Abstract
This paper describes noise radiation from open rotors. The starting point is the expression for noise radiation due to either the blade loading distribution or the blade thickness distribution. These expressions, involving integrals over the blade surface, are themselves obtained from the Ffowcs WIlliams-Hawkings equation. Using the assumption that the number of blades is large, it is possible to evaluate the integrals asymptotically and to show precisely how the farfield radiation is dominated by contributions from a few regions of the blade. At subsonic conditions, the noise is, of course, completely tip generated. At supersonic conditions the radiation is, to leading order, dominated by the Mach radius: the radius that approaches the observer at precisely sonic speed. At supersonic conditions there is also a significant lower order term, originating at the blade tip. The results for all cases show the explicit dependence on tip Mach number, harmonic number and radiation angle. Excellent agreement is obtained between the asymptotic results and full numerical evaluation of the acoustic field. The formulas are also extended to predict the acoustic benefit of sweep.
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