On the influence of biomimetic shark skin in dynamic flow separation

Abstract

The effect of shark skin on the boundary-layer separation process under dynamic conditions (maneuvers) has been studied experimentally. We use a foil covered with biomimetic shark skin to explore how this type of surface impacts boundary-layer dynamics in both steady and accelerating conditions. The effect of denticles is assessed via particle image velocimetry in the wake. It is shown that dynamic conditions and small-scale disturbances can mitigate boundary-layer separation through instantaneous modification of the local pressure-gradient distribution. For instance, the region of favourable pressure gradient can be extended by accelerating the foil. The acceleration results in a thinner separated shear layer on the foil surface when compared to the steady reference case. This remarkable difference indicates that local roughness (introduced through for instance biomimetic shark skin) may trigger an interaction with relatively large-scale structures in the boundary layer for effective boundary-layer control during unsteady propulsion and maneuvering.

Dynamic separation on an accelerating prolate spheroid

“…For all θ, acceleration causes the pressure-gradient field to be transformed from APG to FPG. Furthermore, the strength of the streamwise FPG is increased with increasing a * , which is in good agreement with the theoretical predictions based on potential theory for an accelerating sphere (Fernando et al 2017) and for an accelerating flat plate at incidence (Guo et al 2021), where the strength of the FPG was shown to be linearly dependent on the acceleration magnitude. In contrast, the pressure-gradient field is still covered by the streamwise APG for all decelerations and the strength of the APG increases also with increasing deceleration magnitude.…”

“…2017) and for an accelerating flat plate at incidence (Guo et al. 2021), where the strength of the FPG was shown to be linearly dependent on the acceleration magnitude. In contrast, the pressure-gradient field is still covered by the streamwise APG for all decelerations and the strength of the APG increases also with increasing deceleration magnitude.…”

Focus on Bioinspired Textured Surfaces toward Fluid Drag Reduction: Recent Progresses and Challenges

“…In contrast to smooth walls, the shark skin delays the flow separation point under laminar and turbulent conditions. [37,38] A scanning electron microscope and confocal microscope were adopted to survey the specific parts of dermal denticles by Patricia et al [39] Based on the ratio of length to width, three kinds of microstructures were defined to probe the influence of different shapes and inclination angles on drag. As a result, the higher is the denticle tilt at the same flow rate, the lower is the velocity field above the tips.…”

Characterisation of unsteady separation in a turbulent boundary layer: Reynolds stresses and flow dynamics

“…In many geophysical and cardiovascular flows, and in flows around flying or swimming animals, the turbulent boundary layer (TBL) formed on the surface of an object is subject to a time-varying, non-zero pressure gradient (Li et al 2007;Momen & Bou-Zeid 2017;Guo et al 2021). Changes in curvature or variation of the freestream pressure distribution might cause the boundary layer to thicken and detach from the surface under the effect of a strong enough deceleration (or adverse pressure gradient, APG).…”

“…2007; Momen & Bou-Zeid 2017; Guo et al. 2021). Changes in curvature or variation of the freestream pressure distribution might cause the boundary layer to thicken and detach from the surface under the effect of a strong enough deceleration (or adverse pressure gradient, APG).…”