Abstract:
Nature has long been an inspiration for research in engineering. In particular, the biological surfaces of aquatic swimmers have been studied for their potential as drag reducing surfaces. The hydrodynamic benefit of riblets, or grooves embedded parallel to the flow, which appear on many aquatic biological surfaces, have been well documented and implemented in practical engineering applications. However the skin of dolphins is embedded with grooves that run perpendicular to the flow of water over their bodies. It is theorized that the transverse grooves present on dolphin skin trap vortices between them, creating a partial slip condition over the surface and inducing turbulence augmentation in the boundary layer, thus controlling boundary layer separation over the dolphin's skin. Similarly, sharks are covered with scales that are flexible at the base and capable of bristling, forming grooves running transverse to the flow. It is theorized that the scales bristle when encountering a reversing flow, thereby trapping vortices between the scales and, similarly, delaying boundary layer separation. In an attempt to test this hypothesis and study these affects, a spinning cylinder was used in a water tunnel to induce separation over a flat plate with 2 mm, rectangular transverse grooves and sinusoidal grooves of similar scaling. The results were compared to tripped, turbulent boundary layer separation occurring over a flat plate without grooves using time-resolved particle image velocimetry. The strength of the adverse pressure gradient was varied, and the observed delay in flow separation and other affects upon the boundary layer are discussed.
