Numerical simulations of unsteady flows in solid rocket motors

Solid rockets are known to be prone to fluid dynamic and combustion instabilities. The coupling of flow instabilities with acoustic resonance in internal flows can result in self-sustained oscillations. The shear layer generated in the corner of the rearward-facing step is unstable and leads to periodic vortex shedding which is the primary cause of aero acoustic oscillations in the rocket motor. If frequency of this shedding coincides with the natural acoustic modes of the geometry, then the oscillations would be self-sustaining and the amplitudes of these oscillations would grow. In the present work, numerical calculations of the unsteady flow in a scale model rocket motor chamber experimentally studied by Shanboughe et al. (AIAA Paper 2003-4632, 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 2003) have been carried out using the commercial software FLUENT. The main objective of this work is to find out how well (if at all) the frequencies of the oscillations can be predicted by numerical simulations. The flow is assumed to be incompressible, and axi-symmetric. All the results reported here are second order accurate in space and time. FFT of the pressure-time data has been performed to extract the dominant frequencies of the oscillations. Values for the dominant frequencies (as well as the second dominant one, in some cases) obtained from the numerical calculations are compared with the experimental values. The agreement is found to be good. Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.