Spectral analysis of unsteady interactions between a supersonic jet and a plate

The flow field in the stagnation point region of a supersonic jet that impinges on a plate can be highly unsteady and impingement tones are formed. A parameter governing the unsteady behavior is the distance between the exit of the nozzle from which the jet issues and the plate. At certain nozzle-to-plate distances the bow shock formed ahead of the plate oscillates strongly. The motion of the bow shock has been studied using a multi-exposure visualization technique. Synchronized pressure measurements performed at the stagnation point on the plate have shown that the pressure variations are strongly correlated with the oscillating shock. These features are connected with the formation and destruction of a recirculation bubble in the impingement region of the supersonic jet. Main frequencies of the unsteady flow field are determined by a spectral analysis of the pressure signal obtained at the stagnation point on the plate. Spectra obtained for different nozzle-to-plate distances are plotted as multi-configuration spectra using the nozzle-to-plate distance as a parameter. This is repeated for different pressure ratios of the nozzle, i.e., for under-, ideally, and overexpanded jets