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Journal ArticleDOI

Experiments with three-dimensional riblets as an idealized model of shark skin

03 May 2000-Experiments in Fluids (Springer-Verlag)-Vol. 28, Iss: 5, pp 403-412
TL;DR: In this article, a 3D-riblet surface with sharp-edged fin-shaped elements arranged in an interlocking array was investigated and the turbulent wall shear stress on this surface was measured using direct force balances.
Abstract: The skin of fast sharks exhibits a rather intriguing three-dimensional rib pattern. Therefore, the question arises whether or not such three-dimensional riblet surfaces may produce an equivalent or even higher drag reduction than straight two-dimensional riblets. Previously, the latter have been shown to reduce turbulent wall shear stress by up to 10%. Hence, the drag reduction by three-dimensional riblet surfaces is investigated experimentally. Our idealized 3D-surface consists of sharp-edged fin-shaped elements arranged in an interlocking array. The turbulent wall shear stress on this surface is measured using direct force balances. In a first attempt, wind tunnel experiments with about 365,000 tiny fin elements per test surface have been carried out. Due to the complexity of the surface manufacturing process, a comprehensive parametric study was not possible. These initial wind tunnel data, however, hinted at an appreciable drag reduction. Subsequently, in order to have a better judgement on the potential of these 3D-surfaces, oil channel experiments are carried out. In our new oil channel, the geometrical dimensions of the fins can be magnified 10 times in size as compared to the initial wind tunnel experiments, i.e., from typically 0.5 mm to 5 mm. For these latter oil channel experiments, novel test plates with variable fin configuration have been manufactured, with 1,920–4,000 fins. This enhanced variability permits measurements with a comparatively large parameter range. As a result of our measurements, it can be concluded, that 3D-riblet surfaces do indeed produce an appreciable drag reduction. We found as much as 7.3% decreased turbulent shear stress, as compared to a smooth reference plate. However, in direct comparison with 2D riblets, the performance of 3D-riblets is still inferior by about 1.7%. On the other hand, it appears conceivable, with a careful design of the fin shape (possibly supported by theory), that this inferiority in performance might be reduced. Nevertheless, at present, it seems to be rather unlikely, that 3D-riblets can significantly outperform 2D-riblets. Finally, one interesting finding remains to be mentioned: The optimum drag reduction for short 3D-riblets occurs at a lower rib height than for longer 3D-riblets or for infinitely long 2D-riblets. The same observation had been made previously on shark scales of different species with differing rib lengths, but no explanation could be given.
Citations
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Journal ArticleDOI
TL;DR: In this paper, the theoretical mechanisms of the wetting of rough surfaces are presented followed by the characterization of natural leaf surfaces and a comprehensive review is presented on artificial super-hydrophobic surfaces fabricated using various fabrication techniques and the influence of micro-, nano-and hierarchical structures on superhydrophobicity, self-cleaning, low adhesion, and drag reduction.

1,610 citations


Cites background from "Experiments with three-dimensional ..."

  • ...These grooved scales reduce the formation of vortices present on a smooth surface, resulting in water moving efficiently over their surface [12]....

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Journal ArticleDOI
TL;DR: In this article, the authors review the experimental evidence on turbulent flows over rough walls and discuss some ideas on how rough walls can be modeled without the detailed computation of the flow around the roughness element.
Abstract: ▪ AbstractWe review the experimental evidence on turbulent flows over rough walls. Two parameters are important: the roughness Reynolds number ks+, which measures the effect of the roughness on the buffer layer, and the ratio of the boundary layer thickness to the roughness height, which determines whether a logarithmic layer survives. The behavior of transitionally rough surfaces with low ks+ depends a lot on their geometry. Riblets and other drag-reducing cases belong to this regime. In flows with δ/k ≲ 50, the effect of the roughness extends across the boundary layer, and is also variable. There is little left of the original wall-flow dynamics in these flows, which can perhaps be better described as flows over obstacles. We also review the evidence for the phenomenon of d-roughness. The theoretical arguments are sound, but the experimental evidence is inconclusive. Finally, we discuss some ideas on how rough walls can be modeled without the detailed computation of the flow around the roughness element...

1,389 citations

Journal ArticleDOI
TL;DR: In this paper, an overview of self-cleaning surfaces inspired by nine biological objects is provided: lotus leaves, rice leaves, cicada wings, butterfly wings, snail shell, fish scale, shark skin, pitcher plant, and photosynthesis.
Abstract: Self-cleaning methods currently employed are based on understanding of the functions, structures, and principles of various objects found in living nature. Three types of surfaces, including superhydrophobic, superoleophobic, and superhydrophilic, offer solutions to keep a surface clean. In this review article, an overview of self-cleaning surfaces inspired by nine biological objects is provided: lotus leaves, rice leaves, cicada wings, butterfly wings, snail shell, fish scale, shark skin, pitcher plant, and photosynthesis. These surfaces exhibit special properties such as low adhesion, low drag, anisotropic wetting, anti-reflection, directional adhesion, anti-fouling, photocatalysis, self-sterilizing, and anti-fogging. We discuss the differences between the superhydrophobic and superhydrophilic surfaces and perspectives for self-cleaning surfaces in the future.

672 citations

Journal ArticleDOI
TL;DR: The skin of fast-swimming sharks exhibits riblet structures aligned in the direction of flow that are known to reduce skin friction drag in the turbulent-flow regime.
Abstract: The skin of fast-swimming sharks exhibits riblet structures aligned in the direction of flow that are known to reduce skin friction drag in the turbulent-flow regime. Structures have been fabricated for study and application that replicate and improve upon the natural shape of the shark-skin riblets, providing a maximum drag reduction of nearly 10 per cent. Mechanisms of fluid drag in turbulent flow and riblet-drag reduction theories from experiment and simulation are discussed. A review of riblet-performance studies is given, and optimal riblet geometries are defined. A survey of studies experimenting with riblet-topped shark-scale replicas is also given. A method for selecting optimal riblet dimensions based on fluid-flow characteristics is detailed, and current manufacturing techniques are outlined. Due to the presence of small amounts of mucus on the skin of a shark, it is expected that the localized application of hydrophobic materials will alter the flow field around the riblets in some way beneficial to the goals of increased drag reduction.

551 citations

Journal ArticleDOI
TL;DR: A biophysical model of the interactions between bacterial cells and cicada wing surface structures is proposed, and it is shown that mechanical properties are key factors in determining bacterial resistance/sensitivity to the bactericidal nature of the wing surface.

482 citations


Cites background from "Experiments with three-dimensional ..."

  • ..., sharks (6,7), cicadae (8), butterflies (9), termites (10), mosquitos (11), and geckos (12)) and plants (e....

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References
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Journal ArticleDOI
TL;DR: In this article, NASA-Langley research efforts planned at NASA Langley in view of results obtained to date in passive turbulent drag reduction experiments are discussed, including heat transfer-augmentation, noise-reduction, turboprop/fuselage interaction noise reduction, and other advantages.

107 citations