Numerical simulation of pressure oscillations and instabilities in a solid propellant subscale motor

It has been broadly reported in the literature that the large segmented solid rocket motors (SRMs), such as Ariane 5 solid booster (P230) and Space Shuttle RSRM (Re-designed SRM), experience undesirable pressure oscillations that lead to thrust oscillations during combustion due to a complex coupling between the combustion and the internal aerodynamics of the combustion chamber. These fluctuations adversely affect the rocket motor performance, could damage the payload and eventually cause mission failure. To investigate these instabilities, experiments on full-size motors are very expensive, complicated and time consuming, and are therefore not really suited for research. In this paper, LP6 motor, the 1/15- scale axisymmetric model of the Ariane 5 booster, is used to study the pressure oscillations and the vortex-shedding induced pressure instabilities. Computational Fluid Dynamics (CFD) simulations were carried out on a quadrilateral mesh of the LP6 geometry at the time of 6.4 s after ignition when the pressure oscillations were found to be almost peaked. Pressure oscillation frequencies and amplitudes obtained from CFD simulations are found to be in good agreement with experimental results obtained at ONERA (French National Aerospace Research Centre). From CFD simulations, it was also found that the potential source of instabilities and pressure oscillations in LP6 motor having no inhibitor rings or restrictors is the surface vortex shedding mechanism which is a phenomenon that has also been observed in the experiments. On the basis of the present study and results, CFD simulations can also be carried out for different geometric configurations and flow conditions in other SRMs to investigate the instabilities and pressure oscillations occurring in their combustion chambers.