Numerical study on unsteady backward-facing step flow and pressure oscillations

There was an axisymmetric backward-facing step structure in the burned out tactical solid rocket motor with finocyl grain. The vortex shedding from the step could cause hydrodynamic instability to drive acoustical oscillations. The hydrodynamic stability analysis was illustrated on a simplified motor geometry to predict longitudinal acoustic mode driven by vertex. LES turbulence model was also used to simulate the vortex generation process and acquire pressure data of the key monitoring point in the flow field. The hydrodynamic stability analysis results were consistent with the LES. The maximal pressure amplitude was in the head-end position with the first few frequencies of the chamber longitudinal acoustical mode. In the reattachment region, a new broad pressure peak below fundamental frequency which was influenced by the motion of the large-scale vertex was produced. There was a multi-peak phenomenon of the fundamental frequency if it was close to the broad peak. The pressure oscillations would be excited by the vortex shedding phenomenon when the formation frequency of the small vertex near the step is close to a nature frequency of the sound field.