Experimental study of a round jet impinging on a flat surface: Flow field and vortex characteristics in the wall jet

Impinging jets are widely used in cooling applications. Here, particle image velocimetry measurements were performed to study the flow field (focusing on the wall jet) and vortex characteristics of a round air jet, impinging on a flat surface at three Reynolds numbers, Re = 1,300, 6,260 and 12,354 (based on nozzle diameter, D, and jet exit velocity), and stand-off distance, 4.75D. In the wall jet, self-similarity (outer layer scaling) of the mean radial velocity, rms values of velocity fluctuations and Reynolds shear stress was obtained for Re = 12,354. At Re = 1,300, impinging primary vortices generated highly coherent primary-secondary vortex pairs that were convected along the wall. In contrast, at the two highest Re, primary vortices broke-up into small-scale structures prior to impingement and vortex pairs were only revealed after conditionally averaging the data. Their strengths, areas and numbers were analyzed using the instantaneous swirling strength and vorticity distributions. Primary vortex strength peaked at break-up or impingement (Re = 1,300) and reduced during interaction with the secondary vortex. Analyzing the different contributions to the averaged vorticity equation revealed that stretching and realignment due to the mean flow always strengthened the vortices while turbulent diffusion mainly weakened them.