The directionality and the propagation of blade-vortex interaction noise are investigated in this paper. Results indicate that blade-vortex interaction noise is governed by the radiation of in-phase BVI-generated acoustic wavefronts. In-phase BVI noise sources are identified by supersonic trace Mach numbers at the BVIs. The resulting radiated noise directionality can be efficiently traced and predicted using the radiation cone concept. When supersonic trace Mach number occurs, highly focused sound waves known as caustics are formed in the near-field. Further away from the rotor in the mid-field, the propagation of Mach cone wavefronts dominates and are found to decay at a much slower rate than the inverse square law. For the one-bladed rotor example studied in this paper, the far-field (which obeys the inverse square law) is reached after a vertical distance of seven rotor radii from the rotor disc plane. These findings demonstrate the complexity of the BVI noise environment. In particular, the different regimes of BVI sound propagation and the more distant BVI far-field have serious implications on the methodologies and techniques used to acquire helicopter far-field noise information from wind tunnel experiments.
Investigation of acoustics propagation in blade-vortex interaction noise
Untersuchung der Kaustikausbreitung im Rotorblatt-Wirbel-Kopplungsgeräusch
Forum, Annual Forum, 53 ; 1 ; 42-55
1998
14 Seiten, 14 Bilder, 13 Quellen
Conference paper
English
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