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KMT description of atmospheric shockwaves, anyone?

  1. Sep 23, 2008 #1
    Can anyone give me a layman's description of what happens to air molecules interacting with a supersonic object based on Kinetic Molecular Theory?

    Perhaps another question, how does sound waves look like at the molecular level? How does sound propagate through the random bouncing of air molecules? If anyone's got a java applet for this scenario, I think that will be awesome! Thanks!
     
  2. jcsd
  3. Sep 24, 2008 #2
    Sound is just simply the propagation of a pressure gradient through time and space of a compressible medium. In other words, as a "sound wave" passes a region of space, the NET momentum of the collection of gas particles causes an the applied pressure gradient to exist. So, think of the collection of particles as having a linear combination of 2 momentum vectors in 3 space: one is the random vector caused by thermal agitation, the other is the vector that represents a time/space varying pressure gradient. It is this time varying pressure/momentum vector that the medium particles propagate as sound.

    A good anaolgy might be a room full of bumper cars. Normally, the cars all bounce off one another randomly. All of a sudden, the wall of the room strikes a few of them, imparting momentum in a particular direction to the adjacent bumper cars. These cars will then strike the other bumper cars, and the momentum wave wil eventually propagate though the whole room (and possibly reflect, etc.).

    A shock wave is kind of a special case in acoustics. What you have is a discontinuity in the pressure/density/temperature of the medium; this discontinuity is the shock envelope. With a "regular" sound wave, there is no discontinuity, the pressure gradient is continuous and smooth.

    I hope that helps.
     
  4. Sep 25, 2008 #3
    Thanks for the explanation, Diatreme! Just that shockwaves, like how can it form a vertical wave at speed of sound and wave sweeps back at supersonic speeds. The former I don't know if it's really vertical wave at speed of sound. If it is, it will be quite mind boggling how can that happen
     
  5. Sep 26, 2008 #4
    The shape of a shockwave for a craft/object traveling through the atmosphere is a cone. Basically, the frustrum (tip) of the cone starts just at the leading edge of the object; in the case of a fighter jet, it would start at the antenna or nosecone.

    This front shock is called a bow shock, and it results from the fact that the air simply cannot "get out of the way" of the approaching craft fast enough to form a streamtube over the craft. This is because air particles cannot move faster, in bulk, than the local sound speed (or sonic velocity, c).

    At the trailing edge of the craft, there will be an expansion shock; this is due to the fact that behind the aircraft, there will be a large, very low pressure zone, as the air inside the first shock has so much momentum that it cannot "turn the corner" fast enough to fill this void space.

    As a result, all "sonic booms" are actually heard and felt as two, very close together boom-booms...one is the bow shock, the other the expansion fan shock.

    Again, shock waves are simply discontinuities in the pressure/temperature/density of a medium, in this instance a gas (air). While shocks are cool, there is nothing "exotic" about them at all; every time a meteor hits the atmosphere, it creates a very powerful shockwave; it's only because the object burns up high in the atmosphere (100-50 km up) and ceases to produce a shock for long that we on the ground rarely hear these natural sources of a "sonic boom". When a meteor is large enough to crash to earth, it DOES produce a tremendous sonic boom; the impact with the ground is NOT the source of this intense shockwave (unless it is a VERY large meteor).
     
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