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 1,000 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 for the system 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) the frontal rods concentrated most of the hydrodynamic loads; (4) extended the formation length; (5) and, 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 analyse the more accentuated streamwise vortices produced by the presence of the free surface in case iii compared with case ii.