Influence of local ultrasonic forcing on a turbulent boundary layer |
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Authors: | Young Soo Park Hyung Jin Sung |
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Institution: | (1) Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kuseong-dong Yuseong-gu, Daejeon, 305-701, Korea |
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Abstract: | An experimental study was carried out to investigate the effect of local ultrasonic forcing on a turbulent boundary layer.
The ultrasonic forcing system was constructed by adhering six ultrasonic transducers to a flat plate over which water was
flowed. In this system, the ultrasonic waves projected into the water by the transducers caused cavitation, giving rise to
an enormous number of tiny water-vapor bubbles. Stereoscopic particle image velocimetry (SPIV) was used to probe the flow
characteristics. The SPIV results showed that imposition of the ultrasonic forcing caused a substantial increase in the mean
wall-normal velocity but a decrease in the mean streamwise velocity. The ultrasonic forcing reduced the skin friction coefficient
by up to 60% immediately downstream of the transducers; this effect gradually dissipated with moving downstream. The streamwise
turbulence intensity was reduced near the wall but increased away from the wall, whereas the wall-normal turbulence intensity
was not much affected near the wall but increased away from the wall. The Reynolds shear stress and the production of turbulent
kinetic energy were reduced near the wall. Imposition of the ultrasonic forcing shifted the streamwise vortical structures
away from the wall, leading to a reduction in skin friction. |
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Keywords: | Drag reduction Local forcing Microbubble Ultrasonic |
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