Staging Behavior and Large Coherent Structure of Oscillatory Supersonic Impinging Jets

Abstract

Present study simulates oscillatory supersonic impinging jet flows using an axisymmetric Navier -Stokes code. We focus on the staging behavior of the oscillation frequency of the impinging surface pressure associated with the variation of the nozzle -to -plate distance and nozzle pressure ratio. These two distinct staging behaviors of the oscillat ion frequency are found to be well correlated when the frequency and the distance are normalized by the length of the first shock cell. We show that the staging behavior is in line with the changing pattern of large coherent structures of the jet shear lay er . have clearly established that the self -sustained, highly unsteady behavior o f the jet and the resulting impinging tones is governed by a feedback mechanism. The energy of the feedback loop is provided by the instability waves in the shear layer of the jet. This instability wave usually generates large -scale vortical structures in the shear layer. The feedback loop is the coupling between the downstream -traveling instability waves (coherent structures) and the upstream -propagating acoustic waves. It has been reported that the frequencies of the dominant peak in both of sound and pre ssure signals of a supersonic impinging jet coincide. It is also well known that the frequencies of the impinging tone and the surface pressure oscillation exhibit staging behavior as the distance from the nozzle exit to the plate gradually increases. 1,4 This staging behavior for moderately underexpanded jets is typical of the class where disturbances from the jet interaction feed back to the nozzle, and so cause periodic self -induced oscillations. To predict the frequency and amplitude of the tones, many t heoretical modeling by using linear stability analysis have been attempted. However, the amplitude prediction and staging feature of the frequencies still remains elusive. 1-4