Passive Flow Control with Shark Skin and the Relevance of Low Speed Streaks to Denticle Actuation
The shortfin mako shark (Isurus oxyrinchus) is one of the ocean's fastest and most agile predators. A mako's speed can reach as high as 40 m.s-1 [1]. Its exceptional speeds are due to many biological factors including its dermal denticles known as scales, which may be crucial to delaying flow separation. The teeth-like denticle morphology and flexibility of the scales measured by bristling angle vary along the shark's streamlined body [1]. Scales with narrow and long crowns have higher bristling angles at strategic locations, such as the flank and trailing edge of the pectoral fins which are regions more prone to flow separation. Since separation is more likely to occur when the boundary layer must flow against an adverse pressure gradient, the bristling angle may also be related to the local boundary layer thickness, such that a thicker boundary layer requires a higher bristling angle, to achieve flow control. It is hypothesized that the scales act as a passive separation control mechanism that is flow-actuated in that patches of reversing flow (occurring in the low-speed streaks of a turbulent boundary layer) are induced to form due to the presence of an adverse pressure gradient that is likely to exists around a point of maximum girth on the shark's body and its caudal base. The reversing flow bristles the scales which impedes the reversed patches of flow from traveling further upstream, favoring the oncoming flow to gain momentum, prolonging attachment, and delaying separation [2]. Previous experimental studies, using time-resolved digital particle image velocimetry (DPIV), have shown that the scales (samples of real shark skin) can control flow separation for flow over a hydrofoil at high angle of attack or a in a region of separation induced over a flat plate [3-7]. The current investigation focuses on the scales' ability to control flow separation from two regions with different morphology and bristling angles when placed in the separation or reattachment region of a turbulent boundary layer separation bubble occurring on a flat plate. Furthermore, these experiments study the scales' effects on boundary layer behavior, skin friction, backflow coefficient, turbulent kinetic energy, Reynolds stress, and quadrant analysis.This study also examines the reversed streaks (low-speed streaks) that occur in a turbulent boundary layer on the verge of separation, as it is believed that this particular component of the flow is responsible for inducing scale bristling. Streak widths and velocities for a flow over the shark scales are compared to those forming upstream of a separated flow over a smooth flat plate, and a difference is expected. The spanwise distance between streaks is also analyzed to validate that the scales can control the flow. A vortex identification method was also developed to help identify coherent flow structures responsible for the formation of low-speed streaks.These findings suggest the potential for shortfin mako scales to effectively control flow separation, shedding light on the intricate mechanisms such as flow ejection [8] , drag reduction due an increase in the thickness of the viscous sublayer [9], and shear stress reduction due to the decrease of cross-flow fluctuation [10] underlying the hydrodynamics of fast-swimming sharks and providing insight for potential biomimetic designs that can utilize the same passive flow control mechanism to delay flow separation in both water and air applications.
Boundary Layer, Flow Control, Passive mechanism, Separation Control, Shark Skin, Turbulent Flow
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