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Model Forum / Radio Controlled / Air Models / August 2005Sound barrier
Here you will see a picture of a jet breaking the sound barrier. I am told we are seeing water vapor due to a difference in pressure. True? How does this come about? <http://www.portlandrcflyingclub.org/> Cheers - LeeH Dear LeeH - You might have Googled a bit first. If you had you would find <http://www.wilk4.com/misc/soundbreak.htm> with its associated references which would lead to some fairly clear and perhaps not-so-clear explanations. Toodle-oo for now... LeeH -- > Here you will see a picture of a jet breaking the sound barrier. I am > told we are seeing water vapor due to a difference in pressure. True? > How does this come about? <http://www.portlandrcflyingclub.org/> > Cheers - > LeeH > Here you will see a picture of a jet breaking the sound barrier. I am > told we are seeing water vapor due to a difference in pressure. True? > How does this come about? <http://www.portlandrcflyingclub.org/> > Cheers - > LeeH Air is heated as it is compressed. When the air passes a certain point, it expands. Expansion in a gas leads to a cooling phenomenon, the basis of airconditionig. As the air molecules cool, they form droplets, which then become visible. Ed Cregger > Air is heated as it is compressed. When the air passes a certain point, it > expands. Expansion in a gas leads to a cooling phenomenon, the basis of > airconditionig. As the air molecules cool, they form droplets, which then > become visible. Droplets of air... fascinating. Also the closer the air is to the dew point (or 100% humidity) the more readily the shock cloud or contrail will form. Bill(oc) > > Air is heated as it is compressed. When the air passes a certain point, it > > expands. Expansion in a gas leads to a cooling phenomenon, the basis of > > airconditionig. As the air molecules cool, they form droplets, which then > > become visible. > > Droplets of air... fascinating. Oops! Since air is a composite of various other chemicals/compounds/gasses, it is safe to assume that the water contained in air could also be construed as being a part of the air - agreed? But that wasn't the point, was it? 8>) Ed Cregger The equations for temp and pressure of air before a shock wave and after a shock wave are different. If the air before the shock is near 100% relative humidity, after the shock wave the relative humidity will be over 100% causing some of the water vapor to condense into droplets forming the "cloud" . The shape of the cloud indicates the shape of the shock wave. Ray S. > Here you will see a picture of a jet breaking the sound barrier. I am > told we are seeing water vapor due to a difference in pressure. True? > How does this come about? <http://www.portlandrcflyingclub.org/> > Cheers - > LeeH The equations for temp and pressure of air before a shock wave and after a shock wave are different. If the air before the shock is near 100% relative humidity, after the shock wave the relative humidity will be over 100% causing some of the water vapor to condense into droplets forming the "cloud" . The shape of the cloud indicates the shape of the shock wave. Ray S. > Here you will see a picture of a jet breaking the sound barrier. I am > told we are seeing water vapor due to a difference in pressure. True? > How does this come about? <http://www.portlandrcflyingclub.org/> > Cheers - > LeeH The equations for temp and pressure of air before a shock wave and after a shock wave are different. If the air before the shock is near 100% relative humidity, after the shock wave the relative humidity will be over 100% causing some of the water vapor to condense into droplets forming the "cloud" . The shape of the cloud indicates the shape of the shock wave. Ray S. > Here you will see a picture of a jet breaking the sound barrier. I am > told we are seeing water vapor due to a difference in pressure. True? > How does this come about? <http://www.portlandrcflyingclub.org/> > Cheers - > LeeH > Here you will see a picture of a jet breaking the sound barrier. I am > told we are seeing water vapor due to a difference in pressure. True? > How does this come about? <http://www.portlandrcflyingclub.org/> The presence of the visible vapor itself has absolutely nothing to do with the sound barrier. When the conditions are right, the vapor will be visible well below the Mach speed. On the Net there are numerous photographs of passenger airliners, which demonstrate the same effect, and, I hope, there's no need to explain that these aircraft never fly even close to the speed of sound. When the vapor effect is mentioned in connection with the sound barrier, what it really refers to is the specific _shape_ of the vapor cloud - the so called "sonic egg" - not the presence of the vapor itself. Unfortunately, I don't see the picture you are talking about in your original post and can't tell whether it has anything to do with the sound barrier.  SignatureBest regards, Andrey Tarasevich > When the vapor effect is mentioned in connection with the sound barrier, > what it really refers to is the specific _shape_ of the vapor cloud - [quoted text clipped - 3 lines] > original post and can't tell whether it has anything to do with the > sound barrier. Photo of a Tomcat breaking the sound barrier, with a well-defined "sonic egg:" http://pip.rubberfeet.org/desktop_pics/Sonic_boom/Sonic_boom1152x768.jpg >>When the vapor effect is mentioned in connection with the sound barrier, >>what it really refers to is the specific _shape_ of the vapor cloud - [quoted text clipped - 8 lines] > > http://pip.rubberfeet.org/desktop_pics/Sonic_boom/Sonic_boom1152x768.jpg Very cute. SignatureTim Wescott Wescott Design Services http://www.wescottdesign.com >The presence of the visible vapor itself has >absolutely nothing to do >with the sound barrier. When the conditions are >right, the vapor will be >visible well below the Mach speed. On the Net >there are numerous >photographs of passenger airliners, which >demonstrate the same effect, >and, I hope, there's no need to explain that these >aircraft never fly >even close to the speed of sound. Any time an airplane passes through air it disturbs it, including changing the pressures in the local airflows. Basic aerodynamics. Near or at the speed of sound, compression and rarification of the air can easily be extreme enough to change its temperature and vary its dewpoint (or relative humidity) and water vapor will condense and become visible. It's the same effect seen in other pictures with wingtips or props leaving vapor trails. Shock waves extend a considerable distance from a transsonic airframe and the effect can be startling. Dan >>The presence of the visible vapor itself has >absolutely nothing to do >>with the sound barrier. When the conditions are >right, the vapor will be [quoted text clipped - 12 lines] > considerable distance from a transsonic airframe and the effect can be > startling. There are some good web pages and very cool photos on the web: http://www.galleryoffluidmechanics.com/conden/pg_sing.htm http://fluidmech.net/tutorials/sonic/prandtl-glauert-clouds.htm http://chamorrobible.org/gpw/gpw-20040817.htm SignatureGrant Edwards grante Yow! Yow! I just went at below the poverty line! visi.com | Near or at the speed of sound, compression and rarification of the | air can easily be extreme enough Of course, at the other end of the spectrum, well below the speed of sound, basic aerodynamics is based on the premise that the air is not compressed or rarified at all -- instead it's just moved out of the way. Granted, it does get compressed and rarified a bit, but only a little bit, at least until you start approaching the speed of sound. (I'm not disagreeing with you, just adding to it.) SignatureDoug McLaren, dougmc@frenzy.com `If one studies too zealously, one easily loses his pants.' -- Albert Einstein |
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