Text: J. Acoust. Soc. Am., Vol. 81, No. 5, May 1987 Pages 1323 - 1326 A criterion for an energy vortex in a sound field R. V. Waterhouse David Taylor Naval Ship R&D Center, Bethesda, Maryland 20084 and Physics Department, American University, Washington, DC 20016 D. G. Crighton Department of Applied Mathematics, Cambridge University, Cambridge CB3 9EW, England J. E. Ffowcs-Williams Department of Engineering, Cambridge University, Cambridge CB2 1PZ, England (Received 1 December 1986; accepted for publication 26 January 1987) Measurements with intensity meters have shown that energy vortices exist in certain sound fields. In these vortices, sound energy flows around closed paths, in the steady state. Vortices occur in some sound fields (e.g., that of a point source near a reflecting edge), but not in others (e.g., that of a plane rigid piston in a plane rigid baffle). It is shown that in a two-dimensional or axisymmetric sound field, a necessary and sufficient condition for a vortex to exist is the presence of an isolated maximum or minimum in the stream function. Two examples are given for a vortex in (a) a duct of square cross section, and (b) the field of two monopole sources, one of which just extinguishes the other. Some relations for the interaction between two monopole sources are also given.

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