Experimental and Computational Investigation of Stacked Rotor Broadband Noise in Hover

Abstract

Acoustic measurements of a 2m diameter stacked rotor in hover were taken. The stacked rotor was comprised of two sets of opposing blades, which had a variable angle, referred to as the index angle, between them. The periodic, or tonal noise, was filtered from the acoustic spectra, allowing for analysis of the separate contributions of tonal and broadband noise. Accompanying simulations were done of the same rotors and conditions using the Rotorcraft Comprehensive Analysis System (RCAS) with a viscous vortex particle method and PSU-WOPWOP with the Brooks, Pope, and Marcolini (BPM) method for predicting broadband noise. Broadband and tonal noise measured experimentally were shown to contribute similar magnitudes to the overall sound pressure level (SPL) both near and far away from the rotor. The broadband A-weighted SPL was larger than that of the tonal noise. The tonal noise of a stacked rotor with no axial spacing decreased by 9 dB as the azimuthal spacing increased from the smallest value (28.125◦) to 90◦due to the tonal noise drastically decreasing at specific multiples of the blade passage frequencies; a stacked rotor with axial spacing of 1.5 chords, however, exhibited smaller reduction in tonal noise as the azimuthal spacing approached 90◦. The simulations showed many of the same trends as the experimental measurements, however the higher harmonic tonal noise and the broadband noise were underpredicted by up to 30 dB and 15 dB, respectively.