Drag Reduction of Circular Cylinder in the High-Reynolds-Number Range

A Study on the effect of a V-groove on the drag reduction of a circular cylinder in the high Reynolds number range (5×104 ≤ Re ≤ 107) was made. The pressure distribution along the surface of the cylinder was measured and drag coefficient was calculated. With reference to the results of research about riblet, which was made on a flat plate placed parallel to the stream, it clarified clear that the grooves in the circumferential direction are effective in the reduction of drag, if their depth and width have proper dimensions. The effects can be classified into two cases. One is the original riblet's effects. The base pressure coefficient becomes higher and the angle at which flow separation arises, becomes larger. The V-groove, in this case, contributes to retard the development of the turbulent boundary layer and to reduce the surface drag. In the other case, the pressure distribution around the surface of θ = 30∼90°closely resembles that of potential flow. The angle at which the pressure coefficient is zero becomes smaller and the value of C- becomes lower and the angle θcorresponding to C- becomes larger.