Immersed Boundary Simulations of Aeroacoustics Phenomena in Solid Rocket Motors
Creators: name:  M. CIMINI 
name:   F. STELLA 
name:   E. CAVALLINI 
name:   A. DI MASCIO 
name:   A. NERI 
name:   E. MARTELLI 
name:   M. BERNARDINI 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
SOLID ROCKET MOTOR

description: This work aims at investigating the flow dynamics in solid rocket motors (SRMs), which are still of a great interest for space launchers. The internal ballistics of SRMs is generally characterized by the interaction of the hydrodynamic instabilities that develop from shear layers in different parts of the motor and the acoustic resonant modes of the combustion chamber. This possible coupling can trigger pressure oscillations, which are able to even jeopardize the motor structures, the thrust vector control system and the payload comfort. The problem of pressure oscillations in SRMs has received considerable attention in the last decades. Given the intrinsic limitation of simplified methods to predict instabilities in SRMs [1], several studies have been carried out to characterize the flow behavior in the chamber [2-3] and assess the capability of full numerical simulations to predict vortex shedding and pressure oscillations. Most of the numerical works available in literature are based on quasi-1D models or 2D simulations, whereas only recently 3D high-fidelity numerical simulations have appeared [2]. The latter study, clearly highlighted the importance of accurately simulating the turbulent flow field in the combustion chamber in order to correctly predict the level of pressure oscillations. In this work, we propose to adopt the immersed boundary method to perform high-fidelity 3D simulations of SRMs. This technique offers several advantages with respect to standard method based on standard body-fitted grids and allows to easy handle complex geometries. In this work, unsteady 3D immersed boundary simulations of SRMs flow will be carried out. The first geometry that will be analyzed is the ONERA C1xb (see Fig. 1), which is a simple laboratory-scale solid rocket motor model carefully designed to investigate the aeroacoustic coupling. The study will reproduce the single-phase motor at a firing time corresponding to a 13.5 mm grain burned layer and will be used to assess the capability of this method to predict vortex shedding and pressure oscillations in the SRM. The second geometry under investigation is inspired to a typical large-scale aft-finocyl SRM used in space launchers. The performed simulations will be used to investigate those parts of the flow not accessible in experiments and to assess the effects of turbulence on the development of vortical hydrodynamic instabilities inside the chamber, in order to characterize the unsteadiness of the resulting aeroacoustics loads of SRMs. Furthermore, the unsteady pressure signature on the grain wall, will be analyzed through: i) the Fourier-based spectral analysis performed in time and in the azimuthal wavenumber space, which is used to determine the dominant frequency modes, ii) the inspection of the space-time correlation coefficient to better understand the propagation of pressure disturbances in the SRM chamber and iii) the wavelet transform analysis by decomposing the time series of data into time-frequency space, which is used to determine the time evolution of the energy associated to each mode. References [1] Flandro, G., and Majdalani, J., Aeroacoustic Instability in Rockets, AIAA J., Vol. 41, No. 3, 2003, pp. 485497. [2] M. Bernardini, M. Cimini, F. Stella, E. Cavallini, A. Di Mascio, A. Neri, and E. Martelli, Large-Eddy Simulation of Vortex Shedding and Pressure Oscillations in Solid Rocket Motors, AIAA J., Vol. 58, No. 12, 2020, pp. 5191-5201. [3] Apte, S., and Yang, V., A Large-Eddy Simulation Study of Transition and Flow Instability in a Porous-Walled Chamber with Mass Injection, J. Fluid Mech., Vol. 477, 2003, pp. 215225.