Measurements and Computations of Experimentally Simulated Blade-Vortex Interaction Noise

A single-bladed AH-1 helicopter rotor blade has been tested to high tip Mach numbers (M-T = 0.8) in a hovering acoustically treated test chamber in the presence of vertical gust fields. The vertical gust velocity field has been tailored to closely replicate the impulsive vortex-induced velocity field, thus simulating a single blade-vortex interaction event. The controlled vertical gust disturbance is tailored to simulate the acoustic phasing of parallel, oblique, and a specially designed curved blade-vortex interaction that causes aerodynamic singularities to occur during the interaction process. Far-field acoustic signatures have been measured using a microphone grid in the near in-plane as well as out-of-plane observer regions. Comparisons with theoretical acoustic calculations using computational-fluid-dynamics inputs have yielded good results for the out-of-plane measurement locations, generally validating the use of computational fluid dynamics coupled with linear acoustic theory. At near-in-plane observer regions, comparison with linear acoustic theory calculations showed major discrepancies, indicating that a more in-depth analysis of both the theoretical and experimental approaches is required.