Rotation mechanism of ultrasonically levitated cylinders

Abstract

The rotation of levitated object in the ultrasonic levitation experiment is a common phenomenon. This instability may give rise to many difficulties in locating and detecting the levitated object and even cause the experiment to fail. However, the relevant research of the rotation mechanism of levitated object is seldom carried out. In this work, the rotation mechanism of cylinder in a single-axis ultrasonic levitator is investigated experimentally and theoretically. In the ultrasonic levitation experiment, the cylinder begins to rotate about an axis along the vertical direction as it is levitated at the node between the emitter and reflector. The rotation speed of cylinder tends to a stable value due to the effect of the air resistance, and the final rotation direction is determined by its initial rotation state. Experimental results demonstrate that the rotation speed increases with the decreases of density and length-to-diameter ratio of the cylinder. In order to analyze the rotation mechanism, the finite element method is used to calculate the distribution of acoustic pressure field and the torque acting on the cylinder for each of three different cases. Numerical results reveal that the position offsets of the cylinder and the reflector as well as the tilt of the emitter can all result in the nonaxisymmetrical distribution of acoustic pressure field. Hence, a nonzero torque acting on the cylinder may be generated and the rotation state of the levitated cylinder is subsequently affected. The position offset of the cylinder can produce a torque driving itself to rotate and the torque increases with the increase of the deviation degree. A restoring torque suppressing the rotation of cylinder can be generated by deviating the reflector from the horizontal direction. The cylinder eventually keeps stationary state with its axis perpendicular to the offset direction of the reflector, showing good accordance with the experimental results. In addition, it is predicted that tilting the emitter can also offer a restoring torque which makes cylinder eventually static with its axis perpendicular to the plane through the axes of the emitter and the reflector. However, this restoring torque is approximately three orders of magnitude smaller than that generated by deviating the reflector. In the end, both experimental results and numerical simulations show that the rotation of the cylinder can be effectively suppressed under the disturbance of two fixed cylinders when the emitter and the reflector are coaxial. The cylinder eventually stays still and keeps coaxial with the two fixed cylinders.