Effect of ventilation on the flowfield around a sphere

The flowfield around a sphere with and without ventilation was investigated in a wind tunnel over a range of Reynolds numbers in an incompressible flow. At supercritical Re, the pressure drag of a sphere can be nearly nullified by venting only 2% of the frontal area of the sphere to the base through a smooth internal duct. The drag reduction is achieved by increased pressures in the separated flow region close to the base. At high Re, the vent flow breaks through the near wake and brings about symmetry in the global flowfield. When the internal shear is increased by using a rough internal duct, the base pressure is unchanged, but the external flow is accelerated to velocities beyond that achieved by the potential flow around the basic sphere. The findings can be explained by a flow model in which the near wake is aerodynamically streamlined by a pair of counterrotating vortex rings at the base. A roughness element can be made to partially destroy the vortex system at the base and result in a steady asymmetric wake. A 1.2 mm diameter wire placed at 70° was found to overtrip the boundary layer and completely destroy the vortex system. Simultaneously, the turbulent separation is advanced and the drag increased.At subcritical Re, ventilation marginally increases static pressures all over the surface. Since the large pressure differential between the windward and leeward sides is not reduced, the internal flow has a rapid acceleration to a velocity close to that of the free stream. The reverse flow associated with the near wake forces the vent flow to rest within itself and the wake profile is unchanged. The main features of subcritical flow around the basic sphere are retained in spite of ventilation. The upstream effects of ventilation are greater for subcritical flow than for supercritical flow.The work reported was carried out under a study grant from the German Academic Exchange Service (DAAD) in Bonn. The authors wish to thank the Director of DAAD in Bonn for the same. Thanks are due to Dr. F. R. Grosche and colleagues at DLR in Göttingen who assisted in the design, fabrication and wind tunnel testing of the sphere model. Thanks are also due to Prof. D. G. Mabey, visiting Professor, Imperial College, London for useful discussions. The many useful discussions with the research advisors of the first author viz., Dr. P. R. Viswanath of National Aerospace Laboratories and Prof. A. Prabhu of Indian Institute of Science, Bangalore are acknowledged with thanks. The support given by the Head, Experimental Aerodynamics Division, National Aerospace Laboratories is thankfully acknowledged.