Force and velocity fluctuations over hydrofoils with surface roughness

There have been significant efforts to understand the hydrodynamic function of surface roughness due to large penalties in drag or potential enhancements in lift forces. Often, the research on this topic uses roughness elements with highly complex geometries, for example, by replicating morphological properties of shark denticles. However, it is not clear whether the main contributor in the reported force generation benefits is the highly complex geometry of the roughness elements or the rearrangement of the flow due to presence of roughness itself.

In this study, we aim to characterise the effects of roughness on the force generation from static and unsteady propulsive surfaces. To do so, we used roughness elements with a simpler geometry and conducted a series of force and flow measurements with NACA0012 hydrofoils covered with spherical-cap roughness elements. The area coverage ratio of the roughness elements is varied and a parameter sweep is performed for a chord-based Reynolds number range of 10,000<Re<50,000 and varying angles of attack. Using flow visualization and force measurements, the results from this parameter sweep will be presented in comparison with the forces and flow over a smooth surface foil with no roughness elements present.