Go fast through the air, and you get wind chill effect. Go REAL fast, and you get heat friction (like on re-entry).
At what point does wind chill stop getting cold and start to get hot?
I don’t know the speed but it’s not friction. The heat is due to air compression. Experts may be along to elaborate. The speed you’re looking for depends on the altitude.
Actually, it’s both friction and compression, but CookingWithGas is correct that compression dominates at high speed. You always get heat generated by viscous effects in the boundary layer, the frictional heating. At supersonic speeds, you also have shock waves in front of the vehicle. A shock wave is just a near-discontinuous jump in fluid properties, which means the pressure, temperature and density change dramatically. This is the dominant heating mechanism in high-speed flight, especially for a blunt body which has normal detached shocks (as opposed to attached oblique shocks) because the shock effects are much stronger.
To answer the OP, I don’t think there’s any right answer. In all cases, the heat transfer mechanism is forced convection - the air flowing past the body transfers heat either to or away from the vehicle. If the body is warm and the air is cold, we call this “wind chill”. If the body is colder than the air, it will gain heat. At higher speeds when the compression effects raise the temperature of the air significantly, the heat transfer to the body goes up accordingly.
One elaboration on my previous note: in addition to forced convection, there is also a radiative heat transfer mechanism at work. This is typically very small, but it scales with T[sup]4[/sup] so it can dominate when the gas temperatures get very high. In a reentry vehicle like the shuttle which has a region of gas behind the shock which is hot enough to ionize, the radiation mechanism can dominate the heat transfer.
Good answers. Thanks folks.
Also, note that wind chill cannot cool objects which are already at air temperature. Wind chill can only have a cooling effect on an object which was previously heated up.
Unless the object has some liquid on it, in which case the evaporation will further cool the object.
We beat the whole wind chill vs. evaporation thing to death not too long ago:
http://boards.straightdope.com/sdmb/showthread.php?threadid=157732