At what point does air moving fast enough cause things to combust?
 

Like a meteor hitting the atmosphere? Several thousand miles per hour or so? It'd work in reverse too right? If we somehow had a jet engine that could make a stream of air fast enough it could melt rock, right?

I don't know at exactly what speed range most things are combustible - it would vary greatly depending on air density and the heat-dissipating characteristics of your object. I know the SR-71 Blackbird required the leading edges of its surfaces to be made of titanium - everything else melted. Its cruising speed is +/- 2,100 MPH. So the melting "speed" of most rocks would be somewhere south of that.

But your wind generator already exists. The LENS-X wind tunnel operated by Cal-span University at Buffalo Research Center can generate wind speeds up to 25,000 MPH - that's about 8,000 MPH faster than the Space Shuttle's re-entry speed.

WOW :eek:

If it was moving that fast wouldn't basically be a giant flame?

What generates the heat is not the speed of the air, but the friction of the air moving against an object. Same principle as rubbing your hands together, only with air. So as long as the walls of the wind tunnel whatever happens to be in it are sufficiently low in friction then, no it won't generate a fireball.

As far as your original question though, yes you could melt a rock with air if you got the air moving fast enough and the rock was rough enough.

Here's a little info on how it works, from discovery.com (from Jan 2008):

Moving air (i.e. wind) can actually heat itself through the friction of the air molecules themselves. However, not everything will combust (in general on earth , oxidise). At the speeds generated by the Lens-X wind tunnel, the air itself will become hot enough to ionise relatively quickly, resulting in a plasma. This does not happen within the air tunnel itself (I believe) due to the short bursts of the actual wind blasts. On the other hand, there is a plasma created around objects entering the earth's atmosphere (such as meteors and space shuttles). This is part of what causes the "radio blackou" on re-entry.

As well, as mentioned above, the SR-71 flew so fast that it would expand by about 12" - 18" just from the friction of the air. As a result, they never came up with any seals that could totally contain the fuel within the plane. Fueled in a hangar, they would have to put pans under it to catch the dripping fuel. Once in the air, they would fly at speed until the expansion compressed the seal sufficiently to block the leakage, then re-fuel in air, then fly the missions.

err ya, the first part of what grey wolf said. About the air friction heating itself.

Also, why does plasma cause a radio blackout?

Am I the only one sick enough to wonder what would happen to a human in that insane wind tunnel...?

The same thing that would happen if you were standing on the surface of the sun; minus the intense gravity and radiation, of course. But you'd be vaporized, just the same. Oh - and you'd lose cell phone reception.

A plasma is a state of matter where the material has all become ionised. The ionised material is radio opaque in much the same way that water or earth is, only more so. The electrically charged plasma ions interfer with radio waves extremely effectively. Think of the the problems with radio reception in the far north during a strong aurora, caused by the flow of charged (ionised) particles from the sun streaming into the magnetic poles. The ionisation field around a re-entering space ship is orders of magnitude worse.

But would you go into flames briefly? Would you see the skeleton just before it vaporizes? Would you just poof? I've always been curious since movies have done similar things (with radiation/heat not air speed).

Given the set up on that tunnel, I don't thinkk that a person in it would burst into flames. They would, however, be exploded all over the back wall when the pressure wave hit them. Barring DNA tests, I don't think there would be enough left intact to even tell if it was human, let alone who it was.