Quote:
Originally Posted by IsaacMTSU
So you are saying there is never smooth laminar flow on car's surface?
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It's impossible to have a laminar boundary layer do to the nature of ambient air in ground proximity,the size of the vehicle,and the critical vehicle velocity for generation of large Reynolds number,but that's a good thing for autos.
The early transition to a turbulent boundary layer allows kinetic energy from the outer flow to be fed into the boundary layer beyond the point of maximum body cross-section,where the boundary layer is no longer held against the body in a positive pressure gradient,but rather in an adverse pressure regime.The addition of energy into the turbulent boundary layer allows the separation point to be moved reward as long as there is enough sectional density to the body.This will allow laminar flow in the region beyond the boundary layer.
If the body contracts too quickly,the pressure rises too suddenly,which would require a deceleration of the flow adjacent to the body( under the law of conservation of momentum),but because of viscosity,that flow is already at zero velocity and cannot 'slow' anymore,and the boundary layer will separate from the boundary (body),leading to reversed eddy flow,followed soon by full-blown turbulence.
This would only happen on a wing at very high angle of attack.
On cars,where the flow separates,establishes the base pressure of the wake for the car and its pressure drag.
The further back we can move the separation point,the higher the base pressure of the wake.If we can eliminate separation all together,the 'wake',or lack there of, will be at the highest pressure possible,the delta-P as measured up to the forward stagnation point will be the lowest,leading to the lowest pressure drag,which is the fundamental premise of road vehicle aerodynamic streamlining.