Shark skin has fascinated biologists, engineers, and physicists for decades due to its highly intricate drag-reducing structure, which has motivated a plethora of research into bio-inspired hydrodynamically efficient surfaces. Throughout this thesis the effect of shark scales on the boundary layer is investigated, with a particular focus on the role of riblets in combination with denticles. In addition to examining flows over shark scales studies are also presented investigating the behaviour of Reynolds Averaged Navier-Stokes (RANS) models close to solid boundaries, and the scaling and driving mechanisms of secondary flows over ribletted surfaces.
Extensive numerical and analytical studies are carried out to determine the sensitivity of eleven turbulence closures to the near-wall grid resolution, and their consistency with asymptotic solutions. Results inform the choice of turbulence models adopted for simulations of wall bounded flows, particularly where numerical errors must be minimised.
Secondary flows over longitudinal riblets are found to be driven by Reynolds stress anisotropy, consistent with Prandtl’s second type of secondary flow. The strength of the vorticity field is heavily dependent on the inner-scaled riblet spacing s+ where two distinct regimes arise; a viscous regime where vorticity production is balanced by molecular viscous diffusion, and an inertial regime where an effective turbulent viscosity balances
anisotropic production. The transition between these regimes occurs when riblet tips protrude into the buffer layer and cause increased turbulent mixing (s+ ≈ 30), such that vorticity reaches its maximum before reducing as s+ increases further.
Riblets in combination with shark scales do not operate as they do when applied to smooth walls. Experimental and numerical studies reveal that riblets act to reduce pressure drag acting on roughness elements, rather than the viscous forces typically associated with longitudinal riblets. The mechanisms leading to this behaviour are driven by the ability of riblets to restrict spanwise motion and maintain streamwise-aligned near-wall flow. By doing so riblets protect downstream denticles from high momentum impinging fluid, and reduce high magnitude swirl generated at the exposed denticle edges, which can otherwise lead to increased turbulent production and enhanced momentum transfer through the roughness sub-layer. These mechanisms lead to a significantly more efficient rough surface than smooth denticles, although do not necessarily lead to an overall reduced drag compared to a flat plate. These studies conclude that riblets have evolved as a mechanism to reduce or eliminate the skin friction increase due
to the presence of scales. The combination of scales and riblets appears to be relatively hydrodynamically efficient in terms of skin-friction drag, whilst also acting to maintain boundary layer attachment and providing the other advantages associated with scales such as anti-fouling, abrasion resistance, and defence against parasites.
