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Originally Posted by s_t
Thanks. Been looking at more modelling and theory to try and understand it, and seems I was wrong about the 'high pressure' just above the windscreen. It's high velocity; which is low pressure I understand. But the transition between roof and windscreen would have to be rounded enough to prevent turbulence? Which i gather is what shedding vortexes are? Any guidelines on acceptable radii in this area?
To avoid taking this thread off-topic I'll reply to this on my other thread (35515).
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The big dogs will tell you that there is no 'magic' radius for forebody edges.
4% of the square-root of the frontal area of the body itself is still a good guide.
In the forebody,up to the location of maximum body cross-section,there is a positive pressure gradient as the vehicle attacks the air in front of it.The air is essentially held against the body by this impingement.
A look at a 1975 VW Rabbit/Golf will show you a working minimum for radii.There is fully-attached flow everywhere on the VW's forebody.
It's the aft-body where the challenge comes,as all this area is in a unfavorable pressure regime,with no reason for the flow to remain attached,unless the body cross-section varies as it would in a streamline body.Hence the Template.
Any vehicle manufactured after 1982 would have respected Hucho's shape optimization techniques published in 1978.
Since forebody drag constitutes less than 6% of the overall drag,it's better to spend your energy and money on the aft-body.