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When an airfoil is tested in a wind tunnel the speed of the wind behind the airfoil is less than speed of the wind in front of it. That means the wind loses kinetic energy. The reason for that is drag and since drag is a type of friction the loss of kinetic energy is converted to thermal energy.

Is all of the lost kinetic energy converted to thermal energy or is also a part of it converted to another form?

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  • $\begingroup$ What other 'form' would you propose? How would you convert it? I'm being pedantic here on purpose. For a dumb example: suppose the drag was due to a windmill on the airfoil which generated electricity. You could say that was where some of the energy "went," but it wouldn't add much to the discussion. $\endgroup$ Jan 21, 2015 at 12:56
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    $\begingroup$ Perhaps takes place some ionization of air. $\endgroup$ Jan 21, 2015 at 14:22
  • $\begingroup$ @CarlWitthoft: I can't imagine any other energy form that the kinetic energy is converted into, but I wasn't totally sure either. A windwill on top of the airfoil is another thing; I am talking about a "pure" airfoil. $\endgroup$
    – Chris
    Jan 21, 2015 at 15:22
  • $\begingroup$ Well, again going off into the wild, suppose your airfoil was made of one of those 'memory metals' which return to a predetermined shape upon heating. Then suppose the drag-friction both heats and deforms the airfoil. Allow the airfoil to cool and it's holding some potential energy in its molecular structure (similar to a restrained spring). $\endgroup$ Jan 21, 2015 at 17:01

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The flow past the airfoil has less speed in the original direction and more orthogonal to it. Some of this orthogonal flow is concentrated in the wake, the part of air which formed the boundary layer when it was over the airfoil. In most conditions this boundary layer had a laminar-turbulent transition while flowing around the airfoil. Turbulence means that flow speeds orthogonal to the initial flow are amplified.

Some of the orthogonal speed increase is due to lift and affects a much greater amount of the air flowing off the airfoil. Lift means momentum transfer between airfoil and air, so the airflow behind a lift-creating airfoil is deflected away from the direction of this lift.

Some of the turbulence increases the noise radiating from the flow. In the end, noise is nothing more than pressure fluctuations and will slightly heat the material which absorbs it.

Eventually, all of this turbulence is damped down by itself, by flow guides and by the wind tunnel walls and is converted to heat.

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Drag on a wing is generally lumped into two categories: parasitic and induced.

Parasitic drag is the part due to friction. It generates heat, which warms up the wing and the air.

Induced drag is due to the wing deflecting the air down and, to some extent, forward. It is purely a case of momentum transfer, just like bouncing a ball off of a surface. The higher the angle of attack, the greater the induced drag.

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