Abstract
The present study aims at understanding the effect of external pulsations on the flow characteristics in the downhill region of an oblique jet impingement with the help of particle image velocimetry. The distance between nozzle exit and the wall is kept constant at L = 4D (D is the diameter of the nozzle) and Re = 1900 (based on nozzle diameter and the average jet exit velocity). The impingement angle is varied from & theta; = 90 degrees (normal impingement) to 30 degrees and the Strouhal number (St = fD/U, where f is the external frequency and U is the average velocity) from 0 to 0.9. The wall jet corresponding to St = 0.44 shows highest jet growth as compared to other jet pulsations. The wall jet at St = 0.9 is observed to show similar behavior to that of the steady jet (St = 0). A quadrant analysis shows that instability causes ejection motion to occur nearest to the impingement region when St = 0.44. The ejection motion leads to an expansion of the wall jet region in the wall-normal direction which is also observed in the increase in jet half-width. Further, it is observed that the maximum wall jet velocity increases with decreasing & theta;. The location of the occurrence of the ejection motion depends strongly on & theta;, shifting downstream of the wall as & theta; decreases. A proper orthogonal decomposition analysis reveals that the dominant structures of the wall jet flow occur at the location where the ejection motion is initiated. A higher jet momentum is responsible for delay in the ejection motion and development of coherent structures in the case of a wall jet with lower impingement angles. These results provide some relatively important insights on the behavior of the wall jet under different pulsations in the case of oblique jet impingements which can be of practical interest for heat transfer studies as well as heat transfer applications involving jet impingement cooling and mixing.