Effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics of a sphere

Experiments were conducted in water and wind tunnels on spheres in the Reyn olds number range 6 x 10(3) to 6.5 x 10(5) to study the effect of natural v entilation on the boundary layer separation and near-wake Vortex shedding c haracteristics. In the subcritical range of Re (<2 x 10(5)), ventilation ca used a marginal downstream shift in the location of laminar boundary layer separation; there was only a small change in the vortex shedding frequency. In the supercritical range (Re > 4 x 10(5)), ventilation caused a downstre am shift in the mean locations of boundary layer separation and reattachmen t; these lines showed significant axisymmetry in the presence of venting. N o distinct vortex shedding frequency was found. Instead, a dramatic reducti on occurred in the wake unsteadiness at all frequencies. The reduction of w ake unsteadiness is consistent with the reduction in total drag already rep orted. Based on the present results and those reported earlier, the effects of natural ventilation on the flow past a sphere can be categorized in two broad regimes, viz., weak and strong interaction regimes. In the weak inte raction regime (subcritical Re), the broad features of the basic sphere are largely unaltered despite the large addition of mass in the near wake. Str ong interaction is promoted by the closer proximity of the inner and outer shear layers at supercritical Re. This results in a modified and steady nea r-wake flow, characterized by reduced unsteadiness and small drag.