Andreas Acrivos Dissertation Award Talk: Geometry Mediated Drag Reduction Using Riblets and Streamwise Surface Textures

The surfaces of many plants and animals are covered with a variety of microtextures which can control surface-mediated properties. Inspired by the drag reducing ability of the ribs on shark denticles, passive drag reduction strategies such as riblet surfaces have been studied both experimentally and numerically. Here theoretical and numerical modeling were used to study the evolution of boundary layers in the presence of the riblets in high Reynolds number laminar flow. We explore the role of aspect ratio of the grooves as well as the length of the wetted surface in the streamwise direction. We show that riblets reduce the shear stress by retarding the flow inside the grooves. The total reduction is a non-monotonic function of the aspect ratio of the riblets and to achieve overall drag reduction, the riblet wall needs to be longer than a critical length. To eliminate the role of entrance effects, we investigate the effect of riblets on the torque exerted on the rotor in a Taylor-Couette (TC) flow. We perform experimental studies and time-dependent numerical simulations in both the laminar Couette and the Taylor vortex regime. We explore the effect of the size of the riblets with respect to the geometry of the TC cell, and the aspect ratio of the riblets. The textured rotors are fabricated using 3D printing and the TC cell is custom built using CNC machining. The test cell is aligned and mounted on a rheometer to measure the velocity and the torque on the rotor. We observe a non-monotonic behavior for the reduction in torque as a function of the aspect ratio and discuss the effect of changing the geometry of the flow and the riblets on the changes in the torque. When viewed holistically the results of these two studies show that, through careful design, a net reduction in drag force can be robustly realized on micro-textured surfaces in high Reynolds number laminar flows through complex changes in near-wall velocity profiles even in the absence of turbulent effects.