Fluid Mechanics and Passive Control of the Flow-Excited Helmholtz Resonator

A flow excited Helmholtz resonator was investigated experimentally and theoretically. The analysis was focused on a simplified momentum balance integrated over the region of the orifice. The resulting expressions were used to guide an experimental program designed to obtain measurements of the resonator pressure under flow excitation, as well as the dynamics of the shear layer in the orifice using Particle Image Velocimetry. The PIV results provided a detailed representation of the shear layer vorticity field, as well as the equivalent hydrodynamic forcing of the resonator. The forcing magnitude was found to increase with speed over a range of flow speeds.

A model was proposed that provides a prediction of the resonator pressure fluctuations based on the thickness of the approach boundary layer, the free stream speed, the acoustic properties of the resonator and the spatial growth rate of the shear layer across the orifice. The model was shown to provide an accurate representation of the resonating frequency as well as the magnitude of the resonance to within a few dB.

Various passive flow control methods were examined to reduce the flow-excited resonance. Foam and tuned absorbers were employed to control the acoustic properties of the resonator. Both methods succeeded in reducing the flow-excited resonance. The hydrodynamic forcing was controlled through both changes to the orifice geometry and with the disruption of the approach flow. Most changes to the orifice geometry resulted in significant decreases in the magnitude of the resonance. Thickening and rounding the upstream and down stream edges of the orifice was found to increase the resonance. Obstructions placed upstream of the orifice to disrupt the approach flow decreased the resonance to varying levels of success.

Comparisons were made to a full-scale vehicle. Both microphone and PIV measurements were acquired for a full-scale vehicle and compared to simplified small scale models. The fundamental flow physics were found to be consistent between the full-scale vehicle and simplified small scale models.