On oscillating shock layers of supersonic impinging jets

Shock layers of supersonic jets impinging on a plate are studied by means of multi-exposure photography and synchronized pressure measurements performed at the stagnation point on the plate. At certain nozzle-to-plate distances the bow shock formed ahead of the plate oscillates strongly. The pressure variations are strongly correlated with the oscillating shock. Peak frequencies are determined by a spectral analysis of the pressure signal. The spectra show a characteristic dependence of the peak frequencies on the nozzle-to-plate distance. The behavior of the primary peaks depends strongly on the characteristics of the supersonic jet (over/underexpansion). Frequencies of secondary peaks that decrease about inversely with the nozzle-to-plate distance, depend only little on the characteristics of the jet. In addition, low frequency bursts that occur for certain nozzle-to-plate distances are traced to the formation and destruction of a recirculation bubble.