Riblet Surface Research Articles

Hierarchical Surface Patterns via Global Wrinkling on Curved Substrate for Fluid Drag Control

AbstractIn this paper, the results of an investigation on the global wrinkling of stiff coatings resting on curved compliant substrates are presented. It is found that the overall stiffness of the corrugated coatings can dominate the deformation mode and induce the global wrinkling when the coating thickness exceeds certain critical value. It is demonstrated that hierarchical wrinkling patterns with small wrinkles superposing on large wrinkles, either parallel or perpendicularly, can be generated via the global wrinkling processes. Through a combination of theoretical model, experimental studies, and numerical simulations, the formation mechanism and controlling parameters of the hierarchical wrinkling patterns are systematically investigated. The quantitative expression of the critical loading strain, critical wavelength, and critical coating thickness of the global wrinkling as a function of the initial substrate curvatures and loading conditions are revealed. Inspired by the resemblance between the hierarchical surface patterns and the surface riblets on sharks and dolphins, the potential application of utilizing the hierarchical wrinkling patterns to reduce fluid drag through computational fluid dynamics (CFD) studies is further investigated. Reduced drag resistance is observed on the hierarchical wrinkled surface with small wrinkles and large wrinkles perpendicular to each other.

A review of drag reduction and heat transfer enhancement by riblet surfaces in closed and open channel flow

Turbulence and heat transfer are two critical factors in the chain of world energy consumption. Hence, development of more efficient energy transfer techniques in engineering applications through control and optimization of the thermal turbulent boundary layer and turbulence continues to be a subject of intense interest. This is why reviewing a passive flow control technique with a focus on unique characteristics of heat transfer in devices called riblets is beneficial as technology pushes to revolutionize design and manufacturing of heat exchanger equipment. Riblet surfaces are considered beneficial to many commercial applications as they reduce forces between surface and fluid, and in some cases can enhance heat transfer in the turbulent flow regime. As there is no literature study that focuses on heat transfer and riblets, the current study reports experimental and numerical investigations reviewing both drag reduction and heat transfer using riblets in closed and open channels. Drag reduction studies are limited to water and oil flow studies, and heat transfer studies are expanded to air flow studies as there are none on water or oil flow. In addition, physical mechanisms of fluid drag and convective heat transfer are discussed. This paper outlines the challenges of fabricating riblets and provides a design summary for optimal drag reduction and heat transfer. This review also looks at applied numerical methods and their accuracy in evaluating drag reduction and heat transfer over riblet surfaces. The important findings and a possible future optimization approach on riblet surfaces are highlighted as well.

Biomimicry: An Overview of Structures, Designs and Materials Inspired from Nature

Background: Nature has always played a vital role in the evolution of life forms. The design of products in accordance with nature’s design, popularly known as biomimicry, had played a vital role in pushing the technology and product effectiveness to the next level. Humans have long sought to mimic not just the design, but also the methodology adopted by certain animals. For example, the walking technique of vertebrates has been effectively mimicked for a quadruped robot to make a system more efficient by consuming less power. Thus indirectly, nature acts as a driving factor in pushing technological growth. Methods: The principle objective of this paper is to provide an overview of popular bio mimicked products inspired by nature. This paper emphasizes a wide variety of products developed in the field of materials inspired by nature. Results: Wall-climbing robots, Sonar, X-ray imaging, Sandwich and Honeycomb structures are some of the popular products and designs inspired by nature. They have resulted in better designs, better products with improved efficiency and thus have proven to be better alternatives. Some products and designs such as Samara drone, Riblet surfaces, DSSCs, Biomimetic Drills and Water turbines have plenty of scopes to replace conventional products and designs. Conclusion: While plenty of products, structures and designs have successfully replaced older alternatives, there is still a large scope for biomimicry where it could potentially replace conventional products and designs to offer better efficiency.

Effect of Nano-Titanium Dioxide Contained in Titania-Polyurea Coating on Marina Biofouling and Drag Reduction.

Marine structures often suffer from biofouling, which may lead to macrofouling by marine animals like marine worms and barnacles, weighing down the structures and increasing the drag. This paper analyses the effect of the newly fabricated biological anti-adhesion Titania-Polyurea spray coating, which can effectively reduce biofouling from enriching on the surface. Through the surface characterization, bioassays and micro-channel drag-reduction test, the antibacterial effect caused by the nano-titanium dioxide is systematically studied. Compared to the different weight percentages of nano-TiO₂ in the coating system, the photocatalytic activity, riblet surface structure and hydrophobic wettability are supposed to be the key factors to reduce the flow resistance at a drag reduction rate of 3.0% and further enhance the anti-biofouling performance under dark conditions.

Stereoscopic PIV measurements of flow over a riblet surface at high Reynolds number

The effect of drag reducing riblets on the flow structure was examined experimentally for a turbulent boundary layer at Reθ = 9890 and riblet spacing s+ = 13.4. Trapezoidal riblets were used, which were attached to the water tunnel wall as a coating. Force measurements were performed to quantify the amount of drag reduction. Then, the mechanism underlying this reduction was investigated by stereo-PIV measurements in the cross-stream plane. To determine the effect of the drag reducing riblets, the results were compared with the smooth flat plate. Time-averaged turbulent statistics such as turbulent kinetic energy and Reynolds shear stress were found to be lower over the riblets compared to the flat surface. Two-point correlations of the fluctuating velocity components were calculated to elucidate the average flow structure size and strength, where riblets significantly suppressed the turbulent structures. Quadrant analysis of the Reynolds shear stress was performed to assess the change in ejection and sweep events and the results were found to be in correspondence with previous works.

A numerical study of drag reduction performance of simplified shell surface microstructures

Antifouling and drag reduction are two critical issues for the shipping industry. Previous studies have revealed that biomimetic shell surfaces have distinct antifouling features. In this study, ten simplified surface microstructures with the same cross-section area are designed based on the antifouling scale of the shell surface morphology. Turbulent flow over the riblet structures is simulated using a finite volume based CFD solver with the Shear Stress Transport k-ω model to examine the effects of the riblet structures on skin friction with mean flow velocities ranging from 1 to 7 m/s. The corresponding dimensionless square root of the groove cross-section area is lg+≈4.5–25.6. Considering the predicted drag performance and issues concerning riblet tip rounding and fabrication, an optimal geometry with both antifouling and drag reduction features has been identified. The present study confirms that the riblets lift off streamwise vortices, pushing the high turbulent kinetic energy region away from the surface, and impede the spanwise movement of streamwise vortices, leading to skin-friction reduction. Further, the investigation of the effect of the riblet interval on the drag behaviour indicates the existence of a balance point between the wetted perimeter and the total area of the riblet surface for minimizing drag.

Turbulent structure effects due to ordered surface roughness

The effects of ordered surface feature in the form of converging-diverging riblets are investigated experimentally on an airfoil. Riblet sheets are applied onto the surface of NACA 0026 airfoil at a yaw angle of α=0°,±10°, Riblet height of h = 1 mm, and riblet spacing of s = 2 mm. The length of the riblets strip is approximately 7.5% in chord percentage. At a low friction Reynolds number of Reτ ≈ 200, at an adverse pressure gradient parameter of β ≈ 0.5–0.8, the riblet surfaces cause the boundary layer and turbulence intensity profiles to change drastically. Upon exposure to riblets, both the boundary layer and turbulence intensity profiles closely follow that of the favourable pressure gradient (FPG) flows. Without riblets, the velocity profile shows a very high wake, the flow has a very thin layer of the logarithmic region and a highly energized inner region. From the spectral analysis, without riblets, the inner region contains energized λx+ = 1000 features and large-scale features measuring 20δ, (δ is the boundary layer thickness). The spectral analysis reveals that riblets of any type break these 20δ features in the near-wall into smaller features 3–4δ. The analysis further reveals that the large-scale events of riblet surfaces are similar, but not for the smooth surface.

Modeling Surface Riblets Skin Friction Reduction Effect with the Use of Computational Fluid Dynamics

Towards the minimization of fuel consumption and aicraft emissions reduction, many disruptive technologies have been proposed in aviation industry regarding the improvement of aircraft aerodynamic performance. Among others, these technologies involve innovative passive and active flow control methodologies and techniques. A passive flow control methodology which is studied in the current work is the implementation of riblets, which are inspired by shark skin morphology, on a NACA 0012 airfoil. The main purpose of riblets is to alter the boundary layer characteristics near the wall region in such a way that the total skin friction decreases, resulting in an overall drag reduction of the aircraft with a straightforward impact on fuel consumption and thus emissions. The riblets are implemented in the Reynolds Averaged Navier Stokes equations with appropriate source terms in the turbulent dissipation transport equation. Two turbulence models are used, the k-? SST linear eddy viscosity model and the Baseline Reynolds Stress model which also uses the transport equation of the specific rate of turbulence dissipation (?). The computational results are compared with available experimental data of NACA 0012 with surface riblets attached. Drag and skin friction coefficients are presented for various angles of attack and the maximum potential benefit from riblet use is evaluated regarding aerodynamic performance and consequently reduction of emissions and fuel consumption.

Numerical Investigation of Aerodynamic Noise Reduction of Nonpneumatic Tire Using Nonsmooth Riblet Surface.

Unlike conventional pneumatic tires, the nonpneumatic tires (NPT) are explosion proof and simple to maintain and provide low rolling resistance. At high vehicle speeds, however, the complex airflow produced by the open flexible-spoke structure of NPT yields high aerodynamic noise, which contributes to sound pollution in the vehicular traffic environment. Inspired by the idea that a nonsmooth riblet structure can affect fluid flow and offer noise reduction, the analyses of the effect of the nonsmooth riblet surface on the aerodynamic noise of an NPT and noise reduction mechanism were presented in this paper. First, computational fluid dynamics (CFD) was used to analyze the surface pressure coefficient characteristics of a smooth flexible-spoke tire rolling at a speed of 80 km/h and subsequently validating the numerical simulation results by comparing them with published test results. Secondly, large eddy simulation (LES) and the Ffowcs Williams–Hawkings (FW-H) method were, respectively, used to determine the transient flow and far-field aerodynamic noise. Then, the mechanism of noise reduction was investigated using a vortex theory. Based on the vortex theory, the positions and strengths of noise sources were determined using the Lamb vector. Finally, according to the fluid boundary layer theory, a nonsmooth riblet surface was arranged on the surface of the spokes, and the influences of the riblet structure parameters, including size, position, and direction, on aerodynamic noise were analyzed. Based on the vortex theory, it was found that the nonsmooth riblet structure can reduce the Lamb vector, suppress the generation of flow vortices, decrease acoustic source strength, and effectively decrease noise up to 5.18 dB using the optimized riblet structure. The study results provide a theoretical basis for the structural design of a new low-noise NPT.

Large eddy simulation of near-wall turbulent flow over streamlined riblet-structured surface for drag reduction in a rectangular channel

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.

Optimization of Drag Reducing Shark Inspired Blade-Shape Riblet Surfaces in External Flow

The main goal of this research is to study the drag reduction capabilities of blade-shape riblet surfaces in external flows. For this purpose, the ability of riblet surfaces for drag reduction of an underwater hydrodynamic model has been investigated. The surface geometry has been modified by applying shark skin inspired blade-shape riblets on the surface. These riblets have been modeled in various dimensions and applied on the exterior surface of an underwater hydrodynamic model, and their effects on the exerted drag force, at different flow velocities, have been studied numerically. For validating the numerical solution, the simulation results have been compared with the experimental data obtained by testing an underwater hydrodynamic model in a towing tank laboratory; and the validity of the numerical solution results has been confirmed. The results indicated that, riblet spacing has a significant effect on the reduction of drag force. Furthermore, by increasing the riblet spacing, the drag force is increased rather than decreased. Also, as the velocity increases, the performance of riblets in reducing the drag force is enhanced. In order to minimize the drag force applied on the underwater hydrodynamic model, by analyzing the numerical results, the most optimum riblet spacing has been obtained; at which the drag force is reduced by 7%. The achieved distance is a limit value; and at distances smaller or larger than this optimal distance, the effectiveness of the blade-shape riblet surface in reducing the drag force diminishes.

Analysis of Shark Skin Riblet Surfaces for Fluid Drag Reduction in Turbulent Flow Regime

The integument of fast swimming shark exhibits riblet inspired micro- structured surfaces oriented in the path of flow that will help to make lesser the wall drag in the tempestuous-flow system (turbulent flow). Design have been made for study and utilization, that has been recreate and refine as same as of the shark-skin riblets, presuming an extreme drag depletion of nearly 10% (percent). Mechanism of fluid drag in tempestuous flow and riblet drag depletion theories from experiments and simulations are examined. An examination of riblet intrepratation are discussed and the stellar riblet sizes are defined. An assessment of studies experimenting with riblets-topped shark scale replicas is also discussed. A method for preferring stellar riblet dimensions based on fluid-flow attributes is briefed and current manufacturing approaches are summarized. Due to the existence of little amounts of mucus/booger membranes on the integument of the shark, it is presumed that the constrained application of aqua phobic materials will recast the flow field around the riblets in some way favorable to the goals of augmented drag depletion

Evaluation of Skin Friction Drag Reduction in the Turbulent Boundary Layer Using Riblets

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.

Characteristics of large- and small-scale structures in the turbulent boundary layer over a drag-reducing riblet surface

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.

Evaluation method of riblets effects and application on a missile surface

To overcome the difficulties of numerical simulation for riblets drag reduction effects on large-scale aircraft model, a riblet-equivalent boundary condition is introduced by modifying wall ω value, which makes the riblets effects can be calculated with a conventional smooth wall by RANS computation. Firstly, an empirical formula is deduced by analyzing various experiment results, and the function relationship between nondimensional geometry factor h+ and wall ω value is established. Secondly, the verifications for riblet-equivalent boundary condition are conducted on both zero and adverse pressure flow, by comparing the numerical simulation results with experimental data, which indicates a good reliability of this new method. Finally, an application of riblet-equivalent boundary condition is conducted on a typical configuration missile. Three riblets surface cases are introduced, with various riblets geometry factors. Flow parameters are analyzed for presenting the flow structure and performance of riblets surface. The positive conclusions are obtained that a 2.4% total drag reduction and a 4.6% skin friction reduction are gained.

Effects of surface shapes on properties of turbulent/non-turbulent interface in turbulent boundary layers

The effects of directional riblets surfaces on the turbulent/non-turbulent (T/NT) interface in turbulent boundary layers are experimentally investigated using two-dimensional time-resolved particle image velocimetry (PIV). The velocity field of streamwise—wall-normal plane for the smooth surface, converging and diverging riblets surfaces are measured. The interface is detected using the criterion of local kinetic energy. The statistical properties of interface height and conditional averaged velocity for different surfaces are analyzed. It is shown that, the converging and diverging riblets surfaces have little effect on the fractal dimension of the T/NT interface, but they cause the intermittency profile deviate from error function and the probability distribution of interface height deviate from Gaussian function. To be specific, the distribution of interface height for the converging riblets surface shows a positive skewness while it shows a negative skewness for the diverging riblets surface. Moreover, the conditional averaged streamwise velocity and spanwise vorticity across the interface are analyzed, and it is found that their self-similarities are preserved for different surfaces when normalized with respective friction velocity. The correlation analysis reveals that near-wall streamwise velocity fluctuation and interface height show a negative correlation.

Effect of drag reducing riblet surface on coherent structure in turbulent boundary layer

The characteristics of turbulent boundary layer over streamwise aligned drag reducing riblet surface under zero-pressure gradient are investigated using particle image velocimetry. The formation and distribution of large-scale coherent structures and their effect on momentum partition are analyzed using two-point correlation and probability density function. Compared with smooth surface, the streamwise riblets reduce the friction velocity and Reynolds stress in the turbulent boundary layer, indicating the drag reduction effect. Strong correlation has been found between the occurrence of hairpin vortices and the momentum distribution. The number and streamwise length scale of hairpin vortices decrease over streamwise riblet surface. The correlation between number of uniform momentum zones and Reynolds number remains the same as smooth surface.

Geometric optimization of riblet-textured surfaces for drag reduction in laminar boundary layer flows

Micro-scale riblets are shown to systematically modify viscous skin friction in laminar flows at high Reynolds numbers. The textured denticles of native sharkskin are widely cited as a natural example of this passive drag reduction mechanism. Since the structure of a viscous boundary layer evolves along the plate, the local frictional drag changes are known empirically to be a function of the length of the plate in the flow direction, as well as the riblet spacing, and the ratio of the height to spacing of the riblets. Here, we aim to establish a canonical theory for high Reynolds number laminar flow over V-groove riblets to explore the self-similarity of the velocity profiles and the evolution of the total frictional drag exerted on plates of different lengths. Scaling analysis, conformal mapping, and numerical calculations are combined to show that the potential drag reduction achieved using riblet surfaces depends on an appropriately rescaled form of the Reynolds number and on the aspect ratio of the riblets (defined in terms of the ratio of the height to spacing of the texture). We show that riblet surfaces require a scaled Reynolds number lower than a maximum threshold to be drag-reducing and that the change in drag is a nonmonotonic function of the aspect ratio of the riblet texture. This physical scaling and the computational results presented here can be used to explain the underlying physical mechanism of this mode of passive drag reduction to rationalize the geometric dimensions of shark denticles, as well as the results of experiments with shark denticle replicas of various sizes, and guide designs for optimizing the textural parameters that result in friction-reducing surfaces.

High-efficiency laser fabrication of drag reducing riblet surfaces on pre-heated Teflon

Bio-inspired surfaces are able to decrease friction with fluids and gases. The most recognizable are shark-skin-like riblet surface structures. Such bio-inspired surfaces can be formed by the laser ablation technique. In this work, bio-inspired riblet surfaces with grooves were formed using picosecond ultraviolet laser ablation on pre-heated polytetrafluoroethylene (PTFE) at various sample temperatures. The ablation of hot PTFE was found to be 30% more efficient than the conventional laser structuring at the room temperature. The friction of structured PTFE surfaces with the flowing air was investigated by using drag a measurement setup. Results show the decrease of friction force by 6% with dimensionless riblet spacing around 14–20.

Two-point statistics of coherent structures in turbulent flow over riblet-mounted surfaces

Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.