Abstract
We investigate the influence of smooth and ribletted shark skin on a turbulent boundary layer flow. Through laser Doppler anemometry (LDA) the role of riblets in combination with the shark skin denticle is established for the first time. Our results show that smooth denticles behave like a typical rough surface when exposed to an attached boundary layer. Drag is increased for the full range of tested dimensionless denticle widths, w + ≈ 25–80, where w + is the denticle width, w, scaled by the friction velocity, u τ , and the kinematic viscosity, ν. However, when riblets are added to the denticle crown we demonstrate there is a significant reduction in drag, relative to the smooth denticles. We obtain a modest maximum drag reduction of 2% for the ribletted denticles when compared to the flat plate, but when compared to the smooth denticles the difference in drag is in excess of 20% for w + ≈ 80. This study enables a new conclusion that riblets have evolved as a mechanism to reduce or eliminate the skin friction increase due to the presence of scales (denticles). The combination of scales and riblets is hydrodynamically efficient in terms of skin-friction drag, while also acting to maintain flow attachment, and providing the other advantages associated with scales, e.g. anti-fouling, abrasion resistance, and defence against parasites.
Highlights
Ac pte an us cri pt ce sample, highlighting key features of typical dermal denticles
An field measurements or adopted numerical techniques in order to establish which fluid dynamic mechanisms lead to increased/decreased drag (e.g. Lee and Lee, 2001; Garcia-Mayoral and Jimenez, 2011). To address these issues we present the first flow field measurements of a boundary layer flow over arrays of replica shark skin denticles, using two-component
While typically used for identifying wall effects for hot wire anemometry, we found the technique useful in identifying regions of the boundary layer which were affected by near wall reflections
Summary
Moulding techniques have seen recent success for the fabrication of shark skin surfaces: Tightly packed and overlapping denticles have been moulded and cast by Zhang et al (2011a), Zhang et al (2011b), and Chen et al (2014), leading to a maximum drag reduction of 8 % to 12 %. Two types of denticle were created using Blender (2017) CAD software; one based on Poracanthodes sp., an early fossil ancestor of sharks (Brazeau, 2009), and another based on the same denticle but with makoinspired riblets added to its crown. While loosely packed scales are still common on sharks
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