Laser-Doppler measurements of impinging jet flows through a crossflow

Abstract

The flowfield resulting from the impingement of a single axisymmetric jet against a wall after penetrating a confined crossflow has been studied experimentally using laser-Doppler anemometry. For sufficiently high jet velocities and small distances between the jet exit and the ground plane to produce impingement, two regions of the flow are seen to be of particular interest: the impinging region and the ground vortex caused by the interaction between the upstream wall jet and the crossflow. The latter consists of a vortical structure that wraps around the impinging jet like a scarf and should develop further downstream through a pair of streamwise vortices. The present work reports a study aimed at characterising the influence of the jet-to-crossflow velocity ratio on the structure of the ground vortex. The experiments were performed for Reynolds numbers based on the jet exit conditions of between 60,000 and 105,000 corresponding to jet-to-crossflow velocity ratios from 30 to 73 and for an impinging height of five jet diameters. The shape, size and location of the ground vortex were found to be dependent on the velocity ratio, and two different regimes were identified. For the higher velocity ratio regime, the downstream wall jet it is not strongly affected by the ground vortex, and the velocity profiles become similar. In both regimes, the crossflow acceleration over the ground vortex was found to be directly connected with the jet exit velocity. This indicates that the influence of the upstream wall jet is not confined to the ground vortex but spreads until the upper wall by a mechanism that needs further investigation.