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Old 08-07-2019, 11:09 AM   #10 (permalink)
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transition length

Originally Posted by Vman455 View Post
Just a terminology note: "laminar" =/= "attached." That far back on any car at any appreciable speed*, the flow is turbulent--but attached over the rear window as you observed.

*Flow characteristics are related to the Reynolds number, which is the ratio of the inertial forces acting on the fluid particles to the viscous forces. When the inertial forces outweigh the viscous forces, i.e. Reynolds number is large, the flow will be turbulent. And since the inertial forces are the product of fluid velocity and test length, as speed increases or the point of investigation moves further along the body, Reynolds number increases. Since the kinematic viscosity of air is very small, 1.46x10^-5 m^2/s, it doesn't take much speed or much length for the flow over a car to transition from laminar to turbulent.
There was a Master's thesis on a RAM pickup,which included a CFD study.According to the numerical model,at supercritical Reynolds number,the pickup had 30mm of laminar boundary layer at the nose.The rest was turbulent.
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