PARAMETRIC COMPUTATIONAL STUDY OF ISOLATED BLADE-VORTEX INTERACTION NOISE

A parametric study was performed to quantitatively describe how the vortex core size, the vortex location, and the hover-tip Mach number M(h) affect blade-vortex interaction (BVI) noise. The effects of these parameters on BVI noise are determined using a rotor acoustic prediction program. The acoustic prediction program is based on the well-known Ffowcs Williams and Hawkings equation for acoustic pressure, wherein noncompact monopole terms model rotor blade thickness and distributed dipoles model local blade Surface pressure. The dipole strengths are determined by an unsteady, three-dimensional, full-potential rotor code that models the aerodynamic interactions between a nonlifting rotor and a tip vortex generated by an upstream wing. The acoustic pressures were calculated for several observer positions in regions of intense acoustic radiation for a variety of blade-vortex proximities, orientations, hover-tip Mach numbers, and vortex-core sizes. This study has quantified the sensitivity of BVI noise to the dominant vortex and aerodynamic parameters. The sound pressure level (SPL) falls off as the inverse of the square of the miss distance between the vortex core and the rotor blade when the miss distance is greater than the core radius. Increasing the core radius is not as effective as increasing the miss distance when attempting to reduce BVI noise. As expected, this study shows that SPL decreases with an increase in the obliqueness of interaction. The calculations performed in this work indicate that SPL increases approximately as M(h)(8). These results can be used to guide future research of BVI noise reduction.