Modeling nonlinear acoustic damping due to flow separation over a baffle blade

Abstract

Nonlinear acoustic damping has been observed in many high-amplitude acoustic systems as a result of flow separation and shear layer vortical motion, eventually transforming some of the acoustical energy into heat. The amount of nonlinear acoustic damping helps determine the nonlinear limit cycle amplitude, e.g., damping caused by baffle blades in a liquid rocket engine to reduce combustion instabilities. The damping mechanism is dependent on both the location and phase of flow separation. Flow separation is a function of both the boundary layer growth and the acoustically imposed pressure gradient. When the acoustic pressure gradient is adverse, the boundary layer is more prone to separation. Using this as a basis, a model can be created, applicable to general geometry, which can approximate nonlinear acoustic damping in flow over baffle blades. The constructed model will be compared to established cases, such as an orifice in a duct, to validate the model. Once validated, this model can approximate the nonlinear acoustic damping caused by a baffle blade, for both standing and traveling waves, and will be compared to experimental results to test accuracy. This model could result in designing rocket engines based on engine specific damping requirements rather than past successful designs.