Experimental study on oscillation mode of underexpanded impinging jet and behavior of plate shock waves

In order to investigate the oscillation mode of an underexpanded impinging jet on a flat plate and the behavior of the plate shock waves formed in the vicinity of the plate, the authors visualize the flow field and measure the pressure on the plate. Visualization is made using Mach-Zehnder interferometry and density contours are drawn by analyzing fringe shifts obtained from interferograms under the assumption of axisymmetric flow. At a nozzle pressure ratio of 3.0, the shape of the plate shock wave changes repeatedly from mormal to curved, from curved to normal etc., with nozzle-plate spacings. The shape of the plate shock depends on the location of the plate and strength of the shock wave. When the spacing exceeds a certain value, the mode of oscillation changes markedly. Pressure correlations show that the jet structure under oscillation has four different types of flow patterns: quasi-steady, axisymmetric, whirling and random.