Physics of a strongly oscillating axisymmetric air-water interface with a fixed boundary condition

Abstract

In this work, we experimentally investigate the physics of a strongly oscillating, millimeter-sized, axisymmetric air-water interface with a fixed contact line boundary condition. Many previous studies focused on the regime of small oscillation amplitude, e.g., R = d/D ≪ 1, where d is the oscillation amplitude and D is the characteristic size of the air-water interface. The current investigation instead focuses on a less-studied oscillation regime with large R that is up to 0.33. The dynamic oscillations induce different steady streaming patterns, such as a low-speed streaming vortex or a fast-speed streaming jet. The steady streaming jet, in particular, was not much studied previously and is the major focus of this work. The streaming jet is only generated when the oscillating interface exhibits the higher-order axisymmetric oscillation modes with a large oscillation amplitude, which correspond to the regime with large R and large Weber number (We). The streaming jet has a larger Reynolds number [Re ∼ O(100)] than the typical streaming motions induced by an oscillating interface [Re ∼ O(1)]. In addition, the streaming jet has a high ratio of the streaming velocity versus the oscillatory velocity, which suggests a high efficiency in generating steady streaming motion. The dynamic velocity and vorticity field of the streaming jet in both the initiation stage and the quasisteady stage are presented, which demonstrates that the streaming jet onset process is a consequence of vorticity generation, transportation, and accumulation happening at the oscillating interface. As a zero-mass-flux jet, the streaming jet is sustained by entraining fluid mass flux from the circumferential regions through a process similar to the classical Stokes drift. It is further found that a streaming jet can be induced by an oscillating elastic no-slip membrane as well, when the oscillation has large R and We. The extraordinary characters of the streaming jet can be employed in many engineering applications.

Files

Additional details