Drag Reduction of a Sphere at Supercritical Reynolds Numbers using passive Ventilation

Abstract

This paper describles an experimental investigation of drag reduction of a sphere at supercritical Reynolds numbers up to Re = 4.5x1000000 using passive ventilation. This ventilation was created by a channel connecting the stagnation region with the wake region. Passiv ventilation is a very effective method of decreasing drag of smooth spheres in the named Reynolds numbers range. A drag reduction of about 40%-60% can be achieved with a cross-section area of the venting channel of about 2% of the sphere. Pressure distributions on the sphere show clear changes of base pressure and point of separation. In addition, the flow on the vented sphere showed smaller oscillations at supercritical Reynolds numbers than those on the basic sphere. The reduction in oscillation amplitude came basically from a stabilization of the wake. Flow visualization in a water tunnel was made for the basic and vented spheres. The visualization of the vented sphere showed more stabilization of the wake than that of the basic sphere