Wave-Flow Interactions and Acoustic Streaming
The interaction of waves and flows is a challenging topic where a complete resolution has been frustrated by the essential nonlinear features in the hydrodynamic case. Even in the case of EM waves in flowing media, the results are subtle. For a simple shear flow of constant n fluid, incident radiation is shown to be reflected and refracted in an analogous manner to Snell's law. However, the beam intensities differ and the system has an asymmetry in that an internal reflection gap opens at steep incident angles nearly oriented with the shear. For EM waves these effects are generally negligible in real systems but they introduce the topic at a reduced level of complexity of the more interesting acoustic case. Acoustic streaming is suggested, both from theory and experimental data, to be associated with vorticity generation at the driver itself. Bounds on the vorticity in bulk and nonlinear effects demonstrate that the bulk sources, even with attenuation, cannot drive such a strong flow. A review of the velocity scales in the problem suggest that a variation of the Scholte wave at the driver-fluid interface with local cavitation on the decompression phase creates a lateral flow of fluid that generates the stream and imparts the vorticity from the driver plate. In the case of Darcy flow in sintered media, an analysis of data suggests that acoustically enhanced flow may be the result of surface effects that narrow channels that are cleared by ultrasonic cavitation.