Acoustic Damping of a Helmholtz Resonator with an Oscillating Volume

Abstract

Combustion instabilities are caused by a coupling between acoustic waves and unsteady heat release. Helmholtz resonators are widely used as acoustic dampers to stabilize unstable combustion systems. Such dampers are typically subjected to a low Mach number grazing flow and are normally only effective over a narrow frequency range, close to the resonant frequency. To increase the effective frequency range, a Helmholtz resonator with an oscillating volume, implemented via an electromagnetic shaker and vibrating backplate, was designed and experimentally tested at the University of Loughborough. It was found that volume oscillation can either increase or decrease the acoustic power being absorbed by the resonator, depending on the phase with which it is driven. A nonlinear numerical model of a Helmholtz resonator with an oscillating volume was then developed to simulate the experiments. Excellent agreement between the numerical and experimental results is found. Furthermore, insight into how to obtain maximum power absorption was provided by the numerical model and validated by the experiments. Finally, to optimize the phase in real time (by minimizing the amplitude of the pressure oscillations), active control of the backplate vibration was experimentally investigated. For the low Mach number grazing-flow regime investigated, this was found to give increased damping and to increase the effective frequency range of the resonator.