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

Passive technologies for the control of the flow past bluff bodies have been widely studied. Most of these works have focused on high-aspect-ratio, wall-mounted, or infinitely long cylinders, leaving a gap for low-aspect-ratio bodies fully submerged or under the effect of a free water surface. This is the object of the present work. Detached-eddy simulations at a Reynolds number of 1000 have been carried out for infinitely long (case i) and low-aspect-ratio bodies. For the finite case, two configurations were investigated: bodies fully submerged in the flow, away from surface effects (case ii) and that involving a free surface, here represented by a fixed slip-allowing plane (case iii). These computations were conducted for the bare cylinder and the system is comprised of the same main body fitted with eight wake-control rods uniformly distributed around its perimeter. Results showed that the low-aspect-ratio cases relative to infinitely long structures (1) lowered mean drag and root mean square lift, (2) presented a less coherent wake topology, (3) had frontal rods that concentrated most of the hydrodynamic loads, (4) extended the formation length, and (5) although all cases developed larger hydrodynamic loads when the rods were fitted to the main body compared to the plain cylinder, the finite cases produced a lower increase. We show that these results are intrinsically related to end effects and associated with a less correlated wake lacking coherent vortical structures. Furthermore, we analyze the more accentuated streamwise vortices produced by the presence of the free surface in case iii compared with case ii.