Flow pattern in a channel during application of ultrasonic vibration

In the present study, the flow pattern in a channel during application of ultrasonic vibration is investigated for the purpose of fluid control without contact. An ultrasonic transducer is fixed at the center of the bottom of a square test channel, and a standing wave field is formed in this channel. The velocity of the fluid is measured by a laser-Doppler velocimeter (LDV). By applying ultrasonic vibration to the channel flow, cavitation bubbles are generated at the sound pressure antinode in the channel when Re-d = 2000. Consequently, the cavitation bubbles act on the fluid as the barrier, causing the separation of the flow, and the velocity of the fluid is reduced locally. The generation of cavitation is active at the point where: the sound pressure of the standing wave formed by ultrasonic vibration is high. Moreover, the cavitation bubbles move toward to the antinode and are trapped by radiation pressure because the bubbles are of much lower density than water. Thus, the profile of fluid velocity is strongly influenced by the standing wave formed in the channel. Since the wavelength is shorter at higher frequencies, this technique can be applied to flows of various scales.