Quote:
Originally Posted by JulianEdgar
Assume the boundary layer against the smooth car underside growing to, what, 30mm? I don't think that 'boundary layer' in this context makes sense for the ground (because the ground boundary layer might be a few metres thick, and the ambient air is ostensibly still ie not moving like the air past the car). That would seem to me to give about 70mm (typical car with 100mm ground clearance) of 'free air'.
But you do make a very interesting point. I've not seen, that I can think of, the velocity gradient plotted under the car on a vertical axis. Does anyone know of such data?
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I looked, and for the most part, it appears that the road surface is basically treated as if it has zero boundary layer, only the bottom of the vehicle, and then only if it has a smooth underside.
Wind tunnels will suction off any boundary layer reaching a test vehicle, or lift the vehicle up even with the top of it.
' lifting of the vehicle by the amount of the displacement thickness of the ground boundary layer ( in the wind tunnel ) has no effect upon the flow around the vehicle and the effective forces.' Hucho, page-412.