Spectral Peaks due to ‘Trapped Waves’ in Jets from Plug Nozzles

An experimental study is underway 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. Experiments are planned to further examine the relationship between the exit gap width and the TW frequencies which will be presented in the final paper.