Influence of thermal inhibitor position and temperature on vortex-shedding-driven pressure oscillations

Vortex-acoustic coupling is one of the most important potential sources of combustion instability in solid rocket motors （SRMs）. Based on the Von Karman Institute for Fluid Dynamics （VKI） experimental motor, the influence of the thermal inhibitor position and temperature on vortex-shedding-driven pressure oscillations is numerically studied via the large eddy simulation （LES） method. The simulation results demonstrate that vortex shedding is a periodic process and its accurate frequency can be numerically obtained. Acoustic modes could be easily excited by vortex shedding. The vortex shedding frequency and second acoustic frequency dominate the pressure oscillation characteristics in the chamber. Thermal inhibitor position and gas temperature have little effect on vortex shedding frequency, but have great impact on pressure oscillation amplitude. Pressure amplitude is much higher when the thermal inhibitor locates at the acoustic velocity anti-nodes. The farther the thermal inhibitor is to the nozzle head, the more vortex energy would be dissipated by the turbulence. Therefore, the vortex shedding amplitude at the second acoustic velocity antinode near 3/4L （L is chamber length） is larger than those of others. Besides, the natural acoustic frequencies increase with the gas temperature. As the vortex shedding frequency departs from the natural acoustic frequency, the vortex-acoustic feedback loop is decoupled. Consequently, both the vortex shedding and acoustic amplitudes decrease rapidly.

Effect of the head cavity on pressure oscillation suppression characteristics in large solid rocket motors

In order to discover the effect of head cavity on resonance damping characteristics in solid rocket motors, large-eddy simulations with wall-adapting-local-eddy-viscosity subgrid scale turbulent model are implemented to study the oscillation flow field induced by vortex shedding based on the VKI （yon Karman Institute） experimental motor. Firstly, mesh sensitivity analysis and grid-independent analysis are carried out for the computer code validation. Then, the numerical method is further validated by comparing the calculated results and experimental data. Thirdly, the effects of head-end cavity on the pressure oscillation am-plitudes are studied in this paper. The results indicate that cavity volume, location and configuration have a cooperative ef- fect on the oscillation amplitude. It is proved that Rayleigh criterion can be used as a guiding principle for the design of reso- nance damping cavity. The change of the head-end cavity breaks the balance between the mass flux and acoustic energy. Therefore, the pressure oscillation characteristics change accordingly. It is concluded that a large mass flux added at the pres- sure antinode could attribute to significant amplitude. Meanwhile, the damping effect of the cavity is stronger when the dis- tance between cavity and pressure antinode becomes shorter. Finally, this method is applied to the modification of an engi- neering solid rocket motor. The static test of solid rocket motor reflects that the oscillations can be effectively suppressed by a head-end cavity.