Wake structures behind a cylindrical rotator with asymmetric protrusions were investigated through a laboratory experiment. Such a cylindrical rotator could prevent a flood disaster due to the accumulation of driftwood at bridges. In this study, a rotator composed of a cylinder with five quarter-cylindrical protrusions was placed in a unidirectional flow. The rotator could rotate freely under hydrodynamic forces. The results indicate that vortices shed from the rotator affected flow structures

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... ehind the rotator. The transverse distribution of the time-average longitudinal velocity of the flow with respect to the rotator was asymmetric about the longitudinal centerline. In the accelerating area with rotation of the rotator, five vortices were obviously shed from each quarter-cylindrical protrusion in each period. Where the flow and rotation were in opposite directions, vortices formation was disrupted. A longitudinal series of vortices merged downstream, ultimately becoming a single vortex. Therefore, the wake flow becomes asymmetric in an area behind the rotator where a significant torque acts.