A method for the reduction of bluff body drag

Although several techniques for the reduction of bluff body drag have been proposed in the literature by previous investigators, most of these investigations did not estimate the drag of the entire system consisting of bluff body and drag-reducing device. In the present study, we have devised a method of not only reducing the drag of the bluff body, but reducing the drag of the entire system to values well below that of the bluff body alone. Our method, which utilizes a small flat plate placed upstream of and parallel to the cylinder (with the plate oriented normal to the free stream): has yielded an optimal geometrical configuration consisting of a plate height one-third the cylinder diameter placed 1.5 diameters upstream of the cylinder, and produces a system drag that is 38% that of the bare cylinder alone. The reduction in drag is manifested through two distinct modes of the flow - a cavity mode observed for small values of the plate-cylinder gap, and a wake-impingement mode observed for larger values of this gap. The cavity mode is found to deliver a larger reduction in drag due to a substantial increase in thrust associated with the front surface of the cylinder.