Manipulation of the reverse-flow region downstream of a fence by spanwise vortices

Abstract

The experimental investigation of a turbulent separated flow over a fence is presented. By introducing a periodic disturbance upstream of the separation region in front of the fence, the time averaged length of the separation region downstream of the fence was reduced by as much as 40%. Two types of flow manipulation were applied: an oscillating cross-flow with zero net mass-flux through a spanwise slot in the floor of the test section and a spanwise oriented, oscillating spoiler. The cross-flow was generated by a loudspeaker system connected to a chamber underneath the spanwise slot. Both types of flow manipulation generate spanwise vortices at the fence that convect into the region downstream of the fence where they enhance the mixing in the shear layer and reduce the time mean length of the reverse-flow region downstream of the fence. Velocity profiles phase averaged with respect to the forcing frequency and phase triggered flow visualisations show that the spanwise vortices cause the long reverse-flow region of the unmanipulated flow to break up into separate smaller regions. While the time mean length of the reverse-flow region is reduced in the manipulated case, the length of the region where instantaneous reverse-flow occurs is not changed. The data presented include wall pulsed-wire measurements of the wall shear-stress and its turbulent fluctuations, and LDA measurements of the streamwise and the wall-normal velocity components and turbulent stresses.