Sesquipoles in aeroacoustics

This paper describes a class of sound fields generated at a sharp edge by fluid flow. These fields, half-way between monopoles and dipoles in both amplitude and directivity, are sesquipoles (‘sesqui’ being a term for ‘one and a half’). Such fields are generated with large amplitude when a high-frequency gust strikes the leading edge of an aerofoil or fan blade, and are important in aeroacoustics. Four basic sesquipoles are identified, the two- and three-dimensional single-frequency sesquipoles, and the two- and three-dimensional impulsive sesquipoles; and four related sesquipoles are constructed from sources whose strength has a Gaussian or top-hat profile in time or along the span. Of these eight fields, seven are new, the previously known field being the two-dimensional single-frequency sesquipole, for which R. Martinez and S.E. Widnall gave an explicit formula in 1983. The results are applicable to studies of turbofan noise.