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As far as I know, the movement of air molecules can result in what we call wind, sound, or heat (and maybe some other things I'm not thinking of), depending on the assumptions about its motion. Wind generally occurs when air molecules all move in a group in the same direction. Sound occurs when air molecules (or little packets of air), are locally compressed. And I believe heat moves when air molecules vibrate faster than their neighbors do, and some how that energy is moved.

This figure from "Sound and heat revolutions in phonics" in Nature helps setup what I'm curious about:

from "Sound and heat revolutions in phonics"

So where do we draw the dividing lines between wind, sound, and heat? How does the air "know what to do", given a certain stimulus? If I move a mass, and its position is given by a unit step function, what happens?

Unit Step Function

I understand that by Fourier Analysis, I should be exciting the air around the mass at infinite frequencies, and if this is that case, would this create wind, sound, and heat? And if so, where are the boundaries between these phenomena? Are they just related to the frequency of the motion, or something more complex like coupling or impedance matching? Would the wind, sound, and heat travel out at different speeds?

Thank you!

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Yes you will create wind sound and heat. The figure you show is a simplification of the actual interactions that occur to allow students to learn visually.

Consider a jet aircraft as an example of 'accelerated mass'. if you've ever stood near a runway as a plane is taking off you'll know how much 'wind' is generated by its passage. A supersonic jet in flight gets so hot it actually expands measurably (Concord was a full foot longer in flight that on the ground due to heat expansion) and whilst its quiet you can hear the passage of a gliding aircraft or, more accurately, the passage of the air over the wings.

The phenomena you are asking about don't happen in neat stages. Wind is the first thing to be generated as air is displaced by the passage of an object and it continues to be created as the frequency of the air molecules oscillations reaches the point where audible sound is generated. If the magnitude and frequency of the displacement becomes high enough, heat will also start to be generated.

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JonT provided a nice answer, but I'd also like to add that heat is always generated whenever wind is. When turbulence is generated (and there is always some level of turbulence generated when one generates wind), the energy in each eddy cascades down to smaller eddies (with very little energy lost) and eventually the eddies reach a size where viscous forces cannot allow this cascade to continue (the Kolmogorov scale). At this point, the eddies dissipate into heat.

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  • $\begingroup$ Wait,wind always have turbulence in it? I was thinking turbulence occurs only if the the wind is above certain velocity,I believe that at low velocty,flow can be purely laminar. $\endgroup$ – wav scientist Mar 13 '18 at 21:54

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