Passive drag reduction in cross-flow rigid plates near the free surface

Flat plate models of fixed aspect ratio incorporating either serrations or holes at the upper and lower plate edges, were towed normal to the flow and in proximity to the free surface. The hydrodynamics and the drag generated by the plates has been studied in detail for several configurations. In general, the models with holes presented greater drag reduction than those with serrations, with the hole angle playing an important role on the total drag measured. The fluid dynamic sources for drag reduction at low submergence depths have been investigated using quantitative flow visualization, and they have been related to the changes in the separating shear layers, as a result of the different edge features imposed to the plates.