by Nitpicker » Tue Feb 11, 2014 1:37 am
Chris Peterson wrote:Robert Boyle wrote:The heat is generated by the compression of the air in front of a falling object.
Is it, indeed? Please explain in a few words how does that work.
Nothing moving through the atmosphere is heated by friction. There is a heating process with some analogs to friction, but it's a collisional process that only applies when the moving body is small compared with the mean path between collisions. That means for small particles (<1 cm) at high altitudes (>50 km). Larger bodies simply compress the air in front of them, producing what can be very high ram pressures (megapascals). As always when you compress a gas, you increase its temperature.
In this case, the pressures were low and not much heat was generated. He reached a maximum velocity of only around 350 m/s. You need to be moving more than ten times faster to generate enough heat to start melting and ablating typical refractive materials. An astronaut trying to do this from an orbit would enter at over 8000 m/s, and would burn up with any current technology we have. Jumping from a balloon, which is stationary with respect to the ground, is a trivial problem compared with jumping from low-Earth orbit.
Thanks Chris. Of course, there are still drag forces, including skin friction components, which although not generating heat, are still something I'd want protection from, depending on the relative air speed.
I've never really thought about atmospheric heating arising from the air being compressed in front of the moving object. I've always considered it to be caused by the object slowing down the air within the boundary layer surrounding the object (which is necessary for continuity -- the speed of the air at the object's surface much be zero, relative to the object). Whenever you slow down a fluid, you increase its pressure, as per Bernoulli's principle (the same principle which explains aircraft lift and other cool stuff). And, as per the gas laws, by increasing the pressure, you increase the temperature, which is transferred from the boundary layer to the object via forced convection or possibly conduction (I'm not sure). Of course, both ways of looking at it amount to the same thing, but I believe one could argue that the heat is ultimately generated by viscous forces within the fluid, due to friction between neighbouring fluid particles.
Edit: actually, I may be misunderstanding the gas laws. The relationship between pressure and temperature does not depend on viscosity, so I would appear to be wrong about viscous forces generating heat. It is rather that the heat increases the viscous forces. Poop.[quote="Chris Peterson"][quote="Robert Boyle"][quote]The heat is generated by the compression of the air in front of a falling object.[/quote]
Is it, indeed? Please explain in a few words how does that work. [/quote]
Nothing moving through the atmosphere is heated by friction. There is a heating process with some analogs to friction, but it's a collisional process that only applies when the moving body is small compared with the mean path between collisions. That means for small particles (<1 cm) at high altitudes (>50 km). Larger bodies simply compress the air in front of them, producing what can be very high ram pressures (megapascals). As always when you compress a gas, you increase its temperature.
In this case, the pressures were low and not much heat was generated. He reached a maximum velocity of only around 350 m/s. You need to be moving more than ten times faster to generate enough heat to start melting and ablating typical refractive materials. An astronaut trying to do this from an orbit would enter at over 8000 m/s, and would burn up with any current technology we have. Jumping from a balloon, which is stationary with respect to the ground, is a trivial problem compared with jumping from low-Earth orbit.[/quote]
Thanks Chris. Of course, there are still drag forces, including skin friction components, which although not generating heat, are still something I'd want protection from, depending on the relative air speed.
I've never really thought about atmospheric heating arising from the air being compressed in front of the moving object. I've always considered it to be caused by the object slowing down the air within the boundary layer surrounding the object (which is necessary for continuity -- the speed of the air at the object's surface much be zero, relative to the object). Whenever you slow down a fluid, you increase its pressure, as per Bernoulli's principle (the same principle which explains aircraft lift and other cool stuff). And, as per the gas laws, by increasing the pressure, you increase the temperature, which is transferred from the boundary layer to the object via forced convection or possibly conduction (I'm not sure). Of course, both ways of looking at it amount to the same thing, but I believe one could argue that the heat is ultimately generated by viscous forces within the fluid, due to friction between neighbouring fluid particles. [i]Edit: actually, I may be misunderstanding the gas laws. The relationship between pressure and temperature does not depend on viscosity, so I would appear to be wrong about viscous forces generating heat. It is rather that the heat increases the viscous forces. Poop.[/i]