A new look at the screech tone mechanism of underexpanded jets

An experimental investigation of the screech tone mechanism has been undertaken using the shallow water analogy, The analogy offers the possibility of making detailed measurements of the waterheight distribution within the jet with non-intrusive techniques. The mean value and the fluctuations of the static pressure in the corresponding compressible gaseous flow can be determined. It is found that the intensity of the screech tone is strongly linked to a local static pressure gradient at the end of the second shock-cell, This gradient is a function of the pressure ratio of the jet. The screech frequency is found to be determined by pressure waves. The screech loop consists of a wave generated at the end of the second cell moving in the upstream direction at the ambient speed of sound and a wave moving in the dowstream direction at a speed equal to the addition of the local values of the speed of sound and the flow velocity. The observed modulation of the screech tone intensity is used to determine the velocity of the downstream moving wave. The screech frequency calculated from this model is in good agreement with the measured value.