Thread: dragging dust
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Old 08-06-2019, 05:44 PM   #7 (permalink)
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Originally Posted by Ecky View Post
My Insight gets virtually no dust on the hatch top window, which should have nearly laminar flow, but gets a lot of dirt on the vertical window at the back, where there is a lot of turbulence.
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.

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