The Effect of Turbulent Shear on the Damping of Pressure Waves

Abstract

Combustion instability is the positive feedback between heat release and pressure in a combustion system. Combustion instability occurs in the both air breathing and rocket propulsion devices, frequently resulting in high amplitude spinning waves. If unchecked, the resultant pressure fluctuations can cause significant damage. Tools for the prediction of combustion instability typically include models for the heat release, the wave propagation and damping. Many wave propagation models for propulsion systems assume negligible flow, resulting in the wave equation. In this research the effect of flow on wave propagation was studied both numerically and experimentally. An experiential rig was constructed with varying axial flow to study longitudinal waves. The rig was excited with speakers and the resultant pressure was measured simultaneously at many locations on the test section. From the pressure data Frequency Response Function data were obtained. Turbulent spectrum data were also taken at the exit of the rig. The turbulent spectrum and Frequency Response Function data were compared and it was concluded that turbulent shear extracts energy in such a way that as the mode number increases the damping in the respective mode decreases. Also concluded was as the system damping increases for a given flow rate the resonant frequency of a mode increases. This result is counter to typical systems where the resonant frequency decreases in with systems of increasing damping