EXPERIMENTAL-STUDY OF STREAMING FLOWS ASSOCIATED WITH ULTRASONIC LEVITATORS

Steady-state acoustic streaming flow patterns have been observed durin g the operation of a variety of resonant single-axis ultrasonic levita tors in a gaseous environment and in-the 20-37 kHz frequency range. Li ght sheet illumination and scattering from smoke particles have reveal ed primary streaming flows which display different characteristics at low and high sound pressure levels. Secondary macroscopic streaming ce lls around levitated samples are superimposed on the primary streaming flow pattern generated by the standing wave. These recorded flows are quite reproducible, and are qualitatively the same for a variety of l evitator physical geometries. An onset of flow instability can also be recorded in nonisothermal systems, such as levitated spot-heated samp les when the resonance conditions are not exactly Satisfied. A prelimi nary qualitative interpretation of these experimental results-is prese nted in terms of the superposition of three discrete sets of circulati on cells operating an different spatial scales. These relevant length scales are the acoustic wavelength, the levitated sample size, and fin ally the acoustic boundary layer thickness. This approach fails, howev er, to explain the streaming flow-field morphology around liquid drops levitated on Earth. Observation of the interaction between the flows cells and the levitated samples also suggests the existence of a stead y-state torque induced by the streaming flows.