We present a computational study of nonlinear combustion instability of a mult i-inject or rocket engine. The study is conducted for a choked nozzle and a combustion chamber coupling with 10- and 19- co-axial injector ports. Computations using three-dimensional unsteady k-ω-SST-DDES provide detailed time-resolved information about the combustion instability. The spontaneous longitudinal mode instability is observed for the 10- and 19-injector geometries with a combustion chamber diameter of 28cm. The triggered tangential and longitudinal instability modes are obtained for the 19-injector geometry with combustion chamber diameter 43cm by pulsing the injector mass flux. It is shown that an oscillating combustion chamber flow can be triggered to a new mode with a larger amplitude. Streamwise vorticity created by the instability shows indication of affecting the heat release in a manner that supports the instability.
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