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From what I understand, Bernoulli's equation should be applied in the reference frame of still fluid.

Using Bernoulli's equation to a wind tunnel, we find that the pressure inside a wind tunnel should be lower than that of atmosphere. The air in the wind tunnel is moving, after all. Wouldn't this make something in a wind tunnel an inaccurate representation of something moving through the atmosphere?

For example, because pressure is lower in a wind tunnel, the density of air is also lower, which reduces drag forces compared to the atmosphere. So if one is using a wind tunnel to test how aerodynamics a car or a plane is, they would get an inaccurate answer.

A car staying through the wind tunnel thus faces lower pressure/drag forces than a car actually moving outside. Ditto for airplanes.

So, is pressure in a wind tunnel lower than that of atmosphere? Does this present an inaccuracy when using a wind tunnel as a simulation for something moving through still air?

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  • $\begingroup$ What is the pressure of "still air?" Also, do you suppose it depends on whether the blower is positioned to force air in to one end of the tunnel or, to pull air out from the opposite end? $\endgroup$
    – Solomon Slow
    Commented Nov 20 at 18:23
  • $\begingroup$ The pressure of still air is 1 atmosphere on Earth. Bernoulli's equation does not care about where the pump is, as far as I know. $\endgroup$
    – Fluidized Pigeon Reactor
    Commented Nov 21 at 2:19
  • $\begingroup$ > "we find that the pressure inside a wind tunnel should be lower than that of atmosphere. " No, because you can't apply the Bernoulli equation across an active fan. Wind tunnel has a big fan, which accelerates air, and can create both fast flow and increased pressure downstream (this effect is called compression and it is used e.g. in jet engine). $\endgroup$
    – Ján Lalinský
    Commented Nov 21 at 2:56
  • $\begingroup$ @FluidizedPigeonReactor, Yes, but "1 atmosphere on Earth where? The pressure of still air in Denver, Colorado is only about 80% of the pressure of still air on a beach somewhere. And, that's really only an average figure because air pressure in both of those places varies with the weather. $\endgroup$
    – Solomon Slow
    Commented Nov 21 at 14:23
  • $\begingroup$ @FluidizedPigeonReactor, Bernoulli's equation doesn't care about the location of the blower, but the blower moves air by creating a pressure differential. Pressure on the inlet side of the blower is significantly less than the pressure on the outlet side. If you create a wind tunnel with the blower positioned to force air in to the tunnel, then switching it on will raise the pressure of the air in the tunnel. (Bernoulli says, it won't raise it by as much as if the outlet were blocked so that the air cannot flow.) If the blower sucks air out from the tunnel, then that reduces the pressure. $\endgroup$
    – Solomon Slow
    Commented Nov 21 at 14:32

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