Drag reduction for D-shape and I-shape cylinders (aerodynamic mechanism of reduction of drag)

The aerodynamic mechanisms for the reduction of drag for a D-shape cylinder, wherein a front face of a circular cylinder is cut off, and an I-shape cylinder, wherein front and rear faces are cut off, are investigated. For the D-shape and I-shape cylinders with a cutting angle of 50 - 53 degrees and for Reynolds number Re > 2.3 x 10(4), the shear layer separated from the front edge reattaches on the circular arc of the cylinder, and a transition in the boundary layer as well as turbulent separation occur. As a result, the wake width decreases and the vortex formation region goes downstream. The Strouhal number increases beyond 0.28, the base pressure coefficient rises, and the drag coefficient of the cylinders decreases to half the value for a circular cylinder. The conditions of the above phenomena are clarified.