Sound directivity generated by helicopter rotors using wave tracing concepts

Results are presented from a study of directivity and sound focusing effect s generated by helicopter rotors encountering parallel and oblique blade vo rtex interactions (BVI). The primary analysis is performed by using wave tr acing to determine ray conesand acoustic lines from source points on the ro tor with supersonic trace (phase) velocities. The results are compared and contrasted to predictions made from a numerical solution of the Ffowcs-Will iams Hawkins equation. Sample problems considered include parallel and obli que BVI with an isolated line vortex, and interactions with self-generated epicycloidal vortices in forward flight. It is confirmed that the BVI proce ss can produce strong directivity and clusters of focused sound waves in th e far field. The trace velocity and wave tracing technique is shown to have potential applications for studies in noise reduction and/or noise directi vity modifications using passive devices such as blade tip sweep. It is als o shown that the numerically efficient nature of determining the primary ac oustic lines with the trace velocity method can allow regions with strong d irectivity to be efficiently mapped out using redistributive observer point techniques. (C) 1999 Academic Press.