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Bio-Inspired Passive Drag Reduction Techniques: A Review Hayder A. Abdulbari [1,2], *, Hassan D. Mahammed [1] , Zulkefli B. Y. Hassan [1] www.ChemBioEngRev.de ª 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ChemBioEng Rev 2015, 2, No. 3, 1–20 1 Abstract It was believed that fluid flow and the laminar to turbulent transition delay were more easily con- trolled on smooth surfaces until the discovery of the grooved shark skin surface that changed the whole idea of how smooth the surface should be to have high flow in submerged surfaces. Riblets have gained renewed interest in academic fields of study and in industry due to several advantages in manip- ulating the turbulence boundary layer. Drag reduc- tion using small, longitudinally grooved surface pro- vides up to 10 % lower energy consumption in several applications. This review provides an overview of the mechanism of drag reduction with riblets, the differ- ent geometries and types, and the latest develop- ments in drag reduction riblet technology. Keywords: Drag reduction, Geometry, Riblets, Skin friction Received: December 07, 2014; revised: January 24, 2015; accepted: January 29, 2015 DOI: 10.1002/cben.201400033 1 Introduction In fluid dynamics applications, energy is used to either main- tain the motion of a fluid over a solid surface or to move a solid body through a fluid. In both cases, a substantial amount of energy is expended to overcome the drag force. Since the ener- gy crisis in 1973, increased interest in energy conservation has spurred researchers to seek alternative means to reduce energy consumption. Several studies have been conducted to find solu- tions for enhancing drag reduction for aeroplanes and vehicles, transport pipelines, and other industrial applications [1]. Since the discovery of the drag reduction phenomenon, re- searchers have sought to decrease the effect of the drag force to minimize its impact on the environment and economy. Wall drag force was first explained in 1904 by Prandtl [2] when he proved that all friction losses for fluid flow occur within a thin layer adjacent to a solid boundary. The flow outside this layer can be considered frictionless, and the velocity near the boun- dary is affected only by boundary shear. The boundary layer generates a drag near the wall as a result of the viscous interac- tion between the surface and the fluid [2–5]. This drag is typi- cally referred to as skin friction. The drag force is a component of the resultant force exerted by the fluid over a body and is oriented toward the comparative motion of the fluid. Skin friction drag represents a significant component of the total drag force. It comprises nearly 50 % of the total drag force on commercial aircraft, 90 % of the total drag force on submar- ines, and 100 % of the total drag force in long distance pipelines [6–8]. Pipeline transportation is one of the most convenient meth- ods of conveying fluids due to its safety and cost efficiency. Elongated pipeline systems provide the long-distance transport of water, natural gas, crude oil, and other materials [9–12]. As a result of turbulent flow, a pressure drop is experienced during fluid transport in a pipeline. Flow in the turbulent regime is exemplified by fluctuations in acceleration, pressure, and shear stress, which are all functions of time and position. These fluctuations result in unpredictable convective flow paths, unsteady vortices and eddies and increased skin friction. Because of the increased drag force, the energy required to overcome the viscous drag is spent pumping the fluid to re- build the pressure head. In general, relative to flow in the lami- nar regime, turbulent fluid flow increases the cost of the system [13–17]. A number of studies have investigated various methods to reduce losses from the skin friction drag force. Several recom- mendations and strategies have been adopted, but the most commonly applied technique involves the use of minor quanti- ties of drag reducing agents (DRA), i.e., viscous elastic chemical substance, injected into pipelines to reduce drag reduction. DRA, or flow improvers, have long been used in pipelines to maximize the flow potential by increasing operational flexibil- ity. DRA manipulates fluid behavior at the boundary layer by modifying the characteristics of the fluid to minimize the onset of turbulence. It was discovered accidentally in 1948 by Toms [18] that the addition of a few ppm of a higher molecular ————— [1] Prof. Hayder A. Abdulbari (corresponding author), Dr. Hassan D. Mahammad, Prof. Zulkefli B. Y. Hassan Faculty of Chemical and Natural Resources Engineering, University Malaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia E-Mail: [email protected] [2] Prof. Hayder A. Abdulbari Center of Excellence for Advanced Research in Fluid Flow, Univer- sity Malaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia These are not the final page numbers! &&

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  • Bio-Inspired Passive Drag Reduction Techniques: A Review

    Hayder A. Abdulbari[1,2],*, Hassan D. Mahammed[1], Zulkefli B. Y. Hassan[1]

    www.ChemBioEngRev.de 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ChemBioEng Rev 2015, 2, No. 3, 120 1

    Abstract

    It was believed that fluid flow and the laminar toturbulent transition delay were more easily con-trolled on smooth surfaces until the discovery of thegrooved shark skin surface that changed the wholeidea of how smooth the surface should be to havehigh flow in submerged surfaces. Riblets havegained renewed interest in academic fields of studyand in industry due to several advantages in manip-

    ulating the turbulence boundary layer. Drag reduc-tion using small, longitudinally grooved surface pro-vides up to 10% lower energy consumption in severalapplications. This review provides an overview of themechanism of drag reduction with riblets, the differ-ent geometries and types, and the latest develop-ments in drag reduction riblet technology.

    Keywords: Drag reduction, Geometry, Riblets, Skin friction

    Received: December 07, 2014; revised: January 24, 2015; accepted: January 29, 2015

    DOI: 10.1002/cben.201400033

    1 Introduction

    In fluid dynamics applications, energy is used to either main-tain the motion of a fluid over a solid surface or to move a solidbody through a fluid. In both cases, a substantial amount ofenergy is expended to overcome the drag force. Since the ener-gy crisis in 1973, increased interest in energy conservation hasspurred researchers to seek alternative means to reduce energyconsumption. Several studies have been conducted to find solu-tions for enhancing drag reduction for aeroplanes and vehicles,transport pipelines, and other industrial applications [1].Since the discovery of the drag reduction phenomenon, re-

    searchers have sought to decrease the effect of the drag force tominimize its impact on the environment and economy. Walldrag force was first explained in 1904 by Prandtl [2] when heproved that all friction losses for fluid flow occur within a thinlayer adjacent to a solid boundary. The flow outside this layercan be considered frictionless, and the velocity near the boun-dary is affected only by boundary shear. The boundary layergenerates a drag near the wall as a result of the viscous interac-tion between the surface and the fluid [25]. This drag is typi-cally referred to as skin friction. The drag force is a componentof the resultant force exerted by the fluid over a body and isoriented toward the comparative motion of the fluid.Skin friction drag represents a significant component of the

    total drag force. It comprises nearly 50% of the total drag forceon commercial aircraft, 90% of the total drag force on submar-ines, and 100% of the total drag force in long distance pipelines[68].Pipeline transportation is one of the most convenient meth-

    ods of conveying fluids due to its safety and cost efficiency.Elongated pipeline systems provide the long-distance transportof water, natural gas, crude oil, and other materials [912].

    As a result of turbulent flow, a pressure drop is experiencedduring fluid transport in a pipeline. Flow in the turbulentregime is exemplified by fluctuations in acceleration, pressure,and shear stress, which are all functions of time and position.These fluctuations result in unpredictable convective flowpaths, unsteady vortices and eddies and increased skin friction.Because of the increased drag force, the energy required toovercome the viscous drag is spent pumping the fluid to re-build the pressure head. In general, relative to flow in the lami-nar regime, turbulent fluid flow increases the cost of the system[1317].A number of studies have investigated various methods to

    reduce losses from the skin friction drag force. Several recom-mendations and strategies have been adopted, but the mostcommonly applied technique involves the use of minor quanti-ties of drag reducing agents (DRA), i.e., viscous elastic chemicalsubstance, injected into pipelines to reduce drag reduction.DRA, or flow improvers, have long been used in pipelines tomaximize the flow potential by increasing operational flexibil-ity. DRA manipulates fluid behavior at the boundary layer bymodifying the characteristics of the fluid to minimize the onsetof turbulence. It was discovered accidentally in 1948 by Toms[18] that the addition of a few ppm of a higher molecular

    [1] Prof. Hayder A. Abdulbari (corresponding author), Dr. Hassan

    D. Mahammad, Prof. Zulkefli B. Y. HassanFaculty of Chemical and Natural Resources Engineering, UniversityMalaysia Pahang, Gambang 26300, Kuantan, Pahang,MalaysiaE-Mail: [email protected]

    [2] Prof. Hayder A. AbdulbariCenter of Excellence for Advanced Research in Fluid Flow, Univer-sityMalaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia

    These are not the final page numbers! &&

  • weight polymer into a turbulent flow could significantly reducethe skin friction. DRAs can be classified into three major cate-gories: long chain molecules (polymers), surface active agents(surfactants) and injected micro bubbles, solid particles orfibers (suspended solids) [1, 3, 19, 20].

    2 Foreign Substance Drag ReductionTechniques (Active Means)

    Turbulent skin friction drag can be reduced by introducing var-ious foreign substances. These include long chain polymers,surfactants, micro bubbles, powders or tiny solid particles, andfibers.Numerous drag reduction studies have been conducted using

    polymers as an additive to enhance the flow in pipelines. Allhave concluded that minor concentrations of long chain poly-mers engender drag reductions of over 70% [1, 2123]. Addi-tionally, several other variables, such as the polymer chainlength, particle size, injection technique, and polymer interac-tion within the turbulent flow, were considered [5, 2429].Despite their apparent advantage in reducing drag force, poly-mer-based DRAs cannot be used in a variety of applications.For example, in the pharmaceutical and food and beverageindustries, polymer-based DRAs may alter the original charac-teristics of the fluids [30]. Furthermore, these polymer DRAslose their effectiveness with time due to rapid degradationkinetics at high Reynolds numbers [31, 32].Surfactant-based DRAs modify the surface properties of

    liquids or solids. The majority of surfactants appear to reducethe turbulent skin friction drag only in the presence of electro-lytes. The highest percentage of drag reduction achieved byCho et al. [33] was 80% in a solution containing between 1000and 2000 ppm at 70 C of stearyl amine oxide and betaine. Sur-factant-based DRAs are stable against physical decomposition,which is an advantage over polymer-based DRAs that breakdown when the strain rate is sufficiently high [5, 3337].Fiber suspensions have long been recognized (earlier than

    polymers) as drag reducing agents and were first describedby a small circle of engineers working with paper pulps. Theeffectiveness of these fiber suspensions depends on its prop-erties, such as density and concentration. Drag reduction wasfound to be possible when the concentration is high enoughfor fiber-fiber interactions to occur, but lower than a criticalconcentration above which the suspension viscosity is dra-matically increased [38]. Early observations of suspension-based DRAs used suspensions of natural products, such assediments and wood fiber. The developments of suspension-based DRAs were motivated by the need to provide accuratehydraulic transport criteria. Early attempts to establish thesystematic effects of solids concentration, specific gravity andduct dimensions were unsuccessful, quite possibly becausethe suspended particles were not uniform and did not havereproducible dimensions and surface texture. Several factorsaffect the effectiveness of suspension-based DRAs, includingthe type of internal flows (horizontal or vertical), type of flu-id (liquid or gas) and the type and size of fibrous or non-fibrous particles [20, 39, 40]. The concentration where the

    maximum drag reduction could be achieved using suspen-sion-based DRAs is 4050% [20, 41].

    3 Non-Additive Drag ReductionTechniques (Passive Means)

    In the past decade, comprehensive improvements to the under-standing and perception of turbulent boundary layers havebeen made. Significant efforts have been devoted to find cheap-er and environmentally-friendly alternative methods to reducethe effects of drag force. Extensive reviews can be found inWilkinson et al. [42], Gad-el-Hak [1, 19], Walsh [43], Savill[44], Raupach et al. [45], Meier et al. [46], Bushnell [47], Fishand Lauder [48], and Paul et al. [49]. These studies focused onturbulent drag reduction by passive means. Many techniqueshave been proposed that involve mechanically altering flowcontrol surfaces. The main concept behind this technique is themanipulation of the boundary layer by energy transfer which isaffected by the natural interaction dynamics of the fluid alongthe solid boundary. In other words, the skin friction drag issimply reduced by self-noise, applying these manipulators withcertain dimensions into the surface will shift away the turbu-lence structures from the wall, so the drag force will be reducedby turbulence flow noise. These drag reduction methods do notinvolve the use of additives. The passive means can be classifiedinto two groups based on the local position of the manipulatorsrelative to the turbulent boundary layer. (i) External layermanipulators, or outer layer manipulations, where thin platesare introduced into the external part of the flow, do not effi-ciently reduce the total drag in the turbulent boundary layers.A few devices that are categorized as external layer manipula-tors include outer layer devices (OLDs), boundary layer devices(BLADEs), large eddy breakup devices (LEBUs) and tandemarrayed parallel plate manipulators (TAPPMs). (ii) Internallayer manipulators, or thinner layer manipulators, restructurethe surfaces by altering the wall geometry in the form of smallstream-oriented striations, such as compliant walls, oscillatingwalls, dimples and riblets [1, 20, 5053].Several factors determine the percentage of drag reduction in

    both types of manipulators. For the outer layer devices, thesefactors include the angle of attack, the chord lengths of theaerofoil or flat plate devices, the boundary height, the spacingbetween tandem configurations, the thickness of the flat platedevice and the thickness of the boundary layer at the leadingedge of the device. For the inner layer manipulators, the heightand spacing between geometric striations are the only factorsthat need to be considered in determining the percentage ofdrag reduction [20]. Fig. 1 shows two schematics highlightingthese factors.Most outer layer manipulators follow the same mechanism

    as LEBUs, a popular drag reducing mechanism in the 1980s.However, as shown by Wilkinson et al., LEBUs were not veryefficient at obtaining a definite total drag reduction in turbulentboundary layers [42]. These devices were designed to break uplarge vortices in turbulent flow and inhibit high Reynolds shearstress near the wall. They consisted of metallic ribbons orientedin the direction of flow. LEBUs developed substantial down-

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  • stream regions of reduced skin friction effects and artificialboundary layers. They reduced the characteristic turbulencescale by altering the momentum transferred from the externalflow toward the wall. Particular arrangements of the ribbonsmay reduce the number of turbulent bursts and, hence, the sur-face friction stress. The reductions provided by these devicesnet between 5 and l0% drag reductions [42, 5456]. HoweverSahlin et al. [57] and Lynn et al. [58] depicted that the averageskin friction downstream of the devices decreased by 8%. Find-ings of the study showed albeit the device drag was low, therewas an increase of 3% in net drag. The study further addedthat a decrease in net drag at high Reynolds numbers seemsimplausible. Meanwhile, when the Reynolds number increases,the effectiveness of LEBU decreases and it is all because of thedecrease in size of the most energetic turbulent scales thatrelates to the boundary-layer thickness. This suggests that thevariance in both outer and inner scales disables the LEBUmechanism.In a numerical simulation study conducted by Spalart et al.

    [59], they modified a turbulent flow and weakened the wallpressure fluctuations on an airliner windshield. The resultsshow that the turbulent flow lost most of its sizable voluminousbulges and generally created more minuscule outer-layereddies, which were harmless relative to the original immenselycolossal eddies. Spalart et al. succeeded in reducing the wallfluctuations by simulating LEBUs. However, this was achievedonly over approximately 6 boundary layer thicknesses, whenthe actual effect would need to be sustained over approximately25 boundary-layer thicknesses. Eckhardt et al. [60] investigatedthe possibility of reducing skin friction through the use of flow-aligned vertical LEBUs in which LEBUs were mounted perpen-dicularly to the flat plate surface. The experimental resultsshowed a local drag reduction of approximately 2728%.Moreover, the vertical LEBUs were more effective in reducingdrag compared with the LEBUs directed parallel to the wall.As evidenced by many studies, these devices produce down-

    stream reduced skin frictions of up to 30%. However, not allLEBUs have a net drag reduction due to device-added drag.This finding indicates that LEBUs have no drag reducing abil-ity in channel flow [61].

    A great deal of research has focused on compliant coatingtechniques for drag reduction. The very first study was per-formed by the biologist Gray [62] in 1936, wherein he sug-gested that the malleable skin of a dolphin could have somesort of damping influence on turbulence within the boundarylayer, therefore rendering fluid flow more laminar with lessfriction. However, Kramer [63] was the first to study the use ofcompliant coatings as a possible technique for drag reduction;Grays idea inspired him to use elastic walls to simulate thedrag repelling nature of dolphin skin. The maximum dragreduction found by Kramer was 60%, which was impressiveconsidering that since then, the many studies using compliantwalls to reduce the drag force have never achieved that percent-age [30, 6368].A compliant wall provides a pressure field that tends to

    inhibit the turbulent burst phenomenon [69]. According toKline et al. [70], this sequence of events consists of threephases: lift-up, oscillation and break-up of low speed streaks(i.e., bursts). It is generally assumed that the bursting processalso plays important roles in momentum, heat, and mass trans-fers. In the literature, compliant wall studies have primarilyfocused on delaying the laminar to turbulence transition inapplications for marine vehicles. Compliant wall studies wererarely investigated for other applications such as pipelines, dueto the different mechanism for drag reduction [64, 67, 68, 71].Carpenter and Garrad [72, 73] noticed that Kramers compli-

    ant walls were hypersensitive to pressure gradients and per-formed quite differently under various experimental flow con-ditions. This outcome proved that the reported 60% dragreduction was credible and different experimental conditionswould be plausible explanations for why other researchersfailed to achieve 60%.Even though compliant walls are beneficial for marine ve-

    hicles, it is not applicable for drag reduction in pipes in whichthe flow was already turbulent. However, compliant surfacesare known to have the ability to modify turbulent flow and skinfriction drag and avoid boundary layer separations [64, 74].Dimples are used in many heat transfer applications due to

    the high contact area provided by the structures. Generally,dimples belong to the same family as riblets in that both aremodifications superposed on smooth solid walls; however, theyare different from a design point of view. Most of the researchwork cited in the literature on dimples has focused on heattransfer [7584]. Drag reduction was also addressed in thesemulti-objective studies [8591]. The early findings showed thatimprovements in the heat transfer coefficient also increasedrag. Additionally, the heat transfer performance of the dim-ples was not always as efficient as that of other techniques, suchas fins and turbulators [7881]. The numbers of studies thatfocused on the effect of dimples is very limited and contradic-tory. A few studies claimed drag reduction was achievable whileothers found that drag increased with the addition of dimples[7881]. On the one hand, Rohlfs et al. [92] introduced anexperimental and numerical investigation of the turbulent flowover dimpled surfaces. They observed that the dimples had nosignificant effect on the drag reduction (positive or negative)which led them to conclude that restructuring the surfaces in adimple form will have a significant heat transfer effect and nodrag reduction performance. On the other hand, Yu et al. [93]

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    a)

    b)

    Figure 1. Parameters that control the percentage of drag reduc-tion in (a) outer layer manipulators and (b) inner layer manipula-tors [20].

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  • reported an 18.0% drag reduction when using dimpled surfa-ces (i.e., pin fin-dimple hybrid structures) and Zhong et al. [94]reported a 15.1% drag reduction with direct numerical simula-tions. The reason for this disagreement is the vast differencebetween initial fluid conditions, dimple geometry, measure-ment conditions and evaluation methods. However, there is ageneral agreement that drag reduction is achievable withdimples despite the fact that its heat transfer performance isbetter and more obvious. Lienhart et al. [95] introduced a nu-merical and experimental investigation of the drag reductionusing dimpled surfaces using a channel test rig. They studiedthe effect of the dimples design and arrangement on the dragreduction performance as shown in Fig. 2. They concluded thatusing shallow dimples can have good or acceptable improve-ment in the heat transfer performance without any increase inthe pressure drop. In another words; they concluded that thedimples designs investigated are not effective when it comes tothe drag reduction and when compared to the heat transferperformance.

    Oscillating walls are generally used in channels wherein oneor two walls undergo forced spanwise or streamwise oscillationdriven by a pump. Generally, the oscillation movement of thewalls has been shown to be effective in suppressing the turbu-lent structures of the fluid passing through the channels. Thewall oscillation frequency and amplitude are directly related toits drag reduction performance; there is an expected one pointeffect for each flow rate or degree of turbulence [96100]. Gen-erally, the main mechanism controlling the drag reductioneffect using oscillating walls involves altering the boundarylayer conditions and creating order in a previously chaotic, tur-bulent flow. Choi and Clayton [101] conducted very importantresearch work on the drag reduction using spanwise oscillatingwalls. The study was performed in an open-return, low-speedwind tunnel. They found that 45% drag reduction can beachieved through their design. They stated that the mechanismof drag reduction by spanwise-wall oscillation strongly relatesto the spanwise vorticity where the positive spanwise vorticityreduced the mean velocity gradient of the boundary layer.Akhavan et al. [100] reported a 40% flow enhancement

    using oscillating walls. They concluded that spanwise oscilla-tions have been shown to reduce drag more efficiently thanstreamwise oscillations in the vast majority of cases. However,Quadrio and Ricco [102] suggested that the oscillating wall cre-ates a Stokes boundary layer just above the viscous sub-layer;the Stokes boundary layer holds small localized vortices withspins opposing the oscillation movement. They stated that thepresence of these small vortices altered the stretching of largervortices in the streamwise direction and resulted in a reductionof the mean vortex diameter which reduced the vorticity.Recently, Skote [103] introduced a new theory regarding the

    drag reduction using oscillating walls. They stated that theinner part of the turbulent boundary layer is respondingdirectly to the wall oscillation and the velocity profile is there-fore governed by the new (actual) friction velocity. Throughtheir model derivation, they concluded that the slope of thelogarithmic part of the velocity profile could serve as a measureof how well the outer part of the flow has adjusted to theimposed wall forcing.

    3.1 Riblet Grooves

    Among all passive means, riblets have been shown to be themost widely investigated drag reduction technique. Dragreduction by riblets involves the use of longitudinal micro-grooves oriented on a surface originally designed for skin fric-tion reduction in a fully turbulent boundary layer. Originally,in flow control, it was believed that the smoother a surface, thelonger the delay of the laminar-turbulent transition. However,in the mid-1960s, Liu et al. [104] and Bath [105] consideredusing rough surfaces to reduce the skin friction in turbulentflow near the wall. By modulating the base flow in the spanwisedirection, these practices were observed to delay the transitionto turbulence.The early work on riblets focused on riblet geometry and

    flow conditions to optimize their drag reduction performance[43]. In the late 1970s and early 1980s, Walsh and collaboratorsat NASAs Langley Research Center started to conduct the first

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    Figure 2. Channel flow with multiple dimples at both walls,case B, fine grid simulation, Re = 10 935 [95].

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  • experimental studies on riblets with turbulent drag reductionusing several types and shapes of riblets. Since then, a widevariety of interesting studies have emerged and have focusedon five areas: the effect of riblet geometry on drag, the effect ofheat transfer on wall roughness, the effect of riblet grooves onnon-Newtonian fluid flow, the effect of riblet grooves on lami-nar and transition flows, and simulation analyses and numeri-cal modeling studies of riblets in several applications. More-over, many reviews have been conducted, e.g., Savill [106],Wilkinson et al. [42], Vukoslavcevic et al. [107], Nieuwstadtet al. [108], and Walsh et al. [109, 110].In 1990, Walsh [43] authored a comprehensive review about

    the research on drag reduction using riblets on flat-plate turbu-lent boundary-layer flows. According to Walsh [43], the appro-priate riblet dimensions for drag reduction vary depending ontheir shape and the state of the boundary layer in which theyare placed. Ji-sheng and Heng [111] showed in their study thatthe coherent structure of the wall region directly influenced theproduction of turbulence energy. The authors concluded thatmanipulating this structure can reduce the drag force.The drag reduction mechanisms of fluid flow over a rib sur-

    face are fairly good understood because the structure of turbu-lence flow is at presence well investigated due to numerous the-oretical, experimental and computational works [112114].However, it is well known that rib surfaces only rearrange theturbulence structures near the wall and only influence thatlocalized region near the wall. Additionally, longitudinal ribscan develop a lower shear stress flow than a smooth surface[43, 115, 116].The influence of riblets on the development and structure of

    a boundary layer flow is an important research area. Extensivework has been performed by Choi [117], Baron and Quadrio[118], Park and Wallace [119], and Suzuki and Kasagi [120].A grooved surface (riblets) impedes the cross flow or the

    direction of the flow closest to the wall. Therefore, a groovedsurface prevents random low speed streaks from converginginto an ejection. The riblet protrusion height is an importantparameter and is considered to be the offset between the virtualorigin in the streamwise shear flow and some mean surfacelocation. Compared with a smooth wall, this offset would resultin a greater separation between the wall and the turbulentstreamwise vortices, reducing the momentum exchange at thewall [121].Savill [44] suggested that riblets have two drag reducing

    mechanisms. First, riblets shift the turbulence away from thewall, and second, riblets buffer the spacing between the struc-tures to reduce the vicinity of the maxi-mum reduction position. Researchershave conflicted observations on burstmechanisms, which plays an importantpart in turbulence development [70].Hooshmand et al. [116] observed an in-crease in burst frequency and foundthat the streamwise velocity was inde-pendent of the spanwise position faraway from the surface. However, Walsh[109] and Bacher and Smith [122]found no changes due to bursts. Visual-ization studies conducted by Gallagher

    and Thomas [123] and Savill [106] showed reduced burst fre-quencies of up to 30%. Tardu et al. [124] also conducted visu-alization studies, which showed that the ejection frequencydecreased by 1020%. Furthermore, Tardu [125] observed thatthe riblets shortened the time necessary for the spatio-temporaldevelopment of the instability of low speed streaks in dragreducing configurations. Baron and Quadrio [126] confirmedthat the duration of the ejections tended to be reduced over theriblets and that the reduction compensated for the observedincreased burst frequency. Furthermore, they observed that theejections tended to be more closely grouped over the riblets.Bacher and Smith [122] investigated the interactions of thecounter rotating longitudinal vortices with the resultant smalleddies near the peaks of the riblets. They claimed that the sec-ondary vortices would act to weaken the longitudinal vorticesand maintain the fluid at low speeds within the grooves.Bechert and his co-authors [112, 121, 127, 128] performed

    theoretical and experimental investigations on triangular, tra-pezoidal, blade, semi-circular scalloped, and convex riblets inoil and wind channels. The highest drag reduction percentagewas 8.7% with a riblet spacing of between approximately 2 and10mm in the oil channel. They focused their investigations onthe longitudinal and cross-flow directions. They suggested amechanism dependent on protrusion heights, where the virtualorigin of a riblet surface is located at the extrapolation of thestreamwise velocity profile. The difference between this originand the riblet peak was defined as the protrusion height for thelongitudinal flow (hpl). Then, by extrapolation of the spanwisevelocity profile, they showed that there was a second protrusionheight for the cross flow (hpc). The difference in size of thesetwo heights was dependent on the ratio of height h and spacingof the riblets s where the maximum possible difference was0.132 s. The theory was based on the hypothesis that the ribletsimpeded the instantaneous cross-flow in the viscous sublayer,which was generated by the turbulent motion, and reduced themomentum and the wall shear stress. The net effect of thereduced turbulence intensity was a thickening of the viscoussublayer, which translates into a reduction of the wall shearstress. Fig. 3 shows schematics of the mechanism of fluid flowover the longitudinal and cross-flow directions on a ribbed sur-face.Choi [117] suggested that drag reduction occurs due to the

    structural changes in the near-wall boundary layer caused bythe riblets. The authors explained that the riblets would func-tion as small fences to inhibit the lateral turbulence near thewall surface. This phenomenon would also limit the lateral

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    a) b)

    Figure 3. (a) Longitudinal and (b) cross-flow on a ribbed surface [112].

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  • movement of the hairpin legs to the riblets during the near-wallbursts and lead to premature bursts. When these happen, theintensity and duration of the subsequent bursts decreases. Thespanwise correlation length increases because the bursts occurprematurely before the legs approach one another. The fre-quency of the bursts is increased over the riblet surface. Anoth-er hypothesis was suggested by Djenidi et al. [129] when lowspeed streaks were observed to exist near the wall in turbulentboundary layers. These streaks elongated in the streamwisedirection before lifting off the surface. This result was hypothe-sized to be caused by low speed regions inside the valleys of theriblets. Drag reductions in laminar flow over riblets should alsobe expected. Tang and Clark [124], introduced the concept of ariblets sublayer. Within this layer, the flow characteristics weresignificantly reformed as a result of the presence of the riblets;additional flow features existed which were unique to the layer.Three types of secondary eddy patterns formed in the sporadiccross-flow regions across the riblet peaks, in the longitudinalnear-wall vortices that entered the riblet grooves, and in thesweep flow from the log-layer that further entrained flow intothe riblet grooves.Choi et al. [130] conducted a numerical study on laminar

    and turbulent channel flows. Choi discussed the drag reductionmechanisms in terms of the increased spanwise effective viscos-ity and the thickened viscous sublayer. The rib spacing reducedthe viscous drag by restricting the location of the streamwisevortices above the wetted surface. He suggested a conceptualnear-wall region model which involved the sweep of high mo-mentum fluid toward the wall surface between pairs of mergingcounter-rotating longitudinal vortices. Fig. 4 shows schematicdiagrams of drag increasing and reducing mechanisms. Sup-posedly, the riblets would impede the lateral movement of thelongitudinal vortices during the near-wall sweep, leading to ashorter and lower pre-mature sweep. However, the mechanismof how this process would occur was not provided. Further-more, the proposed mechanism relied on the implication thatlongitudinal vortices near the wall surface were essentiallypaired, even though experimental evidence in support of thisinference was lacking.

    Park and Wallace [119] conducted a comprehensive investi-gation on the spacing between riblets, i.e., V-groove valleys.They concluded that drag reduction was primarily derived

    from greatly reduced wall shear stresses near the bottom of theriblet valley. The authors noted that the turbulence intensitiesof all three velocity components were reduced in the riblet val-leys. The intensities increased over the peaks when the heightwas less than the viscous sublayer thickness.Wang et al. [131] and Ma et al. [132] found in a visualization

    study that riblets effectively reduced the number of the low-speedstreaks and suppressed the formation of vortices (i.e., hairpinvortices, ring vortices and horseshoe vortices) and thereby re-duced the frequency and intensity of turbulence bursts.Boiko et al. [133] conducted an experimental investigation

    on the effect of riblets on streaky flows and found that ribletshave the ability to reduce the velocity gradient of streamwisestreaky flows in the boundary layer.

    3.2 First Inspiration in Nature

    Nature is full of flow control examples and has long providedsubjects of interest in the field of fluid mechanics. From birdsflying in a V-formation to feathers on a bird, to scales on a but-terfly wing, to the skin of marine creatures, such as octopus,fish, dolphins, and sharks, natural solutions to aerodynamicsand hydrodynamics are contrasted from the hard, smooth sur-faces employed in most engineering applications. Fig. 5 showssurface structure examples from nature. However, relatively lit-tle quantitative research has been performed to determine thebenefits that could be gained by the use of these biological sur-face patterns due to their often complex three-dimensionalnature. With the use of flow measurement techniques, such asparticle image velocimetry (PIV), it has become possible to an-alyze fluid flow over such surface geometries and compare theresults with those of more conventional surfaces [134137].As mentioned by Reif [138, 139], the concept of using riblets

    to reduce skin friction drag first came from the observation ofthe swimming technique of sharks. Scientists have known thatsharks are one of the fastest swimming animals with burstspeeds of over 20m s1 [140].Sharks are extremely agile and maneuverable, capable of in-

    stantly changing their direction even at high speeds. The bodyof a shark has a sleek, torpedo-like profile to minimize drag.Additionally, it has been suggested that the scales covering theshark could be a source of further drag reduction and flow sep-aration control. A shark scale contains grooves that run parallelto the flow direction and act as riblets over its surface, therebydecreasing drag by deterring cross flow [128].Fish [141] examined potential mechanisms of drag reduction

    used by dolphins. He noted that the dolphin body shape is thatof a highly streamlined body, similar to a submarine design,and is a likely factor contributing to the lower than expecteddrag. However, Fish noted that during observations of dolphinsswimming, little to no separation showed over their bodies,whereas fluid flow over duplicated dolphin body shapes wereobserved to show flow separations on the surfaces. The authornoted that their body shape cannot be solely responsible fortheir efficiency. It is theorized that dolphin skin also acts as acompliant wall, employing viscous damping to reduce drag.Their skin may absorb perturbations in the boundary layer thatwould ordinarily lead to a transition to turbulent flow. By

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    a) b)

    Figure 4. Schematic diagram of drag increasing and reducingmechanisms of riblets: (a) s+ = 40 drag increase (extensive areaaffected by downwash motion); (b) s+ = 20 drag reduction (lim-ited area affected by downwash motion)[130].

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  • delaying this transition, the dolphin skin reduces drag over itssurface [141].

    4 Experimental Investigation overRiblets in Channels

    It has long been acknowledged that the manipulation of theturbulence boundary layer by riblet grooves enhances dragreduction, and that the riblet geometries (i.e., rib spacing s andrib height h) affect drag reduction.In this comparative study, the most suitable riblet type is

    defined as one that can offer a high percentage of drag reduc-tion. The comparison was based on the percentage of dragreduction of different riblet types having different groove geo-metries. The selection of the riblet type best suited for dragreduction was determined by the coherent structure of the tur-bulent flow, the physical properties of the fluid and the ribletgeometries and shape.Many different types of riblets have been used over the pastfour decades, each of which has been geometrically optimizedto achieve higher drag reduction percentages. Tab. 1 showssome of the riblets shapes that have been tested by NASA.For viscous drag reductions over a flat plate at low speed

    flows, Walsh [109] identified the riblet height and spacing interms of wall units as non-dimensional height h+ and spacing

    s+, which are defined as h h un

    Cf2

    rand s s u

    n

    Cf2

    r,

    respectively, where Cf is the local skin friction coefficient, u isthe free-stream velocity in m s1, h is the kinematic viscosity inm2s1, s is the peak to peak spacing of riblets and h is the valleyto peak height of riblets.Investigations have revealed the features of surface textures

    in many different applications, such as enhancing flow in pipesand reduce the drag surfaces, self-cleaning types of surface,deicing, on commercial aeroplanes, fighter planes, underwater

    vehicles and others applications [143150]. With this widerange of applications, there are significant worldwide interestsin this field.

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    Figure 5. Surface structure examples from nature.

    Table 1. Riblet models tested by NASA [142].

    Model Description

    Symmetric V-groove

    Rectangular

    Spaced triangular

    Right angle rib

    Peak curvature

    Valley curvature

    Peak and valley curvature

    Notched peak

    Spaced V-groove

    Unsymmetrical groove

    Oblique V-groove

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  • Nonetheless, several challenges face the development of sur-face textures and their translation into industrial applications.No single universal riblets design can be used for all applica-tions due to completely random turbulent regime structuresand fluid to fluid differences of the viscous sublayers andstreamwise vortices. For example, the ideal riblet structure forgas turbines compressor blades have spacings of 20 to 120mm[121, 151153]. Furthermore, the ability to accurately manu-facture riblets is another field of study as there are plenty oftechniques that may be used to fabricate micro-grooves[154, 155]. Other challenges include determining the appropri-ate material type and optimizing the type of fabrication proc-ess, such as rolling, grinding, etching, and laser etching [136].NASA machined the first riblets in the late 1970s. These rib-

    lets were made from aluminum sheets for aeroplane applica-tions. Later in the 1980s, NASA started using vinyl rib filmsmanufactured by 3M because of its more cost-effective design.A polyvinylidene fluoride-based thin plastic film with an adhe-sive backing was used in the manufacture of micro-patternedriblets by grinding and rolling processes [156158]. 3M vinylrib films have been used for many test surfaces and variousapplications, such as aerospace surfaces, submerged vehicles,and pipes and channels [159167]. In addition, one of theadvantages of the 3M vinyl rib film is the flexibility of the thinplastic sheets film which can be applied in nearly any orienta-tion, except within small pipes or micro channels, which wouldneed to be split open to apply the 3M vinyl rib films [136].

    4.1 Riblets in Air Flows

    Many studies have focused on wind drag force reductions foraeroplanes by applying particular groove types and sizes onaeroplane bodies. NASA Langley Research Center, Boeing andAirbus have tested aircraft with riblets to investigate their influ-ence on reducing the drag force [20, 168, 169].Because of the high costs in fabricating and testingriblets on aeroplanes, riblets have been applied onreasonably-sized flat plates for their study in windtunnels. Thus, different riblet geometries and typescould be studied for their effects on turbulent windflow. Experimental studies conducted in this man-ner are listed in Tab. 2.From the 1960s to mid-1980s, most of the inves-

    tigations were focused on determining the effect ofriblet shapes and dimension on suppressing near-wall turbulence. As mentioned earlier, Liu [104],was the first who showed that an increase in sur-face roughness suppressed the near-wall turbulenceand confined the turbulent regime by using trans-verse roughness elements made of square bars, withranges of h+ = 45110 and s+ = 190370. The mostobvious effect of an increase in the surface rough-ness was the distinct increase of turbulence produc-tion and the concomitant increase in the eddy vis-cosity. It was shown that this increase in eddyviscosity increased the total thickness of the layerin the same manner that an increase in molecularviscosity would do in a laminar flow. The investiga-

    tion revealed that the low-speed streak in the riblet valleysseemed to disappear. Additionally, the data showed 2025%burst frequency reductions, depending on the spacing of theribs in wall variables. There was a potential drag reduction ofup to 3%; however, the authors did not feel that their data indi-cated sufficient turbulence suppression to result in significantoverall drag reduction.In an experimental study by Furuya et al., conducted over

    rectangular bars placed on a flat plate [170], a series of turbu-lence characteristics were measured (i.e., velocity, turbulenceintensity and wall shear stress distributions). The study showedthat the behavior of the flow was not influenced by changes inthe surface roughness except in the region relatively near theleading edge. The vortex measurements indicated that thelength scale of the secondary flow vortex grew in accordancewith the boundary layer development, while the vorticity,which is more intense in the edge region, decreased in thatdirection. The total frictional drag of the plate on which astreamwise bar is placed was almost the same as that for a flatplate having the same surface area exposed to the flow.Walsh and Weinstein [171] examined the effects of micro

    variations in the surface geometry on altering the near-wallstructure of the turbulent boundary layer. Rectangular and tri-angular-shaped riblets with different geometries were used.Fig. 6 shows the dimensions of the tested ribbed surfaces.The results showed that no net drag reductions were ob-

    served for the rectangular ribs. However, the triangular ribshad smaller drag increases. The drag decreased when the spac-ing between the triangular ribs was reduced, resulting in aV-groove type configuration. The highest percentage of dragreduction, 4%, was recorded for V-groove riblets with 1520height and spacing values in the law of the wall coordinates ofthe ribs. In a later study, Walsh [172] tested additionalV-groove type configurations to optimize the spacing andheights of the grooves and to determine the effect of a free

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    Figure 6. Rectangular and triangular ribbed surfaces tested by Walsh and Wein-stein [171].

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  • stream sweep angle. To improve the riblet drag reduction per-formance, three rib geometries were tested: a peak curvature, avalley curvature, and a notched peak V-groove. The resultsshowed that the maximum drag reduction was 8% for aV-groove geometry with a valley curvature and a V-groovegeometry with sharp peaks and valleys with h+ = 812 and

    s+ = 1520 in the law of the wall coordinates of the ribs. Walshand Lindemann [110] verified the law of the wall scaling forthe riblet height and spacing by testing riblets in boundarylayers with larger Reynolds numbers and optimized the ribletshape to obtain a higher drag reduction percentage. A maxi-mum drag reduction of 78% was observed for V-groove rib-

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    Table 2. List of experimental wind studies over flat plates with different types of riblets.

    Original authors Year Riblet type Riblet dimensions Drag reduction percentage

    Liu [104] 1966 Square h+ = 45110s+ = 190370

    3%

    Furuya et al. [170] 1977 Rectangular h = 60 000mms = 20 000mm

    No drag reduction

    Walsh and Weinstein [171] 1979 V-groove h+ = 25, s+ = 20h = 510mm, s = 250mm

    24%

    Walsh [172] 1983 V-groove h+ = 10s+ = 15

    8%

    Walsh and Lindemann [110] 1984 V-groove h+ = 13s+ = 15

    78%

    Enyutin et al. [173] 1987 Triangular h = 170mms = 250mm

    78%

    Rectangular h = 215mms = 260mm

    79%

    Wilkinson and Lazos [175] 1988 Rectangular h+ =8s+ = 10

    8.5%

    Choi [176] 1988 Trapezoidal groove h+ = 12, s+ = 20h = 1500mm, s = 2500 mm, t =200 mm

    4%

    Nguyen et al. [156] 1990 V-groove h+ = 8, s+ = 20h = 200mm, s = 500mm

    5%

    Enyutin et al. [174] 1991 V-groove h = 500mm, s = 260mmh/s = 0.3

    810%

    Baron and Quadrio [118] 1993 V-groove h+ = s+ = 12h = s = 700 mm

    6%

    Nieuwstadt et al. [108] 1993 V-groove h+ = s+ = 13h = s = 640

    5%

    Park and Wallace [119] 1994 V-groove h+ = 14s+ = 28

    4%

    Gudilin et al. [54] 1995 V-groove s+ = 1215, a = 53h = 140mm, s = 320mm

    89%

    Choi and Orchard [177] 1997 V-groove h = s = 730 1830 mm 6%

    Bechert et al. [128] 2000 Trapezoidal groove s+ = 16, a = 45 6.85%

    Lee and Lee [7] 2001 Semi-circular s+ = 25.2 Drag decrease

    Frohnapfel et al. [178] 2007 Trapezoidal groove h = 150mms = 2h

    25%

    Lee and Choi [180] 2008 V-groove s+ =10.4(h = 178.6 mm s = 300 mm)

    4%

    Jovanovic et al. [179] 2010 Trapezoidal groove h = 150mms = 2h

    25%

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  • lets having h+ = 13 and s+ = 15. The effect of the free streamsweep angle on drag reduction did not change up to 15, andno drag reduction was observed at 30. Enyutin et al. [173]conducted an investigation on two types of riblets (rectangularand triangular) to evaluate the effect of riblet size and shape onskin friction drag. The results showed that for rectangular rib-lets of h = 215 mm and s = 260 mm, the maximum gain was79%. For triangular riblets of h = 170 mm and s = 250mm, themaximum gain was 78%. Enyutin and Walsh and Lindemannused the same type of grooves, but the relative heights of theriblets h/s were 0.54 and 0.45 in the Walsh and Lindemannstudy and 0.68 and 0.8 in the Enyutin study. The frictionadvantage observed by Enyutin was greater than that for Walshand Lindemann as shown in Fig. 7.

    Moreover, Enyutin et al. [174] investigated the influence ofthe free stream sweep angle on the drag reduction over differ-ent ribbing geometries. The results agreed with the data pre-sented by Walsh and Lindemann, in which they observed onlya small difference (12%) for a 15 sweep angle.From the mid-1980s to late-1990s, most experiments were

    conducted to understand the characteristics of turbulent flowover rib surfaces because the coherent structure of turbulenceover a riblet surface is inherently different than that over asmooth surface. Wilkinson and Lazos [175] studied the effectof streamwise near-wall thin element riblets under turbulentflow conditions. The net effects of the thin-element riblets onturbulent, viscous skin friction resulted in up to 8.5% dragreduction, which was similar to other riblet geometries in pre-vious studies.Choi [117, 176] measured wall pressure fluctuations of tur-

    bulent boundary layers over smooth and ribbed surfaces andobserved a substantial increase in the burst frequency but adrop in burst duration of almost 50%. Additionally, the fluctu-ation intensity of the wall shear stress was considerably lowerin the riblet valleys than over flat walls and contained numer-ous periods of very low quiescent fluctuation intensities.Nguyen et al. [156] study a symmetrical V-shaped riblet at

    riblet angles of 60 and 30, and heights and spacings of

    200 mm and 500 mm, respectively. Net drag reductions were cal-culated using momentum thicknesses at the beginning and theend of both the rib and smooth plates under the same flowconditions. The highest percentage of drag reduction, 5%, wasobtained for a free stream velocity of 16m s1.Baron and Quadrio [118] performed experiments in a low

    speed wind tunnel with triangular grooves of 700mm height and700mm spacing, i.e., h+ = 12 and s+ = 12 wall units. By testing thevelocity profile over the surface, the main velocity profile showedan upward movement when plotted in the law of the wall struc-ture. This is a well-known characteristic of drag reduction sys-tems and is directly related to an increase in the thickness of theviscous sublayer. A similar behavior was reported by Choi [117],in which the author stated that the drag reducing effectiveness ofriblets was not due to only a purely viscous effect, but likelydue also to a direct interaction with the turbulence-producingstructures of the turbulent boundary layer.The effectiveness of riblets on skin friction drag reduction

    under the influence of both zero-pressure and adverse pres-sure gradients was investigated by Nieuwstadt et al. [108].Their experiments were conducted using two triangular rib-lets having the same hight h and space s dimensions (640and 360 mm). It was found that the maximum skin frictionreduction was 5% at s+ = 13 with a zero pressure gradient.However, with an adverse pressure gradient, the resultsshowed a decrease in the total drag, which indicates therewas no significant difference.To study the characteristics and effect of the flatness factor on

    ribbed surfaces, Park andWallace [119]measured the streamwisevelocity profiles over a V-groove riblet with dimensions h+ = 14and s+ = 28 using a hot-wire probe. The results showed that themaximum drag reduction was 4% compared with that on asmooth surface. Grooved riblets reduced the vertical flux of astreamwise flow momentum within the riblet valleys. In addi-tion, there remains a small probability of a negative axial flow ve-locity component within the valleys. The vertical flux of stream-wise momentumwas greatly reduced within the riblet, even for adimensionless size somewhat above the drag-reducing range.Gudilin et al. [54] used a surface with isosceles ribs having avertex angle 53, a rib height h= 140 mm, and a distancebetween vertices s= 320 mm. The maximum drag reduction forthe ribbed surface was 89%.Choi and Orchard [177] studied the effect of riblets on heat

    transfer and drag reduction on the lower wall of a flat plate in alow speed wind tunnel. A triangular riblet with dimensionss/h= 1 and s= 183mm was tested. The results showed anincrease of 10% in the heat transfer coefficient relative to thatof a smooth surface. Also, a 6% increase in drag reduction wasobserved, compared with a smooth wall.Bechert et al. [128] found that a turbulent boundary layer on

    a shark skin surface can help to reduce turbulent shear stress.The authors duplicated shark skin using three dimensionaltrapezoidal riblets to cover 64% of the surface area of a400mm 500mm test plate. In their study, the highest dragreduction of 6.85% was observed at s+ = 16 and a fin height ofh= 0.5 s.Lee and Lee [7] analyzed the flow field of a turbulent boun-

    dary layer inside a semi-circular riblet valley using a smoke-wire technique for flow visualization and a PIV for velocity

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    Figure 7. Friction drag reduction with (1) triangular riblets(h= 170mm and s= 250 mm) (2) smooth plate, (3) Walsh and Lin-demann work [173].

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  • measurements. Fig. 8 shows flow visualization images ofstreamwise vortices over rib surface, where the investigationwas conducted on surfaces with semi-circular grooves of ribletsspaced at s+ = 25.2 and 40.6 to understand the different dragdecreasing and increasing rib configurations. For drag decreas-ing configurations, the riblet dimension was h+ = 12.6 ands+ = 25.2. The experiments showed that the streamwise vorticesand the spanwise motion appeared to be confined above theriblets and between the tips, respectively. For the drag increas-ing configurations, the riblet dimension was h+ = 20.3 ands+ = 40.6, and the experiments showed that the grooves of thespaced riblets were wider than the average width of the stream-wise vortices. Unlike the vortices in the drag reducing configu-rations, the vortex centers were located between the valleys andtheir concentrations were reduced.

    Frohnapfel et al. [178] and Jovanovic et al. [179] based theirstudies on the numerical simulations conducted by Frohnapfel[178], wherein the drag force reduction results only for imper-ceptible grooves that were not capable of producing secondaryflows. Several variables determined the groove dimensions,such as the wall shear velocity and the kinematic viscosity ofthe flow medium. Based on this study, the square cross-sectiongave a maximum drag reduction of 33%. The spacing betweenthe grooves was designed to avoid local peaks of high wallshear stresses close to the edges of the grooves. The minimumgroove dimension suggested was h 150mm, with a separationof 2 h 300mm. The suggested grooves were tested; the maxi-mum drag reduction observed was up to 25% compared withthat of flow over a smooth wall at low Reynolds numbers.Lee and Choi [180] investigated the effect of spanwise vorti-

    ces in a turbulent flow over V-groove riblets using a PIV sys-tem. The authors visualized the sectional images of the stream-wise vortices and measured the velocity fluctuation over theriblet surface. The riblet had dimensions of 176.8mm heightand 300mm spacing between each V-groove riblet.The results show that small-scale spanwise vortices were

    reduced in the near-wall region, and therefore, the thickness ofthe viscous sublayer increased.

    4.2 Riblets in Liquid Flows

    Using different types of V-groove riblets on a flat plate, a num-ber of water channel experiments observed maximum dragreductions of approximately 10% as shown in Tab. 3.Gallagher and Thomas [123] conducted flow visualizations

    to study turbulent boundary layer characteristics over stream-wise grooves with dimensions h = s = 1650mm on a flat plate.They reported a low skin friction flow in the valleys of thegrooves and a 30% reduction in the measured bursting fre-quency for the riblet surface. However, the total drag for bothgrooved and smooth surfaces were the same. To determine thechanges in the turbulent flow structure precipitated by the rib-let surface relative to a conventional flat-plate, Bacher andSmith [122, 181] conducted experiments using flow visualiza-tions in a water channel, with non-dimensional V-groove ribletheight and spacing h+ = s+ = 15. They stated that the key to theriblet drag reduction performance was the difference betweenspanwise and streamwise velocity components. The spanwiseflow was always separated over the riblet peaks, while thestreamwise flow was always attached. Fig. 9 shows a streamwisevortex interaction with riblet surface via viscous effects. Finally,they concluded that the most effective riblet design forenhanced drag reduction contains spanwise and streamwiseprotrusion heights.

    Reidy [160] and Reidy and Anderson [159] evaluated thedrag reduction properties of riblets in turbulent boundarylayers. The first experiment used V-groove riblets over a flatplate in a water tunnel, and the second experiment was con-ducted in a 6-inch diameter pipeline. 3M vinyl riblets wereused in both systems with a height and spacing of h= s= 76.2mm. The effects of the riblets on the water turbulent boundarylayer correlated with the previously reported aerodynamicresults, and resulted in a maximum drag reduction of 8.1%.Pulles et al. [182] conducted experiments in both water

    channels and wind tunnels to study the effect of riblets on thestructure of the turbulent boundary layer. The results showedthat the Reynolds shear stress was noticeably reduced in thelog-law region for a riblet-mounted surface, and the maximumreduction was approximately 10.6% for longitudinally-orientedgroove riblets with h = 2500mm and s = 5000mm.Walsh [183] conducted an experiment in a water towing

    tank to study the effect of riblet films on turbulence flow, andthe effect of peak and valley curvatures on grooved riblets per-

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    Figure 8. Flow visualization images of streamwise vortices overrib surface, with (a) drag decreasing configuration and (b) dragincreasing configuration [7].

    Figure 9. Schematic of streamwise vortex interaction with ribletsurface via viscous effects [181].

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  • formance. The V-groove riblets with dimensions h = 35.56 mmand s = 40.89 mm showed an approximate 6% drag reduction.Small deviations in the riblet peak geometry reduced the ribletdrag reduction performance by as much as 40%, whereas thevalley curvature was not critical to the riblet performance.Neumann and Dinkelacker [184] performed an experiment

    in water with flow velocities of up to 9m s1 to investigate thefriction drag on a cylindrical body. Triangular-shaped ribletswith h= s= 152 mm dimensions were used. A 9% turbulentdrag reduction was achieved relative to the flow over a smoothsurface. Additionally, a 13% drag reduction was observed inthe laminar to turbulent flow transition region. The flow tran-sition on a cylindrical body was delayed when the body wascovered with riblets of a certain size.Rohr et al. [164] and Liu [169] investigated the perform-

    ance of 3M vinyl film riblets in internal and external turbu-lent flows. The maximum drag reduction observed was 9%for h+ = s+ = 12.2. No significant change in drag was observedfor the laminar flow region; additionally, the transition wassubstantially delayed by riblets.Parker and Sayers [185] used V-groove riblets of h/s of 0.22

    and 1 for h of 0.25 and 0.5 mm, respectively, to reduce the vis-cous drag on a body. The riblets were machined longitudinallyonto the surface of a smooth plate. The resulting effect on thedrag force of the plate showed that the V-groove riblets reducedthe turbulent skin friction drag by up to 7%, depending on thesize of the riblets. A boundary layer analysis of the turbulentflow characteristics over both the smooth and riblet-machinedsurfaces indicated an increase in the laminar sublayer thickness

    and local Reynolds number and a reduced boundary layerthickness for the ribbed surfaces. A maximum drag reductionof 6.83% was recorded for the surface covered with the sym-metric riblets (i.e., h/s = 1), at a Reynolds number of 1.17 105

    as in Fig. 10.

    The authors concluded that the riblets interrupted themomentum and turbulent energy exchanges from regions ofhigh velocity to lower-velocity regions. The riblets impeded thecross-flow of streamwise vortices that prevailed in the viscoussublayer of the turbulent boundary layer. By suppressing these

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    Table 3. List of experimental studies conducted in water on flat plates with different types of riblets.

    Original authors Year Riblet type Riblet dimensions Drag reduction percentage

    Gallagher and Thomas [123] 1984 V-groove h+ = 15h = s = 1650 mm

    No drag reduction

    Bacher and Smith [122] 1986 V-groove h+ = s+ = 15 Drag decrease

    Reidy [160]Reidy and Anderson [186]

    19871988

    V-groove s+ = 13.1h = s = 76.2 mm

    8.1%

    Djenidi et al. [129] 1989 V-groove h+ = s+ = 15s = h = 2000 mm

    4%

    Pulles et al. [182] 1989 Triangular h = 2500mm 10.6%

    Walsh [183] 1990 V-groove h = 35.56mms = 40.89mm

    6%

    Liu [187] 1990 V-groove s+ = 13 7%

    Neumann and Dinkelacker [184] 1991 Triangular h+ = s+ = 1015h = s = 152 mm

    13%

    Rohr et al. [164] 1992 V-groove h+ = s+ = 12.2h = s = 72.6 mm

    9%

    Parker and Sayers [185] 1999 V-groove h/s = 0.221 6.83%

    Wang et al. [131] 2000 Trapezoidal grooves h = 1000mms = 2000 mm

    Dean and Bhushan [188] 2012 V-groove h = 254mmh/s = 0.3,0.5,0.7

    Figure 10. Direct drag data for the V-groove ribbed, with ribletsurface 1 (h/s = 0.22, h = 0.25mm) and riblets surface 2 (h/s = 1,h = 0.5mm) [185].

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  • streamwise vortices, turbulent mixing and hence, turbulentshear stresses were reduced.Several challenges confronted the investigations of drag

    reduction over ribbed surfaces in wind and water channels. Inwind channels, because the viscous sublayer was less than0.1mm thick, the turbulent shear stress was extremely smalland was difficult to accurately measure by force balance. Addi-tionally, convection issues inside a wind tunnel resulted in poorexperimental conditions. In water channels, the experimentalconditions were similar or worse. At the required very lowspeeds, the measuring times became so large that accurateshear stress measurements were virtually impossible [189].Because the dimensions of drag reducing structures are as

    small as the thickness of the viscous sublayer, a channel with aliquid viscosity greater than those of water and wind would beunrealistic to model. Therefore, an oil channel was designedand fabricated by Bechert et al. [189] to investigate the dragreduction and the dynamics of the oil viscous sublayer near thewall on smooth and ribbed surfaces. It was found that thethickness of the viscous sublayer could be varied between14mm. The shear stress data from the oil channel had anaccuracy of +0.2%. Additionally, the spacing between ribletscould be as large as 310 mm compared with the 0.5mm ribspacing for wind channels.Bechert et al. [112], conducted extensive investigations using

    an oil channel to study the effects of the sizes and shapes of rib-lets and the ratios of the riblet spacing to height (Tab. 4). Babyoil was used in a rectangular channel (25 cm width and 85 cmheight). The results show that for V-groove riblets with dimen-sions a = 60, s = 3.034mm, and s+ = 15, the maximum dragreduction was approximately 5%. For the semicircular scal-loped riblets, the highest drag reduction was 7.5% at s+ = 14.For blade riblets, which are thin plates aligned in the flow di-rection and perpendicular to the wall, the maximum drag re-duction achieved was nearly 10% at h = 0.5 s.Bechert et al. [128] similarly tested the channel three-dimen-

    sional trapezoidal riblets which covered 64% of a test plate.The maximum drag reduction reported was 6.85% for ribletswith dimensions h/s = 0.85, a = 45, s = 0.46 cm, and s+ = 16.Two-dimensional riblets produced larger drag reductions thanthree-dimensional riblets.Gruneberger and Hage [190] optimized the shape of riblets

    based on results available from many laboratories. Differentshapes, including the triangular, rectangular, trapezoidal, saw-tooth, and scalloped cross-sections, were investigated throughexperiments and simulations.

    5 Combination Studies of Riblets andPolymer/Surfactant

    In this section, the literature that studied the effects of combin-ing riblets with active means on reducing the drag force is sum-marized. Reidy and Anderson[159], as mentioned earlier, con-ducted investigations over a flat plate system and in a pipelinesystem with smooth and ribbed surfaces. They also studied theeffect of adding 2wppm of polyacrylamide to the water flowover the ribbed surface in the pipeline system. The resultantmaximum drag reduction was 28%, which is approximatelyequal to the sum of the drag reductions of the two techniquesused separately. Rohr et al. [192] conducted experimental stud-ies at low Reynolds numbers with 2.5, 10 and 40wppm solu-tions of polyethyleneoxide on smooth and ribbed surfaces in apipe. 3M V-groove riblets with dimensions h = s = 76 mm wereused in a 12.7mm pipe. The riblets had no observable effect onthe polymer solutions, as the calculated drag for the combinedsystem was the same as the drag reduction ratio of the polymersolution alone.Christodoulou et al. [193] investigated the effects of the

    combined systems of V-groove riblets with dimensions ofh= s= 110 mm and 250wppm of Polyox 301 or polyacryl-amide in a 25.4mm pipe. At low concentrations of either poly-mer, i.e., 210 wppm, and with h+ < 10, the maximum dragreduction achieved was 4%. Anderson et al. [165] conductedan experiment at low polymer concentrations over ribbedsurfaces in turbulent pipe flow. Polyox 301 (at 2, 4, and8wppm) and guar gum (at 100wppm) solutions withV-groove riblets heights (4 < h+ < 90) were used in this study.It was found that at low concentrations of polymers, 57%drag reduction was observed for a riblet dimension of h+ = 12.Bewersdorff and Thiel [194] investigated the friction behav-

    ior of diluted polymer and surfactant-based DRAs in a smoothpipe and two rough pipes, with k and d-type roughness. Thesurfactant solution exhibited a drag reduction in the smoothand rough pipes; the polymer and surfactant solutions had noinfluence on the roughness of the solutions.Koury and Virk [195] used aqueous solutions of a polyethy-

    leneoxide polymer (3 to 100wppm) as a drag reducing agentwith 150 mm wide V-groove riblets in a pipe at Reynolds num-bers of 300 to 150 000. It was found that the drag reduction inthe ribbed pipe was 1.6-factors higher than that of smoothpipes when using 3wppm polyethyleneoxide polymer. Theyobserved that with an increasing polymer concentration, thedrag reduction by the riblets decreased to 0.8 for smooth pipes

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    Table 4. List of experimental studies conducted in oil on flat plates with different types of riblets.

    Original authors Year Riblet type Riblet dimensions Drag reduction percentage

    Bechert et al. [112] 1997 Blade riblets h = 0.5 s 9.9%

    Bechert et al. [128] 2000 Trapezoidal 3D-riblets s+ = 16h/s = 0.85

    7.3%

    Gruneberger and Hage [190] 2011 Trapezoidal groove h+ = 8.5s+ = 17

    7.6%

    Buttner and Schulz [191] 2011 Blade riblets s+ = 20.5 4.9%

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  • (at 10wppm polymer). The authors concluded that the ribletsand polymers reduced drag by separate mechanisms. In termsof the turbulent burst cycle, the riblets inhibited the growthstage and reduced the burst strength, whereas the polymersaltered the breakdown stage and reduced the axial to transverseenergy transfer.Recent studies were conducted by Chen et al. [196], Zhao

    et al. [197], and Zhang et al. [198, 199], in which they proposedand investigated a new technique based on replication to pro-duce micro-structured, precise, shark skin-like surfaces. Theprocess prepared replicated fresh shark skin. The best piece offresh shark skin was selected based on intact micro-riblets.Then, the skin was cleaned with distilled water and baked in anoven at 60 C for 3 h to increase its rigidity. A mold of the sharkskin was then cast using unsaturated polyester resin under vac-uum. Multilayer fiberglass was added to prevent distortions inthe riblets and to prevent the mold from cracking. Those inves-tigations showed a 24.6% maximum drag reduction. Addition-ally, the group investigated a drag reduction combination ofbioreplicated shark skin and a polymer additive and found thatdrag reduction rates reached up to 80%.

    6 Conclusion

    The first inspiration from nature motivated large numbers ofresearchers to simulate the effect of the evenly structured sharkskin profile to enhance the flow in conduits, pipes and sub-merged surfaces as well. Despite all these efforts, the achieveddrag reduction using riblets is still low (around 10% as an aver-age) and that might be due to the shapes and dimensions opti-mizations that limit exploring the real effect of such phenom-ena. One of the difficulties facing the researchers working withriblets is the cost of conducting experiments covering all the in-teractive and complex variables involved. This is why a largenumber of simulation works (CFD works) where observed inthe literatures using two dimensional models to evaluate theflow behavior over riblets. Such efforts were successful in intro-ducing close pictures of the real fluid flow behavior over thestructures surfaces, but fail to optimize completely the dimen-sions-flow relation (which was obvious from the high differen-ces between the experimental and theoretical results). For allthat, the need for the three dimensional modeling is a need atthis stage to narrow the gap between the experimental and the-oretical works. The three dimensional model can give a newinsight towards highlighting the effect of the reblits structure(shape and design) and length which will create a three dimen-sional turbulence structure and that can visualize clearly thereal effective factors controlling the flow behavior over suchstructured surfaces.It is a fact that most of the instability occurs in the buffer

    layer over the structured surfaces and the laminar sub layerwas not examined properly and most of the works done onthese two layers was through the simulation using CFD andsome experimental tests that never revealed the real effect.This is why it is important to go down to the micro-scale ex-periments by implementing the micro-channels experimentswhere riblets can be built-up using photonic lithography andthe flow observation can be done through microscope PIV.

    Such an approach will help in understanding the real flowbehavior over the structured surfaces and to suggest real op-timized dimensions for the riblets designs especially whentesting liquids with different physical properties (especiallyviscosity). This will contribute significantly to the optimiza-tion efforts because it will reveal the effect of these propertieson the flow behavior in the micro-channels by eliminatingthe flow core bulk effect.

    Acknowledgment

    This work was supported financially by the University MalaysiaPahang (UMP) through the Fundamental Research Grant.

    Hayder A. Abdulbari is thedirector of the Center of Excel-lence for Advanced Researchin Fluid Flow at the UniversityMalaysia Pahang. He was ap-pointed to the University ofBaghdad and University ofTechnology from 2000 to 2007and then joined UniversityMalaysia Pahang in 2007 untilnow. He has authored andpublished a large number ofpapers in the fields of dragreduction, pipeline flow sys-

    tems, lubricants, water treatment systems, and emulsions.The research works carried out by him and his co-research-ers was internationally recognized (22 patents) and hereceived many international awards from USA, Korea,Malaysia, Germany and more.

    Zulkefli B. Y. Hassan gradu-ated from Camborne School ofMines with a B.Sc. (Hons) inMining Engineering in 1984.He obtained his M.Sc. in Natu-ral Gas Engineering fromTexas A&M University in1989. In 1997, he gained hisPh.D. from Salford Universityfor his research thesis titledModeling and Simulation ofTransient Gas Flow. At theend of 2011, he started histenure at the University Ma-

    laysia Pahang in the Faculty of Chemical and NaturalResources Engineering. He is is currently the Director of theIndustry Partnership and Community Relation of the Uni-versity and has authored and published numerous papers injournals and conferences. His research interest include oiland gas flow technologies.

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  • Hassan D. Mahammad stud-ied Chemical Engineering atthe University of TechnologyIraq from 20052009 and re-ceived his M.Sc. in BioprocessEngineering from the Univer-sity Malaysia Perlis in 2013.He then moved to Pahang andstarted his Ph.D. at the Univer-sity Malaysia Pahang in the re-search group of Dr. HayderA. Abdulabari in Chemical En-gineering. His current researchinterest is mostly focused ondrag reduction in pipeline.

    Symbols used

    h [mm] riblet heighth+ [] non-dimensional riblet heighthpc [mm] height for the cross flowhpl [mm] height for the longitudinal flows [mm] riblet spacings+ [] non-dimensional riblet spacing

    Abbreviations

    BLAD boundary layer deviceDRA drag reducing agentLEBU large eddy breakup deviceOLD outer layer devicePIV particle image velocimeterTAPPM tandem arrayed parallel plate manipulator

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