Dynamics of Flow Past Shark-Inspired Textured Surfaces

Speaker: Dr. Shabnam Raayai - Harvard University, Cambridge, MA
Date: Sep 23, 2022; Time: 2:30 PM Location: UTEB 150

Abstract: Skin of fast swimming shark species such as Mako are packed with overlapping micro-scale denticles where each denticle is covered with 3-7 ribs. These textures allow sharks to swim faster than other animals in the ocean. Inspired by this capability, two-dimensional symmetric and periodic textures have been considered for the purpose of drag control and reductions of between 7-10% have been reported. Previous research on 2D textures have focused on the effect of the height and spacing of the grooves on the flow and concentrated on V-grooves (triangular grooves). However, the cross-sections of the ribs on shark denticles are concave and the few reported experiments and simulations of textures with curved profiles show that the response of these surfaces cannot be explained as a function of height and spacing alone, and other geometric features play important roles. In addition, 2D textures are simplified models of the shark scales, missing the effect of the overlaps among the denticles.

In this talk, I will examine the effect of the geometric profile of the cross-sections of 2D textures aligned in the flow direction in two cases: first in a small-scale internal flow (Taylor-Couette) and then in a larger scale external flow (boundary layer) setting. I will present the results of the experiments performed using textured covered rotors in a Taylor-Couette cell in the Couette Flow and early transition to Taylor vortex regimes, as well as textured flat samples in a water tunnel in high Reynolds number laminar flows. The custom-designed experiments involve a combination of load/torque measurements parallel with particle image velocimetry of the flow in the vicinity of the textures. I will explore the response of different profiles, and the effect of convex vs. concave cross-sectional shapes, as well as overlaps, on the ability of textures in altering the flow field, frictional loading, and flow instabilities as a function of the geometric features and flow dynamics (i.e. the Reynolds number). I will show that, overall, when compared with the well-known V-grooves, concave profiles (similar to the cross-section of the shark ribs) with height-to-half-spacing less than or equal to unity can enhance the drag reducing ability of textures while convex textures reduce the level of drag reduction.

Biographical Sketch: Shabnam Raayai is a Rowland Fellow and principal investigator at Rowland Institute at Harvard University where her lab is focused on the study of flow around textured and complex geometries. Prior to her current role, she was a postdoctoral associate at the department of civil and environmental engineering at MIT. She received her SM and PhD in mechanical engineering from MIT and have won multiple awards including the outstanding teaching assistant award from the department of mechanical engineering at MIT and Andreas Acrivos Dissertation Award in fluid dynamics from the American Physical Society.