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So I asked this question, and I wanted to know if my guess below was correct. I got an answer in the comments, but the reason for asking this question is because I was still a bit confused if my thinking was correct, if that makes sense. (the below is copy/pasted from the linked question) Also, if this question is considered a duplicate or goes against any guidelines, I'll happily delete it. (I did read this page, on what questions to ask and what to avoid.)

My guess of what happens with density at higher and lower speeds is the following (my guess is that it's a non-linear relationship) : The air density doesn't change much until you reach Mach ~0.3, where Bernoulli's equation isn't valid anymore, so the density can't change but so much, or Bernoulli's equation would be false, even below Mach 0.3.

When you go faster, there are less air molecules to 'bounce' off of the surrounding body and each other (because at high speed, the surrounding air is less dense), so changing speed doesn't do as much as it would at a lower speed, if that makes sense. (I might be wrong about some of those statements.)

I think if I explain my guess of why density doesn't change much up until Mach 0.3, it might clarify things. So if you have normal-density air (like still air at room temp) and you start to speed it up, a few things happen.

The air molecules bounce off of each other less, because they all have a forward speed component now, and they 'bounce' less on the surrounding container/body, both creating a lower density air. Doing this at lower speeds, there are more molecules, and because there are so many, speeding up doesn't prevent the 'bouncing' as much as it does at higher speeds. (Again, I might be wrong about some of those statements.)

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  • $\begingroup$ Going back to basics may help here. If air molecules "bounce off each other less", the air is cooler. Pressure is a function of Density and Temperature. I would not conflate this with Trans-sonic "Gedanken experiments" for now. $\endgroup$
    – Robert DiGiovanni
    Commented Jan 16 at 10:02
  • $\begingroup$ @RobertDiGiovanni ah okay I see. So I’m kind of confused about how the Gedankin experiment answer ‘works’ because you made a good point about pressure vs. temperature. $\endgroup$
    – Wyatt
    Commented Jan 16 at 16:34
  • $\begingroup$ I know it can be confusing because, even in a wind tunnel, "wind flow" is not entirely accurately portrayed (but good enough to get some solid research done). In reality it's the air reacting with the wing in motion. This is why they say air isn't compressed (at lower Mach), it just gets moved out of the way. The Gedanken experiment seemed to deal with pressure differences behind the shockwave at transonic speeds. This took a lot of years for many people to figure out. $\endgroup$
    – Robert DiGiovanni
    Commented Jan 16 at 18:35
  • $\begingroup$ @RobertDiGiovanni hmm okay. I came across this answer, and if you look at the bottom of it, you'll see a part about density. Could you explain what it means, because it says "A speed increase is always coupled to a decrease in density." which is confusing because at this point I'm not sure if density increases or decreases with speed. Thanks $\endgroup$
    – Wyatt
    Commented Jan 16 at 18:52
  • $\begingroup$ density where? I believe they are talking about phenomena behind the shockwave. But is density vs velocity a function of air itself? Or is it (as pressure, Bernoulli) that a moving air (or water) stream draws air away from a surface, creating lower pressure, ergo density? Air simply moves from high pressure to low. To have a significant change in density (along the path of the airstream, within a few meters) would require one heck of a pressure differential. $\endgroup$
    – Robert DiGiovanni
    Commented Jan 16 at 21:19

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The density doesn't go down. If anything it goes up. Here's some intuition for the process:

The aircraft is constantly trying to mush the air particles together, which would increase the air density.

When the aircraft is very subsonic, the particles around the ones being mushed together get out of the way. This keeps the density from increasing much at all. But as it gets faster and faster the particles have less chance to get out of the way, so they bunch up instead, which results in increased density.

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  • $\begingroup$ ... to add this answer, the ability of the particles to "get out of the way" is a function of their average molecular motion (think how fast they are moving around in the fluid from just bumping into one another, the motion due to their heat or temperature), and this remains the overriding determinant up to and until the aircraft true airspeed exceeds the average thermal velocity, (transonic, or about 0.92Mach), where shock waves begin to form. Shock waves are the result of the particles inability to "get out of the way" of particles bumping into them. $\endgroup$
    – Charles Bretana
    Commented Jan 15 at 12:17
  • $\begingroup$ Okay, thanks. Also what if there wasn’t a body or an object in the flow? It would still decrease density as it sped up, right? $\endgroup$
    – Wyatt
    Commented Jan 15 at 14:29
  • $\begingroup$ @Wyatt No, it doesn't decrease whether a body is in the way or not. Density is a function of how many air molecules are in a given volume. Speeding air up has no effect on its density. $\endgroup$
    – Chris
    Commented Jan 15 at 14:58
  • $\begingroup$ @Chris oh okay. Could you read the 3rd paragraph of this answer to see why I thought density decreases with speed? You might see why I’m a little confused because of what the linked answer says. Thanks. $\endgroup$
    – Wyatt
    Commented Jan 15 at 16:10
  • $\begingroup$ Also if you see the first answer in the link I provided in the question, you might be able to see where I'm coming from more (plus the link in my previous comment) $\endgroup$
    – Wyatt
    Commented Jan 15 at 23:53

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