A turbomachinery duct using turbine exhaust gas as a working fluid has been applied as a space launch vehicle component. Excessive duct vibrations were observed when the operation exceeded a certain power level. Investigations were conducted to identify the root cause of the observed vibrations. Using computational fluid dynamics (CFD), it is found that the annular exhaust gas exit condition combined with the duct bend in close proximity creates a complex vortex system that convects downstream in the duct. Similar to the von Karman vortex, the oscillating asymmetric vortices are generated by a non-swirling parallel upstream flow on a smooth surface. The vortex generation and the oscillatory dynamics are self sustained. Leaving the supersonic nozzle of the duct, the vortices induce unsteady fluctuating thrust forces that could generate structural vibrations. The main purpose of this paper is to report the investigations on the formation of this self sustained vortex system. Comparisons of the CFD results with those of ground tests and flight data of pressures, thrusts, and power spectral densities are also presented.
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