Riblet Surface Research Articles

Drag Reduction Performance Analysis of Chlamys nobilis Ribbed Surface

AbstractConsidering the fact that the living condition of the shell is similar to the significant defiled environment of a ship, it is selected as a bionic object to study drag reduction for ship hulls. Previous studies have demonstrated that the special physical structure of the shell surface results in preventing biological adhesion. In order to improve ship operation efficiency, collaborative research on the physical structure was first carried out by studying the drag reduction performance of the actual Chlamys nobilis surface among the antifouling ranges of the structural dimensions without simplification. Thereafter, the drag reduction mechanism was analyzed by simulation analysis of the surface structures on the antifouling region of the C. nobilis surface. Simulation results demonstrated that a pair of counter-rotating vortices appeared at the tip of the bionic riblet surface. The secondary vortex, induced by the counter-rotating vortices, was the main reason for the drag reduction effect of the bionic riblet surface. Moreover, analysis of the drag reduction performance indicated that the maximum drag reduction rate could reach up to 8.53% at the speed of 3 m/s. This study reveals the drag reduction effect of the C. nobilis surface, which will provide a reference for combining drag reduction and antifouling.

Effect of flexible riblet surface on turbulent transition

Effect of flexible riblet surface on turbulent transition

A reduced order model to simulate compressible flows over an actuated riblet surface

Experimental investigations confirmed a significant drag reduction effect by means of either riblet structured or actuated surfaces. Combining both passive and active mechanisms might even further improve this effect. To determine appropriate physical parameter ranges computations must be highly efficient. For this purpose, a model order reduction method is developed. By means of an upscaling strategy time dependent effective boundary conditions on a virtually smooth actuated wall are derived, modeling the influence of the riblets without resolving them. Numerical simulations for the configuration of a compressible flow over an actuated riblet surface verify the efficiency of the reduced model.

Turbulent structures in a statistically three-dimensional boundary layer

We investigate the behaviour of large-scale coherent structures in a spanwise-heterogeneous turbulent boundary layer, using particle image velocimetry on multiple orthogonal planes. The statistical three-dimensionality is imposed by a herringbone riblet surface, although the key results presented here will be common to many cases of wall turbulence with embedded secondary flows in the form of mean streamwise vortices. Instantaneous velocity fields in the logarithmic layer reveal elongated low-momentum streaks located over the upwash-flow region, where their spanwise spacing is forced by the$2\unicode[STIX]{x1D6FF}$periodicity of the herringbone pattern. These streaks largely resemble the turbulence structures that occur naturally (and randomly located) in spanwise-homogeneous smooth-/rough-wall boundary layers, although here they are directly formed by the roughness pattern. In the far outer region, the large spanwise spacing permits the streaks to aggressively meander. The mean secondary flows are the time-averaged artefact of the unsteady and spanwise asymmetric large-scale roll modes that accompany these meandering streaks. Interestingly, this meandering, or instability, gives rise to a pronounced streamwise periodicity (i.e. an alternating coherent pattern) in the spatial statistics, at wavelengths of approximately 4.5$\unicode[STIX]{x1D6FF}$. Overall, the observed behaviours largely resemble the streak-instability model that has been proposed for the buffer region, only here at a much larger scale and at a forced spanwise spacing. This observation further confirms recent observations that such features may occur at an entire hierarchy of scales throughout the turbulent boundary layer.

Dual-plane stereoscopic PIV measurement of vortical structure in turbulent channel flow on sinusoidal riblet surface

Abstract Vortical structure in wall turbulence over a sinusoidal riblet surface is investigated by means of a dual-plane stereoscopic particle image velocimetry (DPS-PIV) measurement. The experiment is made in a channel flow at a friction Reynolds number of 150. The lateral spacing of the adjacent walls of the sinusoidal riblet varies in the streamwise direction and 12% of the drag reduction rate has been confirmed. The DPS-PIV measurement system consists of four high-speed CCD cameras. The laser sheets are provided on streamwise and wall-normal planes and separated 0.5 mm in the spanwise direction to each other. The profiles of the velocity statistics in the flat case show a good agreement with previous data. Since all velocity components can be measured on adjacent laser sheets simultaneously, an instantaneous velocity deformation tensor can be obtained and vortical structures can be identified by a second invariant of the tensor i.e., the Q value. The probability of Q value in the riblet side is almost unchanged from that of the flat side. The analysis of the tracking of vortical structures by using the Q value is performed. As similar to a pathline analysis in a previous study, we confirmed that the riblet surface prevents the vortical structure hitting the wall and vortical structures follow the upward and downward flows induced by the sinusoidal riblet surface.

Experimental study on deposition enhancement of ultrafine particles in a duct flow by riblets

High filtration for ultrafine particles (UFPs) has many potential energy-saving and environmental applications. In this study, riblet surfaces with different geometries were investigated for their enhanced UFPs deposition in duct flow conditions for the first time. Polydisperse sodium chloride (NaCl) particles as UFPs were used and tested in a wide range of Reynolds numbers, ranging from laminar to fully turbulent flow. The upstream and downstream size-resolved particle concentrations of the NaCl particles were measured. Experimental results show that deposition velocity increased with Reynolds numbers and decreased with particle size for both empty duct and duct furnished with riblet surfaces. Duct with riblets surface had the highest enhanced deposition velocity ratio for UFPs in the laminar regime, which could reach about 7.0 times for particles less than 20 nm, and about 4.8 times for particle size between 20 nm and 50 nm. The experimental results reveal that the friction factors of the duct with riblet surfaces were close to that of the empty duct in the turbulent flow, while higher pressure drop penalty was observed for riblet surfaces in the laminar flow. The performance index of riblet surfaces by combining enhanced UFPs deposition and high frictional drag was also evaluated. The results show that the highest performance ratio of riblets could be up to 4.0. In addition, the riblet surface with largest protrusion height generally give the best performance.

Drag-reducing riblets with fouling-release properties: development and testing

The manufacture and preliminary testing of a drag-reducing riblet texture with fouling-control properties is presented. The commercial fouling-release product Intersleek® 1100SR was modified to manufacture riblet-textured coatings with an embossing technology. Hydrodynamic drag measurements in a Taylor–Couette set-up showed that the modified Intersleek® riblets reduced drag by up to 6% compared to a smooth surface. Barnacle settlement assays demonstrated that the riblets did not substantially reduce the ability of Intersleek® 1100SR to prevent fouling by cyprids of Balanus amphitrite. Diatom adhesion tests revealed significantly higher diatom attachment on the riblet surface compared to smooth Intersleek® 1100SR. However, after exposure to flow, the final cell density was similar to the smooth surface. Statically immersed panels in natural seawater showed an increase of biofilm cover due to the riblets. However, the release of semi-natural biofilms grown in a multi-species biofilm culturing reactor was largely unaffected by the presence of a riblet texture.

CFD analysis of effects on fluid flow resistance of metallic wavy structures

Microscale wrinkles can be utilized to enhance the characteristics of products. For example, a shark skin having riblet surface can move faster underwater, minimizing fluid flow resistance. We introduce a novel approach to fabricate microscale metallic wrinkles using a soft lithography and electroforming process; we also evaluated the effects of wrinkles by computational fluid dynamics (CFD) analyses. For the generation of metallic wrinkles, a UV-curable resin, NOA68T was used to fabricate a master wrinkling pattern by mechanism of compressive forces on a skin of the weakly polymerized resin layer. The master pattern was molded and replicated as metallic wrinkles on a surface via soft lithography and electroforming processes. To understand the advantages of a wavy surface on reduction of flow resistance, we carried out two- and three-dimensional (2D and 3D) CFD analyses. The drag coefficient of a wrinkled 2D square model was decreased about 17.1 %, and a 3D real wrinkle model showed 4.9 to 7.3 % reduction of it compared to without wrinkle models. We believe that it is possible to reduce the fluid flow resistance using wavy surfaces that can easily be generated selectively or wholly on an arbitrary surface.

Turbulent channel flow over riblets with superhydrophobic coating

The performance of riblet surfaces after applying a superhydrophobic coating (SHC) is evaluated by planar particle image velocimetry (PIV) measurement at riblet tip spacing of s+ = 8.6, 17.3, and 34.6 (normalized using wall unit). The three riblet sizes correspond to an undersized (small drag reduction), an optimum (maximum drag reduction), and an oversized (drag increase) riblet, respectively. All the experiments are carried out in a turbulent water channel flow at constant ReH = 4360 (based on channel height H and average velocity). The superhydrophobic layer is formed by spray coating of micro/nano particles with a thickness of ∼1λ (wall unit). The results show smaller mean velocity over the s+ = 8.6 and s+ = 17.3 riblets when coated with the superhydrophobic layer at near-wall region of y+ < 15 while the mean velocity over the s+ = 34.6 riblet with SHC is larger relative to the non-coated counterpart. The SHC increased 〈u2〉 over the s+ = 8.6 and 17.3 surfaces while 〈u2〉 reduced over the s+ = 34.6 surface in the near wall region of y+ < 40. A smaller 〈v2〉 value is observed in the near-wall region (y+ < 50) of all three riblets upon applying the SHC while the reduction is smaller for smaller riblets. The 〈v2〉 peak also shifts away from the wall upon coating the s+ = 34.6 riblet. The Reynolds shear stress over s+ = 8.6 and s+ = 17.3 riblets is not considerably different relative to the superhydrophobic coated counterparts while a large reduction of 〈uv〉 is observed at y+ < 30 after coating the s+ = 34.6 riblet. The estimation of drag reduction (DR) based on weighted integral of 〈uv〉 shows 6.0% and 10.1% reduction of drag over the s+ = 8.6 and s+ = 17.3 riblets after the SHC process, respectively. SHC on the oversized s+ = 34.6 riblet improves the performance from 9.0% drag increase (DI) over the non-coated surface to 1.2% DR, equivalent to 10.2% reduction of drag upon coating the riblet. The larger improvement of oversized riblets (s+ > 30) is associated with the effectiveness of the SHC in the larger riblet valley and consequently attenuation of ejection and sweep motions. The SHC broadens the operation range of larger riblets, which are easier to manufacture.

Drag reduction using wrinkled surfaces in high Reynolds number laminar boundary layer flows

Inspired by the design of the ribbed structure of shark skin, passive drag reduction methods using stream-wise riblet surfaces have previously been developed and tested over a wide range of flow conditions. Such textures aligned in the flow direction have been shown to be able to reduce skin friction drag by 4%–8%. Here, we explore the effects of periodic sinusoidal riblet surfaces aligned in the flow direction (also known as a “wrinkled” texture) on the evolution of a laminar boundary layer flow. Using numerical analysis with the open source Computational Fluid Dynamics solver OpenFOAM, boundary layer flow over sinusoidal wrinkled plates with a range of wavelength to plate length ratios (λ/L), aspect ratios (2A/λ), and inlet velocities are examined. It is shown that in the laminar boundary layer regime, the riblets are able to retard the viscous flow inside the grooves creating a cushion of stagnant fluid that the high-speed fluid above can partially slide over, thus reducing the shear stress inside the grooves and the total integrated viscous drag force on the plate. Additionally, we explore how the boundary layer thickness, local average shear stress distribution, and total drag force on the wrinkled plate vary with the aspect ratio of the riblets as well as the length of the plate. We show that riblets with an aspect ratio of close to unity lead to the highest reduction in the total drag, and that because of the interplay between the local stress distribution on the plate and stream-wise evolution of the boundary layer the plate has to exceed a critical length to give a net decrease in the total drag force.

Characteristics and mechanism investigation on drag reduction of oblique riblets

This work primarily focuses on the drag reduction characteristics and mechanism investigation of oblique riblets. First, a calculation model of the oblique riblets surface is established, and Reynolds stress model (RSM) turbulence model is used for numerical simulation of the oblique riblets flow field. Subsequently, the influence of inclination angle between flow direction and arrangement direction of riblets on friction resistance and drag reduction rate is further analyzed. Through the investigation of the distribution of shear stress, pressure stress and velocity in oblique riblets boundary layer, the oblique riblets drag reduction mechanism is finally revealed. Results show that, with increase of velocity and inclination angle, the pressure resistance increases obviously, along with the decreasing of the viscous resistance distinctly. The maximum drag reduction rate of the oblique riblets is 7.33%. Moreover, when the inclination angle increases, the wall shear stress reduces on oblique riblets surface; while differential pressure increases at both sides of oblique riblets tips. In addition, when inclination angle is small, the secondary vortex at oblique riblets tips will disappear soon. New vortices will be formed inside the oblique riblets and cause the decrease of viscosity resistance. Thus, oblique riblets show a better drag reduction effect and have an effective control on boundary layer.

Parametric Study on a Sinusoidal Riblet for Drag Reduction by Direct Numerical Simulation

The direct numerical simulation of fully developed turbulent channel flow with a sinusoidal riblet surface has been carried out at the friction Reynolds number of 110. Lateral spacing of adjacent walls in a sinusoidal riblet is varied sinusoidally in the streamwise direction. The average lateral spacing of a sinusoidal riblet is larger than the diameter of a quasi-streamwise vortex and its wetted area is smaller than that of ordinary straight-type riblets. We investigate the effect of sinusoidal riblet design parameters on the drag reduction rate and flow statistics in this paper. The parametric study shows that the maximum total drag reduction rate is approximately 9.8% at a friction Reynolds number of 110. The riblet induces downward and upward flows in the expanded and contracted regions, respectively, which contribute to periodic Reynolds shear stress. However, the random Reynolds shear stress decreases drastically as compared with the flat surface case, resulting in the reduction of total drag owing to the sinusoidal riblet. We also performed vortex tracking to discuss the motion of the vortical structure traveling over the sinusoidal riblet surface. Vortex tracking and probability analysis for the core of the vortical structure show that the vortical structure is attenuated owing to the sinusoidal riblet and follows the characteristic flow. These results show that the high skin-friction region on the channel wall is localized at the expanded region of the riblet walls. In consequence, the wetted area of the riblet decreases, resulting in the drag-reduction effect.

Three-dimensional measurement of turbulent flow over a riblet surface

Measurement of three-dimensional (3D) turbulence over riblet surfaces is challenging due to the small size of the grooves and the requirement for measurement in the inner layer. The capability of two-dimensional (2D) and 3D particle image velocimetry (PIV) and particle tracking velocimetry (PTV) for characterization of the 3D structure of turbulent flow over a riblet surface with groove spacing of 750μm at Reτ=147 (based on friction velocity and half channel height) is investigated. The 2D measurements were carried out using standard planar PIV and high-magnification long-range microscopic PTV (micro-PTV). The investigated 3D techniques include tomographic PIV (tomo-PIV) and 3D-PTV. The results are evaluated in comparison with measurement over a smooth surface and also with direct numerical simulation (DNS) of channel flow by Tsukahara et al. (2005) at Reτ=150. The reflection of the laser light from the smooth and riblet surfaces is significantly different in spite of the wall-parallel illumination. This resulted in biased near-wall (y/H<0.05) measurement using planar PIV. The shortcoming was fulfilled by micro-PTV which could measure the mean velocity profile within the linear viscous sublayer (2<y+<5) and showed a 6.1% reduction of the skin-friction over the riblet surface. Micro-PTV also accurately measured the location of the 〈u2〉 peak and its magnitude reduction over the riblet surface compared with planar PIV. The Planar PIV measured 〈v2〉 peak which is further away from the wall at y/H=0.15 and also the 〈uv〉 profile in the outer layer. The 〈uv〉 profile showed 7.4% reduction of wall shear stress over the riblet surface. 3D-PTV showed a 9.4% reduction of the 〈w2〉 peak and attenuation of v and w fluctuations over the riblet surface compared to the smooth surface through quadrant analysis. The three components of fluctuating vorticity measured by tomo-PIV showed negligible variation over the two surfaces due to the random noise and lack of spatial resolution. Quadrant analysis using planar PIV showed attenuation of the sweep and ejection events near the riblets, which indicates weaker streamwise vortices. Two-point correlation of PIV measurement also demonstrated increase of the coherence of the low and high-speed steaks over the riblets.

Marine drag reduction of shark skin inspired riblet surfaces

Abstract Shark skin inspired riblet surfaces have been known to have drag reduction effect for the over past 40 years. It first drew the attention from the aircraft industry. With the property of low drag and self-cleaning (antifouling), shark skin inspired riblet surfaces can also be used on navigation objects. In this paper, different marine drag reduction technologies are discussed, and a review of riblet performance studies is also given. Experimental parameters include riblet geometry, continuous and segmented configurations, fluid velocity (laminar and turbulent flow), fluid viscosity (water, oil and gas), and wettability are analyzed. However, force is obtained by area-weighted integral of shear stress distributions. So area of riblet surfaces is a crucial factor which has not been considered in many previous studies. An experiment is given to discuss the impact of area. This paper aims not only to contribute to a better understanding of marine drag reduction, but also to offer new perspectives to improve the current evaluation criteria of riblet drag reduction.

Cross-stream stereoscopic particle image velocimetry of a modified turbulent boundary layer over directional surface pattern

A turbulent boundary layer developed over a herringbone patterned riblet surface is investigated using stereoscopic particle image velocimetry in the cross-stream plane at$Re_{\unicode[STIX]{x1D70F}}\approx 3900$. The three velocity components resulting from this experiment reveal a pronounced spanwise periodicity in all single-point velocity statistics. Consistent with previous hot-wire studies over similar-type riblets, we observe a weak time-average secondary flow in the form of$\unicode[STIX]{x1D6FF}$-filling streamwise vortices. The observed differences in the surface and secondary flow characteristics, compared to other heterogeneous-roughness studies, may suggest that different mechanisms are responsible for the flow modifications in this case. Observations of instantaneous velocity fields reveal modified and rearranged turbulence structures. The instantaneous snapshots also suggest that the time-average secondary flow may be an artefact arising from superpositions of much stronger instantaneous turbulent events enhanced by the surface texture. In addition, the observed instantaneous secondary motions seem to have promoted a free-stream-engulfing behaviour in the outer layer, which would indicate an increase turbulent/non-turbulent flow mixing. It is overall demonstrated that the presence of large-scale directionality in transitional surface roughness can cause a modification throughout the entire boundary layer, even when the roughness height is 0.5 % of the layer thickness.

Applicability Evaluation of a Laser Light-Mater Interaction Based Computational Tool on Status Identification of Applied Micro-Structured Coatings

The current work aims at evaluating a proposed method based on a computational tool developed using Object-Oriented Programming to identify the status of micro-structured surfaces. In this case, these are micro-structured coatings with riblet microstructure developed by Fraunhofer Institute–IFAM, by building a graphical reproduction of the analyzed surface and calculating an expected laser reflection intensity acquired by a laser sensor device, the proposed method is assessed by using the simplest case: a flat surface, and an optimal case: an intact riblet surface. The results corroborate the calculations to be applied to further steps from more complex cases of degradation and to diverse riblets geometries. Based on Huygens-Fresnel and Fraunhofer diffraction theories, the calculations developed and demonstrated in this paper improved the nondestructive tests to support the status identification of the micro structured coatings, e.g. riblet structures based on shark skin used in shipping and aerospace industries. This work is assured required quality of the riblet coating identifying the number of structures and expected geometry using implemented calculations to foresee the laser reflection intensity acquired by a laser sensor device with 3 detectors, for instance, a riblet structure could be graphically reproduced, analyzed and completely identified based on the application of the theoretical optics applied on this work.

Ordered roughness effects on NACA 0026 airfoil

The effects of highly-ordered rough surface - riblets, applied onto the surface of a NACA 0026 airfoil, are investigated experimentally using wind tunnel. The riblets are arranged in directionally converging - diverging pattern with dimensions of height, h = 1 mm, pitch or spacing, s = 1 mm, yaw angle α = 0o and 10o The airfoil with external geometry of 500 mm span, 600 mm chord and 156 mm thickness has been built using mostly woods and aluminium. Turbulence quantities are collected using hotwire anemometry. Hotwire measurements show that flows past converging and diverging pattern inherit similar patterns in the near-wall region for both mean velocity and turbulence intensities profiles. The mean velocity profiles in logarithmic regions for both flows past converging and diverging riblet pattern are lower than that with yaw angle α = 0o. Converging riblets cause the boundary layer to thicken and the flow with yaw angle α = 0o produces the thinnest boundary layer. Both the converging and diverging riblets cause pronounced outer peaks in the turbulence intensities profiles. Most importantly, flows past converging and diverging pattern experience 30% skin friction reductions. Higher order statistics show that riblet surfaces produce similar effects due to adverse pressure gradient. It is concluded that a small strip of different ordered roughness features applied at a leading edge of an airfoil can change the turbulence characteristics dramatically.

Investigation on an integrated approach to design and micro fly-cutting of micro-structured riblet surfaces

Drag reduction in wall-bounded flows can be achieved by the passive flow control technique through the application of bio-inspired ribleted surfaces. In this paper, innovative design and manufacturing of serrate-semi-circular ribleted surfaces are presented with application to friction and drag reduction on engineering surfaces. Firstly, the design of the ribleted surfaces is described particularly focusing on the serrate-semi-circular shaped structures. Secondly, machining of ribleted surfaces by fly-cutting is investigated, covering the utilization of bespoke CVD diamond tools on a micro-milling machine and the corresponding micro fly-cutting processes. Metrology measurement results show good agreement achieved between the designed and machined surface features. Experiment conducted in wind tunnel shows the machined surface can produce 7% in drag reduction. Compared with conventional micro milling, the micro fly-cutting technique resulted from this research illustrates the unique advantage and industrial significance, particularly for manufacturing micro-structured surfaces in an industrial scale.

Material selection and manufacturing of riblets for drag reduction: An updated review

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.

航空機にリブレットを適用した際の簡便な抵抗低減率推算手法

This paper proposes a simple estimation technique for drag-reducing performance of riblet applied to the whole body of an aircraft. The point of this technique is how to estimate the wall shear stress on the aircraft body covered with riblets. First, RANS analysis of flow around an aircraft which is not covered with riblets is conducted to obtain the whole-body distributions of wall shear stress. The distributions of wall shear stress of an aircraft covered with riblet are obtained from the combination of the RANS results and drag reduction rate graphs obtained from wind-tunnel test. Finally, drag reduction rates are estimated from surface integration of the wall shear stress on both the riblet surface and smooth surface. As a result, the estimated drag reduction rates are close to that obtained by the previous flight tests, which indicates that this technique is reasonable. As a future plan, the effects of pressure gradient, angle between riblet and local flow will be considered.