Drag Reduction Properties of Riblet Surfaces
When aligned with the direction of flow, riblets were found to contribute significantly to the reduction of skin friction in turbulent boundary layers, with optimal geometries obtaining drag reductions of up to 7-8%.
Drag reduction is achieved by impeding the cross-stream translation of streamwise vortices in the viscous sublayer whilst elevating the high-velocity vortices above the surface, thus reducing the shear stress and momentum transfer. By lifting high-velocity vortices above the ridges of the riblets, the surface area experiencing high shear stresses is reduced, allowing low-velocity fluid flow to dominate the riblet valleys.
Riblets are one mere instance of biomimicry – an approach to innovation where solutions to human problems are found by emulating systems and elements of nature. Birds and sharks alike were found to possess such features.
Birds – Herringbone riblets line the shafts of primary and secondary bird features, and they often have narrow smooth edges intended for drag reduction purposes.
Sharks – Dermal denticles are ridged structures embedded in the skin of sharks, and serve numerous hydrodynamic functions.
Riblet Geometries
We decided to focus on 4 primary riblet geometries amongst the many existing configurations. The riblets were aligned parallel to the flow direction, and the 4 variations are listed as follows – blade, v-shaped, trapezoidal and scalloped.
Amongst the four aforementioned geometries, we chose to focus solely on blade riblets as they were not only proven to be an optimal drag-reducing surface in water by existing literature, but the blades were also the most feasible option to manufacture. The test cylinder with this surface is explored in greater detail at ‘Design Specifications’.
Drag Reduction Properties of Dimpled Surfaces
Apart from riblet surfaces, we decided to study the aerodynamic characteristics of dimpled surfaces as well.
As observed in the picture, golf balls are covered in surface indentations, and these are known as ‘dimples’ – features intended to reduce drag and optimise flight paths. For instance, studies have shown that a smooth golf ball hit by a professional golfer would only travel approximately half as far as a dimpled golf ball.
When a golf ball is hit from the tee, the movement results in a high-pressure area on its front. Air flows smoothly over the contours of the golf ball’s front, creating a turbulent region where air flow is agitated and a lower-pressure area in its wake. Dimples aid in creating the turbulent boundary layer of air at the golf ball’s surface, resulting in a smaller wake at the back of the ball, thus reducing drag.
Not only do dimples reduce drag, they also enable the golf ball to experiment more lift. As the golf ball moves through the air, the dimples create a region of high pressure above the ball, as well as one of low pressure below the ball. This provides additional lift that enables the ball to stay airborne for a longer duration.
Even though the dimples on a golf ball are not all of a single standardised size, we have translated this pattern into a uniform biomimetic surface on our test cylinder found at ‘Design Specifications’.