Design Aspects of Riblets for Fluid Drag Reduction: A Review - Article Example

 Design Aspects of Riblets for Fluid Drag Reduction: A Review ABSTRACT There has been a strong belief that the concept of controlling the flow while delaying the transition from laminar to turbulence was based on the surface smoothness until groove riblets proved the opposite. Riblets are now gaining renewed attention, not only from researchers but also from industries, due to several advantages over manipulated turbulence boundary layer. This is partly because drag reduction has lower energy consumptions of up to 10% for several applications with small but longitudinally grooved surface, and partly because of environmental concerns regarding to active ways. This paper tries to elucidate and provides an overview on the effect of riblets shapes on reducing the effects of drag force. Contents ABSTRACT 1 Introduction 2 Analysis of the shark skin 4 Reduction of skin friction 5 Review of Riblet Grooves as a technique of reducing skin friction 7 Conclusion 13 References 14 Introduction Most of fluid dynamic applications rely on the use of energy to either facilitate movement of solid or solid bodies through fluids as well as maintaining fluid motion over the surface of a solid body (Rohr, Anderson and Reidy, 1989).The two cases require energy in overcoming the available drag force (Walsh, 1990). Due to the increased demand of energy to facilitate the success of various dynamic applications, the world is witnessing increased interest of promoting energy conservation activities, with researchers being on the front line to identify alternative means of reducing consumption (Breus et al, 1993). As a result, the world is experiencing increased research activities to determine and recommend appropriate ways of reducing drag effects in transport pipelines, vehicles and airplanes as well as in other industrial applications (Hage, Bechert&Bruse, 2000, Walsh, 1990). The discovery of the phenomena to reduce drag force boosted the effort among researchers involved in actualizing its effects (Virk, 1971). Many researchers have been trying to come up with appropriate measures that will enable in reducing the impacts of drag force on both the economy and the environment (Alfredsson and Johansson,1989).Many researchers have presented their explanation concerning the effects and cause of drag force between a fluid and solid boundary (El-Samni, Chun and Yoon, 2007). Findings by many researchers associate drag force with friction losses taking place as a result of fluid flow in a thin layer that is adjacent to a given solid boundary (Itoh, 2006, Walsh, M. J. &Lindemann, 1984). Any flow taking place outside the layer in consideration is considered frictionless, since the velocity around it is only affected by boundary shear (Bushnell, 2003). Drag is generated by the boundary layer near the wall due to viscous interaction that exists between the fluid and the surface (Lumley, 1972). The drag generated is the one referred to as skin friction (Jiménez, 1994).In other words, drag is the component of the actual resultant force exerted by fluid over a body aligned towards the comparative motion of the fluid (Suzuki &Kasagi, 1994). A large portion of the total drag force is comprised of skin friction.For instance, (Garcıa-Mayoral revealed that the force comprises 90% of the submarine total drag force, 100% of the long distance total drag force as well as 50% of the drag force experienced in commercial aircrafts (2011).Pipeline transportation is preferred for transporting fluids over long distances, as the method is safe and cost effective (Jiménez &Pinelli, 1999).The flow inside the pipe is mostly turbulent leading to pressure drop, during fluid transportation (Jiménez et al, 2001). Turbulent flow is characterized by fluctuations in pressure, acceleration and shear stress, all associated with position and time (Garcıa-Mayoral, 2011). The fluctuation is responsible for unfavorable flow conditions such as eddies, unsteady vortices, convective flow paths as well as increased skin friction (Finnigan, 2000). Therefore, the increased drag force reverses the role of energy from overcoming viscous drag to pumping the fluid so as to rebuild pressure head (Garcıa-Mayoral, 2011). Thus, presence of turbulent fluid flow will increase the system’s overall cost. Researchers have been actively investigating possible methods that can help reduce losses arising from skin friction (Squire and Savill, 1989). There have been several recommendations, with most of them getting approval for adoption (Bandyopadhyay1986).For example, the technique that has been widely in use is the addition of small quantities of substances referred to as drag reducing agents (Jiménez, 2004). The agents are mainly elastic chemicals that are viscous (Breugem, Boersma and Uittenbogaard, 2006). They are injected into pipelines so as to reduce the effects of drag force, by modifying fluid characteristics so as to reduce onset turbulence (Clark, 1989). This paper focus on the shape of those micro grooves and their effects on reducing the skin friction (drag force). Thus, the problem of losses arising from drag force will be solved by applying micro grooves riblets following critical observation and analysis of the shark skin (Clauser, 1956). Analysis of the shark skin The interest to study effects of riblets on reducing impacts of drag force in fluid flow emanated from a keen observation of the swimming ability of a shark. Following observation of the shark’s swimming techniques; scientists concluded that the fish is among fastest swimming animals (Reidy and Anderson, 1988). The agility of a shark enables it to maneuver and change direction instantly, regardless of its speed (Reed and Weinstein, 1988). It has got a sleek and torpedo-like body profile that helps in minimizing drag (Ladd and Hendricks, 1985). Various studies on the Shark’s skin concluded that the scales could be another source of drag reduction as well as flow separation control (Bacher and Smith, 1985). One of the reasons leading to the study and analysis of the shark’s skin is the grooves in the scales that run parallel with respect to the flow direction, and acts like riblets that decreases drag through determent of the cross flow over the surface (Bechert and Bartenwerfer, 1989). According to the information presented by Bechert, Bruse and Hage, wall shear stress and highest flow velocity is experienced at the fins of the shark (2000). In addition, maximum drag reduction of a shark is attained with the shortest riblets, lower riblet heights. Reduction of skin friction The need to determine affordable and environmentally friendly methods of reducing skin friction can, also, be identified as one of the reasons contributing towards the study of effectiveness of riblet shapes.(Gaudet, 1989).There are many studies conducted with regard to the use of riblets, as a non-additive techniques, to reduce drag effects between a fluid and solid boundary (Beibeiet al, 2011).The techniques developed following extensive research entailed mechanical alteration of the flow control over riblets surfaces (Jung, Mangiavacchi, &Akhavan, 1992). The boundary layer is manipulated by energy transfer under the effects of the fluids natural dynamic interactions along a solid boundary (Gillcrist and Reidy, 1989; Choi, 1989). It means that skin friction drag undergoes a self-noise reduction by shifting turbulence structures away from the wall, thereby reducing drag force through turbulence flow noise (Orlandi& Jiménez, 1994). Passive drag reduction techniques are farther classified into two sub-groups, with regard to local position of manipulators against turbulent boundary layer (Smith and Metzler,1983; White, F W. 1979). The two subgroups are external layer manipulators and internal layer manipulators (Bechert et al, 1997). External layer manipulators involve introduction of thin layers into external part of flow, their reduction effect of drag force is low as compared to internal layer manipulators (Kline SJ and McClintock).According to the research done by Smith and Metzler, the “…external layer manipulators include outer layer devices (OLDs), boundary layer devices (BLADEs), large eddy break up devices (LEBUs) and tandem arrayed parallel plate manipulators (TAPPMs).” (1983). On the other hand, internal layer manipulators are capable of restructuring the surfaces through alteration of the wall geometry into small stream-oriented striations like compliant walls, dimples, oscillating walls and riblets (Koeltzsch, Dinkelacker&Grundmann, 2002). The degree of success of all techniques of drag reduction is determined by various factors. Performance of outer layer manipulators is determined by the aerofoil’s chord length, angle of attack, boundary height, space separating tandem configurations, flat plate devise thickness as well as thickness of the boundary layer (Park & Wallace, 1994).Outer layer manipulators have not been able to obtain total drag reduction among turbulent boundary layers, as their designs facilitated break up of large vortices existing in turbulent flows capable of inhibiting Reynold shear stress next to the walls (Green, Anderson and Edmonds, 1985).Outer layer manipulators consist of ribbons being oriented parallel to the direction of flow (Szodruch, 1991). The devices produced drag reduction net of between 5% and 10% . Reduction of down-stream skin frictions stands at 30%, because not all outer layer manipulators are capable of producing net drag reduction as a result of devise-added drag (Peet, Sagaut and Charron, 2010). Therefore, outer layer manipulators are not able to reduce drag effects in channel flow (Johansen and Smith, 1985). Various researchers have been investigating the effectiveness of using compliant coating and dimples in reducing impacts of drag force on a fluid flow. At the beginning, investigations targeted damping influence of a Dolphin’s malleable skin on turbulence within a given boundary layer (Tang and Clark,1993). The outcome revealed a laminar flow within a fluid flow, with drag force reduction effect of 60% (Goldstein, Handler &Sirovich, 1995). Drag force reduction process takes place in three stages, which include lift-up, followed by oscillation and finely break-up of all low speed streaks (Launder and Li 1991). Also, a lot of effort has been dedicated towards studying effectiveness of using dimples to reduce drag force effects in heat transfer applications (Raupach, Finnigan& Brunet, 1996). Both dimples and riblets belong to the same category of inner layer manipulators (Choi, 1999). Also they have similar characteristics, as they both base on modification of the smooth solid walls (Tang and Clark,1993). However, findings on dimples revealed that their performance concerning heat transfer is less compared to that of fins and tabulators (Py, de Langre&Moulia, 2006).In addition, the findings concerning dimples have been contradictory (McLean, J. D.; George-Falvey and Sullivan, 1987). There are other investigators whose conclusion suggested that dimples help in reducing drag force effect in heat transfer while others differed with the findings by proposing that the use of dimples resulted into an increase of drag force (Launder and Li, 1993). Only shallow dimples were effective in ensuring improvement of heat transfer performance with no increase in pressure drop (Schlichting, 1979). Review of Riblet Grooves as a technique of reducing skin friction Riblets have been the most investigated technique of reducing drag force effects. Drag reduction effects of the riblets entail using longitudinal micro-grooves which are oriented on a surface initially designed for reducing skin friction within a turbulent boundary layer (Baron, 1993). Despite initial research findings recommending the use of smooth surface in delaying transition from laminar to turbulent flow, recent studies provided a contradictory conclusion (Choi, 2000). Available evidence recommended the use of rough surfaces for reducing skin friction within turbulent flow close to the wall (Luchini, 1995). In most cases studies on the use of riblets focus on flow conditions and the geometry in place, to optimize drag reduction performance (Lee & Lee, 2001).Following the increased interest in determining the effectiveness of using riblets, analysis process had to be split into five key areas (Chu and Karniadakis, 1993). The areas included effects caused by riblets geometry on the drag, groove effects on non-Newtonian flow of fluids, groove effects on laminar and the transition flows, simulation analyses as well as modeling studies regarding riblets in various applications (Lee & Jang, 2005). Conclusions arrived at in many discussions about riblets shapes suggest that “…the appropriate riblet dimensions for drag reduction vary depending on their shape and the state of the boundary layer in which they are placed.” (Lee & Lee, 2001). The coherent structure that a particular wall has presents a direct influence on the turbulence energy produced (Crawford 1996). Thus, manipulation of the structure enables reduction of drag force effects. Available information reveal that rib surfaces have a capability of rearranging turbulent structures close to the wall (Goldstein & Tuan, 1998). Thereby their effects are felt around the localized region of the wall (Roskam, 1987). Application of riblets for reducing skin friction favor the use of longitudinal ribs as they are capable of developing lower shear stress unlike a smooth surface (Lowson, Bates and Jewson, 1989). It has been documented that the use of riblets impedes the direction of flow or cross flow near to the wall (Luchini, Manzo&Pozzi, 1991). Thus, random streaks at a lower speed will not be able to converge into an ejection (Dubief, Djenidi and Antonia, 1997).The ejection frequencies reduce by 10 to 20%, while being closely grouped around the riblets. The area of concern, while investigating important parameters associated with drag reduction, has been riblet protrusion height as it offers the offset between virtual origins of the stream wise flow and mean surface location (Coustols&Savill, 1992). The offset is the main distinguishing factor from the smooth surface and it separates turbulent stream wise vortices from the wall, thereby reducing momentum exchange. Also, many suggestions have been publicized recommending that riblets have two mechanisms of reducing drag (Nitschke, 1983). The first mechanism entails capability to shifty away turbulent from the nearest wall (Luchini and Trombetta, 1995). The second mechanism is the provision of a buffer space between structures which promotes reduction of the vicinity of maximum reduction position. Many analysis made, both theoretical and experimental, have been focusing on semi-circular scalloped, triangular, convex, and trapezoidal as well as blade riblets in wind and oil tunnels (Bandyopadhyay 1987). Evidence from the research presented drag reduction value of 8.7%, following riblet spacing of between 2-10mm of an oil tunnel. The investigation focused on both cross-flow and longitudinal flow directions (Ueda and Hinze,1975). The mechanism employed, during the study, depended on the protrusion heights, by locating the virtual origin at the point of extrapolation of stream wise velocity profile (Debisschop&Nieuwstadt, 1996). The difference between the defined height and the riblet peak provided the protrusion height of the longitudinal flow (Rosenhead, 1963). Then extrapolation of span wise velocity profile provided a second protrusion height of the cross-flow (Nakagawa and Nezu, 1981).The difference in size of the two heights depended on the ratio between height (h) and spacing(s) of the riblets (Duan and Choudhari, 2012). Fig 1: a) Longitudinal protrusion height and b) Cross-flow protrusion height (Bechert, Bruse and Hage, 2000). According to the research conducted by Choi et al concerning turbulent and laminar channel flows, rib spacing has the ability to reduce viscous drag through restriction of stream wisevortices’ location above wetted region (1987). By using a fluid of high momentum sweep towards wall surface, choi and his team were able to demonstrate cases of drag reducing and increasing mechanism (Viswanath, 2002).Riblets impeded lateral movement of longitudinal vortices as it swept close to the wall resulting into a lower and shorter premature sweep (Sagrado, 2007). Information presented by Dean supported research findings by Choi, by stating that riblets are capable of reducing velocity gradients among stream-wise streak flows within a boundary layer (Vukoslavcevic, Wallace, Balint, 1992). Also, there is enough evidence to prove the ability of riblets in reducing the number of low-speed streaks by suppressing formation of vortices (Dean &Bhushan, 2010). The end result is a reduction in the intensity and frequency of turbulent bursts. Having attained satisfactory conclusion on reduction of intensity and frequency, investigation process shifted to identification of appropriate theories to explain performance of riblets. One of the theories proposed give more attention on the cross flow of fluids over riblets. The theory postulated that the ability to generate secondary vortices by the use of riblets valleys will result into weakening of the stream-wise vortices that are located above the riblets (Akinlade, 2005, Walsh, 1990). Starting from Choi (1989), Hooshmand et al. (1983), Thomas (1984) and Walsh (1982) and Smith et al, they also suggested that riblets are capable of causing interference to the span-wise motion among low speed streak occurring next to the wall. Choi (1987), also confirmed the findings and argued that riblets have the capability to reduce skin friction by interfering with span-wise movements among longitudinal vortices. This is because the paired vortices on top of riblets surface seem shorter as opposed to those on smooth surface (Choi, Moin, Kim 1993). Also, their span-wise spacing have been found to be shorter thereby confirming suggestions that skin friction in fluid flow is reduced by span-wise movements over riblets. According to Luchin et al, skin friction can be effectively reduced through maximization of the difference in riblets’ protrusion height of longitudinal flow from that of cross flow (1995). Investigation of the boundary layer stability due to riblets leads to Tollmien-Schlichting (T-S) waves to be excited, even at a lower value of Reynolds number. The effect was successfully observed over triangular riblets. Evidence presented by García-Mayoral and Jiménez (2012) attributed the success of riblets in reducing drag force to their new length and groove area. The argument presented outlined that degradation experienced by large riblets belonging to linear regime has no connection with stroke breakdown of longitudinal velocity taking place along riblet grooves (Allen and Tudor, 1969). Available information regarding contribution of geometry to the riblets’ ability to reduce skin friction during a fluid flow, provide approval to the use of triangular profile groove (Nikuradse, 1933). For instance, investigation on the ability of rectangular profile groove to reduce drag force in a fluid flow did not have any effect on the turbulence (Djenidi, Squire and Savill, 1990). On the other hand, the use of a triangular profile grove presented smaller increase in drag force (Sawyer and Winter, 1987). Redesigning of the triangle, by reducing spacing of the riblets produced a V-groove configuration. A test on the V-groove with a height of 15-20 resulted into a drag reduction of 4% (Beauchamp and Philips, 1988). The positive results obtained after conducting tests on V-groove paved way for farther studies, as researchers begun to investigate effectiveness of V-groove in reducing skin friction of the fluid flow (Djenidi, 1991). The growing interest resulted in refinement of the test parameters into three rib geometries (Neumann and Dinkelacker, 1991).The three geometries were peak curvature, valley curvature and notched-peak V-groove (Dong et al, 1982). After the tests, maximum drag reduction of 8% was attained while using v-groove configuration with valley curvature and sharp peak of h+89 to 20, while spacing being set between 15 and 20 (Wallace, 1987). Changing the spacing between V-groove down to 13 to 15 resulted in reduction of drag force of between 7-8% (Dong and Ding,1990). The use of riblets to reduce impact of drag force has been on the rise. Many engineering applications, such as flight testing, have been successful following consideration of riblets’ shapes during the design stage. According to the information presented by Saravi and Cheng, aircraft testing exercise by Boeng, Airbus and NASA was successful due to the effectiveness of riblets in reducing drag force. For instance, Airbus 320 applied 70% of riblets on its surface leading to drag force reduction of 2%. Sareen conducted another test using wind turbines airfoil. The results revealed drag force reduction of 2-4%, having used different sizes of sawtooth riblets with wide range of Reynold’s number (García-Mayoral and Jiménez, 2012). The optimum size during the test was 62 (Saravi and Cheng, 2013). Similar effects were reported by Lee, after performing the test using riblets with optimum spacing between 30 and 70 (2005). According to the research conducted by Bogard and Tiederman, events that characterize Reynold stress production can either be grouped or single with regard to critical time between main events (1986). Another research by Tang and Clark employed the same technique and produced similar findings (1992). Flow visualization is another technique applied to measure the flow of fluids over riblets while paying attention to the behavior of the fluid flow in the valleys of riblets (Saravi and Cheng, 2013). The research by Bacher and Smith (1985) as well as another by Gallager and Thomas (1984) identified a quiescent pooling characterized by low speed of flow in water marked with dye in the valleys of V-groove riblets (García-Mayoral & Jiménez, 2007). Also Clark (1989) and Hooshmand et al. (1983) identified similar results while observing air flows marked with smoke over riblets with sizes 2h+=s+=30 and 2h+=s+=16. Conclusion Many engineering applications such as submarine hulls, piping systems, heat exchangers, channels, ducts and flat plates have demonstrated the effects of surface roughness towards heat transfer and skin friction. Long objects experience drag force as a result of turbulence experienced at the wall. The findings confirm that riblets will automatically have an appreciable effect.It has also been revealed that riblets have a capability of reducing skin friction up to 10%, due to the geometry optimization. Also, it has been determined that the buffer layer experiences instability problem, as long as the flow is continuously turbulent. Therefore, the problem of losses arising from drag force can be solved by applying micro grooves riblets. References 1. 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