Active control of vortex shedding past finite cylinders under the effect of a free surface

This paper presents the analysis of the active flow control promoted by low-aspect-ratio cylinders under the effect of a free surface at a low Froude number, modeled as a slip-allowing plane. To advance the literature in this merit, that is scarce compared with infinitely long and surface-mounted bodies, we carry out Detached-eddy simulations at Reynolds number of 1000 to investigate the active control provided by eight spinning rods surrounding a larger body. One of the ends of this system was immersed in the free stream, while the other was in contact with a free water surface. Our results reveal that when the rods spun with sufficiently large angular velocities, the (non-Karman) vortex street was progressively organized and the part of the wake associated with the mechanism of vortex formation described by Gerrard ["The mechanics of the formation region of vortices behind bluff bodies," J. Fluid Mech. 25, 401-413 (1966)] was eliminated. Nevertheless, tip-vortices prevailed throughout the examined range of spinning velocities. We also contrasted drag mitigation with power loss due to viscous traction and found that to reduce the mean drag on the system to a lower value than that of the bare cylinder necessarily required power expenditure. Steady reduction of mean drag and less significant mitigation of root mean square of lift and mean side force were verified to occur for the entire system and for the central body. However, the side force proved less affected by the wake-control mechanism. We demonstrate this to be associated with a novel ring-like vortex that prevailed throughout the simulations. Vortex dynamics and formation of these turbulent structures are explored.