Acoustohydrodynamic phenomena near a small acoustic dipole operating in a viscous incompressible liquid in a wide range of Reynolds numbers

Oscillations of a small acoustic dipole in a viscous incompressible liquid in free space and near a hard wall have been modeled. It is established that an alternating-sign pressure field appears in the gap between the dipole and hard wall. The magnitude of rarefaction depends on the Reynolds number, relative width of the gap (expressed in units of the boundary-layer thickness), and the ratio of the dipole size to gap width. This pressure field corresponds to a field of instantaneous velocities in the form of oscillating potential flows. In addition, viscous waves are generated at the sites of nonzero tangential velocity of the dipole, the interaction of which with the medium leads to the development of acoustic-vortex flows. The spatiotemporal structure and dynamics of these waves have been numerically simulated. The structure of vortex flows proved to be analogous to that observed previously in native experiments. The problem studied is analogous to those arising in some technological acoustic processes in small volumes of viscous liquids at low ultrasonic frequencies.