Impingement tones and associated shock instabilities in supersonic plug nozzle flows

An investigation into a supersonic impinging jet emitted from a truncated plug nozzle is presented for a nozzle pressure ratio of 3.4. High-resolution schlieren and acoustic measurements verify that the jet behavior close to the nozzle is significantly different from round jets. Impingement tones are not observed for plate distances less than 3.2 nozzle diameters from the throat. Sufficiently far downstream from the nozzle, the jet shock structure is similar to that of a round jet. Ultrahigh-speed schlieren is used to track the motion of the standoff shock located above the impingement plate. The shock-oscillation frequency matches the impingement-tone frequency within statistical uncertainty. The average shock standoff distance and oscillation amplitude are related to the relative position of the impingement plate with respect to the shock structure in the equivalent freejet. Shock splitting is directly observed for certain plate distances.