Effect of roughness on rollup of tip vortices on a rectangular hydrofoil

Experiments focusing on the development of a tip vortex on a rectangular NACA-66 have been performed in a towing tank. The study consists of flow visualisation by illumination with a laser sheet and by distributing fluorescing dye in the water. It also includes surface pressure measurements, particularly around the tip. The experiments have been performed with a smooth surface, and by painting a layer of roughness elements with characteristic sizes of 250 and 500 microns. The observations demonstrate that the physical dimensions of the tip vortex are not affected significantly by surface roughness, whereas the surface pressure, particularly just under the tip vortex, changes substantially. This observation leads to the conclusion that an increase in the surface roughness reduces the strength of the tip vortex. The flow visualisation experiments also demonstrate that for all the roughness sizes, the tip vortex dimensions increase with the incidence angle and decrease as the velocity is increased. In addition, the study demonstrates that the flow is unsteady and that the tip region is dominated by multiple secondary vortex structures. A series of sample photographs demonstrating these phenomena is provided.