Experimental study of streaming flows associated with ultrasonic levitators

Abstract

Steady-state acoustic streaming flow patterns have been observed during the operation of a variety of resonant single-axis ultrasonic levitators in a gaseous environment and in the 20–37 kHz frequency range. Light sheet illumination and scattering from smoke particles have revealed primary streaming flows which display different characteristics at low and high sound pressure levels. Secondary macroscopic streaming cells 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 levitator physical geometries. An onset of flow instability can also be recorded in nonisothermal systems, such as levitated spot-heated samples when the resonance conditions are not exactly satisfied. A preliminary qualitative interpretation of these experimental results is presented in terms of the superposition of three discrete sets of circulation cells operating on different spatial scales. These relevant length scales are the acoustic wavelength, the levitated sample size, and finally the acoustic boundary layer thickness. This approach fails, however, 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 steady-state torque induced by the streaming flows.

Acoustic/steady streaming from a motionless boundary and related phenomena: generalized treatment of the inner streaming and examples

“…Similar diagrams have been obtained by Gopinath & Trinh (2000). Note though that, in real acoustic levitators, such a localized streaming from a particle as considered here ("microstreaming") may get distorted by the streaming on a scale of the levitator as a whole, existing even in the absence of the particle, as shown experimentally by Trinh & Robey (1994). For a simple model accounting for this effect, see Rednikov & Riley (2002).…”

Single-Droplet Evaporation Kinetics and Particle Formation in an Acoustic Levitator. Part 1: Evaporation of Water Microdroplets Assessed Using Boundary-Layer and Acoustic Levitation Theories

“…An array of toroidally-shaped vortices is formed around a droplet levitated within a constrained space (i.e., the levitator chamber) in an acoustic field. 18 These vortices can trap water vapor 18 and thus cause P 1 in Eq. 1 increase.…”

Numerical simulation of oscillating flow field including a droplet

“…It is, however, shown in the average flow field that the steady vortex flow appears. The steady flow in the oscillating pressure field is known as the acoustic streaming, and observed in the experiments [9,10] and in the theoretical analysis [11]. The recirculation flow region on the droplet was also predicted by the theoretical treatment, though it is not clear in the experiments and in this study.…”

“…The time evolution of φ is given by (8) In this study, the ALE method [13] is applied, and the computational grid is assumed to be oscillating in the vertical direction with the same velocity as the boundary velocity. The substantial derivative terms in Eqns (2) and (8) are thus defined by (9) where U is the velocity of the computational grid or the oscillating boundary velocity. In order to maintain the level set function as a distance function, reinitialization of the level set function by solving the following equation is proposed [8]: (10) where τ is an artificial time and sign (φ 0 ) indicates the sign of the level set function at the beginning of the reinitialization procedure.…”

“…It is thus important to know the oscillating flow field including a droplet. Streaming flows associated with ultrasonic levitator have been studied experimentally [9,10] and theoretically [11]. Although steady-state flows in and around the droplet were studied, transient flow fields have not yet been discussed well.…”