Discussion
Air flows with a freestream velocity U over four different bodies having same frontal area facing the flow direction, as shown in the figure. Which one of the following bodies has the lowest pressure (form) drag force for Reynolds number, \(Re \ge 10^{4}\)?
Answer: D
The lowest pressure drag is on Body S.
At Reynolds numbers of \(10^{4}\) and above, form drag is set mainly by flow separation at the rear. A rounded nose lets the flow follow the surface at the front, and a long, gradually tapering tail lets the flow decelerate slowly along it, so it stays attached and the low-pressure wake is narrow. Body S has both, a streamlined shape.
Body P, a blunt block, separates at its sharp front corners and leaves a wide wake, the highest drag. Body Q rounds the nose but still ends in a blunt base. Body R rounds the rear corners too, but its rear is still bluff compared with S's long taper, so its wake is wider than S's.
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