BLUFF-BODY DRAG REDUCTION BY PASSIVE VENTILATION

The drag of a sphere at high Re can be reduced to more than half its v alue by passive ventilation from the stagnation region to the base. Si multaneously, the flow field around the base is stabilized and made sy mmetric, leading to reduction of unsteady aerodynamic forces. At high Re, the vent flow breaks through the dead water region associated with the near wake and aerodynamically streamlines the base. The streamlin ing is done by virtue of a base-vortex-ring beyond the point of turbul ent boundary layer separation. A mean flow model for the flow around t he vented sphere is proposed. Smoke flow visualized on a laser light s creen placed at two diameters behind the base of the sphere shows the effectiveness of the method in suppressing the flow oscillations. The drag reduction achieved is very sensitive to the quality of the extern al surface and relatively insensitive to disturbances in the internal flow. Surface roughness or boundary layer tripping wire on the externa l flow can completely offset the benefit obtained.