Numerical simulation of 3D unsteady flowfield in aft-finocyl solid rocket motor

The flowfield inside the second solid stage of the European launch vehicle VEGA is simulated with a full 3D unsteady flow solver in order to characterize the unsteadinesses, aero-acoustics and dynamics loads resulting from the growth of complex vorticity patterns in the internal flow of the aft-finocyl solid rocket motor. The analysis considers different configurations during the firing of the solid stage: the first one corresponds to an advanced stage of combustion, when the aft-part of the propellant grain is almost completely burnt out and the aft-end cavity is almost empty of propellant; the second one corresponds to an early stage of combustion, with a large burning surface and a small internal volume available for the flow. The first configuration is computed for different motor geometrical conditions: a symmetric case, a case with an imposed small angle of nozzle gimbaling and a case with an imposed propellant grain offset with respect to motor assembly. The second configuration is analyzed considering a small angle of nozzle gimbaling. The rotation of the gimbaled nozzle, as well as the propellant grain offset are imposed in such a way to remove any symmetry in the motor geometry.The motor configuration advanced into the firing shows the presence of low-level limit cycle oscillations of the flowfield at high frequency, resulting into the generation of lateral force oscillations of the order of one percentage of the generated thrust. This peculiar behavior is not altered by the perturbation of the motor geometry due to the imposed nozzle gimbaling; whereas it is slightly modified in terms of lower characteristic frequencies and lower amplitude of the oscillations, for the case with the imposed propellant grain offset. The presence of such flowfield fluctuations is caused by the onset of unstable sound generating vortical structures along the motor axis, that produce pressure fluctuations and that are coupled with acoustic waves. The motor configuration early into the firing, instead, does not show any significant of flow instability.