Acoustic radiation torque on a compressible spheroid

Starting with the theoretical framework for calculating the acoustic radiation force on a compressible spheroid [Jerome et al., J. Acoust. Soc. Am. 148, 2403-2415 (2020)], the present work develops a model for the corresponding acoustic radiation torque. A general result is obtained that may be applied to an object of arbitrary size, shape, and impedance in an arbitrary incident sound field. Like for the radiation force, the general result for the radiation torque is a summation of terms involving products of the coefficients in spherical wave expansions of the incident and scattered fields. For the compressible spheroid under consideration, spheroidal wave expansions are employed to satisfy the boundary conditions on the surface of the spheroid to obtain the scattering coefficients. Results are presented for the radiation torque exerted on a compressible spheroid by a progressive or standing incident plane wave. The results illustrate the dependence of the radiation torque on the size, aspect ratio, and impedance of the spheroid and on its orientation with respect to the incident wave field.