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Marine drag reduction of shark skin inspired riblet surfaces

Y.F. Fua,b, C.Q. Yuana,b,n, X.Q. Baia,b

aReliability Engineering Institute, National Engineering Research Center for Water Transportation Safety, Wuhan 430063, ChinabKey Laboratory of Marine Power Engineering & Technology (Ministry of Transport), Wuhan University of Technology, Wuhan 430063, China

Abstract

Shark skin inspired riblet surfaces have been known to have drag reduction effect for the over past 40 years. It first drew the attention from theaircraft industry. With the property of low drag and self-cleaning (antifouling), shark skin inspired riblet surfaces can also be used on navigationobjects. In this paper, different marine drag reduction technologies are discussed, and a review of riblet performance studies is also given.Experimental parameters include riblet geometry, continuous and segmented configurations, fluid velocity (laminar and turbulent flow), fluidviscosity (water, oil and gas), and wettability are analyzed. However, force is obtained by area-weighted integral of shear stress distributions. Soarea of riblet surfaces is a crucial factor which has not been considered in many previous studies. An experiment is given to discuss the impact ofarea. This paper aims not only to contribute to a better understanding of marine drag reduction, but also to offer new perspectives to improve thecurrent evaluation criteria of riblet drag reduction.& 2017 Southwest Jiaotong University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Marine; Shark skin; Drag reduction; Riblet; Area

1. Introduction

Navigation objects in the ocean, such as ship, submarine andtorpedo play important roles in the development and defense ofthe ocean economy. Traveling speed and energy consumptionare significant parameters which determine the operatingperformance of navigation objects. It is very crucial forwarship to improve speed, and it is economical for themerchant marine to save energy consumption. Because theconsumption of fossil fuel has been increasing with economicgrowth, severe energy crisis and environmental pollution havebecome intractable social problems in China during recentyears. As an important mode of transportation, navigationobjects consume a large amount of heavy fuel oil (HFO). In2011, International Maritime Organization (IMO) adopted twostandards Energy Efficiency Design Index (EEDI) and ShipEnergy Efficiency Management Plan (SEEMP), which

10.1016/j.bsbt.2017.02.00117 Southwest Jiaotong University. Production and hosting by Elsemmons.org/licenses/by-nc-nd/4.0/).

nce to: Reliability Engineering Institute, National Engineeringr for Water Transportation Safety, Wuhan 430063, China.549879.ss: [email protected] (C.Q. Yuan).nder responsibility of Southwest Jiaotong University.

article as: Y.F. Fu, et al., Marine drag reduction of shark skin in2017.02.001

sparkplug the concept of ‘green ship’. Marine vessel resistancecontains pressure drag, skin friction (viscous) and wave-making resistance. Friction resistance of high speed shipsaccounts for 40%-50% of total resistance, while it will increaseto 70%-80% for low speed ships. So it is of significance toemphasize the researches directed to viscous drag reduction,which will can realize speed improvement and energy con-servation at the same time.Biologically inspired design, adaptation, or derivation from

nature is referred to as ‘biomimetics’, which means mimickingbiology or nature [1]. Many creatures in nature have gonethrough evolution over the past millions of years, so they havegood adaptability to the environment. By studying and under-standing the mechanisms of natural phenomena, human maybe able to reproduce these phenomena on demand [2]. Sharksare dangerous predators, as they swim very fast in the ocean.Inspired by this, researchers found that the shark skin is notsmooth surface. It is a kind of riblet surfaces. On the surfacesof shark skin, there are many micro-scales called dermaldenticles. It was reported that the riblets can prevent theformation of vortices or keep the vortices off the surfaces,which result in water moving easily over the shark skin.Moreover, the structures can also protect skin surfaces from

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Fig. 1. Different marine drag reduction technologies.

Y.F. Fu et al. / Biosurface and Biotribology ] (]]]]) ]]]–]]]2

marine fouling and play a role in the defense against adhesionand growth of marine organisms. In general a smooth surfacehas less fouling than a rough one [3]. This is due to the lowersurface area of a smooth surface, which causes the forcesbetween the fouling species and the substratum to be weaker,but the smoothness of the underlying substratum has no impactafter the initial layer of fouling film has settled. As biofoulingis another problem in the shipping industry, low drag andantifouling applications are crucial for ships [4].

It can be seen that the characteristics of shark skin are ofgreat scientific and technological relevance. On the basis of theunderstanding of shark skin, a lot of studies and imitations ofthe riblets, which are referred to as ‘biomimetics’, have beencarried out by researchers [5]. Initial studies on riblets wereconducted by Walsh [6,7] at NASA Langley Research Center,where riblet technology were first machined on aluminumsheets for use on aircrafts. He focused on optimizing the ribletsize and shape for a maximum drag reduction. Different shapesincluding the triangular, rectangular, trapezoidal, sawtooth andscalloped were also studied. Despite the ongoing research onriblets, challenges such as finding the optimal riblet size andantifouling still remain.

As drag force is equal to integrating shear stress with area,both of the two factors, shear stress and area, should be takeninto consideration. In previous studies, many researchers triedto suppress the turbulence intensity and reduce the averageshear stress. In fact, with the decrease of average shear stress,the area of riblet surface will increase. On the other hand,different parts of the riblet surface have different shear stresses.Since the higher velocity vortices interact only with a smallsurface area at the riblet tips, only this localized areaexperiences high shear stresses. The low velocity fluid flowin the valleys of the riblets produces very low shear stressesacross the majority of the surface of the riblet. By keeping thevortices above the riblet tips, the cross-stream velocityfluctuations inside the riblet valleys are much lower than thecross-stream velocity fluctuations above a flat plate. Thisdifference in cross-stream velocity fluctuations is evidence ofa reduction in shear stress and momentum transfer near thesurface, which minimizes the effect of the increased surfacearea [8].

Please cite this article as: Y.F. Fu, et al., Marine drag reduction of shark skin in10.1016/j.bsbt.2017.02.001

In this paper, firstly, various kinds of marine drag reductiontechnologies are summarized. Secondly, different factorseffected the drag reduction performance of riblet surfaces areexplored and discussed from different aspects. Lastly, anexperiment is presented to investigate the effect of area ondrag reduction of riblet surfaces.

2. Marine drag reduction technologies

Drag of marine vessel has three main forms, includingpressure drag, skin friction (viscous drag) and wave-makingresistance. In general, pressure drag depends on the stream-lined geometry of the ship traveling through the fluid, whileskin friction drag depends on the surface morphology. Andwave-making resistance has a close relationship with shipwaves and velocity. Among them, skin friction accounts for agreat proportion of the total drag. Friction or viscous drag iscaused by the interactions between the fluid and a surfaceparallel to the flow, as well as the attraction between moleculesof the fluid. The drag on an object is, in fact, a measure of theenergy required to transfer momentum between the fluid andthe object to create a velocity gradient in the fluid layerbetween the object and undisturbed fluid away from theobject’s surface [9].In order to reduce the drag, many technologies are applied in

this field, including active drag reduction, passive dragreduction and composite drag reduction, as shown in Fig. 1.

2.1. Bionic jet surface drag reduction

Sharks have the ability to move very fast especially whenthey are preying, and the burst speed of deep-sea sharks canreach up to 10–20 m/s. It is observed that the front body sidegills of sharks are lined with large panels with each sidebearing 5–7 gill slits. When sharks swim, they inhale waterthrough the half-open mouth, and discharges water from thegills for gas exchange. This will not only affect the process ofbreathing, but also help to reduce the resistance of motion. Thejet surface helps to increase the thickness of the low flow field,and the corresponding speed gradient causes reduction of thelower shear stress and friction. Subsequently, as a vortex






Y.F. Fu et al. / Biosurface and Biotribology ] (]]]]) ]]]–]]] 3

appears downstream of the jet hole at a low swirl rate, theviscous layer of the boundary increases and the shear stressdecreases, which shows the drag reduction state [10]. The dragreduction characteristics of bionic jet surface have beenverified according to the experimental view, and numericalsimulation results were also compared and evaluated. Thebionic jet surface can be used on mini underwater vessel toachieve drag reduction.

2.2. Heating wall drag reduction

In 1942, Frick [11] found that transition of boundary layerfrom laminar flow to turbulent flow on heating wall becameearlier in the wind tunnel experiment. Inspired by thisphenomenon, it is assumed that heating wall can have dragreduction effect conversely. Because the viscosity of waternear the wall will reduce, and the velocity distribution near thewall can become more stable and uniform.

In living nature, it is also believed that there is a closerelationship between low drag dolphin surface and its body heat,which will result in the laminarization of the boundary layer. In1968 and 1970, Wazzan, Okamun and Smith [12] verified thisassumption by analytic computing. Ross [13] and Nice [14]validated the numerical solution from Orr-Sommerfeld equation.The American Institute of Navy Vessels Research and Develop-ment Center also made a large number of researcher on heatingwall drag reduction with the cooperation of Case WesternReserve University. With the application of heating wall, thetorpedo had a drag reduction ratio of 42.3% in the velocity of26 m/s. However, as there is a huge area of the ship hull, heatingthe ship hull will also consume a considerable amount of energy.So this will offset a part of drag reduction effect.

2.3. Wall vibration drag reduction

Since the first study by Jung et al. [15], the action of the wallhas been correctly recognized as being effective in thesuppression of the burst-sweep activity. Laadhari et al. [16]were the first ones to postulate that the reduction of turbulencelevel caused by the disruption of the coherent interactionsbetween the quasi-streamwise vortices and the underlying lowvelocity streaks. In the spirit of these pioneering investigations,the interest on the subject has amply grown in the past fewyears, and the picture of the physical modifications induced bythe wall movement has been subsequently supported, con-firmed and expanded [17,18]. Both experimental and numer-ical results confirmed the oscillating wall was an effectivevehicle for producing a drag reduction effect. According tomany authors, the mean streamwise velocity gradient at thewall can be reduced by as much as 40%. In the case ofstreamwise homogeneous wall motion, the skin friction wasobserved to drop by a maximum of around 35%.

2.4. Air bubbles drag reduction

From the surface of an object in a fluid flow, each fluid layerhas higher velocity than another until a layer is reached where


the fluid has velocity equal to the mean flow. The boundaryhas an important effect on the fluid drag. Fluids of higherviscosity—the attraction between molecules—have higherapparent friction between fluid layers, which increases thethickness of the fluid layer distorted by an object in a fluidflow. For this reason, more viscous fluids have relativelyhigher drag than less viscous fluids. The viscosity of air ismuch less than that of water. Such property has promisingapplication in vessel speed improvement and energy consump-tion saving [19]. The formation of a layer of gas at or near theboundary between solid and liquid via using the gas injection[20,21] or other methods [22–25] can achieve effective dragreduction.Three physically different air bubble drag reduction techni-

ques have been identified: small bubble, air film and air cavitymethods. The small bubble method, which utilizes functions ofinjected micro- to sub-millimeter bubbles. It has two advan-tages. One is that without requiring any change in the form ofthe ship’s hull. Another one is its large impact to the dragreduction ratio per void fraction supplied into the boundarylayer. It reported approximately 10–15% saving of the totalenergy consumption for an experimental ship using smallbubble methods. Air film is hard to maintain for high speedvessel under high pressure, so it gets less attention. Air cavityis used to improve the navigation speed of underwaterprojectile and torpedo. With the appearance of high-speedsupercavitation torpedo developed in Russia, breaking thelimitations of the conventional methods, successfully enhan-cing the speed of a submerged navigation body, and a verygratifying effect is completed. The speed was up to 200 knots.For active approaches, however, extra gas-providing devices

or energy are essential for an effective drag reduction, whichraises costs and limits their applications. So the superhydro-phobic surface has the instinctive capability of holding airpockets in gaps at the solid–liquid interface when it issubmerged in a liquid even without adding any extra energyand gas-providing devices. This method is a kind of compositedrag reduction, which combines structured hydrophobic sur-faces and air bubbles drag reduction. However, it is hard tosteadily sustain air pockets, particularly in the condition ofliquid owing at a high speed and a high liquid pressure [19].

2.5. Compliant wall drag reduction

The first experimental studies were carried out by Kramer[26], who examined the possibility that the compliance ofdolphins’ skins could result in delay of the laminar to turbulenttransition and realize drag reduction. It is found that a turbulentboundary layer can result in more than double the drag of alaminar one, so one way of achieving drag reduction is tosuppress the laminar to turbulent transition of the boundarylayers. Theoretical studies were carried out by Benjamin andLandahl, who used a linear stability analysis to examinewhether transition could be delayed. The compliant wall usedin some studies was a soft coating such as PVC plastisol. Thethickness of the coating and its modulus of rigidity, indicatinga coupling between the flow dynamics and the dynamics of the







surface all have an effect on drag reduction. However,compliant surfaces have the advantages of being passive,simple, inexpensive and easy to retrofit to existing vessels[27]. The challenge is that it is hard to keep the compliant wallwork for a long time. When the compliant wall hardens, it willlose its drag reduction performance.

2.6. Hydrophobic/ super hydrophobic coating

Over the past few decades, researchers have consistentlymade efforts to devise ways by which this skin-friction dragcan be effectively reduced as the fluid passes over a solidsurface. A suitable way to generate slip-velocity on the walls isby making use of super-hydrophobic surfaces in both laminarand turbulent flow regimes [28]. Many biological surfaces,particularly some plant leaves, exhibit remarkable non-wettingcharacteristics. The well-known super-hydrophobicity of lotusleaves have attracted a lot of attention and have generated greatinterest in fundamental research, as well as in industrialapplications. Static contact angle and contact angle hysteresisof a lotus leaf are about 1641 and 31. It is well-known thatthere are two major factors influencing the wettability of asolid surface: surface chemistry, and surface topology [29].Super-hydrophobic surfaces are engineered by taking materialswith micron-level surfaces roughness in form of rectangularridges and square posts and then coating them with hydro-phobic chemicals. Through mimicking lotus leaf structure,Rediniotis and Balasubramanian [30] have developed a hydro-phobic surface coating and experimentally investigated itseffect on drag reduction in flow over a flat plate, flow insidea pipe and flow over an elliptic model. They have measuredvelocity by means of PIV and reported that velocity is not zeroat the vicinity of the plate's coated surface; therefore, there is aslip velocity condition rather than a no-slip condition on thesuper-hydrophobic surface. This slip velocity causes the dragreduction [31].

2.7. Riblet surfaces drag reduction

Shark skin is designed with microstructured features thateffectively control naturally occurring turbulent vortices, whichleads to less momentum transfer and shear stress, and thusreduces drag. Different potential factors generate sharkskin dragreduction effect [32].The sharkskin scales can erect to somedegree during shark’s swimming, which has a thrust function,just as the oar of the boat. On one hand, the micromorphologyon the sharkskin can decrease the viscous force, and on the otherhand, the erected scales have the thrust effect like the oar.Therefore, the combined function can explain the high drag-reducing effect. Therefore much attention has been given toactual shark skin and the mechanisms at work. The most famousapplication of sharkskin in fluid engineering is sharkskinswimming suit. It is reported that Spitz was awarded seven goldmedals wearing briefs and Phelps earned 14 gold medals inAthens and Beijing in a hightech all-over bodysuit, and in 2010high-technology swimsuits were banned from competition assignificantly improving performance. In the 1984 Los Angeles


Olympics and 1987 American’s Cup, 3 M riblets were applied toU.S. boats, which presumably helped to secure victories. Manyresearches have proven the riblet surfaces can reduce drag up to10% in water.

3. Effects on drag reduction of riblet surfaces

There are many factors affecting the drag reduction perfor-mance of riblet surfaces. Each of them is analyzed below.

3.1. Turbulent and laminar

Flow regime has an important effect on drag reduction ofriblet surfaces. Many studies have shown that riblet surfacesonly have drag reduction effect in turbulent-flow regime. Oneclassical cause of increased drag that riblet surfaces exhibit isan increase in wetted surface area. At the same time, the ribletstructure will disturb the laminar and cause large stress. Asshown in Fig. 2, in the laminar-flow regime, fluid dragtypically increases dramatically with an increase in surfacearea owing to the shear stress at the surface acting across thenew and larger surface area. However, turbulent-flow regimeas vortices form above a riblet surface, they remain above theriblets, interacting with the tips only and rarely causing anyhigh-velocity flow in the valleys of the riblets. Since the highervelocity vortices interact only with a small surface area at theriblet tips, only this localized area experiences high-shearstresses. The low-velocity fluid flow in the valleys of theriblets produces very low-shear stresses across the majority ofthe surface of the riblet. By keeping the vortices above theriblet tips, the cross-stream velocity fluctuations inside theriblet valleys are much lower than the cross-stream velocityfluctuations above a flat plate [33]. It is clear in Fig. 2 that theshear stresses in the valleys are much lower than that of thetops. This difference in cross-stream velocity fluctuations is theevidence of a reduction in shear stress and momentum transfernear the surface, which minimizes the effect of the increasedsurface area. Though the vortices remain above the riblet tips,some secondary vortex formations do occur that enter the ribletvalleys transiently. The flow velocities of these transientsecondary vortices are such that the increase in shear stresscaused by their interaction with the surface of the riblet valleysis small [9].The numerical simulations and the fluid experiments have

confirmed that if the thickness of the viscous sublayer isgreater than the height of riblets, all small convex parts can becompletely incorporated into the viscous sublayer. Under sucha condition, the interface can be called hydrodynamicallysmooth, the friction force will be transformed into the viscousresistance, which cannot be reduced further both in turbulentand laminar regime, and the traditional concept cannot workany longer.

3.2. Geometries and configurations

Real shark skin has complex three dimensional geometriesand configurations, so in previous studies the shark skin is






Fig. 2. Counters of wall shear stress of riblet surfaces and smooth surfaces in different flow regime.


simplified as artificial riblet samples include various combina-tions of V, L, U, \ , space-V geometries with continuous andsegmented (aligned and staggered) configurations [4]. Thesizes of riblets vary from species to species and from locationto location. For example, the Spiny Dogfish shark Squalusacanthias has triangular shaped riblets with a base width of100–300 mm, summit radius of about 15 mm, height of 200–500 mm, and spacing of 100–300 mm [34]. In the study the sizeof riblets are among the range. Some typical geometries andconfigurations are shown in Figs. 3 and 4. Dimensions ofinterest include riblet height (h), spacing (s), and thickness (t).

In order to compare the difference between the 5 kinds ofriblet geometries, numerical simulation are conducted by usingFLUENT. The same parameters of grids, feature size, initialconditions and boundary conditions are set. The initial velocityU0 is 5 m/s, and riblet spacing (s) is 0.1 mm. The counters ofturbulent kinetic energy distribution are shown in Fig. 5.

According to the numerical results, the average turbulentkinetic energy of smooth surface is 0.2413 m2/s2. It is obviousthat V, L, U, \ , space-V geometry surfaces have differentturbulent kinetic energy on riblet tips and riblet valleys (Table1). The turbulent kinetic energy on riblet tips is much higherthan that on riblet valleys. Only riblet tips of \ geometrysurfaces have higher value than smooth surfaces.

Table 2 shows that with the same s and U0 (s¼0.1 mm,U0¼5 m/s), the L geometry surface has best drag reductionperformance. While the \ geometry surface has increaseddrag. However, the L geometry surface is difficult to befabricated and to maintain. So the V geometry surface is morepractical and has high drag reduction ratio at the same time.The results have a good agreement with previous studies. Thebest values have been achieved with infinitely-thin bladeriblets giving about 10% reduction for an optimized config-uration; while more realistic cross-sections for industrial use,such as triangular, V-groove or scalloped, usually account for4–8% drag reduction [36–39].

In order to understand the effects of riblet configurations,namely continuous and aligned segmented as shown in Fig. 4,a series of experiments were conducted. The results show the


continuous sample provide higher drag due to the increasedwetted surface area. Each experiment with the continuoussample exhibits a higher drag as compared to the segmentedsample [4].Straight and sinusoidal riblets are also discussed in some

literature [40,41]. If sinusoidal-like riblets are used instead ofconventional straight riblets, the drag reduction ratio can beincreased by at least 50%. It is found that drag reduction withsinusoidal riblets depend strongly on the wavelength, showinga benefit over straight riblets for a larger value of thewavelength, and an opposite trend for a smaller value.With the development of bio-replication and computational

fluid dynamics (CFD), both numerical simulation and experi-mental studies of drag reduction surface of a real shark skin areconducted [8,42–47]. It is well known that the cross sectionalshape of riblets on fast swimming sharks varies greatly, even atdifferent location on the same shark. Fig. 6 shows the differentbetween the separated blade riblets on the tail of a Mako with thescale-grouped reblets on its front section, as well as themorphology that exists on various other fast swimming sharks.So the size of real shark skin is hard to decide. There are stilldifficult in building numerical model of complex real shark skinsurface and creating high precision bio-replication shark skin.The drag reduction of bio-replication shark skin has a large rangeand the maximal drag reduction rate is up to 24.6%, which ismuch higher than traditional riblet surface [45]. Some recentstudy of foils made of shark skin showed that results obtainedunder rigid conditions are substantially different from thoseobtained when shark skin is allowed to flex and curve duringswimming [47,48], suggesting that hydrodynamic effects of sharkskin not only may result from the complex 3D denticle geometrybut also are likely influenced substantially by skin deformationdue to its flexibility. So the fluid solid coupling should beconsidered in the numerical simulation study.

3.3. Viscosities (oil, water and gas)

In fact, riblet surfaces are not only used in water, but also inoil and gas. However, the dynamic viscosities of oil, water and






Fig. 4. Contious, aligned segmented, and staggered segmented configurations.

Fig. 3. Typical shark skin inspired riblet geometries that have been computa-tional evaluated.


gas are quite different. This will lead to different dragreduction performances of riblets in different fluid media.Fluids of higher dynamic viscosity have higher apparentfriction between fluid layers, which increases the thicknessof the fluid layer distorted by an object in a fluid flow. For thisreason, more viscous fluids have relatively higher drag thanless viscous fluids. A similar increase in drag occurs as fluiddynamic velocity increases. The drag on an object is in fact ameasure of the energy required to transfer momentum betweenthe fluid and the object to create a velocity gradient in the fluidlayer between the object and undisturbed fluid away from theobject’s surface [8].

So in general, object in the oil has larger drag than that inthe water. Object in the gas has lowest drag. In the field of longdistance oil pipeline, over 90% energy is lost in the form offriction loss. With the application of riblet surfaces, drag


reduction can be achieved and energy can be saved in oilpipeline. Another important application of bio-inspired shark-skin surface is the navigation, and for ship. The direct bio-replicated method has been put forward to fabricate the drag-reducing surface covering biological sharkskin morphology,and the testing results in water tunnel showed that the wallresistance could be reduced more than 12% compared to thesmooth surface [45,49]. For example, the famous swimmingathlete Ian Thorpe wearing shark-skin like swimsuit got threegold medals on Olympic Games in Sydney, since then theswimsuit has been well known in the world. Experimentalresults concerning the performance of 3M riblets on airfoils,wings and wing-body or aircraft configurations at differentspeed regimes are conducted [50]. Airbus Company affixedmicrogroove films on the 70% surface of A320 experimentalaircraft, and achieved 1–2% of the fuel-saving effect [51].Another issue is the size of riblets, which has a close

relationship with Reynolds number, speed and kinematicviscosity. V-riblets is general geometry. The use of non-dimensional characteristic dimensions for riblet studies,namely non-dimensional spacing, sþ , is important for compar-ison between studies performed under different flow conditions[9]. The optimal sþ is around 15, while the real spacings indifferent fluid media are varied. Studies in an oil channel arecarried out in flow that is both highly viscous and slowermoving. This allows for riblets to be made with spacing in the3–10 mm range [52]. Conversely, riblets used in water havemuch smaller spacing. With the velocity of 3–10 m/s, thecorresponding spacing is among 0.08–0.34 mm. While ribletsused in air flow require spacing at or below 1 mm owing to thelow kinematic viscosity of air and the high speed at whichwind tunnels must operate to create accurately measurableshear stresses on a test surface [9].

3.4. Flow direction

In fact, the riblets of real shark skin are not parallel to theflow direction totally. So some studies focus on the effect offlow direction on riblet drag reduction. Figs. 7 and 8 showstreamwise, spanwise and herringbone riblets. The fact thatribbed surfaces (riblets) aligned in the streamwise direction doreduce turbulent skin friction has been established beyond anyreasonable doubt. There are a large number of independentmeasurements available from many laboratories. These datahave been collected by various direct measurements with shear






Fig. 5. Counter of turbulent kinetic energy distribution. (a) Geometry of V (b) Geometry of L (c) Geometry of U (d) Geometry of \ (e) Geometry of Space-V.


Please cite this article as: Y.F. Fu, et al., Marine drag reduction of shark skin inspired riblet surfaces, Biosurface and Biotribology (2017), http://dx.doi.org/10.1016/j.bsbt.2017.02.001






stress balances, with pressure loss measurements in tubes andin aircraft flight experiments.

Spanwise means that the direction of riblets is vertical withthe direction of flow direction. In this case, the existence ofvortex in the riblet valley can reduce the drag. It function asantifriction bearing as shown in Fig. 9, which can reduce the

Table 1Turbulent kinetic energy distribution of different riblet geometry.

Different geometry Turbulent kinetic energy

Riblet tips(m2/s2) Riblet valleys(m2/s2)

V 0.2232 0.0182L 0.2183 0.0189U 0.2332 0.0193\ 0.2894 0.0201Space-V 0.2391 0.0194

Table 2Comparison of drag reduction ratio of different riblet geometry (s¼0.1mm,U0¼5m/s) [35].

Different geometry F-smooth F-riblets Drag reduction ratio（%）

V 0.0040836 0.0038739 5.135U 0.0041032 0.0039751 3.122L 0.0040134 0.0037872 5.636\ 0.0039823 0.0041837 -5.057Space-V 0.0039412 0.0038653 1.926

Fig. 6. Scale patterns o


friction. It is clear in Fig. 10 that the shear stresses in the valleysare much lower than that of the tops and the smooth surfaces.Based on quantitative identification of feather structure,

Chen [54] proposed one novel biomimetic drag reductionsurface structure, i.e. herringbone riblets with narrow smoothedge. The drag reduction rate of such surface with herringboneriblets was experimentally prove to be about 16%, higher thantraditional micro-groove riblet, flat plate and herringbonewithout narrow smooth edge. The shear stress and fluidvelocity vector across the surface of herringbone riblets andtraditional micro-groove riblets are shown in Fig. 11. Compar-ing herringbone riblet surface and traditional microgrooveriblet surface, it is clear that the vortices above the ribletsurface interact with the tips only, and rarely cause high-velocity flow in the valleys of riblets. Especially, the shearstress of herringbone riblets remarkably concentrates at theconverging region of herringbone riblets. Although the max-imum shear stress is the highest in the herringbone riblets,surface area experiencing high-shear stresses is just localized atimmensely small surface area around the tips of herringboneriblets.

3.5. Wettability (superhydrophobic, both micro-groove andnano-long chain drag reduction interface)

Wettability has a relationship with both material and micro-scale surface topography. Now many hydrophobic materialsare applied to the field of drag reduction. Synthetic riblet

f fast sharks [42].







surface means the combination of micro-riblets with super-hydrophobic material, which has a very large contact angle.The relative velocity between a solid wall and liquid isbelieved to be zero at the solid–liquid interface, which is theso called no-slip boundary condition. However, for hydro-phobic surfaces, fluid film exhibits a phenomenon known asslipping, which means that the fluid velocity near the solidsurface is not equal to the velocity of the solid surface. No slipwas observed on hydrophilic surfaces. Theoretical studies andexperimental studies suggest that the presence of nano-bubblesat the solid-liquid interface is responsible for boundary slip onhydrophobic surfaces [1].

With a large contact angle and the presence of nano-bubblesat the solid-liquid interface, the superhydrophobic structuredsurface can also decrease the contact area between liquid andsolid. However, despite the superhydrophobic structured sur-face is promising to hold air pockets in riblet structures, it ishard to steadily sustain air pockets, particularly in the condi-tion of liquid owing at a high speed and a high liquid pressure.As shown in Fig. 12, the inset I is the magnified figure ofregion I, and there are arcs on the edges of microgrooves withthe contact angle of θ0 and the intruding angle of β. Case Aand Case B are two different liquid–gas interfaces. The inset IIis the state of case Wang [19] has designed and fabricated a

Fig. 7. Streamwise and spanwise of riblets.

Fig. 8. Herringbon


dense (12 μm pitch) and deep (13 μm) hydrophobic transversemicrogrooved (HTM) surface, which has experimentallydemonstrated a steady drag reduction rate of 13.5870. 96%at a flow speed of 11.6 m/s.

4. Effect of area on drag reduction of riblet surfaces

As analyzed above, many factors have an effect on the dragreduction performance of riblet surfaces. In fact, force isobtained by area-weighted integral of shear stress distributions.The above researcher are mainly focused on reducing shearstress. However, area of riblet surfaces is a crucial factor whichhas not been considered in many previous studies. In the nextsection, effect of area on drag reduction of riblet surfaces isanalyzed.

e riblets [53].

Fig. 9. Schematic diagram of spanwise riblets.

Fig. 10. Comparison of wall shear stress of smooth and riblet surfaces.






Fig. 11. Shear stress and velocity vector of herringbone riblets [54].

Fig. 12. Liquid meniscus over hydrophobic microgroove that have a crosssection of a trapezoidal shape with the height of H, the slope angle of α, thegroove width of D, the intruding depth of h, and the curvature radii ofmeniscus of R [19].

Fig. 13. Schematic of the riblet structures under consideration with the heightof h, the spacing of s, the interval width of w.



4.1. Design and fabrication of riblet surfaces

To investigate the effect of area on drag reduction, ribletstructure with different interval w was designed to achieve thegoal as shown in Fig. 13. It shows the schematic of the riblet







structures under consideration with the height of h, the spacingof s, the interval width of w. The size of riblets has aremarkable impact on the effect of drag reduction. So valuesof peak-to-peak non-dimensional spacing (sþ ) and height (hþ )are among the range which have drag reduction. In this paper,graphite plate (100 mm� 100 mm� 3 mm) was chosen as themother board, and the method of laser ablation was adopted tofabricate the samples. Specific geometric parameters consid-ered in this study are shown in Table 3. Under the effect ofheat affected zone, the output power and scanning times aremost important during the laser processing, and it is difficult toobtain the completely accurate designed size of groove.However, through the reasonable setting of those parameters,the dimensional error can be controlled in a reasonable range.Fig. 14 shows the three-dimensional surface topographymeasurement of microriblets measured by Keyence digitalmicroscope VHX-5000. Table 3 gives the details of geometricparameters. Among them, h and s are constants, while w isvariable. That is the total area will decrease with the increaseof w.

4.2. Experiment method

Experiment was conducted in Qingdao Branch of LuoyangShip Material Research Institute. Fig. 15 shows the schematicof the testing device, which is mainly consist of pump, watertank, pressure differential gauge, relief valve, check valve, anti-revolving section, honeycomb, flowmeter, test section, electriccabinet and so on. The drag resistance of different samples canbe determined by pressure difference between two measure-ment points.

During the experiment, water was injected into the disposi-tion chamber through the pipe line. When the chamber wasfilled up with water, there would be a uniform water flow outof the disposition chamber along the upper line. One of thefunctions of the disposition chamber was to allow the water to

Fig. 14. Three-dimensional surface topog

Table 3Specific geometric parameters considered in this study.

Wall Type h (μm) s (μm) w (μm) sþw (μm)

SP-1 130 220 80 300SP-2 130 220 180 400SP-3 130 220 280 500SP-4 130 220 380 600


settle in the chamber and thus eliminated any disturbancescoming from the upstream. The velocities used for theexperiments were 2.5, 3.5, 4.5, 5.5 and 6.5 m/s respectively,corresponding to speed of a ship ranging from 5 to 12 knots.The flow distance from the disposition chamber to the testchamber, where the sample surface was mounted, wasapproximately 1.5 m. Accordingly, the Reynolds number atthe upstream end of the sample plate was estimated to bebetween 1.05–4.2� 106, resulting in fully turbulent flow overthe sample surface. When the system was in steady operation,the pressure drop was measured every 3 s, and in total 50 setsof data were collected (i.e. over a duration of 150 s) in eachexperiment. The average of the 50 sets of data was thencalculated, which was used to calculate the shear stress anddrag acting on the plate. The uncertainty of the pressures wasestimated to be around 3.5%.

4.3. Result and discussion

Drag reduction ratios of SP-1, SP-2, SP-3, SP-4 between thevelocity from 2.5 to 6.5 m/s are shown in Fig. 16. Dragreduction ratios have a non-linear relationship with thevelocity. It can be seen that the drag reduction rations of ribletsurfaces fluctuate marginally. Except SP-3, other riblet sur-faces SP-1, SP-2, SP-4 all get their maximal drag reductionratio at the velocity of 6.5 m/s. It is clear that the dragreduction ratio of SP-2 is higher than other riblet surfaces indifferent velocities. While SP-1 has the worst drag reductionperformance. That is to say, from SP-1 to SP-4, with theincrease of riblet interval w, the drag reduction ratio firstlyincreases, then decrease. So the drag reduction ratio does nothave a linear relation with the area.As is well-known, the wetted area of the plate with riblets is

greater than that of the flat plate. For continuous riblet surface,the wetted area of the plate with riblets is sec(a) (where01oao901, a is the included angle between hypotenuse ofriblet and level line) times greater than that of the flat plate. Andwith the increase of the riblet interval w, the wetted area of theplate with riblets will decrease. In this case, if the averagediameter of the streamwise vortices above the wall are smallerthan the spacing of the riblets, the streamwise vortices can movefreely and can be found inside the riblet valleys. This exposes alarger surface area of the riblets to the sweep motion that theyinduce. So the drag will increase. On the other hand, when theaverage diameter of the streamwise vortices above the plate are

raphy measurement of microriblets.






Fig. 15. Schematic of runner-type marine biological adhesion testing device.

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.016

17

18

19

20

21

22

23

24

25

26

27

28

29

Dra

g re

duct

ion(

%)

V (m/s)

SP-1 SP-2 SP-3 SP-4

Fig. 16. Drag reduction rations of different riblet surfaces.

Fig. 17. Schematic diagram of drag increase and reduction mechanisms byriblets.


larger than the spacing of the riblets, most streamwise vorticeswill stay above the riblets, and only a limited area of the riblettips is exposed to their induced sweep, as shown in Fig. 17. Soin order to realize drag reduction, wetted area must be taken intoconsideration. Only when a small part of the wetted surface isexposed, resulting in a net drag reduction, can the significantincrease of the wetted area be offset.

In this experiment, with the increase of the riblets interval w,the wetted area decreased, while the drag reduction effect ofriblet valleys decreased at the same time. As both of the factorshave an impact on the total drag, it is hard to say whether thedrag reduction ratio will increase or not. There must be a


balance between the two factors, so an optimal drag reductionratio will be there.

5. Summary and outlook

In tribology, the concept of effective contact area is putforward, which means only the effective contact area has effecton the force of friction. In physical significance non-contactarea contributes to nominally lubricated area. Because of theexistence of riblets, the velocity gradient at the bottom of thevalleys is considered negligible, which reduces the wallaverage velocity gradient. This means that the riblets workas an anti-friction coating (similar to lubricant): once the ribletvalleys are filled up with still water, the flow slips more easilyover the ribbed surfaces. In other words, with the increase oftotal area, the effective contact area may not increase. Forexample, retention of gases in transverse hydrophobic ribletsurfaces can achieve a drag reducing efficiency of more than13%. The existence of both gas and vortex in the riblet valleycan contribute to a slippage at liquid–gas interface whichdecreases the velocity gradient of the boundary layer and thearea of solid–liquid interface when water flows over thissurface.With different drag reduction technologies based on the

boundary layer control in turbulence discussed above, it canpredicted that if two or more technologies are combinedtogether, the drag reduction effect may be improved dramati-cally. The best way to realize drag reduction is to suppress theturbulence intensity and reduce the effective contact area at thesame time. So in the future, the hydrophobic materialcombined with micro-structured surfaces can work as sharkskin or other biomimetics inspired surfaces, which can haveboth drag reduction and antifouling properties.Sharks are study subject, while other creature like shell,

lotus leaves, and bird feather can also be studied to reduce dragof marine vessel. Nowadays, there are still many engineeringproblem needing to be solved before riblet surfaces dragreduction technology coming into practical application state.Not only because the cost of riblet surface is high, but alsobecause of the biofouling by biological and organic contami-nants. Now, the riblet surface can be used on rowing, smallunderwater vessel and torpedo. For large-scale marine vessel,only commercial coating or film can be applied in the future.







Acknowledgement

This work was supported by the National Natural ScienceFoundation of China under the projects (51379166 and 51422507).The technical assistance from Qingdao Branch of Luoyang ShipMaterial Research Institute and the assistance from Zhang Xuanwith the numerical simulation are also acknowledged.

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