Abstract
Results are presented from a study of directivity and sound focusing effects generated by helicopter rotors encountering parallel and oblique blade vortex interactions (BVI). The primary analysis is performed by using wave tracing to determine ray cones and acoustic lines from source points on the rotor with supersonic trace (phase) velocities. The results are compared and contrasted to predictions made from a numerical solution of the Ffowcs-Williams Hawkins equation. Sample problems considered include parallel and oblique BVI with an isolated line vortex, and interactions with self-generated epicycloidal vortices in forward flight. It is confirmed that the BVI process can produce strong directivity and clusters of focused sound waves in the far field. The trace velocity and wave tracing technique is shown to have potential applications for studies in noise reduction and/or noise directivity modifications using passive devices such as blade tip sweep. It is also shown that the numerically efficient nature of determining the primary acoustic lines with the trace velocity method can allow regions with strong directivity to be efficiently mapped out using redistributive observer point techniques.
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