…velocity you're using Yes, the velocity that you're using over the speed of sound is the same in both cases. That's called the Mach number. And when you're getting near the speed of sound or above, you must make sure that both the Reynolds number and…

…Mach number. And when you're getting near the speed of sound or above, you must make sure that both the Reynolds number and the Mach number are equal for the two situations. But if we're way below the speed of sound, then we don't have to worry about…

…number are equal for the two situations. But if we're way below the speed of sound, then we don't have to worry about the Mach number as long as it's small. All right, then. We now discuss the results of experiment on the case of the flow past the…

…altitudes, because the speed of sound is lower there. It's true. No, it's true. Many times they argue about how many Mach numbers they've achieved. And the reason is partly because they went to low temperature. One other thing that's rather interesting, you…