‘Trapped Wave’ Resonances in Jets From Plug Nozzles

An experimental study is conducted investigating the characteristics of ‘trapped wave’ (TW) resonance tones occurring with nozzles having a center plug. The pressure fluctuation spectral peaks due to these ‘guided’ or ‘trapped’ waves when the nozzle contains a center plug occur at nondimensional frequencies that are an order of magnitude larger than those noted with simple round nozzles. It appears that the exit gap width of the plug nozzle is the characteristic length-scale that dictates the corresponding TW frequencies. The results of this study further our understanding of this phenomenon as it relates to other complex nozzle geometries.