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Old 03-04-2011, 10:35 AM   #1 (permalink)
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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?
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Old 03-04-2011, 01:52 PM   #2 (permalink)
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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.
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Last edited by yournamehere; 03-04-2011 at 02:06 PM..
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Old 03-04-2011, 02:12 PM   #3 (permalink)
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Originally Posted by yournamehere View Post

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

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

Last edited by Zeraph; 03-04-2011 at 02:15 PM..
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Old 03-04-2011, 04:02 PM   #4 (permalink)
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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.
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Old 03-04-2011, 04:08 PM   #5 (permalink)
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Here's a little info on how it works, from discovery.com (from Jan 2008):
Code:
While the Space Shuttle might be up for the highest bidder, engineers at NASA are
working hard on the next generation manned spacecraft, the Orion.
Part of that work involves testing scaled down models in the world's largest wind tunnel in Buffalo, N.Y.,
which was recently upgraded to achieve air speeds up
to Mach 30, or about 30 times the speed of sound.

"We just got it up and running and started its research life," said
Michael Holden, vice president of hypersonics at Cubrc. "The original wind tunnel was developed by NASA, 
but we have built it much bigger and can get
much more information with our new tunnel."

It takes a "one-two punch," as Holden puts it, to create wind speeds
of 30,000 miles an hour in the 100-foot-plus long, eight-foot wide tunnel.

First, helium or hydrogen gas is compressed to 3,000 pounds per square inch (psi)
and 1,000 degrees Fahrenheit, which breaks a seal. The shock wave created by
the influx of hot gas raises the the pressure inside a second chamber to over
20,000 psi, which breaks a second diaphragm, and releases the compressed air
into the expansion chamber, where it flows over the model craft in the chamber.

The wind tunnel doesn't operate for long. At its most powerful state, the tunnel "blows wind,"
 as Holden says, for only two milliseconds. It's not much time,
but its enough to simulate 60 feet of airflow. Any longer and the facility would literally melt down; those speeds
create temperatures hotter than the sun. . . 
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Old 03-04-2011, 04:21 PM   #6 (permalink)
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Originally Posted by Hektore View Post
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.
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.
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Old 03-04-2011, 05:28 PM   #7 (permalink)
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err ya, the first part of what grey wolf said. About the air friction heating itself.
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Old 03-05-2011, 11:21 AM   #8 (permalink)
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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...?
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Old 03-06-2011, 12:09 PM   #9 (permalink)
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. . . 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.
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Old 03-06-2011, 01:37 PM   #10 (permalink)
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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.
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Old 03-06-2011, 06:12 PM   #11 (permalink)
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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.
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).
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Old 03-22-2011, 05:38 PM   #12 (permalink)
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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.
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