Passive Blade-Vortex Interaction Noise Reduction Using In-Blade Resonator Cavities

A novel passive blade-vortex interaction noise reduction concept is proposed which integrates open-closed resonator cavities into the rotor blades. These cavities reduce blade-vortex interaction through two mechanisms: by reducing the magnitude of unsteady surface pressure fluctuations and by “defocusing” the acoustic effect in the far-field. The effectiveness of the concept is assessed using a midfidelity modeling method. An optimization is performed to design several acoustic treatments that reduce blade-vortex interaction noise. One approach uses a multiple-degree-of-freedom resonator design that is applied uniformly across the outer portion of the blade span. The other staggers a single-degree-of-freedom resonator design across various portions of the outer blade span. Both treatment designs are effective in “dephasing” the focusing of blade-vortex interaction noise, resulting in approximately 6 dB of noise reduction in comparison to the baseline untreated blades. Dissipation of the blade-vortex interaction pressure fluctuations is secondary to the effect of “dephasing” for both design approaches. No additional benefits were observed when these approaches are combined, since both approaches rely on the same physical mechanism of reducing blade-vortex interaction noise.