Technically it's not "friction" because "friction" has to be between two solids. But arguably that's just being pedantic. The air gets in the way of the object moving through it and slows it down, but energy is conserved, as always, so the energy ends up as heat. Knowing that the correct term is "air resistance" rather than "friction" doesn't seem to provide any insight into what's happening, so I don't understand the obsession with correcting people online who call it "friction".

I've been told you can bring water to the boil with a food mixer and a bit of patience. That's not friction either. It's water getting in the way of and slowing down the blades.

> so I don't understand the obsession with correcting people online who call it "friction".

It lets dumb people feel intelligent. The type of person that confuses trivia for insight.

No, actually friction is closer to the truth. It's air running into air at a shock, a process that generates entropy, just as friction does.

Compression is a process that is adiabatic, generating no entropy. And in an ideal gas the density increase across a shock reaches a finite limit even as the mach number goes to infinity.

I can't make sense of your claim. Compression could be done adiabatically, or not, but that doesn't mean it's isothermal or somehow inherently adiabatic. In fact, for it to be isothermal would require the transport away of compression heat, which would be decidedly non-adiabatic.

The parent is correct.

I didn't say the compression was isothermal.

The parent is not correct. Compression occurs at a shock, but this is not heating due (solely) to adiabatic compression. This is clearly seen by considering the case where mach number goes to infinity: in that limit, the fraction of heating due to compression goes to zero (as the gas density remains below some finite limiting value.) What is causing heating is a dissipative process in the shock itself. This dissipative process is more akin to friction than it is to compression.

Parent just said "compression heating" which no one takes as a claim of isentropic compression. Shock-driven compression still qualifies as compression.

This has just defined shock heating as compression heating. But as I argued elsewhere, that makes no sense, since the compression at a shock is bounded by a finite maximum value regardless of the mach number.

This is very interesting, is there a textbook for this?

I haven't seen any of pfdietz' arguments in a book.

But ThenAsNow already recommended the excellent "Modern Compressible flow" by J.D. Anderson, and, in extension, I can recommend "Hypersonic and High-Temperature Gas Dynamics" by the same author.

I am having trouble seeing how compression in a shock would be isentropic? Care to elaborate?

And why passing through a shock (compression) is akin to friction, but not to compression?

Compression due to a shock is not isentropic. Parent is incorrect. This is basic gasdynamics. An accessible reference: Modern Compressible flow, by Anderson.

Replying to myself since I guess there's a thread depth limit. Now what you are saying is making sense, your original post did not. Irreversibility (increase in entropy) across the shock does cause a significant temperature gain (and at re-entry speeds, nonequilibrium flow, radiation, etc.), but isentropic compression does as well. Still, the shock is a compression artifact, and while you can draw an analogy between the irreversibility within a shock and that in a friction process like in a viscous drag, the original poster's categorization of re-entry heating as compression heating is completely consistent with how practitioners in the gasdynamics community would categorize it.

(you have to wait a short time before you can reply to a message; it's not a depth limit)

Practitioners may say that, but it's sloppy and makes no sense when examined closely, as I've argued here. Compression (as the word is properly defined) is finite even as mach number and heating increase without limit.

The basis of the disagreement hinges on your original claim that "compression is a process that is adiabatic, generating no entropy" which doesn't have any valid basis. Nothing about even the definition of compression that you provided above implies that compression is or isn't by definition isentropic (also, adiabatic and isentropic are not the same thing). Are you trying to argue that flow passing through a shock is not compressed by passing through the shock?

I'm also struggling to understand your point about the hypersonic density ratio limit. You keep repeating it but how specifically does it support your argument?

Of course it has a valid basis. The alternative is to say that in any process that reduces the volume of the material, all the heating is due to compression (even though the amount of heating can vary widely in different processes with the same reduction in volume.)

The logical error you are committing is saying "if a process includes compression, then the heating is due to compression".

The semantic error you're making is redefining the general term compression as the more specific "isentropic compression". The process by which the compression takes place is not prescribed just by the term "compression" as you did originally. Just saying "compression" specifies nothing about the process by which it happens. Compression can take place (theoretically) through an isentropic process, or a non-isentropic process, with work being done on the fluid (increasing its total enthalpy) or not.

I get your point that the shock contributes to the heating through irreversibility, and agree that has a significant impact on the sensed temperature. If we want to tie your point back to the original post, one could say that "compression and shockwaves" are the cause of re-entry heating to ensure you are explicitly capturing the effect of the shock.

Whether you consider the shock heating to merit explicit mention or whether you see it as a detail associated with the specific means of compression is largely a matter of perspective & practice.

The gasdynamic basis of shock formation is derived by considering what happens when pressure fields are no longer able to smoothly vary as the speed of a body exceeds the speed at which pressure disturbances can be communicated upstream, namely the speed of sound. The pressure jump across the shock is elemental to its derivation. Practitioners commonly view the shock as the means by which compression takes place and pressure fields reconciled. As I noted above, just saying "compression" doesn't imply "isentropic" and so the entropy-generating processes associated with a shock performing compression are viewed, for example, as thermodynamically similar to viscous heating of the fluid in a gas turbine compressor.

Consider the following extension of this thermodynamic process. The gas is shocked, and then we adiabatically expand it back to its original density.

Because entropy was added, the gas is now hotter than initially. But it is not compressed. The heating wasn't due to compression, it was due to dissipation at the shock. That dissipative process is separate from compression, and you are confusing the two.

The incoherence of your position should have been clear to you. You seem to think that compression, as a cause of heating, doesn't even have a well defined amount of heating. Your explanation isn't even wrong, it's undefined.

Well, but the gas does not reversibly expand again between the shock and the stagnation point. So clearly it is heating up, even if it were compressed isentropically.

The position is far from incoherent. Temperature rise during isentropic compression is well understood, as is the temperature rise and total pressure loss through a shock.

No, it is quite incoherent. Specifically, it fails to even offer what any physical theory must: a quantitative prediction. Compression, in your view, can be allowed to produce variable amounts of heating, for the same change in density. Since you aren't making a quantitative prediction, how is what you are saying even testable?

(The answer is that compression produces a specific amount of heating when it is adiabatic, and additional heating comes from dissipative processes that are occurring alongside the compression, but that are not themselves compression.)

I didn't say compression at a shock was isentropic. I was saying the claim that compression at the shock could not be the cause of (all) the heating, unless you simply define heating at a shock as being due to compression. Instead, I think you should separate out the heating there into two parts: one due to isentropic compression, and one due to injection of additional entropy by dissipation at the shock.

Yes you do, you keep repeating that 'proper' compression is isentropic. That's simply not the case, certainly not across a shock.

The second part of your argument is well defined and makes sense, and no-one argued that.

But explain to me again how this is now 'like friction'?

Requiring compression to be isentropic is just requiring "compression" to be a well-defined term with a specific quantitative result. Otherwise, it's a vague term that offers no quantitative prediction for how much heating it produces for a given change in density. That's not science.

Because compression, as normally considered, does not generate entropy. It causes gas to become hotter because the temperature has to be higher for entropy to remain constant, as the volume is reduced. This is a reversible process. But shocks are inherently irreversible.

But as my last sentence said, as the mach number goes to infinity the density increase across the shock is finite. Therefore, compression cannot be the (primary) source of the heating; dissipation is.

I have never seen compression to be considered inherently reversible, certainly not in the aerospace or hypersonics/reentry communities. I assume you come from a different background?

Care to explain your second argument? Even when the density reaches a limit, the change in pressure over the shock does not, so you can pump in energy arbitrarily (of course your gas would dissociate at some point). The total pressure loss can be complete if your initial Mach number is high enough.

Let's look up a definition.

"Compression: decrease in volume of any object or substance resulting from applied stress."

Compression refers to a reduction in volume, not an increase in pressure. This is finite for gas crossing a shock, even as the mach number goes to infinity.

Exactly. There's no mention at all about the foundation of all your arguments, namely that compression be inherently isentropic. It simply is not. You can add it as an additional constraint, if you want to.

What I say is that even when dV approaches a constant, the pressure still rises, so the product also rises.

[edit: removed snark]

Requiring compression to be isentropic is just requiring "compression" to be a well-defined term with a specific quantitative result. Otherwise, it's a vague term that offers no quantitative prediction for how much heating it produces for a given change in density. That's not science.

Look, where does this idea come in that compression "as normally considered, does not generate entropy"? The inlet of a supersonic aircraft or a ramjets/scramjet performs non-isentropic (shock-based) compression. You're coming up with your own definition of compression that is inconsistent with gasdynamics practice.

Considering compression to be isentropic is just nailing down what compression means enough for its heating to be actually definable. Otherwise, the argument is just "if there's compression occurring, all the heating must be considered as being due to compression" which is just bullshit.