Impingement tones of large aspect ratio supersonic rectangular jets

The frequency structure of large aspect ratio jet impingement tones is analyzed. Feedback loops consisting of downstream propagating instability waves of the jet flow and upstream propagating acoustic waves from the wall to the nozzle lip produce as many as 20 or more tones when a supersonic rectangular jet of large aspect ratio is directed normal on a wall. It is seen that there are only two basic tones. The basic tone of the lower frequency is associated with a symmetric instability feedback mode of the jet, while that of the higher frequency is associated with an antisymmetric instability feedback mode. The remaining tones are combination tones of the two basic frequencies and their harmonics. It is determined that the measured basic impingement tone frequencies fall in the theoretical permissible frequency bands over a wide range of jet Mach number.