Wake Vortices in Jets in Cross-Flow

Abstract

The objective of the current experimental study is to investigate unsteady wake vortices of jets in cross-flow in order to (1) explore the effect of various excitation techniques, their parametric dependence, and impact on the flow field, and (2) provide detailed flow visualizations for a range of velocity ratios. The jet passed through a nozzle and entered the cross-flow with a turbulent boundary layer. While mechanical perturbation did not result in any significant periodic organization of the wake vortices, the database obtained for the unperturbed flow provided an insight into their possible origin and behavior. The key finding was the shedding of wake vortices from the ‘swell’ at the lee side of the jet. The ‘swell’ delineated a region of the flow at the intersection of the jet efflux boundary layer and the cross-flow boundary layer. Separation events released upright vortices and these peeled strands of vorticity convected, twisted and stretched for a few diameters along the wall while still being attached to the bottom of the ‘swell.’ They extended from the tunnel wall and penetrated the jet core where they appeared to burst. In no case were the wake vortices seen to originate either from the wall boundary layer or the jet shear layer at downstream locations. Increasing the jet-to-cross-flow velocity ratio influenced the slope of the wake vortices relative to the jet. The upper part of the vortical strand for VR < 4 was almost perpendicular to the cross-flow. Higher velocity ratios featured a tilt of the vortex structure toward the jet. This angle was particularly pronounced for VR = 8 and 10, and could be observed at all downstream distances. The average convection velocity of the wake vortices for the given cross-flow and all velocity ratios was estimated to be about 0.8 Ucf.