Understanding of the Flow Behaviour on a Helmholtz Resonator Excited by Grazing Flow

Grazing flow passing over the quiescent fluid inside the cavity of a Helmholtz resonator can excite the resonator. When the frequency of the instabilities within the shear layer is near the natural frequency of the resonator then flow-excited resonance will occur. This process causes self-sustained periodic oscillations of the flow over the orifice of the resonator. Quantitative numerical prediction of the characteristics of these fluctuations requires accurate modelling of the grazing flow over the Helmholtz resonator. In the present study a Large Eddy Simulation (LES) of the three dimensional shear flow over the orifice of a threedimensional Helmholtz resonator was implemented at low Mach numbers. The simulations were performed over a wide range of Reynolds numbers to analyse the effect of the inlet flow properties on the excitation condition. It was assumed that the excitation phenomena are associated with external pressure fluctuations within the turbulent boundary layer of the grazing flow and the acoustic response of the cavity. For validation proposes, the results obtained from the numerical simulations have been compared with published experimental data and show that the numerical modelling provides an accurate representation of the pressure fluctuations inside the cavity. The main objective of this paper is an understanding of the flow features over a flow-excited Helmholtz resonator. To this end using the numerical model the interaction of a turbulent boundary layer with a Helmholtz resonator has been considered and the characteristics of the flow inside the resonator and over the orifice for various flow conditions are also analysed.