Effect of orifice geometry on Helmholtz resonator excitation by grazing flow

The characteristics of a Helmholtz resonator embedded under a turbulent boundary layer were experimentally investigated. Thirteen resonators were tested, each with different orifice geometries. The resonance frequency and pressure amplitude in the cavity were observed throughout the freestream velocity range. Orifice size, planform shape, and sidewall shape were all varied. For circular orifices, it was established that the response amplitude decreases with diameter, becoming negligible for diameters less than 5% of the boundary-layer thickness. The plan shape of the orifice affects both resonance frequency and maximum cavity pressure. Results are only roughly correlated by a Strouhal number containing the orifice length in the flow direction. The exact shape of the orifice sidewall also has an important effect on the resonator response. Orifices slanted toward the upstream flow have increased response, and those slanted downstream have negligible response. An orifice with rounded sidewalls both upstream and downstream has an unusually high response.