Vortex shedding in segmented rocket motors: Acoustic mode shifts

This paper describes some experiments performed with cold flow of air at atmospheric pressure in a duct with a single orifice intended to simulate the protruding inhibitors in the inter-segmental regions in large solid rocket motors. The length of the duct was closely varied in most experiments, and tests were performed with the orifice at different locations along the axis of the duct at each length level. Other parameters that were varied include the orifice size and flow velocity. Measurements were mainly in the nature of acoustic diagnostics, with a microphone or a transducer located at different axial locations along the duct in different runs. The combined results indicate that, although no audible sound of distinct frequencies were excited by vortex shedding at the orifice, the mechanism of pressure-coupled feedback-based vortex shedding is prevalent in the geometry adopted. The mechanism manifests in abrupt shifts in frequencies related to the different possible duct acoustic mode, and is accompanied by oscillatory fluctuations in the amplitude of the duct acoustic modes, as the duct length is varied. The results are thought to have implications in the context of abrupt frequency shifts experienced in large rocket motors. © 2000 by the authors. Published by the American institute of Aeronautics and Astronautics, Inc.