Direct Numerical Simulation of Quasispherical Bubble Motion in Ultrasonic Standing Wave Fields

To promote the development of an ultrasonic levitation technique, it is essential to understand the mechanism of bubble motion in ultrasonic standing wave fields. The trajectory of bubble motion, the levitation position, and the accompanying change in the surrounding flow field were investigated. The effects of sound pressure amplitude pa, acoustic frequency f, bubble radius Rb, and gravity level G/g on bubble motion were fully analyzed. It was found that the bubble levitation position y/λ decreases with an increase in pa but it increases with an increase in Rb, f, and G/g. The chaos of flow fields increases with an increase in pa, Rb, and f, but it decreases first and then increases with an increase in G/g. The time required for a bubble to remain in levitation and the flow field to be steady decreases with an increase in pa and Rb, but it increases first and then decreases with an increase in f and G/g. Based on the equilibrium relationship between the time-averaged primary Bjerknes force FBj and buoyancy force Fbuoy, a dimensionless parameter X is proposed to determine whether or not a bubble will remain in levitation, and the equation to predict bubble levitation position is presented.