Fluid flow analysis of a shark-inspired microstructure

Abstract

The scales of fast-swimming sharks contain riblet structures with microgrooves, aligned in the direction of fluid flow, that result in water moving efficiently over the surface. In previous studies, these riblet structures have shown a drag reduction of up to 10 % when compared with a smooth, flat surface. These studies have suggested two prevalent drag-reduction mechanisms which involve the effect of vortices and turbulence fluctuations. To further explore relevant mechanisms and study the effect of riblet geometry and flow properties on drag, vortices and turbulence fluctuations, various shark-skin-inspired riblet structures were created using computational models in which velocity, viscosity, spacing, height and thickness parameters were independently modified. A relevant mechanism of drag reduction is discussed to relate riblet parameters and flow properties to drag change and vortex size. Modelling information will lead to a better understanding of riblets and allow for optimum drag-reducing designs for applications in marine, medical and industrial fields.

Investigation on the Mechanism of Drag Modification over Triangular Riblets

“…This setup corresponded to x-anddomain size (λ + and λ + , normalized by u * and ν), of 3267 and 1045 wall units, satisfying the mini-mum scale requirements (300 and 100 wall units in x-and -directions) for investigating wall turbulent flows (Jim énez and Moin (1991), Martin and Bhushan (2014)). In addition, a smaller computa-tional domain was used to study the effects of ri-blets in previous studies (Choi et al (1993), Martin and Bhushan (2014), Ng et al (2016)). Obviously, it was reasonable that the present domain size was sufficient to properly capture the turbulent flow over smooth and riblet wall surfaces.…”

“…So far, four kinds of explanations about the drag reduction mechanism of riblets have been publically re-ported: (1) spanwise inhibition, i.e., the spanwise movements of streamwise vortices were inhibited by riblets, which impaired the turbulent momentum transport near the wall surface and thereby reduced the skin-friction (Choi (1989), Chu and Karniadakis (1993), Monfared et al (2019)); (2) existence of protrusion height, i.e., the existence of the distance between virtual origin and riblet tip suppressed the spanwise shift of the viscous sublayer where the riblets were submerged, resulting in more stable low-speed streaks and lower bursting frequencies (Bechert and Bartenwerfer (1989), Luchini et al (1991), Gr u neberger and Hage (2011));(3) creation of a secondary vortex, i.e., a couple of secondary vortices generating near the riblet tip weakened the main streamwise vortical flow in the turbulent boundary layer, which retarded the strong interactions among neighboring streaks and subsequently kept a quasistatic flow in rib valley (Chu and Karniadakis (1993), Bacher and R. Smith (1986), Boomsma and Sotiropoulos (2016)); (4) uplifting of streamwise vortex, i.e., the ribs might uplift the streamwise vortices to decrease the ejection and sweeping events which were related with the high shear stress force in turbulent flows. Thus, the viscous drag force was reduced (Lee and Lee (2001), Martin andBhushan (2014), Huang et al (2016)). As for the 3D case, a real shark skin morphology was modeled and the interpretations on the drag reduction mechanism were more complicated, including the inhibition effect of micro/nano structured morphology on the turbulence, the influence of scale's attack angles, nano-long chains and the boundary layer slipping due to the super-hydrophobicity (Chen et al (2014), Wen et al (2014), Boomsma and Sotiropoulos (2016)).…”

“…Even so, the physical mechanism of the drag reduction by riblets has still been inconclusive. One main reason is the fact that most previous research only focused on the understanding of the riblet effect on the total viscous drag over the wall surface, while few discussed the effects of the microscopic flow structures around riblets, especially the vortex structures concerned with the evolving flow field, on the local distribution of skin-friction (Bechert et al (1997), Garcıá-Mayoral and Jiménez (2011), Martin and Bhushan (2014), Benschop and Breugem (2017)). Further investigating these ignored points will help to get a better understanding on the drag reduction mechanism of riblets.…”

Drag reduction in a thermally modulated channel

“…The wall topography, application of suction/blowing and/or use of plasma-, soundor piezo-driven actuators represent commonly used techniques. New ideas are sought from biological systems as natural evolution must have led to the optimization of their performance (Martin & Bhushan 2014).…”

Designing bioinspired superoleophobic surfaces

“…1 Sharkskin scales contain riblets with micro-groves aligned to the direction of flow, which help reduce drag underwater. 2 And there are numerous examples of water-repellent surfaces in plant and animal species. This review will investigate how lessons are being learnt from nature to develop surfaces that are repellent to not just water, but oils as well.…”