Blade‐Vortex Interaction Acoustic Results from a Forty Percent Model Rotor in the DNW

Abstract

Acoustic data from a 40 percent scale model of the 4-bladed BO-105 main rotor, measured in a large test section aeroacaustic wind tunnel, are presented. Rotor blade-vortex interaction (BVI) noise data were acquired using a traversing in-flow microphone array in the low to moderate speed night range. Results are presented which document the flight conditions far maximum BVI impulsive noise; assess the acoustic farfield of BVI noise; map the directivity of BVI impulsive noise in a large plane below and upstream of the rotor; illustrate the temporal characteristics of the BVI signals as a function of advance ratio; and show the existence of retreating side BVI signals. The occurrence of a strongly focused radiation pattern, as opposed to a more omnidirectional pattern, was found to be very dependent on rotor operating condition. Multiple interactions per blade were ohserved at the lowest speed eases, with less sharp impulsive content, while the higher speed cases exhibited fewer interactions per blade, with much sharper, higher amplitude acoustic signals. BVI acoustic signals were measured under the aft retreating quadrant of the rotor and found to originate from the retreating side of the rotor. Notation location to microphone location, zero over tail, 90 degrees over advancing side degrees C, = rotor thrust coefficient p = advance ratio r = radial distance from rotor hub to microphone, m r, = radial distance from BVI source location to microphone, m = rotor t i p -~a t h -~l~~~ angle, degrees, positive nose up sensitive to the rotor design parameters and the helicopter op-9 = polar angle, angle from the tip-path.plane (hub) to erating conditions. When BVI does occur, this noise mechamicrophone location, positive down, degrees nism dominates the acoustic signal and is in the frequency range 8, = polar angle from tip.path.plane (BVI source considered the most important to human subjective response. location) to microphone location, positive down, de-Past experimental work in rotor BVI noise has concentrated pees in two areas: fundamental two-dimensional studies of BVI aeroly = azimuth angle in the tip-path-plane from the rotor hub dynamics and full rotor system BVI noise studies. The full to microphone location, zero over tail, 90 degrees over rotor system noise studies have attempted to define rotor opadvancing side, degrees erating regimes, parametric effects, directivity, and scaling of l y = azimuth angle in the tip-path-plane from BVI source the BVI acoustic signal. Notable efforts in the system noise 37

Modeling aerodynamically generated sound of helicopter rotors

“…In parallel with the (NR) 2 program were several high quality acoustic tests conducted in the newly opened German-Dutch wind tunnel (DNW 7 ) [23][24][25][26][27].…”

Computational and Asymptotic Methods in Aeroacoustics With Applications

“…For instance, NASA/AHS Rotorcraft Noise Reduction Program (NR) [19] improved experimental data and theoretical understanding of sound generation. In parallel with the NR program, there were several high quality acoustic tests conducted in the German-Dutch Wind tunnel (DNW) [20,21]. Farasat and Succi [22] predicted the transonic effects contained in the quadrupole term.…”