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6th International Offshore Industries Conference 4 and 5 May 2015 – Tehran, Sharif University of Technology

Drag Reduction Analysis of Flat Plate Using Riblets

Saeed jaamei1, Alireza heydarian2, Alireza kazemipour3,Atoosa bahrani4

1 Assistant Professor, Persian Gulf University, Faculty of Engineering Department of Marine Engineering Bushehr 75169, Iran; [email protected]

2 Master Student, Persian Gulf University of Bushehr, Iran; [email protected] 3 Undergraduate students, Persian Gulf University of Bushehr, Iran; [email protected]

4Undergraduate students, Persian Gulf University of Bushehr, Iran; [email protected]

AbstractEngineering marvels found throughout living nature continually provide inspiration to researchers solving technical challenges. For example, skin from fast-swimming sharks intrigue researchers since its low-drag riblet microstructure is applicable to many low drag and self-cleaning (antifouling) applications.In recent years, such as land and air transport, sea transport in achieving a high rate of interest is. Given that that major part of the drag exerted on the vessel, the frictional drag, reducing the drag is of particular importance. In this review, nature as a guide to achieve the target is used. By Study the skin a high-speed marine animals, including sharks and dolphins create the riblet surface production. Between various methods of reducing the frictional drag, use riblet and micro level regarding practicality and drag reduction mechanism, it is beneficial to use on large vessels. Drag reduction in turbulent flow using riblet is studied in this project. 3D printing and molding techniques such as micro-particles riblet surfaces of manufacturing methods that have been studied.

Keywords: Drag reduction, shark, Riblet, Micro surface,

IntroductionInspired by designs found throughout living nature, researchers are reverse engineering the world’s flora and fauna to solve technical challenges. Much attention is given to structures and materials since living nature efficiently uses resources and incorporates ingenious designs to survive. Therefore by using lessons from living nature, bioinspired designs are serving as the basis for many new innovations. In living nature, certain flora and fauna benefit significantly from low drag and antifouling. In particular, the skin of fast swimming sharks is especially intriguing due to its low drag and antifouling properties. For instance, in the marine environment whales are often covered with barnacles, however sharks remain clean, as illustrated in Figure 1, Even though these two marine creatures live in the same environment, the shark seems unaffected by the biofouling. It is believed sharks stay clean due to their micro structured riblets, flexion of dermal denticles, and a mucous layer. Lower drag is necessary for shark survival, since it allows sharks to swim faster in order to catch prey. The subsequent increased fluid flow velocity at the skin reduces microorganism settlement time and promotes antifouling. In addition, microorganisms larger than the spacing between riblets are unable to effectively adhere to the surface and ultimately colonize the skin, which further promotes antifouling [1]. Low drag and antifouling surfaces have been the subject of much experimentation using shark skin riblet-inspired micro-textured surfaces. An ideal surface would withstand harsh environments, adhere to a variety of substrates, combine both low drag and antifouling properties, and be relatively inexpensive. Determining the optimal riblet surface morphology for maximum drag reduction has been the focus of many efforts. Experimental results indicate useful information about riblet production and geometries. Previous experiments have utilized a variety of riblet geometries, configurations, materials, fluids, and flow conditions (laminar and turbulent flow).

Figure 1: Biofouling in the marine environment. Images highlight differences between Humpback whales and sharks

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Geometry of riblets include blade, saw tooth, scalloped, and bullnose geometries with continuous and segmented configuration (aligned and staggered). Larger scale drag reducing efforts have included the experimental 3M Corp. (Minneapolis, MN) vinyl riblets as well as the commercially available Speedo brand Fast Skin [13] fabric racing swimsuit. Such riblet technology has captured the attention of the National Aeronautics and Space Administration (NASA), U.S. Department of Energy, U.S. Navy, Airbus, Boeing, as well as Olympic competitors. In the 1984 Los Angeles Olympics and 1987 America’s Cup, 3M riblets were applied to U.S. boats, which presumably helped to secure victories. The 2008 Beijing Olympics witnessed the benefit of the Speedo Fast Skin swimsuit when American Michael Phelps set Olympic records and won several gold medals. Reportedly, the swimsuits reduce drag up to 4% for men and 3% for women [2].Application of riblets for drag reduction The transition between research and application of technologies is often slow, and riblet surfaces have not been different. Because of the limitations of past riblet technologies, both benefit in commercial applications and the methods of application have been limited. Because riblets provide drag reduction on objects where the dominant form of drag is caused by turbulent flow at the surface, only objects of a certain form factor will show any measurable benefit. A large portion of the total drag on long objects with relatively flat sides usually comes from turbulence at the wall, so riblets will have an appreciable effect. However, for objects like automobiles, where pressure drag or flow separation is the dominant form of drag, application of riblets would have minimal effect. Beginning in the mid-1980s, vinyl film sawtooth riblets have been applied to boat hulls for racing. Both an Olympic rowing boat and an Americas Cup sailing yacht have been covered with riblets during competition [3]. Because skin friction of an airplane accounts for as much as 48% of total drag [4], vinyl film riblets have also been applied to test planes of both Boeing and Airbus. These films have not been seen use on standard commercial flights yet, but the benefits seen in testing should not go unmentioned. Application of riblets to an airplane requires that several concessions are made. Several locations that would be covered by riblets must be left uncovered due to environmental factors; windows are not covered for the sake of visibility, several locations where dust and debris contacts the airplane during flight are left bare because the riblets would be eroded during flight, and locations where deicing, fuel, or hydraulic fluid would come in contact with the riblets are left bare. After these concessions, the riblets covering the remaining 70% of the aircraft have provided 3% total drag reduction. This 3% drag reduction correlates to a similar 3% savings in fuel costs.Another large commercial application for riblet technologies is drag reduction in pipe flow. Machining the surface or applying vinyl film riblets proves difficult in the confines of most pipes, and an alternate solution must be used. Experimental application of a scratching technique to the inside surf ace of pipes has created a riblet like roughness that has provided more than 5% drag reduction benefit. Stemming from an old sailors’ belief that ships sail faster when their hulls are sanded in the longitudinal direction, Weiss [5] fabricated these riblets by using a steel brush moved through the pipeline to create a ridged surface. Studies have shown as much as a 10% reduction in fluid flow with the combined effect of cleaning the pipe and ridging the surface. Tests on a 10 mile gas pipeline section have confirmed this benefit during commercial operation.The dominant and perhaps only commercial market where riblet technology for drag reduction is commercially sold is competitive swimwear. The general population became aware of shark skin’s drag reduction benefits with the introduction of the FastSkin® suits by Speedo in 2004. Speedo claimed a drag reduction of several percent in a static test compared to other race suits. However, given the compromises of riblet geometry made during manufacturing, it is hard to believe the full extent of the drag reduction. It is clear that creating surface structures by weaving threads is difficult. As a result, riblet geometries woven. from thread have limited options of feasible riblet shapes [6].

Mechanisms of Fluid Drag

Fluid drag comes in several forms, the most basic of which are pressure drag and friction drag. Pressure or form drag is the drag associated with the energy required to move fluid out from in front of an object in the flow and then back in place behind the object. Much of the drag associated with walking through water is pressure drag, as the water directly in front of a body must be moved out and around the body before the body can move forward. The magnitude of pressure drag can be reduced by creating streamlined shapes. Friction or viscous drag is caused by the interactions between the fluid and a surface parallel to the flow as well as the attraction between molecules of the fluid. Friction drag is similar to the motion of a deck of cards sliding across a table. The frictional interactions between the table and the bottom card, as well as between each successive card mimic the viscous interactions between molecules of fluid. Moving away from the surface of an object in a fluid flow, each fluid layer has a higher velocity until a layer is reached where the fluid has velocity equal to the mean flow. Fluids of higher viscosity the attraction between molecules have higher apparent friction between fluid layers, which increases the thickness of the fluid layer distorted by an object in a fluid flow.For this reason, more viscous fluids have relatively higher drag than less viscous fluids [7]. A similar increase in drag occurs as fluid velocity increases. The drag on an object is in fact a measure of the energy required to transfer

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momentum between the fluid and the object to create a velocity gradient in the fluid layer between the object and undisturbed fluid away from the object’s surface.The above discussion of friction drag assumes all neighboring fluid molecules move in the same relative direction and momentum transfer occurs between layers of fluid flowing at different velocities. Figure 2 shows an image of the transition between laminar flow and turbulent flow, in which molecules move in swirling and cross stream motions such that an average velocity is maintained in the direction of flow. The inclusion of cross-flow and nonparallel relative velocities between molecules in turbulent flow causes a dramatic increase in momentum transfer. The cross flow momentum transfer is of particular interest, as all momentum transferred parallel to the surface of an object results in a corresponding increase in drag. Natural transition occurs from laminar to turbulent flow regimes near a Reynolds number around 4,000 for pipe flow and 500,000 for flow over a flat plate [8].

Figure 2: Transition between laminar and turbulent flow in fluid over a flat plate[2]

The small riblets which cover the skin of fast-swimming sharks are used to reduce the increased drag that exist in turbulent flow in two ways: First by prevention of the cross-stream translation of the stream wise vortices in the viscous sublayer and then by elevating the high-velocity vortices above the surface, reducing the shear stress and momentum transfer. The first mechanism has not been fully understood fully, because the interaction of the riblets with vortex translation, is complicated. On a practical level, impeding the translation of vortices reduces the occurrence of vortex ejection into the outer boundary layers and momentum transfer caused by tangling and twisting of vortices in the outer boundary layers.The addition of riblets protruding from a surface is not an obvious option for the reduction of drag at first. One classical drag-increasing feature which riblets exhibit is an increase in total wetted surface area. In the turbulent flow regime, fluid drag typically increases dramatically with an increase in surface area due to the shear stresses at the surface acting across the new, larger surface area. However, as vortices form above a riblet surface, they are held off of the lower structures by the riblet tips, interacting with the tips only and allowing lower velocity flow to dominate the valleys of the riblets [9]. Since the higher velocity vortices interact only with a small surface area at the riblet tips, only this localized area experiences high shear stresses. The low-velocity fluid flow in the valleys of the riblets produces very low shear stresses across the majority of the surface of the riblet. By keeping the vortices above the riblet tips, the cross-stream velocity fluctuations inside the riblet valleys are much lower than the cross-stream velocity fluctuations above a flat plate.This difference in cross-stream velocity fluctuations is evidence of a reduction in shear stress and momentum transfer near the surface, which minimizes the effect of the increased surface area [10]. Though the vortices remain above the riblet tips, some secondary vortex formations do occur that enter the riblet valleys transiently. The flow velocities of the se transient secondary vortices are such that the increase in shear stress caused by their interaction with the surface of the riblet valleys is small. Additionally, the protrusion of the riblets into the bulk flow increases the overall cross-sectional area of the object which has been covered with riblets. In external flows, this increase in cross-sectional area is minimal, and the drag-increasing effects are easily overcome by the riblets. Protruding into the flow without greatly.

Studies with Various Riblet Geometries:Many types of riblets have been studied, from riblets which have a shape resembling natural riblets to those which bear little resemblance to the riblets on shark skin but sought to test all facets of riblet drag reduction. Sometimes, riblet

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shapes have been chosen for their ease of fabrication. A review of riblet optimization studies has been provided to summarize the findings of various researchers. Figure 3 shows different types of riblets and drag reduction of them.

A B

Figure 3: A-Typical shark skin inspired riblet geometries [2], B- Shear stress for different riblet geometries [9]The size of ribletsIn addition to experimenting with actual shark skin or replicas, research has been conducted by fabricating shark skin inspired artificial riblets. Such riblets represent those found on actual shark skin; however their shape and size differ in order to optimize the tradeoff between drag reduction and feasible manufacturing techniques. Optimizing riblet geometries is twofold; one is to lift and pin vortices, and the other is to minimize drag (skin friction) due to the riblets themselves [2]. Since riblets protrude into the flow channel, the increased surface area equates to increased drag. In order to optimize drag reduction, the rib-lets should lift and pin the vortices as well as encourage anisotropic flow along the surface. Such efficient flow is found by considering each valley between riblets and minimizing the wetted perimeter, since increased wetted perimeter leads to increased drag. Another consideration for riblet optimization includes utilizing dimensionless parameters (measured in wall units). These are denoted by the + symbol, which allow for better comparison of experiments with various riblet geometries and flow conditions. Minimizing energy losses can be accomplished with riblets that effectively lift and presumably pin the turbulent vortices. In general, skin friction or viscous drag increases with higher Reynolds numbers, which is related to energy dissipated through heat loss. Similarly, Reynolds numbers based on riblet parameters can also be calculated, which are useful for riblet optimization. Important riblet parameters include spacing (s), height (h), and thickness (t), which are described below. The Reynolds numbers based on spacing and height of the riblets are defined as [11]:

s+¿=

sV τν

¿

h+¿=

hV τν

¿

The Reynolds numbers based on thickness is being defined here as:

t+¿=

t V τ

ν¿

Where V τ is the wall shear stress velocity, Considering kinetic energy, the wall shear stress expression

τ 0=ρV τ2

Provides the wall shear stress velocity as [12]:

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V τ=( τ0

ρ )12

Wall shear stress can be estimated for round pipe flow using the equationτ 0=0.03955 v1 /4 ρV 7 /4D−1/4

Where V is the average flow velocity and d is the hydraulic diameter.

Numerical analysisIn this simulation in ansys CFX, a flat plate that in by riblet film and without riblet, are placed in fluid flow at various velocity. Drag coefficient and drag force of plates captured on below curves. The dimension of the riblet that applied on the plate h=150μm , s=300 μm. the simulation condition include pressure base because the fluid is incompressible and consider the flow regime is steady state and viscous and turbulency model for simulation is shear stress transport(sst).In this simulation, the working fluid is water at 25 ° C and the boundary conditions are inlet velocity, pressure outlet, and no slip wall. the flat plate is placed in the open channel and they are submerged. The area of simulation plate is 11*10 cm. Based on the geometry of the above, we are creating riblet on the plate. ansys CFX software by solve the continuity equation (1) and Navier-Stokes momentum equation (2) analyzed.

∂ ρ∂t

+∇ . ( ρv )=0 (1)

∂∂ t

( ρv )+∇ . ( ρvv )=−∇ p+∇ .(τ ) (2)

Figure 4: flat plate in rectangular duct

Pressure distribution in simple plate indicated that large area of plate are in high value pressure and it means that the plate received higher force.

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Figure 5: Pressure Contour of smooth flat plate

Pressure distribution in riblet plate shows that the pressure uniformly distributed on the plate and it means that plate received less force.

Figure 6: Pressure Contour of riblet plate

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Figure 7: Drag coefficient based on the Reynolds number for simple and riblet plates at different speeds

The diagram represents the drag coefficient of the Riblet plate less than a simple plate. As a result drag force in this plate less is less than simple plate.

5.11*10^6 7.67*10^6 1.02*10^7 1.28*10^7 1.53*10^70

100

200

300

400

500

600

700

64.23

144.28

256.32

399.82

575.35

37.2182.57

145.6

226.43

324.51

Drag ForceSimple Plate Riblet Plate

Figure 8: Drag force based on the Reynolds number for simple and riblet plates at different speeds

Plots showing numerically simulated flow through a duct with virtual riblet surface on bottom and flat surface on top. A magnified view of a region of interest from the duct is shown below. Virtual riblets have sawtooth cross section with h=180μm , s=300 μm.it was empirically tested that the spacing between the riblets should be around twice as large as the height of the riblets. This results in

hs = 0.5-0.6

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ℜNumber

CD

ℜNumber

Drag Force(N)


6^01*11.5 6^01*76.2 7^01*20.1 7^01*82.1 7^01*35.1100.0-

999999999999999000.0

300.0

500.0

700.0

900.0

110.0

310.0

86010.056110.0 69110.0

35210.0 6210.0

435400.0

421600.0286600.0 249600.0 80700.0

tneiciffeoC tfiLetalp telbiR etalp htoomS

Figure 9: Lift coefficient on the Reynolds number for simple and riblet plates at different speeds

Results and Discussion

Results indicate that the smaller height riblets provide the greatest drag reduction. Larger sized riblets and higher Reynolds numbers provide less drag reduction, and in some cases the drag increases compared to the smooth plate. Research investigating the performance of riblets for viscous drag reduction has received considerable attention during the last two decades. riblet films with adhesive backing manufactured by 3M company, USA have been utilized very widely in riblet research both in wind tunnels and in flight tests. In this review, we have addressed primarily the effectiveness of 3M riblets for turbulent skin friction drag reduction on airfoils, wings and wing-body combinations in different speed regimes; these applications bring in issues of riblet performance in pressure gradients and in the presence of three-dimensionality. Based on the available experimental data, certain broad conclusions are drawn, which are informative both from the point of view of design applications as well as flow features associated with riblets.

Conclusions

The reduction of skin friction in aviation industry helps to save fuel, which is a positive aspect for the environment and could also lead to lower costs. One promising way to decrease the skin friction is the structuring of surfaces with riblets. Due to those riblets, the turbulent momentum transfer at the wall, which is responsible for the skin friction, is hampered. This effect was adopted from the shark skin since sharks also have tiny riblets on their scale which have proven to reduce the skin friction. This effect of friction reduction works for fluidic flows, including gaseous flow Depending on the riblet design, theoretically, a wall shear stress reduction of up to 10 % can be achieved. A reduction up to 8 % is technically feasible with an optimum riblet geometry and with a riblet aspect ratio of approximately 0.5. The wall shear stress reduction of 8 % would lead to several tons of fuel savings per tankful.

References[1]- Bechert, D. W., et al. (2000). "Experiments with three-dimensional riblets as an idealized model of shark skin." Experiments in Fluids 28(5): 403-412. [2]- Bixler, G. D. and B. Bhushan (2013). "Fluid Drag Reduction with Shark-Skin Riblet Inspired Microstructured Surfaces." Advanced Functional Materials 23(36): 4507-4528.

[3]- America's Cup Multihull Battle Set For February 2010". The International Sailing Federation. May 14, 2009.

[4]- Weiss MH (1997) Implementation of drag reduction techniques in natural gas pipelines. Paper presented at 10th European drag reduction working meeting, Berlin, Germany, 19–21 Mar 1997

[5]- Han, M., et al. (2003). Fabrication of a micro-riblet film and drag reduction effects on curved objects. TRANSDUCERS, Solid-State Sensors, Actuators and Microsystems, 12th International Conference on, 2003. [6]- Bhushan, B. (2012). Shark skin Surface for Fluid-Drag Reduction in Turbulent Flow. Biomimetics, Springer Berlin Heidelberg: 227-265. [7]- Ei-Samni, O. A., et al. (2005). "Turbulent flow over thin rectangular riblets." Journal of Mechanical Science and Technology 19(9): 1801-1810 [8]- Lee, S. J. and Y. G. Jang (2005). "Control of flow around a NACA 0012 airfoil with a micro-riblet film." Journal of Fluids and Structures 20(5): 659-672.

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CL

http://www.sailing.org/news/28267.php


[9]- Bechert DW, Bartenwerfer M, Hoppe G, Reif W-E (1986) Drag reduction mechanisms derived from shark skin. In: Proceedings of 15th ICAS congress, vol 2 (A86-48-97624-01), Paper # ICAS-86-1.8.3. AIAA, New York, pp 1044–1068 AIAA, New York

[10]- Gyr, A. and H. W. Bewersdorff (1995). References. SHARK SKIN INSPIRED SURFACES FOR AERODYNAMICALLY OPTIMIZED HIGH TEMPERATURE APPLICATIONS-FABRICATION, OXIDATION, CHARACTERIZATION, Springer Netherlands. 32: 219-229. [11]- Choi, K. S. and D. M. Orchard (1997). "Turbulence management using riblets for heat and momentum transfer." Experimental Thermal and Fluid Science 15(2): 109-124 [12]- Huang, A., et al. (2004). "Microsensors and actuators for macrofluidic control." Sensors Journal, IEEE 4(4): 494-502.

[13]- www.speedo.com

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http://www.speedo.com/

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