1. The analysis of the sound generated by the unsteady loadings due to turbulence fluctuations on unducted subsonic rotors began with the quite general analysis of Homicz and George,15who treat the general case of unsteady forces distributed in space following the Lighthill equation of aeroacoustics with specialization to rotating blades. They devised an analysis for the sound radiated from arbitrarily varying forces on a rotor disk The analysis was then applied to the varying loadings on a rotating blade in the disk The loadings were obtained from an approximate compressible aerodynamic analysis for an inflow of isotropic turbulence defined by the Dryden spectrum Inflow turbulence was seen to be an important noise source over a range of frequencies. Also, the analysis explained the humped or peaked nature of the low frequency part of the spectrum as due to the large scale components of the turbulence inflow These large scale components give nearly periodic disturbances as they are swept through the rotor plane; this leads to nearly periodic but finite bandwidth radiated sound. This analysis is not well suited for high frequencies, since large CPU times are required for the calculations; thus, high frequency analyses were developed by George and Kim16and by Amiet,17and variations on them by Harris and coworkers1819
2. Noise is also produced by the self generated turbulence in a blade's boundary layer passing its trailing edge This was recognized as far back as 195929Various investigators developed very simple models for this noise, but these early models were not complete and were basically useful as bases for correlations. Fink, for example, used such a correlation to predict the 6n-axis noise of a rotor due to boundary layer trailing edge noise.30Complete first principle analyses of rotor trailing edge noise were developed more recently by Kim and George31and by Schlinker and Amiet32Recently, Hubbard, et al33also have proposed an OASPL and spectrum peak correlation for wind turbine rotors
3. The analytical problem of sound radiating from the effect of turbulence being eonvected past a nonrotating trailing edge has been studied intensively since about 1970 A variety of mpdelsi were studied (see the review of Hpwe34), butjhese primarily resulted in sealing laws which needed determination of empirical constants. There also remained a number of questions regarding the details of the modelling and the ef "fects of the Kutta condition. On the other hand Amiet developed a method which is based on solving the problem of a statistically stationary pressure field being eonvected past a trailing edge35>36This result depends only on the pressure spectrum in the boundary layers being known from ex periments. Amiet's method has been compared to the ex perimental findings of Brooks37and found to be consistent