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Question on wing Down wash ?

  1. Oct 2, 2005 #1
    I was wondering if you could help me with a question?

    How does the down wash from an airplane at high altitude (out of ground effect) reach the earth? It seems to me from looking at various airfoil simulation programs that in 2-D flow *bound vortex only* the pressure distribution aft of the airfoil will return the airflow to the free stream direction, in addition, if you go certain distance above or below an airfoil, you will find a straight streamline. In 3-D flow, I know that the wing tip vortices contain down wash, but they will dissipate due to friction (viscosity). Is it a momentum flow or something from the dissipated vortices that provides the down wash at the surface of the earth?

    From conservation of momentum laws, is it required that a down wash reach the earth?

    Thank you for your time
  2. jcsd
  3. Oct 4, 2005 #2


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    No, it's not required that they reach the earth, and you answered the question yourself. Air viscosity will dissipate the downwash entirely if the plane is high enough off the ground.
  4. Oct 5, 2005 #3
    I know that you can dissipate the energy, turning it into heat, but you can not dissipate the momentum.

    Also, I think Prandtl showed that an aircraft at altitude will leave a "footprint" which would be extremely hard to maesure because it would be a really small pressure change over a huge area. I was just wondering what "transport mechanism" brought this "footprint" to earth.
  5. Oct 6, 2005 #4


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    That is incorrect. Momentum and energy are dissipated - or transferred.

    As enigma already mentioned - Air viscosity will dissipate the downwash entirely if the plane is high enough off the ground!

    If a plane is close enough to the ground, the downwash reach the ground where it will become a lateral flow with some upflow as well, which itself will be dissipated.

    Take a look at a B-1 near the surface of a lake - http://www.everything-science.com/index.php?option=com_smf&Itemid=82&topic=1448.msg48989#msg48989 . The effect is quite localized. The disburance is pretty much dissipated at a distance greater than the characteristic dimension, which could be effective diameter or wing span.
  6. Oct 6, 2005 #5

    I thought the rule was that energy is dissipated (turned into heat) by viscosity.

    Momentum is "spread out" by viscosity I think that spread out and transfered mean the same thing here.

    I realize that you could probably spread out the momentum so much that you no longer have a "bulk movement" of fluid. In which case I guess that the momentum would be transfered (spread out) by diffusion (molecular motion) of the gas.

    Does that sound correct?
  7. Oct 7, 2005 #6


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    In free shear flows the momentum is conserved integrating the velocity in all the infinite domain, despites you have the viscous stress acting inside it. But if you have a bounded domain by solid boundaries, the friction against the wall will cause a loose of momentum (transferred to the wall !!!!) and a loose of energy. Boundaries acts as sinks of momentum and energy, when the heat flux and viscosity are enabled. On the other hand, gravity can be a source of momentum, like in a bouyant jet.
  8. Oct 7, 2005 #7

    In a gas, pressure is caused by molecular collisions with each other and with the walls of whatever container they are in. These velocitys are random velocitys in the Vx, Vy, and Vz directions. Temperature is a measure of the average translational kinetic energy.

    If you have two different layers of gas sliding past one another, at different velocitys (velocity of the fluid as a whole), you will have friction/viscosity also caused by molecular collsions

    Is this correct?
    Last edited: Oct 7, 2005
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