High‐speed leading‐edge noise

This paper determines the sound generated at the leading edge of a turbofan blade in a subsonic flow when the blade is struck by a convected gust of arbitrary shape. The gust may be localized in the span direction, i.e. may strike only a limited section of the leading edge, and it may strike the edge for only a limited time. The parameter range considered is the non‐compact limit in which the wavelength of the sound is less than the chord of the blade, so that the leading edge may be considered in isolation. By means of the Wiener‐Hopf technique and the use of aeroacoustic similarity variables, a simple integral expression is found for the unsteady three‐dimensional sound field everywhere in space, including the near field and the leading‐edge region remote from the gust, where the Lloyd's mirror effect reduces the field amplitude. The expression for the field is well suited to numerical evaluation, e.g. of transients, and its far‐field approximation is simple enough that for many gust shapes all integrations may be performed analytically. The results provide a transfer function between the gust and the source term in the acoustic part of a computational fluid dynamics code.