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1
West, Nathan, Karl Sammut, and Youhong Tang. "Material selection and manufacturing of riblets for drag reduction: An updated review." Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications 232, no. 7 (April 1, 2016): 610–22. http://dx.doi.org/10.1177/1464420716641452.
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Riblets are a well-researched and understood passive method for achieving viscous drag reduction. Since the 1970s, researchers have found that, with riblets, viscous drag reduction in the order of 8% is achievable in turbulent air and fluid flows. Most of the relevant literature provides insight into the drag-reductive mechanisms of riblets and the effect of riblet morphological design in varying flow conditions. A few recent studies have begun to investigate the influence of material properties on the drag-reductive ability of riblet surfaces with promising results. We here provide an updated review of material selection and riblet manufacture and show current trends. A brief summary is provided of the theories of riblet drag-reductive ability, riblet surface design, the role of material selection for drag reduction and current manufacturing techniques.
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Bai, Xiuqin, Xuan Zhang, and Chengqing Yuan. "Numerical Analysis of Drag Reduction Performance of Different Shaped Riblet Surfaces." Marine Technology Society Journal 50, no. 1 (January 1, 2016): 62–72. http://dx.doi.org/10.4031/mtsj.50.1.9.
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AbstractAiming to investigate the drag reduction performance of an antifouling shell rough surface, six different geometries of shell surface texture are simplified as V-shaped riblet, U-shaped riblet, space-V-shaped riblet, blunt-V-shaped riblet, L-shaped riblet, and ∩-shaped riblet. Five kinds of riblets have the same geometric features: groove height and grooved spacing. SST-k-ω model is adopted for the turbulence model. The flow field structure above the different shaped riblet surfaces as well as shear stress and turbulent kinetic energy are analyzed. Moreover, how the flow is influenced by different shapes of riblets is discussed. The knowledge gained in this study can provide theoretical reference for optimal groove surface design of a ship hull with drag reduction performance.
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Goldstein, D., R. Handler, and L. Sirovich. "Direct numerical simulation of turbulent flow over a modeled riblet covered surface." Journal of Fluid Mechanics 302 (November 10, 1995): 333–76. http://dx.doi.org/10.1017/s0022112095004125.
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An immersed boundary technique is used to model a riblet covered surface on one wall of a channel bounding fully developed turbulent flow. The conjecture that the beneficial drag reduction effect of riblets is a result of the damping of cross-flow velocity fluctuations is then examined. This possibility has been discussed by others but is unverified. The damping effect is explicitly modelled by applying a cross-flow damping force field in elongated streamwise zones with a height and spacing corresponding to the riblet crests. The same trends are observed in the turbulence profiles above both riblet and damped surfaces, thus supporting cross-flow damping as a beneficial mechanism. It is found in the examples presented that the effect of the riblets on the mean flow field quantities (mean velocity profile, velocity fluctuations, Reynolds shear stress, and low–speed sreak spacing) is small. The riblests cause a relatively small drag reduction of about 4%, a figure that is in rough agreement with experiments and other computations. The simulations also suggest a mechanism for the observed displacement of the turbulence quantities away from the wall.The immersed boundary technique used to model the riblets consists of creating an externally imposed spatially localized body force which opposes the flow velocity and creates a riblet-like surface. For unstead viscous flow the calculation of the force is done with a feedback scheme in which the velocity is used to iteratively determine the desired value. In particular, the surface body force is determined by the relation f(xs, t) = α ∫ t0U(xs,t′)dt′ + βU(xs, t) for surface points xs, velocity U time t and negative constants α and β. All simulations are done with a spectral code in a single computational domain without any mapping of the mesh. The combination of the immersed boundary and spectral techniques can potentially be used to solve other problems having complex geometry and flow physics.
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García-Mayoral, Ricardo, and Javier Jiménez. "Drag reduction by riblets." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369, no. 1940 (April 13, 2011): 1412–27. http://dx.doi.org/10.1098/rsta.2010.0359.
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The interaction of the overlying turbulent flow with riblets, and its impact on their drag reduction properties are analysed. In the so-called viscous regime of vanishing riblet spacing, the drag reduction is proportional to the riblet size, but for larger riblets the proportionality breaks down, and the drag reduction eventually becomes an increase. It is found that the groove cross section A + g is a better characterization of this breakdown than the riblet spacing, with an optimum . It is also found that the breakdown is not associated with the lodging of quasi-streamwise vortices inside the riblet grooves, or with the inapplicability of the Stokes hypothesis to the flow along the grooves, but with the appearance of quasi-two-dimensional spanwise vortices below y + ≈30, with typical streamwise wavelengths . They are connected with a Kelvin–Helmholtz-like instability of the mean velocity profile, also found in flows over plant canopies and other surfaces with transpiration. A simplified stability model for the ribbed surface approximately accounts for the scaling of the viscous breakdown with A + g .
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Zhang, Yufei, and Yuhui Yin. "Study on Riblet Drag Reduction Considering the Effect of Sweep Angle." Energies 12, no. 17 (September 2, 2019): 3386. http://dx.doi.org/10.3390/en12173386.
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This study computationally evaluates the riblet drag reduction effect considering the effect of sweep angle. An implicit large eddy simulation is performed on a channel flow and an infinite swept wing. First, three different inclined angles between the riblets and the flow direction are tested in the channel flow. The results show that with increases in the inclined angle, the friction drag decreases, while the pressure drag increases approximately quadratically. The riblets with a 30° inclined angle increase the total drag of the channel flow. Then, an infinite wing with a 30° swept angle with and without riblets is studied. The riblets demonstrate satisfactory drag reduction efficiency because the cross flow over most parts of the wing is mild. The lift and friction drag follow the relation of the cosine law of a swept wing. Moreover, the cross flow and the turbulence fluctuation are suppressed by the riblets.
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Yang, Yu, Zhang Ming-Ming, and Li Xue-Song. "Numerical investigation of V-shaped riblets and an improved model of riblet effects." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 232, no. 9 (April 24, 2017): 1622–31. http://dx.doi.org/10.1177/0954406217705907.
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Symmetric V-shaped riblets are simulated by using the computational fluid dynamic method to understand the riblet effects on the turbulent boundary layer and the skin friction reduction. Three classical turbulence models, namely Spalart–Allmaras, shear stress transport, and re-normalization group k-epsilon models, are investigated under different grid densities. The re-normalization group model produces good results consistent with the experiment, as compared with the existing theoretical and experimental drag results of the flat plate and the V-shaped riblets with different sizes. Simulating V-shaped riblets yield the unexpected discovery that the shear stress transport model produces large errors, and the Spalart–Allmaras model even produces results of qualitative errors. Another finding is that von Kármán’s constants can no longer meet the requirement of describing velocity profiles in the logarithmic law layer. Aside from the traditional shift of the logarithmic law’s intercept, the slope is also changed by riblet height and spacing. Therefore, an improved model of riblet effects is proposed by redefining von Kármán’s constants.
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Zhang, Zi-Liang, Ming-Ming Zhang, Chang Cai, and Yu Cheng. "Characteristics of large- and small-scale structures in the turbulent boundary layer over a drag-reducing riblet surface." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 234, no. 3 (November 13, 2019): 796–807. http://dx.doi.org/10.1177/0954406219887774.
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Riblet is one of the most promising passive drag reduction techniques in turbulent flows. In this paper, hot-wire measurements on a turbulent boundary layer perturbed by a drag-reducing riblet surface are carried out to further understand the riblet effects on the turbulent flows and the drag reduction mechanism. Compared with the smooth case, different energy variations in the near-wall region and the logarithmic region are observed over riblets. Then, by using a spectral filter of a given wavelength, the time series of the hot-wire data are decomposed into large- and small-scale components. It is indicated that large-scale structures in the logarithmic region impose a footprint (amplitude modulating effect) on the near-wall small-scale structures. By quantifying this footprint, it is found that the interactions between large- and small-scale structures over riblets are weakened in the near-wall region. Furthermore, the bursting process of large and small scales is studied. For both large- and small-scale structures, a shorter bursting duration and a higher bursting frequency are observed over the riblet surface, which indicates that riblets impede the formation of large- and small-scale bursting events. The flow physics behind these phenomena are also discussed in detail.
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Radmanesh, Mohammadreza, Iman Samani, Alireza Amiriyoon, and Mohammad-Reza Tavakoli. "The effects of rectangular riblets on rectangular micro air vehicles for drag reduction." Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 231, no. 2 (August 6, 2016): 364–73. http://dx.doi.org/10.1177/0954410016638868.
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Reduced drag, increased lift and, consequently, increased vital ratio and lift-to-drag coefficients are crucial in almost all efficient micro air vehicles. Riblet geometries use a variety of air vehicles. Further investigation on micro air vehicles is, however, necessary for enhanced development. Rectangular riblets on a rectangular micro air vehicle are computationally investigated. In this study, the governing equation of fluid flow is solved numerically; the turbulent model around the NACA S5020 airfoil section is covered by riblets either on both sides or on the upper side of the wings. Results show a difference of behavior in drag reduction due to the angle of attack on the airfoil. When the lift-to-drag coefficient of an angle of attack is at its maximum, an improvement can be observed, where lift-to-drag ratio increases, and drag decreases. Results for the two-side riblets show an increase in the lift-to-drag ratio as well; although the lift-to-drag coefficient and the drag reduction of riblets on both sides were comparatively less than that for riblets on the upside.
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Denkena, Berend, Thilo Grove, and Jan Harmes. "Grinding of Riblets on Curved Paths." Materials Science Forum 874 (October 2016): 28–33. http://dx.doi.org/10.4028/www.scientific.net/msf.874.28.
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The grinding of riblets with multiple profiled grinding wheels is an efficient method to minimize the fluid friction on surfaces. In turbo machinery components, like pump impellers or compressor blades, the riblets must be ground with a curved tool path since the flow is rarely linear on such surfaces. This leads to angular errors in the generated riblet profiles and therefore requires the use of grinding wheels with smaller diameters. The tool wear increases due to lateral strain on the peaks of the grinding wheel. Consequently, the increased wear and the need of smaller tool diameters decrease the efficiency of the process. In this paper a structuring process with dicing blades was investigated in order to increase the economic viability of this process. A dressing operation for such tools is not necessary and thus reduces the non-productive time of the manufacturing process. Furthermore, profile tip wear has no negative effects on the aspect ratio of the generated riblets since the riblet geometry is determined by the thickness of the dicing blades.
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Dean, Brian, and Bharat Bhushan. "Shark-skin surfaces for fluid-drag reduction in turbulent flow: a review." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368, no. 1929 (October 28, 2010): 4775–806. http://dx.doi.org/10.1098/rsta.2010.0201.
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The skin of fast-swimming sharks exhibits riblet structures aligned in the direction of flow that are known to reduce skin friction drag in the turbulent-flow regime. Structures have been fabricated for study and application that replicate and improve upon the natural shape of the shark-skin riblets, providing a maximum drag reduction of nearly 10 per cent. Mechanisms of fluid drag in turbulent flow and riblet-drag reduction theories from experiment and simulation are discussed. A review of riblet-performance studies is given, and optimal riblet geometries are defined. A survey of studies experimenting with riblet-topped shark-scale replicas is also given. A method for selecting optimal riblet dimensions based on fluid-flow characteristics is detailed, and current manufacturing techniques are outlined. Due to the presence of small amounts of mucus on the skin of a shark, it is expected that the localized application of hydrophobic materials will alter the flow field around the riblets in some way beneficial to the goals of increased drag reduction.
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Parker, K., and A. T. Sayers. "The effect of longitudinal microstriations and their profiles on the drag of flat plates." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 213, no. 8 (August 1, 1999): 775–85. http://dx.doi.org/10.1243/0954406991522392.
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The use of surface modifications as a means of reducing viscous drag on a body has potential aerodynamic and hydrodynamic applications. V-grooves of specific dimensions are machined in a longitudinal direction onto the surface of a smooth plate and the resulting effect on the drag force of the plate is observed. Experiments show that V-grooves (riblets) could reduce turbulent skin friction drag by up to 7 per cent, depending on the size of the groove. The drag-reducing performance of riblets with aspect ratios, h/s, of 0.22 and 1 are examined. A boundary layer analysis of the turbulent flow characteristics over the smooth surface and the riblet surfaces indicated an increase in the laminar sublayer thickness and local Reynolds number while reducing the boundary layer thickness for the ribbed surfaces. A maximum drag reduction of 6.83 per cent was recorded for the surface covered with the symmetric riblet, at a Reynolds number of 117 101. It is felt that riblets hamper the momentum and turbulent energy exchange from regions of high velocity to lower-velocity regions. Riblets impede the cross-flow of stream-wise vortices that prevail in the viscous sublayer of a turbulent boundary layer. By suppressing these streamwise vortices, turbulent mixing and hence turbulent shear stress are reduced. Results obtained agree with results suggested from research elsewhere.
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Anderson, G. W., J. J. Rohr, and S. D. Stanley. "The Combined Drag Effects of Riblets and Polymers in Pipe Flow." Journal of Fluids Engineering 115, no. 2 (June 1, 1993): 213–21. http://dx.doi.org/10.1115/1.2910126.
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The additional skin friction effect produced by a 3M riblet surface, used in conjunction with low concentration polymer solutions, is investigated in fully developed, turbulent pipe flow. Generally at the low concentrations of Polyox 301 and guar gum studied, the absolute drag reduction of the 3M riblets appears to be independent of the polymer presence, with a maximum between 5 and 7 percent occurring around h+ = 12. Comparisons with previous polymer studies with 3M riblets, sand roughened and commercially rough surfaces are made.
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STRAND, JAMES S., and DAVID B. GOLDSTEIN. "Direct numerical simulations of riblets to constrain the growth of turbulent spots." Journal of Fluid Mechanics 668 (January 26, 2011): 267–92. http://dx.doi.org/10.1017/s0022112010005033.
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A spectral direct numerical simulation (DNS) code was used to study the growth and spreading of turbulent spots in a nominally laminar, zero-pressure-gradient boundary layer. In addition to the flat-plate case, the interaction of these spots with riblets was investigated. The flat plate, riblets and initial spot perturbation were simulated via an immersed boundary method, and a ‘suction wall’ allowed the available channel code to model a boundary layer. In both flat-wall and riblet cases, self-similar arrowhead-shaped spots formed. The λ2 variable of Jeong & Hussain (1995) was used to visualize the vortical structures within a spot, and a spot was seen to consist primarily of a multitude of entwined hairpin vortices. The range of scales of the hairpin vortices was found to increase as the spot matures. Ensemble averaging was used to obtain more accurate results for the spot spreading angle, both for the flat-wall case and the riblet case. The spreading angle for the flat-wall spot was 6.3°, in reasonably good agreement with prior DNS work. The spreading angle for the spot over riblets was 5.4°, a decrease of 14% compared with the flat-wall.
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Wang, Z. Y., and J. Jovanovic. "Drag Characteristics of Extra-Thin-Fin-Riblets in an Air Flow Conduit." Journal of Fluids Engineering 115, no. 2 (June 1, 1993): 222–26. http://dx.doi.org/10.1115/1.2910127.
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An experimental study of riblets with extra thin fins (5 μm thick) is presented. A drag reduction of 2–3 percent per quarter conduit wall is indicated when h+ is around 3–15 in a square section of air flow conduit lined with the extra-thin-fin-riblets (ETFR) on one side wall. The pressure distributions along the conduit indicate the influence of the riblet front step on the drag reduction performance in the conduit flow. The measurement methods and the detailed structure of the ETFR are also discussed.
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Nakao, Shin-ichi. "Application of V Shape Riblets to Pipe Flows." Journal of Fluids Engineering 113, no. 4 (December 1, 1991): 587–90. http://dx.doi.org/10.1115/1.2926519.
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Pipes with V shape riblets were tested at Reynolds numbers between 5×103 and 4×104. All riblet pipes indicated some drag reduction. The model with h = 0.55 mm and h/S = 0.483 showed the maximum drag reduction of 8 percent and the widest range of Reynolds number over which the riblet reduces drag. The riblet shape desirable for drag reduction in pipe flows was almost the same as that in flat plate boundary layers, but the value of S+ which provided the maximum drag reduction was quite different; S+ = 23 for pipe flows and S+ = 12 for flat plate boundary layers.
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Chu, Douglas C., and George Em Karniadakis. "A direct numerical simulation of laminar and turbulent flow over riblet-mounted surfaces." Journal of Fluid Mechanics 250 (May 1993): 1–42. http://dx.doi.org/10.1017/s0022112093001363.
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The flow in a channel with its lower wall mounted with streamwise riblets is simulated using a highly efficient spectral element-Fourier method. The range of Reynolds numbers investigated is 500 to 3500, which corresponds to laminar, transitional, and turbulent flow states. A complete study is presented for V-groove riblets; the effect of rounded riblets is also investigated. Our results suggest that in the laminar regime there is no drag reduction, while in the transitional and turbulent regimes drag reduction exists (approximately 6 % at Reynolds number 3500) for the riblet-mounted wall in comparison with the smooth wall of the channel. For the first time, we present detailed turbulent statistics (turbulence intensities, Reynolds shear stresses, skewness and flatness) as well as a temporal analysis using a numerical analog of the VITA technique. The flow structure over the riblet-mounted wall is also analysed in some detail and compared with the corresponding flow over the smooth wall in an attempt to identify the physical mechanisms that cause drag reduction. The accuracy of the computation is established by comparing flow quantities corresponding to the smooth wall with previous direct numerical simulation results as well as with experimental results; on the riblet-mounted wall comparison is made with available experimental results. The agreement is very good for both cases. The current computation is the first direct numerical simulation of turbulence in a complex geometry domain.
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Pöplau, Julia, Sebastian Stille, Thijs Romans, Tilmann Beck, Lorenz Singheiser, and Gerhard Hirt. "The Influence of Process Parameters on the Forming of Riblets during Riblet Rolling." Key Engineering Materials 611-612 (May 2014): 715–22. http://dx.doi.org/10.4028/www.scientific.net/kem.611-612.715.
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In aeronautics, economic and environmental aspects become increasingly important. As those are very much influenced by the frictional drag of the airplane, a reduction of skin friction which causes a major portion of total aerodynamic drag is desirable. One possible approach for passive drag reduction is the application of riblets small longitudinal grooves orientated in flow direction. Through an adapted rolling process, riblets can be brought into metal sheets on a large scale. For this process a thin high-strength steel wire is wound around a work roll to structure it with the negative riblet imprint. In a subsequent step the riblet profile is rolled into the sheet material. Different parameters can influence the process and the quality of the resulting riblet structure. Those parameters that depend on the sheets sheet thickness, material strength, and composition of the sheet are discussed in this paper. Form filling is used as an indicator for riblet quality. It is found that decreasing sheet thickness is beneficial for form filling, but a process dependent minimum sheet thickness exists for which this effect will reverse. Material strength is found to have a much smaller influence on form filling. Nevertheless, harder alloys seem to need a slightly smaller thickness reduction, but higher rolling forces and pressures to achieve desired form filling. Using clad instead of bare materials has a positive influence on form filling and riblet structuring. Furthermore, riblet rolling does not reduce the fatigue strength of the clad material.
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Wu, Nan Huo, You Hong Tang, Cheng Bi Zhao, Wei Lin, Xiao Ming Chen, and Run Heng Li. "Numerical Investigation of a Blade Riblet Surface for Drag Reduction Applications with Large Eddy Simulation Method." Applied Mechanics and Materials 187 (June 2012): 315–19. http://dx.doi.org/10.4028/www.scientific.net/amm.187.315.
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Fully developed turbulent channel flow with a blade riblet surface has been simulated numerically at Reynolds number by Large Eddy Simulations (LES). The blade riblet is shown to provide a total viscous drag reduction approximately 9% with the riblet spacing and the cross section . For the sake of investigating the interaction of the turbulent flow with riblets, the mean velocity profiles, velocity fluctuations, and instantaneous flow visualization have been analyzed. It has been found that the riblet of certain size reduces drag by damping the dynamics and weakening the cross motions in the near-wall boundary layer, revealing beneficial turbulence controlling.
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Takahashi, Hidemi, Hidetoshi Iijima, Mitsuru Kurita, and Seigo Koga. "Evaluation of Skin Friction Drag Reduction in the Turbulent Boundary Layer Using Riblets." Applied Sciences 9, no. 23 (November 29, 2019): 5199. http://dx.doi.org/10.3390/app9235199.
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A unique approach to evaluate the reduction of skin friction drag by riblets was applied to boundary layer profiles measured in wind tunnel experiments. The proposed approach emphasized the turbulent scales based on hot-wire anemometry data obtained at a sampling frequency of 20 kHz in the turbulent boundary layer to evaluate the skin friction drag reduction. Three-dimensional riblet surfaces were fabricated using aviation paint and were applied to a flat-plate model surface. The turbulent statistics, such as the turbulent scales and intensities, in the boundary layer were identified based on the freestream velocity data obtained from the hot-wire anemometry. Those turbulent statistics obtained for the riblet surface were compared to those obtained for a smooth flat plate without riblets. Results indicated that the riblet surface increased the integral scales and decreased the turbulence intensity, which indicated that the turbulent structure became favorable for reducing skin friction drag. The proposed method showed that the current three-dimensional riblet surface reduced skin friction drag by about 2.8% at a chord length of 67% downstream of the model’s leading edge and at a freestream velocity of 41.7 m/s (Mach 0.12). This result is consistent with that obtained by the momentum integration method based on the pitot-rake measurement, which provided a reference dataset of the boundary layer profile.
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Alinovi, Edoardo, Mattia Gribaudo, and Alessandro Bottaro. "Fractal Riblets." AIAA Journal 56, no. 6 (June 2018): 2108–12. http://dx.doi.org/10.2514/1.j056985.
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Choi, Kwing-So. "Near-wall structure of a turbulent boundary layer with riblets." Journal of Fluid Mechanics 208 (November 1989): 417–58. http://dx.doi.org/10.1017/s0022112089002892.
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A detailed wind tunnel study has been carried out on the near-wall turbulence structure over smooth and riblet wall surfaces under zero pressure gradient. Time-average quantities as ‘well as conditionally sampled profiles were obtained using hotwire/film anemometry, along with a simultaneous flow visualization using the smoke-wire technique and a sheet of laser light. The experimental results indicated a significant change of the structure in the turbulent boundary layer near the riblet surface. The change was confined within a small volume of the flow close to the wall surface. A conceptual model for the sequence of the bursts was then proposed based on an extensive study of the flow visualization, and was supported by the results of conditionally sampled velocity fields. A possible mechanism of turbulent drag reduction by riblets is discussed.
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Mizunuma, H., K. Ueda, and Y. Yokouchi. "Synergistic Effects in Turbulent Drag Reduction by Riblets and Polymer Additives." Journal of Fluids Engineering 121, no. 3 (September 1, 1999): 533–40. http://dx.doi.org/10.1115/1.2823501.
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Drag reduction was investigated for the combined system of polymer additives and a riblet pipe. The riblet grooves were V-shaped, the spacing of which was 1.3 mm and the height of which was 1.01 mm. For higher h+, a triangular riblet system including other geometries increases the drag to levels similar to those of normal transient roughness. This drag increase was generally given as a function of h+. The polymer additives were Aronfloc N-110 and Separan AP-30. The critical shear stress τ*, at which N-110 started the drag reduction, was approximately eight times higher than τ* for AP-30. In the combined system, the synergistic drag reduction for higher h+ was discussed under the assumption that the additives suppressed the drag increase resulting from riblets. Since the additives thicken a wall layer covering the region from a viscous sublayer to a buffer layer, the relative height of h to this wall layer thickness is lowered. In addition, the flow enhancement due to additives relatively suppresses the riblet-induced drag increase. The analysis based on velocity profiles indicated that these effects can produce synergistic drag reduction for higher h+.
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Choi, Kwing So. "Smart Flow Control with Riblets." Advanced Materials Research 745 (August 2013): 27–40. http://dx.doi.org/10.4028/www.scientific.net/amr.745.27.
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A review of riblets as a smart flow control device is given. After the state-of-the-art examination of the drag reduction performance, different views on the drag reduction mechanism are put in perspective. Being a smart flow control device, riblets can control not only momentum, but also heat and flow noise. Some of the available results are summarized to encourage the readers to consider other applications of riblets. Although useful for research investigations, riblets films are not industrially friendly. Here, recent efforts in manufacturing the micro-groove surfacein situare reviewed, including an overview of patents relating to the production of riblets.
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Oyewola, M. O. "EFFECT OF RIBLETS AND SUCTION ON A FLAT PLATE TURBULENT BOUNDARY LAYER." Revista de Engenharia Térmica 5, no. 1 (July 31, 2006): 78. http://dx.doi.org/10.5380/reterm.v5i1.61676.
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This work presents hot-wire measurements in a flat plate turbulent boundarylayer, subjected to the combination of riblets and suction. The suction is applied through a porous strip for a range of suction rates. The effect of riblets and suction has been quantified through the measurements of mean velocity and Reynolds stresses downstream of the suction strip on the riblets surface. The results of the mean velocity and Reynolds stresses indicate that there is no significant change in the distributions of riblets and smooth wall. However, there exist some changes with the combination of suction and riblets relative to the smooth surface. These changes arise from the interference of suction with the mechanism of the layer. The results suggest that riblets may not alter the effect suction has on the boundary layer structures.
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Tsunoda, Kazumi, Tomohiko Suzuki, and Toshiaki Asai. "Improvement of the Performance of a Supersonic Nozzle by Riblets." Journal of Fluids Engineering 122, no. 3 (May 2, 2000): 585–91. http://dx.doi.org/10.1115/1.1286991.
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This paper describes an experimental study of supersonic internal flow over a riblet surface mounted on a channel wall to reduce pressure loss and improve the performance of a supersonic nozzle. The magnitude of the static pressure in the pressure-rise region observed in channels with riblet surface became lower than that for a smooth surface, and the significance of its difference was indicated by uncertainty analysis estimated at 95 percent coverage. The Mach number distributions obtained by traversing a Pitot-tube showed that the separation point moved downstream and the size of the separation region became small when using riblets. Furthermore, it was found that the stagnation pressure loss reduction was as large as 56 percent in the uniform supersonic flow field at a Mach number of 2.0, and 29 percent in the separation region. [S0098-2202(00)00103-6]
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Chen, Hua Wei, Fu Gang Rao, De Yuan Zhang, and Xiao Peng Shang. "Drag Reduction Study about Bird Feather Herringbone Riblets." Applied Mechanics and Materials 461 (November 2013): 201–5. http://dx.doi.org/10.4028/www.scientific.net/amm.461.201.
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Flying bird has gradually formed airworthy structures e.g. streamlined shape and hollow shaft of feather to improve flying performance by millions of years natural selection. As typical property of flight feather, herringbone-type riblets can be observed along the shaft of each feather, which caused by perfect alignment of barbs. Why bird feather have such herringbone-type riblets has not been extensively discussed until now. In this paper, microstructures of secondary feathers are investigated through SEM photo of various birds involving adult pigeons, wild goose and magpie. Their structural parameters of herringbone riblets of secondary flight feather are statistically obtained. Based on quantitative analysis of feathers structure, one novel biomimetic herringbone riblets with narrow smooth edge are proposed to reduce surface drag. In comparison with traditional microgroove riblets and other drag reduction structures, the drag reduction rate of the proposed biomimetic herringbone riblets is experimentally clarified up to 15%, much higher than others. Moreover, the drag reduction mechanism of herringbone riblets are also confirmed and exploited by CFD.
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Spalart, Philippe R., and J. Douglas McLean. "Drag reduction: enticing turbulence, and then an industry." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369, no. 1940 (April 13, 2011): 1556–69. http://dx.doi.org/10.1098/rsta.2010.0369.
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We examine drag-reduction proposals, as presented in this volume and in general, first with concrete examples of how to bridge the distance from pure science through engineering to what makes inventions go into service; namely, the value to the public. We point out that the true drag reduction can be markedly different from an estimate based simply on the difference between turbulent and laminar skin friction over the laminarized region, or between the respective skin frictions of the baseline and the riblet-treated flow. In some situations, this difference is favourable, and is due to secondary differences in pressure drag. We reiterate that the benefit of riblets, if it is expressed as a percentage in skin-friction reduction, is unfortunately lower at full-size Reynolds numbers than in a small-scale experiment or simulation. The Reynolds number-independent measure of such benefits is a shift of the logarithmic law, or ‘Δ U + ’. Anticipating the design of a flight test and then a product, we note the relative ease in representing riblets or laminarization in computational fluid dynamics, in contrast with the huge numerical and turbulence-modelling challenge of resolving active flow control systems in a calculation of the full flow field. We discuss in general terms the practical factors that have limited applications of concepts that would appear more than ready after all these years, particularly riblets and laminar-flow control.
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Al-Kayiem, Hussain, Desmond Lim, and Jundika Kurnia. "Large eddy simulation of near-wall turbulent flow over streamlined riblet-structured surface for drag reduction in a rectangular channel." Thermal Science 24, no. 5 Part A (2020): 2793–808. http://dx.doi.org/10.2298/tsci190909059a.
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Sharkskin-inspired riblets are widely adopted as a passive method for drag reduc?tion of flow over surfaces. In this research, large eddy simulation of turbulent flow over riblet-structured surface in a rectangular channel domain were performed at various Reynolds numbers, ranging from 4200-10000, to probe the resultant drag change, compared to smooth surface. The changes of mean streamwise velocity gradient in wall-normal direction at varied locations around riblet structures were also investigated to reduce mechanisms of streamlined riblet in reducing drag. The computational model is validated by comparing the simulation results against analytical and experimental data, for both smooth and riblet surfaces. Results in?dicating that the performance of the proposed streamlined riblet shows 7% drag reduction, as maximum, which is higher than the performance of L-shaped riblet with higher wetted surface area. The mean velocity profile analysis indicates that the streamlined riblet structures help to reduce longitudinal averaged velocity component rate in the normal to surface direction of near-wall region which leads to laminarization process as fluid-flows over riblet structures.
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Ninnemann, Todd, and Wing F. Ng. "Loss Reduction Using Riblets on a Supersonic Through-Flow Fan Blade Cascade." Journal of Fluids Engineering 126, no. 4 (July 1, 2004): 642–49. http://dx.doi.org/10.1115/1.1667883.
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An experimental and computational study to determine the effects of riblets on the performance of the Supersonic Throughflow Fan (STF) cascade blades was performed. The cascade was tested in the Virginia Tech intermittent wind tunnel facility, where the Mach and Reynolds (based on chord) numbers were 2.36 and 4.8×106, respectively. The riblet sheets were symmetric v-grooved type and were applied onto the blade surfaces. Three different riblet heights were tested: 0.023, 0.033, and 0.051 mm. Riblet testing was conducted at design incidence as well as at off-design conditions (incidence angles: +5, −10 deg). Loss coefficients were measured and compared with a control test case where an equivalent thickness of smooth material was applied to the blade. Results show that at the design incidence, the riblet sheet with a height of 0.033 mm provides the optimal benefit, with a reduction of 8.5% in loss coefficient compared to the control case. Smaller effects were measured at the off-design conditions. In addition to the experimental study, a numerical investigation of the riblet effect on the STF cascade was conducted at design incidence. A simple method was developed to model riblet effects due to decrease in turbulent viscous drag and the delay of turbulent transition on the blades. Conclusions from numerical study indicate the 2/3 of the total decrease in losses are the result of delaying the transition location. The final 1/3 decrease in loss coefficient comes from the decrease in turbulent viscous losses.
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Martin, Samuel, and Bharat Bhushan. "Fluid flow analysis of a shark-inspired microstructure." Journal of Fluid Mechanics 756 (September 1, 2014): 5–29. http://dx.doi.org/10.1017/jfm.2014.447.
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AbstractThe scales of fast-swimming sharks contain riblet structures with microgrooves, aligned in the direction of fluid flow, that result in water moving efficiently over the surface. In previous studies, these riblet structures have shown a drag reduction of up to 10 % when compared with a smooth, flat surface. These studies have suggested two prevalent drag-reduction mechanisms which involve the effect of vortices and turbulence fluctuations. To further explore relevant mechanisms and study the effect of riblet geometry and flow properties on drag, vortices and turbulence fluctuations, various shark-skin-inspired riblet structures were created using computational models in which velocity, viscosity, spacing, height and thickness parameters were independently modified. A relevant mechanism of drag reduction is discussed to relate riblet parameters and flow properties to drag change and vortex size. Modelling information will lead to a better understanding of riblets and allow for optimum drag-reducing designs for applications in marine, medical and industrial fields.
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GARCÍA-MAYORAL, RICARDO, and JAVIER JIMÉNEZ. "Hydrodynamic stability and breakdown of the viscous regime over riblets." Journal of Fluid Mechanics 678 (April 19, 2011): 317–47. http://dx.doi.org/10.1017/jfm.2011.114.
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The interaction of the overlying turbulent flow with a riblet surface and its impact on drag reduction are analysed. The ‘viscous regime’ of vanishing riblet spacing, in which the drag reduction produced by the riblets is proportional to their size, is reasonably well understood, but this paper focuses on the behaviour for spacingss+≃ 10–20, expressed in wall units, where the viscous regime breaks down and the reduction eventually becomes an increase. Experimental evidence suggests that the two regimes are largely independent, and, based on a re-evaluation of existing data, it is shown that the optimal rib size is collapsed best by the square root of the groove cross-section, ℓg+=Ag+1/2. The mechanism of the breakdown is investigated by systematic DNSs with increasing riblet sizes. It is found that the breakdown is caused by the appearance of long spanwise rollers belowy+≈ 20, with typical streamwise wavelengths λx+≈ 150, that develop from a two-dimensional Kelvin–Helmholtz-like instability of the mean streamwise flow, similar to those over plant canopies and porous surfaces. They account for the drag breakdown, both qualitatively and quantitatively. It is shown that a simplified linear instability model explains the scaling of the breakdown spacing with ℓg+.
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Ganesan, Surendar, and Balasubramanian Esakki. "Computational fluid dynamic analysis of an unmanned amphibious aerial vehicle for drag reduction." International Journal of Intelligent Unmanned Systems 8, no. 3 (April 17, 2020): 187–200. http://dx.doi.org/10.1108/ijius-01-2019-0003.
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PurposeThe aim of this article is to minimize the drag of an unmanned amphibious aerial vehicle (UAAV) and enhancing the endurance.Design/methodology/approachVarious surface geometrical profiles such as rectangular, semicircular groove, razor blade and V-groove riblets are incorporated into the UAAV, and computational fluid dynamic (CFD) analysis is performed for various angles of attack at diverse vehicle speed conditions to estimate the coefficient of drag considering k–e turbulence model. Comparative evaluation between riblet and blunt body shape methodology is performed. Wind tunnel experiments are conducted to validate the flow characteristics around the UAAV.FindingsIt is observed that V-groove riblet method produced minimal drag in comparison with other profiles. The pressure distributions around UAAV for various geometrical profiles suggested that V-groove profile has achieved minimal vortex region, flow separation and turbulent boundary layer near to the outer profile.Originality/valueThe CFD analysis of UAAV for various riblet configurations and validation with wind tunnel smoke test confirms that UAAV with V-groove riblet provides low drag.
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Li, Xiang, Jun Cai, and De Yuan Zhang. "Study on the Manufacturing Method of the Biomimetic Drag Reducing Morphology Replication Mold." Advanced Materials Research 97-101 (March 2010): 2533–37. http://dx.doi.org/10.4028/www.scientific.net/amr.97-101.2533.
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In order to manufacture the drag reducing morphology replication mold used for biomimetic drag reducing film fabrication, the real shark skin and micro riblets machined with ultrasonic elliptical vibration cutting (UEVC) were used as templates respectively, followed by the casting process. The manufacturing process and the morphology quality of the molds was studied, and the biomimetic drag reducing effect and suitable application field was analyzed. The result indicates that: the bio-replicated forming mold based on shark skin is superior in morphology similarity with the real shark skin; the UEVC mold can achieve simplified riblets with high quality, and is feasible for large area mold manufacture and riblets spacing adjustment. The riblets morphology made with UEVC has wider application field than the shark shin morphology because of its flexibility of spacing adjustment.
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Lv, Hongqing, Zhenqing Wang, Jiahao Chen, and Lei Xu. "The Influence of Boundary Layer Caused by Riblets on the Aircraft Surface." Applied Sciences 10, no. 11 (May 26, 2020): 3686. http://dx.doi.org/10.3390/app10113686.
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Drag reduction of riblets is one of the most important problems in drag reduction of non-smooth surfaces. In the past two decades, the use of riblets arranged along the flow direction to reduce frictional resistance has received considerable attention. In this paper, we study the plates with the triangular concave grooves, triangular protrusion riblets, trapezoidal concave grooves, trapezoidal protrusion riblets, and circular concave grooves. The numerical simulation method is used to calculate five kinds of plates with grooves and riblets under multiple working conditions. The results showed that the plates with grooves and riblets generated vortices inside the grooves, which separated the incoming flow from the wall surface, and by increasing the thickness of the boundary layer, greatly reducing the average velocity gradient of the wall surface, compared with the smooth flat plate, the friction resistance is reduced. But, lateral riblets and grooves cause additional pressure resistance, which is one order of magnitude higher than the friction resistance. Then, the triangular concave grooves are arranged on the suction and pressure sides of the NACA0012 airfoil, respectively. We calculated the aerodynamic parameters of the both airfoils, and the standard NACA0012 airfoil from the −8° attack angle to their respective stall attack angles. The results showed that the NACA0012 airfoil with triangular concave grooves on the suction side reduced the aerodynamic characteristics of the standard NACA0012 at a small angle of attack, but the stall angle of attack of the standard NACA0012 airfoil was improved, because the grooves ensure that some gas can flow normally on the suction side and delay the separation of the boundary layer. The NACA0012 airfoil with triangular concave grooves on the pressure side did not effectively improve the aerodynamic characteristics: lift–drag ratio decreased and stall angle of attack decreased, but it can increase the lift slightly.
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Bharadwaj Ananthan, Varun, R. A. D. Akkermans, and Dragan Kozulovic. "Trailing-edge noise reduction of a wing by a surface modification." INTER-NOISE and NOISE-CON Congress and Conference Proceedings 263, no. 3 (August 1, 2021): 3194–201. http://dx.doi.org/10.3397/in-2021-2326.
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There is an increased emphasis on reducing airframe noise in the last decades. Airframe noise is sound generated by the interaction of a turbulent flow with the aircraft geometry, and significantly contributes to the overall noise production during the landing phase. One examples of airframe noise is the noise generated at a wing's trailing edge, i.e., trailing-edge noise. In this contribution, we numerically explore the local application of riblets for the purpose of trailing-edge noise reduction. Two configurations are studied: i) a clean NACA0012 wing section as a reference, and ii) the same configuration with riblets installed at the wing's aft part. The numerical investigation follows a hybrid computational aeroacoustics approach, where the time-average flow is studied by means of RANS. Noise sources are generated by means of a stochastic approach called Fast Random Particle Mesh method. The results show a deceleration of the flow behind the riblets. Furthermore, the turbulent kinetic energy indicates increased unsteadiness behind the riblets which is shifted away from the wall due to the presence of the riblets. Lastly, the sound sources are investigated by means of the 3D Lamb-vector, which indicates a slight reduction in magnitude near the trailing edge.
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Tardu, Sedat F. "Coherent structures and riblets." Applied Scientific Research 54, no. 4 (June 1995): 349–85. http://dx.doi.org/10.1007/bf00863518.
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GOLDSTEIN, D. B., and T. C. TUAN. "Secondary flow induced by riblets." Journal of Fluid Mechanics 363 (May 25, 1998): 115–51. http://dx.doi.org/10.1017/s0022112098008921.
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The effects of riblets on one wall of a channel bounding fully developed turbulent flow are investigated. Various perturbation elements including wires, fins and slots are modelled in order to understand the effects of riblets. It is found that widely spaced riblets, fins and wires create a substantial increase in turbulent activity just above the element. These elements are also found to produce a remarkable pattern of secondary mean flows consisting of matched pairs of streamwise vortices. The secondary flows occur only if the bulk flow is turbulent and their characteristics depend on element geometry. It is suggested that these secondary flows are strongly linked with the increase in drag experienced by widely spaced riblets in experimental studies. The secondary flows are probably caused by two-dimensional spanwise sloshing of the flow, inherent in a turbulent boundary layer, interacting with the stream-aligned element. This two-dimensional mechanism is investigated with a series of two-dimensional simulations of sloshing flow over isolated elements. Grid resolution and domain size checks are made throughout the investigation.
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Martin, Samuel, and Bharat Bhushan. "Discovery of riblets in a bird beak ( Rynchops ) for low fluid drag." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374, no. 2073 (August 6, 2016): 20160134. http://dx.doi.org/10.1098/rsta.2016.0134.
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Riblet structures found on fast-swimming shark scales, such as those found on a mako shark, have been shown to reduce fluid drag. In previous experimental and modelling studies, riblets have been shown to provide drag reduction by lifting vortices formed in turbulent flow, decreasing overall shear stresses. Skimmer birds ( Rynchops ) are the only birds to catch fish in flight by flying just above the water surface with a submerged beak to fish for food. Because they need to quickly catch prey, reducing drag on their beak is advantageous. For the first time, riblet structures found on the beak of the skimmer bird have been studied experimentally and computationally for low fluid drag properties. In this study, skimmer replicas were studied for drag reduction through pressure drop in closed-channel, turbulent water flow. Pressure drop measurements are compared for black and yellow skimmer beaks in two configurations, and mako shark skin. In addition, two configurations of skimmer beak were modelled to compare drag properties and vortex structures. Results are discussed, and a conceptual model is presented to explain a possible drag reduction mechanism in skimmers. This article is part of the themed issue ‘Bioinspired hierarchically structured surfaces for green science’.
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BECHERT, D. W., M. BRUSE, W. HAGE, J. G. T. VAN DER HOEVEN, and G. HOPPE. "Experiments on drag-reducing surfaces and their optimization with an adjustable geometry." Journal of Fluid Mechanics 338 (May 10, 1997): 59–87. http://dx.doi.org/10.1017/s0022112096004673.
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Previous research has established that surfaces with tiny ribs (riblets) aligned in the streamwise direction can reduce the turbulent wall-shear stress below that of a smooth surface. Typical skin-friction reductions have been found to be about 5%. The results of the present investigation, however, demonstrate a considerable improvement over this value. This improvement is achieved by a systematic experimental optimization which has been guided by theoretical concepts.A key feature of our experiments is the utilization of an oil channel. Previous experiments in wind tunnels had to contend with very small riblet dimensions which typically had a lateral rib spacing of about 0.5 mm or less. By contrast, in our oil channel, the ribs can have a lateral spacing of between about 2 and 10 mm. This increased size of the surface structures enables test surfaces to be manufactured with conventional mechanical methods, and it also enables us to build test surfaces with adjustable geometry. In addition, the Berlin oil channel has a novel shear stress balance with an unprecedented accuracy of ±0.3%. This latter feature is a prerequisite for a systematic experimental optimization.In the present investigation, surfaces with longitudinal ribs and additional slits are studied. The experiments cover a fairly large range of parameters so that the drag reduction potential of a surface with ribs and/or slits is worked out conclusively. A large parameter range is made possible because of the adjustability of the surfaces as well as the automatic operation of the oil channel. In particular, the following tests were run:(i) Shear stress measurements with conventional riblet configurations, i.e. with triangular and semi-circular grooves, have been carried out. These measurements were necessary in order to establish the connection between our oil channel data and previous data from wind tunnels. As was previously established, we found a drag reduction of about 5%.(ii) An adjustable surface with longitudinal blade ribs and with slits was built and tested. Both groove depth and slit width could be varied separately and continuously during the experiment. It turned out, that slits in the surface did not contribute to the drag reduction. Nevertheless, these investigations show how perforated surfaces (e.g. for boundary-layer control) can be designed for minimal parasitic drag. On the other hand, with closed slits, an optimal groove depth for the rib surface could be determined, i.e. half of the lateral rib spacing. For this configuration, we found an 8.7% skin-friction reduction. By carefully eliminating deleterious effects (caused by little gaps, etc.), the skin-friction reduction could be improved to a record value of 9.9%.(iii) A quantitative comparison between theory and experiment was carried out. The theory is based on the assumption that riblets impede the fluctuating turbulent crossflow near the wall. In this way, momentum transfer and shear stress are reduced. The simplified theoretical model proposed by Luchini (1992) is supported by the present experiments.(iv) For technological applications of riblets, e.g. on long-range commercial aircraft, the above thin-blade ribs are not practical. Therefore, we have devised a surface that combines a significantly improved performance (8.2 %) with a geometry which exhibits better durability and enables previously developed manufacturing methods for plastic riblet film production to be used. Our riblet geometry exhibits trapezoidal grooves with wedge-like ribs. The flat floor of the trapezoidal grooves permits an undistorted visibility through the transparent riblet film which is essential for crack inspection on aircraft.
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Grek, G. R., V. V. Kozlov, and S. V. Titarenko. "An experimental study of the influence of riblets on transition." Journal of Fluid Mechanics 315 (May 25, 1996): 31–49. http://dx.doi.org/10.1017/s0022112096002315.
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An experimental study of the effect of riblets on three-dimensional nonlinear structures, the so-called Λ-vortices on laminar-turbulent transition showed that riblets delay the transformation of the Λ-vortices into turbulent spots and shift the point of transition downstream. This result is opposite to the negative influence of such ribbed surfaces on two-dimensional linear Tollmien-Schlichting waves (the linear stage of transition). Thus, the ribbed surface influences laminar-turbulent transition structures differently: a negative influence on the linear-stage transition structures and a positive influence on the nonlinear-stage transition structures. It is demonstrated that transition control by means of riblets requires special attention to be paid to the choice of their location, taking into account the stage of transition.
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Djenidi, L., F. Anselmet, J. Liandrat, and L. Fulachier. "Laminar boundary layer over riblets." Physics of Fluids 6, no. 9 (September 1994): 2993–99. http://dx.doi.org/10.1063/1.868429.
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Hoyer, K., H. W. Bewersdorff, and A. Gyr. "A study on thread riblets." Applied Scientific Research 52, no. 2 (March 1994): 169–72. http://dx.doi.org/10.1007/bf00868058.
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Sanchez Ramirez, Alberto, Manuel Enrique Islán Marcos, Fernando Blaya Haro, Roberto D’Amato, Rodolfo Sant, and José Porras. "Application of FDM technology to reduce aerodynamic drag." Rapid Prototyping Journal 25, no. 4 (May 13, 2019): 781–91. http://dx.doi.org/10.1108/rpj-09-2018-0251.
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Purpose The purpose of this paper is to analyze the aerodynamic improvements obtained in a wing section with a NACA 0018 airfoil manufactured using the fused deposition modeling (FDM) technique with regard to a smooth surface made by milling. The creation of micro-riblets on the surface of the airfoil, due to the deposition of the material layer by layer, improves the general aerodynamic performance of the parts, provided that the riblets are parallel to the flow line. The incidence of the thickness of the thread deposited in each layer – to be the variable on which the geometry of the riblets is based – was studied. Design/methodology/approach The wing section was designed using 3D software. Three different models were designed by rapid prototyping, using additive and subtractive manufacturing. Two of the profiles were manufactured using FDM varying the thickness of the layer to be able to compare the aerodynamic improvements. The third model was manufactured using a subtractive rapid prototyping machine generating a smooth surface profile. These three models were tested inside the wind tunnel to be able to quantify the aerodynamic efficiency according to the geometry and the riblets size. Findings The manufacture of an aerodynamic profile using FDM provides, in addition to the lightness and the ability to design parts with complex geometries, an improvement in the aerodynamic efficiency of 10 per cent compared with profiles with a smooth surface. Practical implications With the aerodynamic advantage gained through the use of FDM positions, the additive manufacturing serves as an excellent alternative for the manufacture of lightweight aerodynamic parts, with low structural loading and with low Reynolds number (∼5·105). This technological advantage would be applied to the UAV (unmanned aerial vehicle) industry. Originality/value The study carried out in this article demonstrates that the use of FDM as a manufacture process of end-used parts that are subject to movement generates an additional advantage that had not been considered. The additive manufacturing allows us to directly manufacture riblets by creating the necessary surface so as to reduce the aerodynamic drag.
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Rastegari, Amirreza, and Rayhaneh Akhavan. "The common mechanism of turbulent skin-friction drag reduction with superhydrophobic longitudinal microgrooves and riblets." Journal of Fluid Mechanics 838 (January 10, 2018): 68–104. http://dx.doi.org/10.1017/jfm.2017.865.
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Turbulent skin-friction drag reduction with superhydrophobic (SH) longitudinal microgrooves and riblets is investigated by direct numerical simulation (DNS), using lattice Boltzmann methods, in channel flow. The liquid/gas interfaces in the SH longitudinal microgrooves were modelled as stationary, curved, shear-free boundaries, with the meniscus shape determined from the solution of the Young–Laplace equation. Interface protrusion angles of $\unicode[STIX]{x1D703}=0^{\circ },-30^{\circ },-60^{\circ },-90^{\circ }$ were investigated. For comparison, the same geometries as those formed by the SH interfaces were also studied as riblets. Drag reductions of up to 61 % and up to 5 % were realized in DNS with SH longitudinal microgrooves and riblets, respectively, in turbulent channel flows at bulk Reynolds numbers of $Re_{b}=3600$ ($Re_{\unicode[STIX]{x1D70F}_{0}}\approx 222$) and $Re_{b}=7860$ ($Re_{\unicode[STIX]{x1D70F}_{0}}\approx 442$), with arrays of SH longitudinal microgrooves or riblets of size $14\lesssim g^{+0}\lesssim 56$ and $g^{+0}/w^{+0}=7$ on both walls, where $g^{+0}$ and $w^{+0}$ denote the widths and spacings of the microgrooves in base flow wall units, respectively. An exact analytical expression is derived which allows the net drag reduction in laminar or turbulent channel flow with any SH or no-slip wall micro-texture to be decomposed into contributions from: (i) the effective slip velocity at the wall, (ii) modifications to the normalized structure of turbulent Reynolds shear stresses due to the presence of this effective slip velocity at the wall, (iii) other modifications to the normalized structure of turbulent Reynolds shear stresses due to the presence of the wall micro-texture, (iv) modifications to the normalized structure of mean flow shear stresses due to the presence of the wall micro-texture and (v) the fraction of the flow rate through the wall micro-texture. Comparison to DNS results shows that SH longitudinal microgrooves and riblets share a common mechanism of drag reduction in which $100\,\%$ of the drag reduction arises from effects (i) and (ii). The contributions from (iii)–(v) were always drag enhancing, and followed a common scaling with SH longitudinal microgrooves and riblets when expressed as a function of the square root of the microgroove cross-sectional area in wall units. Extrapolation of drag reduction data from DNS to high Reynolds number flows of practical interest is discussed. It is shown that, for a given geometry and size of the surface micro-texture in wall units, the drag reduction performance of micro-textured surfaces degrades with increasing bulk Reynolds number of the flow. Curved SH interfaces at low protrusion angle ($\unicode[STIX]{x1D703}=-30^{\circ }$) were found to enhance the drag reduction by up to 3.6 % compared to flat interfaces, while reducing the instantaneous pressure fluctuations on the SH interfaces by up to a factor of two. This suggests that the longevity of SH interfaces in turbulent flow may be improved by embedding the SH surface within the microgrooves of shallow, scalloped riblets.
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Viswanath, P. R. "Aircraft viscous drag reduction using riblets." Progress in Aerospace Sciences 38, no. 6-7 (August 2002): 571–600. http://dx.doi.org/10.1016/s0376-0421(02)00048-9.
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Bonnivard, Matthieu, and Dorin Bucur. "Microshape Control, Riblets, and Drag Minimization." SIAM Journal on Applied Mathematics 73, no. 2 (January 2013): 723–40. http://dx.doi.org/10.1137/100814846.
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Gaudet, L. "Properties of riblets at supersonic speed." Applied Scientific Research 46, no. 3 (July 1989): 245–54. http://dx.doi.org/10.1007/bf00404821.
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Alving, A. E., and P. Freeberg. "The effect of riblets on sails." Experiments in Fluids 19, no. 6 (October 1995): 397–404. http://dx.doi.org/10.1007/bf00190257.
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EI-Samni, O. A., Hyun Sik Yoon, and Ho Hwan Chun. "Turbulent flow over thin rectangular riblets." Journal of Mechanical Science and Technology 19, no. 9 (September 2005): 1801–10. http://dx.doi.org/10.1007/bf02984192.
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Luchini, Paolo, and Giuseppe Trombetta. "Effects of riblets upon flow stability." Applied Scientific Research 54, no. 4 (June 1995): 313–21. http://dx.doi.org/10.1007/bf00863516.
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