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VKI PHDT 2000-03, Experimental and Numerical Study of Aeroacoustic Phenomena in Large Solid Propellant Boosters, Online thesis of Jérôme Anthoine (free download)

Experimental and Numerical Study of Aeroacoustic Phenomena in La

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Experimental and Numerical Study of Aeroacoustic Phenomena in Large Solid Propellant Boosters
with Application to the Ariane 5 Solid Rocket Motor

By Jérôme Anthoine

PhD Thesis from the von Karman Institute/Université Libre de Bruxelles, October 2000, ISBN 978-2-930389-43-5, 237 pgs

Icone PDFTo Download the thesis for free (printing not allowed): Please add the online thesis to your cart and register yourself. You will receive an email with a link to download the thesis.

The present research is an experimental and numerical study of aeroacoustic phenomena occurring in large solid rocket motors (SRM) as the Ariane 5b oosters. The emphasis is given to aeroacoustic instabilities that may lead to pressure and thrust oscillations which reduce the rocket motor performance and could damage the payload. The study is carried out within the framework of a CNES (Centre National d’Etudes Spatiales) research program. Large SRM are composed of a submerged nozzle and segmented propellant grains separated by inhibitors. During propellant combustion, a cavity appears around the nozzle. Vortical flow structures may be formed from the inhibitor (Obstacle Vortex Shedding – OVS) or from natural instability of the radial flow resulting from the propellant combustion (Surface Vortex Shedding – SVS). Such hydrodynamic manifestations drive pressure oscillations in the confined flow established in the motor. When the vortex shedding frequency synchronizes acoustic modes of the motor chamber, resonance may occur and sound pressure can be amplified by vortex-nozzle interaction.

Original analytical models, in particular based on vortex–sound theory, point out the parameters controlling the flow–acoustic coupling and the effect of the nozzle design on sound production. They allow the appropriate definition of experimental tests.

The experiments are conducted on axisymmetric cold flow models respecting the Mach number similarity with the Ariane 5SRM. The test section includes only one inhibitor and a submerged nozzle. The flow is either created by an axial air injection at the forward end or by a radial injection uniformly distributed along chamber porous walls. The internal Mach number can be varied continuously by means of a movable
needle placed in the nozzle throat. Acoustic pressure measurements are taken by means of PCB piezoelectric transducers. A particle image velocimetry technique (PIV) is used to analyze the effect of the acoustic resonance on the mean flow field and vortex properties. An active control loop is exploited to obtain resonant and non-resonant conditions for the same operating point.

Finally, numerical simulations are performed using a time dependent Navier-Stokes solver. The analysis of the unsteady simulations provides pressure spectra, sequence of vorticity fields and average flow field. Comparison to experimental data is conducted.

The OVS and SVS instabilities are identified. The inhibitor parameters, the chamber Mach number and length, and the nozzle geometry are varied to analyze their effect on the flow–acoustic coupling.

The conclusions state that flow-acoustic coupling is mainly observed for nozzles including cavity. The nozzle geometry has an effect on the pressure oscillations through a coupling between the acoustic fluctuations induced by the cavity volume and the vortices travelling in front of the cavity entrance. When resonance occurs, the sound pressure level increases linearly with the chamber Mach number, the frequency and the cavity volume. In absence of cavity, the pressure fluctuations are damped.

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