4 ms·
That's what a several miles per second wind looks like! The heat is not from the friction, but from the compression of the gasses.
by BryanLegend 3y ago
That's what a several miles per second wind looks like!
The heat is not from the friction, but from the compression of the gasses.
- enderfusion 3y agoAll those little molecules trying to get out of the way.
- rbanffy 3y agoThey are screaming, first in infrared, then red, orange, white...
- enderfusion 3y agoThey were not expecting this....
- pfdietz 3y agoThey're so excited, and they just can't hide it.
- pfdietz 3y agoI often see that claim, but I don't like it. The implication is that the heating is like when one compresses air in a bicycle pump, the increase in temperature that comes from adiabatic (reversible, isentropic) compression of a gas. And some compression does occur, so there is some necessary heating from that source (as required by the second law). But entry heating is not reversible. It's fundamentally irreversible, in fact. The gas is going through a shock. Shocks fundamentally cause an increase in entropy as fast gas slams into slow gas over a region whose thickness is on the order of a mean free path of molecules in the gas. And, in fact, the increase in density of gas going through a shock approaches a limit (around 4, IIRC, for air) regardless of the Mach number. So at sufficiently high speed most of the heating is coming from dissipation at the shock (a process akin to friction), over and above the heating implied by adiabatic compression.
- rbanffy 3y agoI prefer to say it's the molecules screaming in agony as they are violently compressed and forced to hit each other at speeds they are mostly uncomfortable with.
- TeMPOraL 3y agoA molecular equivalent of a massive, high-gee car pileup on a highway. (Hope that never actually happens.)
- pfdietz 3y agoShock wave dynamics are observed in the flow of traffic on sufficiently crowded highways. https://en.wikibooks.org/wiki/Fundamentals_of_Transportation/Shockwaves https://en.wikibooks.org/wiki/Fundamentals_of_Transportation...
- pixl97 3y agoI see you've never seen people drive in Texas when there's ice on the road.
- mindfulmark 3y agoSeems like the gases are getting compressed either way and it's just different ways of wording the same effect. As for it being reversible or not, is it not just a matter of whether the energy was actually transferred somewhere? Like you could technically undo the shock the same as you could depressurise air in a pump no? I don't really know what I'm talking about though, fyi.
- pfdietz 3y agoGas is being compressed, but that doesn't mean the heating is from compression. There's a gas heater on the market that works by using rapidly moving vanes to induce shock waves in the gas. The outflow has the nearly the same pressure as the inflow, but the gas has been heated, potentially to a temperature higher than could be achieved by resistive heating elements. EDIT: I mistated this; see below for link. Consider also that once the shock heated air around the reentry vehicle has expanded back to ambient pressure, it will be hotter than it initially was.