Effect of impinging jet pulsation on primary and secondary vortex characteristics

In this study, the effect of impinging air jet pulsation on the generation of toroidal ("primary") vortices near to the nozzle exit, their subsequent downstream evolution, the creation of "secondary" vortices and their interaction along the radial wall jet was investigated. Experiments were performed using particle image velocimetry and data analysis was based on the determined instantaneous vorticity and swirling strength fields. Reynolds numbers based on the average jet exit velocity were Re = 4606, 8024 and 13,513, whereas jet pulsation Strouhal numbers ranged from 2.0 x 10(-3) to 15.6 x 10(-3). The present results showed that jet pulsation significantly affected both the primary and the secondary vortex numbers, areas, and strengths. However, the actual change in vortex strength depended on the phase and the jet Reynolds number. Secondary vortex strengths increased up to twice the value obtained for the steady jet case. However, no enhancement was measured when the Strouhal number exceeded 8.5x10(-3). Conditionally averaged data indicated that secondary vortex generation in the pulsating jet is similar as for the baseline steady jet case. The different contributions to the averaged vorticity equation showed that the mean vortex stretching term strengthened primary and secondary vortices. Further away from the stagnation point, wall-normal diffusion of fluctuating, azimuthal vorticity weakened the vortices. (C) 2020 Elsevier Ltd. All rights reserved.