Vortex-Sound Generation and Thrust Unsteadiness in Aft-Finocyl Solid Rocket Motor

The generation of complex vorticity pattern in aft-finocyl solid rocket motors is inves- tigated in this paper by means of full-3D ILES CFD simulations with a high-order/low- dissipation class of centered numerical schemes with oscillation control and an immersed boundary treatment of the propellant grain surface, treated with a level-set approach. The development of vortical/shear structures is observed both at the motor axis, immediately downstream the igniter and across the finocyl region and in the submergence region. The first ones are rather a relevant finding which characterizes the flowfield structures that develops inside the combustion chamber of an aft-finocyl geometry, that find confirmation in both small scale cold-flow tests and theoretical justification in fundamental works. The second one are instead more classical vortical structures that belong to the class of angle shear-layers, due to the turning flow characteristic of solid rocket motors. These vortical structures are found to induce very low-level, but present, pressure oscillations as due to the coupling of the vorticity pattern and pressure waves. These pressure oscillations result into oscillations of the thrust delivered by the SRM, involving both a longitudinal cham- ber mode excitation (corresponding to the first chamber longitudinal mode) and a lateral chamber mode excitation. The level of such thrust component oscillations is of the order of one percent of the delivered motor thrust, with the uncertainty assessed by both grid convergence analyses and sub-grid model of the ILES approach. These flowfield character- istics are a little dependent upon the motor configuration, and in particular, the angle of gimbaling imposed to the nozzle and a bias-offset of the propellant grain with respect to the motor assembly.