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Post-Script
Patrick raised the question of the origin of the formula I'm using for the surface-drag calculation.
It comes from an example problem published in my college text,"FLUID MECHANICS with Engineering Applications," by Daugherty and Franzini.
On page 62 of the text the authors make the comment:"..a rigid mathematical treatment of turbulent flow is impossible,and instead statistical means of evaluation must be employed."
From the authors' comment,I infer that the drag of the underside of a 'typical' automobile would be 'impossible' to calculate,and that this explains the deletion of the vehicle underside surface area from the surface-drag calculation illustrated in their work.
If your car car has an extremely well detailed,full bellypan,which under flow visualization demonstrates fully attached flow over its entire length then you'd want to include its area in a surface drag calculation.
Short of that,you could just ignore it,keep things simple,which is the spirit of the formula.
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