Fluid Drag Reduction with Shark‐Skin Riblet Inspired Microstructured Surfaces

Abstract

Engineering marvels found throughout living nature continually provide inspiration to researchers solving technical challenges. For example, skin from fast-swimming sharks intrigue researchers since its low-drag riblet microstructure is applicable to many low drag and self-cleaning (antifouling) applications. An overview of shark skin related studies that have been conducted in both open channel (external) and closed channel (internal) fl ow experiments is presented. Signifi cant work has been conducted with the open channel fl ow, and less with closed channel fl ow. The results provide design guidance when developing novel low drag and self-cleaning surfaces for applications in the medical, marine, and industrial fi elds. Experimental parameters include riblet geometry, continuous and segmented confi gurations, fl uid velocity (laminar and turbulent fl ow), fl uid viscosity (water, oil, and air), closed channel height dimensions, wettability, and scalability. The results are discussed and conceptual models are shown suggesting the effect of viscosity, coatings, and the interaction between vortices and riblet surfaces. FEATURE ARTICLEeffectively reduce drag in open and closed channel fl ow, although limited data is available with closed channel. Furthermore, closed channel experiments have been conducted to study the neighboring wall effects by using so-called microsized closed channels. In open channel, drag has been measured using water, [47][48][49][50][51] oil, [52][53][54][55] and air. [ 39 , 53 , 56-64 ] Similarly, in closed channel fl ow, drag has been measured using water, [ 47 , 51 , 65,67 ] oil, [ 68 ] and air. [ 67 , 69,70 ] Both drag (for open channel) and the pressure drop (for closed channel) measurements characterize the riblet drag reduction effi ciency.Previous experiments have utilized a variety of riblet geometries, confi gurations, materials, fl uids, and fl ow conditions (laminar and turbulent fl ow). Geometries include blade, sawtooth, scalloped, and bullnose geometries with continuous and segmented (aligned and staggered) confi gurations in water, oil, and air. Open channel oil experiments with metal riblets show drag reduction of nearly 10%, [ 52 ] whist closed channel order to catch prey. [ 40,41 , 43 ] The subsequent increased fl uid fl ow velocity at the skin reduces microorganism settlement time and promotes antifouling. [ 37,38 , 42 ] In addition, microorganisms larger than the spacing between riblets are unable to effectively adhere to and ultimately colonize the skin, which further promotes antifouling. [ 5 , 44-46 ] Low drag and antifouling surfaces have been the subject of much experimentation using shark skin riblet-inspired microtextured surfaces. An ideal surface would withstand harsh environments, adhere to a variety of substrates, combine both low drag and antifouling properties, and be relatively inexpensive.Determining the optimal riblet surface morphology for maximum drag reduction has been the focus of many efforts. Experimental results indicate that shark skin in...

Rice and Butterfly Wing Effect Inspired Low Drag and Antifouling Surfaces: A Review

“…5,[43][44][45] Sharks remain clean due to their microstructured riblets, flexion of dermal denticles, and a mucous layer. 1,[46][47][48][49][50] Lower drag is necessary for shark survival, since it allows sharks to swim faster in order to catch prey. 47,48,50,51 Increased fluid flow velocity at the skin reduces microorganism settlement time and promotes antifouling, 44,45,49 along with riblet spacing smaller than microorganisms.…”

Discovery of riblets in a bird beak (Rynchops) for low fluid drag

“…Nondimensionalized parameters are denoted with a superscript + [6,40]. Similarly, riblet height can be non-dimensionalized using [15] …”

“…Scientists can use these solutions as inspiration for innovative engineering designs. For example, Nature has Riblets on the skin of a mako shark (adapted from [15]) shown at two magnifications. (c) Vortices generated in turbulent flow shown on a flat and on a shark-inspired riblet surface (adapted from [25]).…”

“…In the industrial field, low-drag surfaces could be applied to gas distribution and petroleum pipelines to decrease the required pumping energy [7][8][9]. The skin of fast-swimming sharks is a design inspiration for low-drag surfaces [6,[10][11][12][13][14][15][16][17][18][19]. Sharks are able to move quickly through turbulent water flow using a low energy input due to the surface features on their scales.…”

“…[15]. The skimmer gets its name from its unique feeding position, known as skimming, where the bird glides very close to the surface of the water with its lower beak submerged [26][27][28]34,35].…”