Passive Control of the Wake of a Sphere by Ventilation

Abstract

Wind tunnel tests carried out on centrally vented smooth spheres show a 50% drag reduction due to venting when the Re is greater than the critical. Tests were conducted in the range of Re &#61 0.6 x 10 sup 5 to 7.5 x 10 sup 5. The vented area was only about 2% of the frontal area of the sphere. Ventilation stabilises the unsteady ans asymmetric wake and thereby reduces the unsteady aerodynamic forces associated with a smooth sphere at high Re. Beyond the critical Re, the vent flow breaks through the dead water region of the sphere and comes out from center of the wake, resultingin an enclosed vortex system which has an effect of aerodynamically streamlining the base. Oil-flow visualisations and laser light sheet visualisations suffest formation of a base vortex ring beyond and above the turbulent separation line. The vortex ring brings the separated external shear layer lowards the center and symmetrises the wake. The width of the wake is simultaneously reduced. Oil-flow visualisation indicates the occurrence of a laminar separation bubble in the range of Re &#61 2.8 x 10 sup 5 to 7.5 x 10 sup 5 with or without venting. Whereas the positions of laminar separation and turbulent reattachment are practically unchanged, a marginal upstream shift of the turbulent separation occurs for the vented sphere.