Interaction of a flow-excited Helmholtz resonator with a grazing turbulent boundary layer

This study focuses on the characteristics of the flow oscillations induced by various Helmholtz resonators which are excited by a fully developed turbulent grazing flow. A full experimental investigation has been conducted in a subsonic wind tunnel with a low turbulence level. Detailed fluid dynamic and acoustic analyses of the flow inside the wall-mounted resonator and within the shear layer have been performed. The results reveal that the degree of excitation of the pressure and velocity fields is related to the resonator geometry, and that the pressure fluctuations within the resonator are most sensitive to the orifice length for the range of geometric parameters considered. Interestingly, the turbulent length scale over the orifice with the minimum diameter was found to be higher than the other cases investigated. This in turn causes a modification to the turbulent structures downstream of the resonator. The turbulence intensity and energy spectra of the velocity fluctuations within the turbulent boundary layer downstream of the resonators demonstrate that in a specific velocity range certain resonators have the potential to modify the flow instabilities within the turbulent boundary layer. The results provide information on the relationship between the pressure and velocity fluctuations within and downstream of the resonator and the geometric characteristics of the resonator and the properties of the grazing flow.