Effect of nozzle cavity on resonance in large SRM:: Numerical Simulations

The nozzle design effect on sound production is investigated to improve the understanding of the aeroacoustic coupling that occurs in solid rocket motors with a submerged nozzle. Earlier analytical and experimental work demonstrated that flow-acoustic coupling is observed only for submerged nozzles for which the sound pressure level increases linearly with the nozzle cavity volume. Numerical simulations of the flow-acoustic coupling phenomena and in particular of the effect of the nozzle cavity volume on the pressure pulsations are performed using the code CPS. The numerical and experimental pressure spectra are compared. The frequencies are well simulated by the numerical code even if the pressure levels are overestimated. The nozzle design effect on sound production is also observed with a reduction of pressure level of 55% when the nozzle cavity is removed. Furthermore, the nozzle cavity modifies the flowfield around the nozzle head. With the cavity, the recirculation bubble is shorter, and the flow close to the nozzle head presents high amplitudes of radial mean velocity and fluctuation. That explains why the vortices break up when interacting with the nozzle head. With the cavity, the vortices shed by the inhibitor move along the border of the recirculation bubble and then pass in front of the cavity entrance, where they generate sound by interacting with the velocity fluctuations induced by the cavity volume. This results in a strong interaction between the vortices and the nozzle, which leads to large pressure oscillations.