Oscillatory modes of interaction of supersonic overexpanded jets with barriers

The self-oscillatory interaction of supersonic jets with barriers has mainly been studied for under-expanded jets. There are only a few experimental studies examining the case of overexpanded jets, with little computational work done in this direction. To fill this gap, we performed numerical simulations of overexpanded supersonic jets with barriers. The calculations were performed by the Godunov method on fine grids using parallel programming techniques. In the course of numerical simulations, the gasdynamic parameters of the jet and the geometric parameter of the barrier were varied. The barrier had the shape of a cylindrical cavity of depth l = (0 − 18)r _a , where r _a is the nozzle exit radius (the case l = 0 corresponds to a flat-end barrier). Based on the results of the numerical simulations, the conclusion on whether the self-oscillation process occurs was drawn and the dependence its characteristics (frequency and amplitude) on the governing gasdynamic and geometric parameters were obtained. Good agreement with experimental data on the fundamental tone frequency was demonstrated. A low-frequency oscillation mode was mostly realized. In this case, the jet experienced periodic suctions into and ejections from the cavity, counter the oncoming jet flow, with the formation of a structure consisting of three discontinuity surfaces (two shock waves and a separating surface contact).