If you compress a gas, even an ideal frictionless gas, its temperature rises. Like I said, that's how refrigerators work. As for compression in flight, that's how pitot tubes measure airspeed. It has nothing to do with friction.

On a wing, the temperature rise is highest at the static point on the leading edge, where there is no motion of the air. The temperature declines from there back as the airflow speeds up. If friction was the source of the heat, you'd expect the temperature to be the lowest at the static point, and would increase towards the trailing edge.

BTW, how a diesel engine works is by compressing the air/fuel mixture until it heats up enough to ignite. A gas engine will sometimes "diesel" on after the ignition is turned off.

> It has nothing to do with friction.

The point I am making is that gas going through a shock is a fundamentally different process than a gas being adiabatically compressed. The former causes an increase in entropy; the latter ideally does not. While a shock isn't "friction" per se, like friction there is dissipation (and at entry speeds, quite a lot of dissipation).

Are you sure? Consider:

https://www.kimerius.com/app/download/5782475760/Calculation...

The paper does talk about heat from friction, but not at the stagnation point. It also talks about the heating of the surface being less than predicted by adiabatic compression, but that is the heating of the surface, not the heating of the air at the stagnation point.

I have argued with @pfdietz about this before. In the zeal to make the point the majority of leading edge heating arises from the dissipative effects of shocks, they make statements that have to be very carefully parsed to be correct.

You are right. Isentropic compression will result in a higher temperature of the compressed fluid. This is undergraduate gasdynamics. In these supersonic flowfields with high shock strength though, the shock acts as a very dissipative compression mechanism. Behind the shock is a compressed fluid that is much hotter than it would be if compressed to the same pressure isentropically. Shocks are dissipative, like friction, but shocks and friction are not the same thing.

So unless you remove heat from a perfect compressible fluid being compressed (i.e., via a non-adiabatic process), it will always be hotter post-compression. It's just that the compression and heating that shocks effect causes much greater heating than the isentropic case.

In what we call "acreage" where the forward-facing area is limited (e.g., upper surfaces of wings and vehicle bodies), other effects, including turbulent skin friction heating, can dominate the local heating rate.

Thanks for the explanation. My undergraduate fluid mechanics class did get into shock waves, but not the temperature implications of them.

That paper doesn't appear to have much to do with what we're talking about here. The transfer of heat to the vehicle from the hot shocked gas is another issue entirely.

Anyway, it's foundational supersonic fluid physics that shocks create entropy.

"Shocks are irreversible occurrences in supersonic flows (i.e. the entropy increases)." -- https://en.wikipedia.org/wiki/Shock_(fluid_dynamics)

Reentry with blunt bodies works because most of the dissipation is occurring at the shock, at some distance away from the vehicle, allowing most of the heat (that must be produced from the kinetic energy of the vehicle as it slows) to be carried away.

Yet, he has explained that this is not what happens, and why.