4 ms·

Or how reentry heating is apparently caused mostly by friction. (Compression of air is the main cause in actuality)

pfdietz 6y ago
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.
- bernulli 6y ago
  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?
  - pfdietz 6y ago
    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.
    - bernulli 6y ago
      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.
      - pfdietz 6y ago
        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.
        bernulli 6y ago
        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]
        pfdietz 6y ago
        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.
    - ThenAsNow 6y ago
      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.
      - pfdietz 6y ago
        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.
  - ThenAsNow 6y ago
    Compression due to a shock is not isentropic. Parent is incorrect. This is basic gasdynamics. An accessible reference: Modern Compressible flow, by Anderson.
    - pfdietz 6y ago
      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.
      - bernulli 6y ago
        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'?
        pfdietz 6y ago
        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.
    - ThenAsNow 6y ago
      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.
- ThenAsNow 6y ago
  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.
  - pfdietz 6y ago
    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.
    - ThenAsNow 6y ago
      Parent just said "compression heating" which no one takes as a claim of isentropic compression. Shock-driven compression still qualifies as compression.
      - pfdietz 6y ago
        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.
    - bollu 6y ago
      This is very interesting, is there a textbook for this?
      - bernulli 6y ago
        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.
bloak 6y ago
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.
- dunce2020 6y ago
  > 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.

bernulli 6y ago
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?
- pfdietz 6y ago
  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.
  - bernulli 6y ago
    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.
    - pfdietz 6y ago
      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.
      - bernulli 6y ago
        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]
        pfdietz 6y ago
        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.
  - ThenAsNow 6y ago
    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.
    - pfdietz 6y ago
      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.
- ThenAsNow 6y ago
  Compression due to a shock is not isentropic. Parent is incorrect. This is basic gasdynamics. An accessible reference: Modern Compressible flow, by Anderson.
  - pfdietz 6y ago
    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.
    - bernulli 6y ago
      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'?
      - pfdietz 6y ago
        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.
  - ThenAsNow 6y ago
    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.
ThenAsNow 6y ago
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.
- pfdietz 6y ago
  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.
  - ThenAsNow 6y ago
    Parent just said "compression heating" which no one takes as a claim of isentropic compression. Shock-driven compression still qualifies as compression.
    - pfdietz 6y ago
      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.
  - bollu 6y ago
    This is very interesting, is there a textbook for this?
    - bernulli 6y ago
      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.

bloak 6y ago

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.

dunce2020 6y ago
> 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.