Vortex shedding in segmented solid rocket motors

Abstract

This article is devoted to the numerical simulation of vortex shedding in the field of solid-propellant rocket motors resulting from the strong coupling between the shear-layer instability and acoustic waves in the chamber. Segmented solid rocket motors tend to develop thrust and pressure oscillations, linked to a periodic vortex shedding. An axisymmetric geometry close to the realistic configuration is computed. The computer code solves the unsteady Navier-Stokes equations by means of an explicit predictor/ corrector MacCormack scheme. Mesh dependence is studied by using three grid levels. In all cases, the vortex shedding process is observed. This mechanism is complex and vortex pairing occurs. The viscosity effect is analyzed by using three different viscosity values. The effect of the shape of diaphragms oh vortex shedding is also studied by comparing sharp and smooth diaphragms. HIS work is part of the overall combustion stability assessment of the Ariane 5 P230 MPS solid motor and has been supported by Centre National d'Etudes Spatiales (CNES) within a research program managed by ONERA. For this motor it is believed that there exists a severe risk for instability. Occurrence of low-amplitude , sustained oscillations pulsating at a frequency associated with one or more acoustic modes of the combustion cavity affects motor performances.13 One mechanism that drives pressure oscillations in rocket motors is the vortex shedding,1'4 which can interact with the chamber acoustics to generate pressure oscillations.5 Because of the segmented design of solid-propell ant rocket motors, shear layers induced by surface discontinuities appear and produce this vortex shedding. The dipole mechanism involving the interaction of the vortices with an impingement surface can be invoked as a typical source of energy transfer from the vortex fluctuations to the acoustic field. The observed periodic vortex shedding in rocket motors2'4 is the result of a strong coupling between the instability of mean shear flow and organ-pipe acoustic modes in the chamber. The feedback from the acoustic waves provides the control signal for the aerodynamic instability. Because of the complexity of the problem an analytical solution to the governing equations with complex boundary conditions does not exist. Numerical simulations may provide important help for understanding the complex physics. Such simulations would naturally couple mean-flow shear layer and acoustic waves. Numerical methods have been performed to isolate and study the interaction between acoustic waves and vortex structures.610 The present work is also concerned by a numerical simulation. The aim of this article is to present the ability of numerical codes to predict the unsteady behavior inside the combustion chambers of solid-propellant rocket motors and to analyze the acoustic and aerodynamic instability interaction. The computational domain corresponds to a 1/15 axisymmetric subscale motor representative of the Ariane 5 P230 solid rocket booster for which experimental results exist.1113 For this subscale motor, the propellant is an unmetallized analog of Ariane 5 propellant and the inhibitors are