The Beauty of Fluid Mechanics In Video and Photos 31
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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Re:How does it follow the jet? (Score:4, Informative)
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Re:How does it follow the jet? (Score:4, Informative)
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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)
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.
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So, u
Re:How does it follow the jet? (Score:5, Interesting)
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
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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
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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
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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)
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?
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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.
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Jets = Mario Brothers (Score:1)
Blue Thunder (Score:2)
May I be the first to say (Score:2)
Here I am, Rocking like a hurricane!
Give me beer and a hot dot baby!
WOOO!!!
Why the Hornet? (Score:1)
I rarely see it on other jets.
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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.
Best images I've seen yet (Score:1)
No link to this video? (Score:2)
No entry! (Score:2)
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