Manipulating the flow over spherical protuberances in a turbulent boundary layer

Means of controlling the flow over a large spherical protuberance were examined. The role of suction around the base of the protuberance in reducing or even eliminating the necklace vortex created by the protuberance was considered. In the absence of suction, this vortex was lifted by the low base pressure existing behind the protuberance into the wake, thus affecting the turbulence level along its path. Large vortex generators placed upstream of the protuberance were able to delay local separation of the flow over the protuberance, thus affecting the symmetry of the wake and the level of turbulence on one side or the other. Observations made using flow visualization were supplemented by hot-wire measurements. The experiments were carried out at low speed at Reynolds numbers that did not exceed 3 × 10 ⁵.