Broadband and discrete frequency radiation from subsonic rotors

A unified analytical technique is presented for the prediction of both the discrete frequency and broadband acoustic signatures generated by aerodynamic rotors. While present applications deal with dipole (force) type radiation from circularly shaped source regions, the approach is formally applicable to other order multipoles and arbitrarily shaped source distributions. The prediction of broadband radiation from a hovering rotor due to inflow turbulence/blade interaction has been analyzed by using this approach. By working in co-ordinates fixed with respect to the observer rather than the blades, the appropriate blade-to-blade load correlations can be handled correctly. A direct relationship between the acoustic and turbulence spectra results. The most important parameter affecting the shape and magnitude of the acoustic spectrum is shown to be the ratio of the time taken by the rotor to complete one revolution to the time needed to convect one integral scale of turbulence through the rotor face. The result indicates no direct dependence of broadband noise on steady thrust level, except through whatever influence it may have on the mean velocity through the rotor face. The method is applied first to a simplified point dipole, quasi-steady aerodynamic blade model for a B -bladed rotor, providing a convenient upper bound estimate to the noise one can expect in a given situation. The principal effects of compressible unsteady aerodynamics and of distributed loading are analyzed and shown to result in reductions in high frequency radiation. The influence of chordwise distributed loading is found to be highly directional in nature, the radiation near the rotor plane being most affected. Comparisons between the present theory and the limited available experimental spectral data are presented, and exhibit reasonable agreement. It appears likely that some noise previously interpreted as discrete tones may actually be the result of interactions with large scale turbulence. Various means to effect reductions in turbulent interaction noise are discussed.

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