A Flow Model for the Effect of a Slanted Base on Drag

Experiments by Morel have shown that slanting the base of a bluff body causes large variations in the drag, as the slant angle changes. For a particular, critical slant angle the drag changes discontinuously. This phenomenon was correlated with a drastic change in the type of base flow. A mechanism to explain this change, and therefore, the discontinuity in drag, is proposed. The basis of the mechanism is breakdown of the side edge vortices. An estimate of the swirl angle in these vortices is obtained using a swept-back mixing zone solution. This swirl angle and the other elements of the theory provide an estimate for the critical slant angle that is entirely consistent with the observed value.