Turbulent structures and associated Reynolds shear stress in an impinging jet

Abstract

The Reynolds shear stress is closely related to the turbulent structures in shear flows. In this paper, the principal turbulent structures and the mechanisms of the associated Reynolds shear stress generation in an impinging jet were examined. A two-dimensional particle image velocimetry system was used to measure the flow field on the axisymmetric plane of the impinging jet. Three Reynolds numbers were considered: 4200, 8179, and 13,926, with the impingement distance fixed at 6 d (d represents the internal diameter of the jet nozzle). The results illustrate that although the inner edge of the free-jet shear layer is dominated by ejection events, the Reynolds shear stress produced by sweep events in this region is relatively large. In the outer edge of the free-jet shear layer, sweep events occur more frequently than ejection events, but the Reynolds shear stress produced by the latter is higher. For the wall-jet shear layer, high-frequency sweep events are observed near the half-width, whereas ejection events produce higher Reynolds shear stress. Conditional averaging of the instantaneous momentum flux and fluctuating velocities around the vortex cores suggests that the observed phenomena are attributed to differences in the intensity of ejection and sweep events induced by vortices in each region. Overall, this study further reveals the turbulent structures in the impinging jet shear layer and provides a deeper understanding of the connection between these turbulent structures and Reynolds shear stress. The results are critical for optimizing the performance of impinging jets in heat and mass transfer applications.