Experimental investigation of turbulence suppression by the imposition of a large-scale vortical control flow

Abstract

Recent numerical work has indicated that largescale streamwise vortices embedded in the turbulent boundary layer could realise substantial skin friction drag reductions. In order to give such a control experimentally micro vortex generators have been immersed in a turbulent boundary layer. Downstream of the vortex generators there is a developing threedimensional regime, with a spanwise variation of skin friction caused by the common flow up and commonflow down associated with the counter-rotating vortex pairs. Overall skin friction reduction is defined by the balance between the increases in local cjcaused by the common flow-up regions and the reductions caused by common flow down across the blade wavelength. Thus far, the vortex generators tested have produced widely varying results, ranging from large skin friction increases to small overall reductions. Two-component velocity data characterising the vortices produced by the arrays seem to indicate that vortex strength (especially the spanwise component w) and downstream development (vorticity spreading and decay) are strongly related to the extent of skin friction reductions. These results tentatively concur with the proposals of Schoppa and Hussain for the existence of an optimum control amplitude (w is 6% of [/«,) although at present our results cannot confirm this exact figure.