Effects of turbulence by ultrasonic vibration on fluid flow in a rectangular channel

The effect of ultrasonic vibration in the Reynolds number (Re) range of 1500 to 6000 on fluid flow in a square channel was investigated experimentally. An ultrasonic transducer was fixed at the center of the bottom of a square test channel, and a standing wave field was formed in this channel. The velocity of the fluid was measured by a laser-Doppler velocimeter (LDV). By applying ultrasonic vibration to the laminar flow, agitation or disturbance of cavitation bubbles was produced in the flow and the transition to turbulent flow from laminar flow was promoted downstream. The ultrasonic vibration velocity near the wall is closely approximated by the expression of the log-law even if there exists a laminar flow region. In high Re regions, turbulence can be restrained by the ultrasonic vibration near the wall. Acoustic cavitation near the wall causes the reduction of turbulence intensity under turbulent regions. Since it is possible to control the fluid flow externally due to the easy transmission of ultrasonic vibration in liquids, this technique can be applied to fluid flow in various channels as a noncontact turbulence promoter.