them but couldnt find the one I remember seeing before..

tm

• #### Re: (Score:1, Interesting)

by Anonymous Coward

Have a careful look at this [galleryoff...hanics.com] famous and popular video of a transonic F-14 Tomcat (best viewed frame by frame): http://www.galleryoffluidmechanics.com/conden/f14. mpeg [galleryoff...hanics.com]

Why does the cloud begin to form above the jet? Are there any insights you can bring that can help solve this mystery? In his explanatory remarks at http://www.galleryoffluidmechanics.com/conden/mpeg f14.htm [galleryoff...hanics.com] Dr. Mark S. Cramer (who wrote the Prandtl-Glauert Condensation Cloud material pointed to in this excellent and very interesting /. arti

• #### Re: (Score:2)

Why does the cloud begin to form above the jet?

Remember, we are dealing with transonic velocities, where the pressure waves are basically generated on top of themselves, since the source is moving close to, at, or just past the same speed they propagate from the location they were created. This causes a shockwave to form, which extends radically from the source (the aircraft). The aircraft itself will actually create two major shocks, one at the leading edge, the bow shock, and one at the trailing edge, t

• #### Re: (Score:3, Interesting)

Because the air is being rapidly compressed, then decompressing, before returning to normal pressure.

Any object moving through air creates sound waves. For an aircraft travelling close to the speed of sound, these sound waves build up into a pressure wave of compressed air in front of the aircraft. If you compress air, the pressure goes up and the air becomes warmer (the faster you go, the more compressed the air becomes). But every compression wave also has a decompression wave behind it. When air pressure
• #### Re:How does it follow the jet? (Score:5, Interesting)

on Monday November 20, 2006 @01:19PM (#16916894)

How/why does the condensation follow the jet, instead of leaving a trail like you would expect? (Or at least like I would expect).

If you read the article, it is explained that the condensation occurs because the air cools in regions of transient extremely low pressure. These regions are "attached" to the aircraft and so the cloud follows it. When the aircraft passes, the air returns to its equilibrium pressure, the temperature increases accordingly and the cloud evaporates.

Also, what the hell is wrong with Slashdot these days when the only articles that get over 500 comments are political or Sony-related and something cool like this gets less than 10 comments?

• #### Re: (Score:1, Insightful)

by Anonymous Coward
One reason could be that, it is not in the main page.
Since not many people see it, not many respond to it.
I saw it just because of the RSS feed I had kept for science.slashdot.

Or maybe that there is no major questions on this.
It is almost fully understood and explained. personal opinions dont have the value here.
• #### Re: (Score:2)

From a tactical standpoint isn't it a major downside in air-to-air combat to have that giant cloud behind you? Wouldn't that completely block your rear view? Also, wouldn't it increase the aircraft's visual profile? What does this do to its radar profile?

• #### Re: (Score:1)

This is an example of mathematical modelling describing several different phenomena correlating with an occurence of a natural event which happens to describe a condition that produces a recognizable, noticeable, and appreciable display. This is a three dimensional zone described similar to x = 0 ... 20 meters, x^2 -3x -18, mirrored across both the x and y axes and spinning around the y axis in which molecules of water present in an airstream over the surface of a jet are compressed to a degree which signi
• #### Jets = Mario Brothers (Score:1)

If you follow the links deep enough, you'll find a B2 Bomber covered in a similar cloud. It looks like the government is taking camo advice from whoever thought up Lakitu from Mario Brothers.
• #### Blue Thunder (Score:2)

As a jr. high kid, I remember being fascinated by the intro sequence to the short-lived Blue Thunder TV series [wikipedia.org] -- where the helicopter flies through the smoke cloud, causing a beautiful double-curl shedding vortex. (And yes, the title sequence was the best part of the show.) However, I think that was one of the reasons I went on to be a mechanical engineer, specializing in fluid mechanics. I still think that stuff is cool.
• #### May I be the first to say (Score:2)

WOOOO, YEAH!!!

Here I am, Rocking like a hurricane!

Give me beer and a hot dot baby!

WOOO!!!
• #### Why the Hornet? (Score:1)

Why does the F/A-18 Hornet always seem to have a condensation cloud?
I rarely see it on other jets.
• #### Re: (Score:1)

Probably because being a Navy aircraft, it operates in a high humidity environment which is conducive to forming these types of clouds. Also, to photograph this, the jet needs to be going really fast, and typically very low--something you can't do freely over land, especially if you happen to slip into supersonic flight while doing the high-speed pass.

Over the ocean, there are no such restrictions.

Here's another link http://www.eng.vt.edu/fluids/msc/gallery/gall.htm [vt.edu] for more cool fluid mechanics photos.

• #### No link to this video? (Score:2)

How can you have a discussion about fluid dynamics and transonic wakes without a link to this [youtube.com] video of an F-14 going transonic at flight desk level parallel to a carrier? That is simply one of the most amazing pieces of video I have ever seen!
• #### No entry! (Score:2)

This sucks - it seems I'm not allowed to look at these :((

"Currently, the playback feature of Google Video isn't available in your country."

"
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* The mirror is brand new and is in the process of being created. This usually only takes a minute or two, so please try your request again.
* The page is expired and no longer mirrored here. We keep mirrors around for o

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