Q1D Modelling of Vortex-Driven Pressure Oscillations in Aft-Finocyl SRMs with Submerged Nozzle Cavity

Abstract

This paper has the aim to investigate the aero-acoustic coupling that occurs typically in large aft-finocyl SRMs with submerged nozzle configurations, leading the onset of sustained pressure and thrust oscillations during the motor firing, by means of a Q1D model of the SRM aero-acoustic and vortex sound generation. The model has been applied successfully for the characterization of the pressure oscillations due to angle vortex shedding and the aero-acoustic coupling of P80 SRM, first stage of the VEGA launcher. The P80 pressure oscillations phases are characterized with a good correlation with the experimental data, identifying the different characteristics of the flowfield conditions for the first two blows (that occur when the aft-region of the SRM burns), with respect to the other two blows (which occur when the aft-region of the SRM is completely burnt out). In particular, the effect of the submergence cavity within the Q1D vortex sound generation model is investigated by means of proposed closures for the characterization of the cavity behaviour and compared by means of a parametric analysis with a benchmark numerical solution and the experimental data of the firings.Results show that for the first two blows, occurring when the submergence cavity burns and is active in terms of mass addition, the role of the cavity model is almost negligible, or very small, and well correlated with the dispersion of the experimental data of the P80 pressure oscillations. Instead, for the third and fourth blows, that occur when the cavity is empty of propellant grain, the cavity model plays a role on the aeroacoustic coupling, showing that the the feedback loop between the angle vortex shedding and the chamber acoustics is influenced by the cavity response.