A new physics-based analytical approach to understanding and reducing near in-plane far-field high speed harmonic noise is developed. Using analytical approximations to the solution of the governing Ffowcs Williams and Hawkings equations, preliminary design estimates have been developed for low to mid frequency near in-plane harmonic noise in hover. These approximations, which work well for high aspect ratio blades up to moderate hover tip Mach numbers (below derealization, MH < 0.85 for 9% thick airfoils), relate the time history and peak of in-plane noise to rotor design and operational parameters explicitly. Trends for both linear thickness noise and loading noise are studied in hover as a function of various governing design and operational parameters in both time and frequency domains. This analytical approach is then used to develop unsteady idealized on-blade point controllers to cancel or reduce the in-plane far-field thickness noise. Both force and volume controllers are investigated in terms of their effectiveness is reducing the peak thickness noise levels in a region around the in-plane target location. Higher frequency controllers are found to require small peak-to-peak inputs but are only effective in a small region near the target observer; while low frequency controllers are found to cancel the noise over a larger region around the target observer but require larger peak-to-peak control inputs.
Understanding far field near-in-plane high speed harmonic helicopter rotor noise in hover: Governing parameters and active acoustic control possibilities
2008
23 Seiten, 7 Quellen
Aufsatz (Konferenz)
Englisch
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