Flow characterization in the uphill region of pulsed oblique round jet

Abstract

An experimental study to investigate the effect of jet pulsations on the wall jet development in the uphill region of an obliquely inclined round water jet has been performed using particle image velocimetry technique. The study has been performed at a constant nozzle to target wall distance, L/D = 4 (D is the diameter of the nozzle) by varying the jet impingement angle θ (=60o, 45o, and 30o), Reynolds numbers (ReD = 1900 and 3280; based on nozzle diameter and average nozzle exit velocity Uavg), and Strouhal number (0 ≤ St ≤ 0.9; St=fDUavg, where f is the frequency of external pulsation). It is observed that the pulsations have no significant effect on the jet in the free jet region when the target plate is kept at a distance less than the potential core length (the potential core extends up to 4D–6D from the nozzle exit toward the impinging plate), and the jet impingement region extends up to 1D from the plate. The location of the stagnation point is observed to depend on all three parameters: the jet pulsation, the Reynolds number, and the jet impingement angle. An increase in Reynolds number creates an adverse pressure gradient toward the downstream direction in the uphill region, resulting in an intrusion of ambient fluid toward the wall jet. The distance between the geometric center and the stagnation point is observed to be minimum for St = 0.44 at both the Reynolds numbers. The wall jet that develops in the uphill region exhibits a maximum velocity decay rate and a jet half width growth rate corresponding to St = 0.44. These parameters are also observed to increase with the increase in the Reynolds number and decrease in the jet impingement angle. The velocity fields reconstructed using proper orthogonal decomposition reveal the dominant modes in the upstream location for St = 0.44 than the other pulsations. Furthermore, we observed that the jet after impingement deviates entirely in the downhill region for θ=30o irrespective of the jet pulsation, suggesting a non-dependence of the critical angle of inclination on jet pulsations for L/D = 4.