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Old 02-22-2008, 02:56 PM   #13 (permalink)
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Aerodynamics is fun. I once had a job doing wind tunnel tests, and it's a tricky business. One thing I learned is that most of the terms like Cd are approximations made up to fool amateurs. Classical fluid dynamics is all steady-state stuff where a body is assumed to reach an equilibrium state with a steady flow around it. "Turbulence" (another loosely defined term) is treated as an untidy exception to that.

One note about your example: that's a supersonic test. The lines are shock waves; the angle suggests it's moving at about Mach 2. You might want to find a subsonic example.

Reynolds number is difficult to explain, but it's basically a scaling factor between the body and the intramolecular distance of the gas. At small scales, turbulence is disproportionately less likely to occur, while large bodies have many more eddies to deal with.

Most of aerodynamics is about flow visualization. There are many tricks to getting smoke trails, pressure sensors, etc., around the model to get useful measurements. The good news is that empirical tests work -- theory is sort of a starter course on where problems occur, but the state of the art is a lot of testing and fiddling.
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