Riblet Surface Research Articles

Hierarchical nested riblet surface for higher drag reduction in turbulent boundary layer

Riblets can be potentially employed to passively reduce the turbulent friction drag. However, the drag reduction performance of riblets does not currently meet expectations, which could assist in emission reduction and energy conservation in green transportation. This study proposes and validates a topological form of hierarchical nested riblets (HNR) that significantly enhances the drag reduction performance. To explore the drag reduction enhancement mechanism, direct numerical simulations are performed for flow simulation on the riblet surface under different Reynolds numbers. The results show that under the riblet dimensionless spacing of the riblet s+≈21, the drag reduction performance of the HNR surface improves by about 70% compared to that of the uniform riblet surface, which is inspired by shark skin. From the perspective of turbulence statistics, the HNR surface reduces the turbulent mixing near the wall, weakening the momentum transfer. Furthermore, the transient flow field shows that the secondary riblet in HNR prevents some turbulent flow and streamwise vortices from entering the groove, considerably weakening the dispersive stress induced by the secondary flow. Moreover, owing to the influence of the secondary riblet, small-scale turbulence develops and strengthens into large-scale turbulent motion, which is advantageous to the boundary layer flow and results in drag reduction improvement.

Analysis of drag reduction and partial setup of riblets surface on the airfoil

Using riblet structures to modulate the turbulent boundary layer to reduce frictional drag is important for vehicle stability and range. The flow on the airfoil surface at different angles of attack is different, and the effect of the riblets structure on the turbulent boundary layer flow will be changed, which in turn affects the drag reduction effect. In this paper, the drag reduction characteristics of V-shaped riblets are investigated experimentally by the particle image velocimetry technique. The distributions of mean velocity, friction coefficient, drag reduction rate, and coherent structure in the boundary layer on the airfoil surface at angles of attack from 0° to 12° are analyzed, and the effects of smooth and riblets surfaces on the turbulent flow characteristics near the wall are compared. Based on these results, the airfoil surface was zonally covered with riblets to investigate further the effects of the riblets' covering positions on the drag reduction of the airfoil at different angles of attack. The results show that the riblets reduce the intensity, distribution, and probability of the eject and sweep events in the turbulence bursts by suppressing the velocity fluctuations at the near-wall surface, thus realizing the reduction of turbulence drag. The zoned arrangement of riblets can effectively regulate the airfoil surface flow and reduce the unfavorable effects of the riblets on the laminar flow region and flow separation region on the surface of the airfoil with different angles of attack, and the results of the study can provide a reference for the influence of the coverage range of the riblets on the drag reduction effect.

A strategy of hob-grinding the cylindrical riblet surface for drag reduction by the grinding wheel with ordered abrasive pattern

A strategy of hob-grinding the cylindrical riblet surface for drag reduction by the grinding wheel with ordered abrasive pattern

Large-scale control in turbulent flows over surface riblets

The drag reduction efficacy of a large-scale flow control over a rough surface is studied via direct numerical simulations of turbulent channels (at friction Reynolds numbers Reτ=180) by combining together wall riblets and streamwise counter-rotating swirls. In particular, the height of triangular riblets is h+≈10 (+indicating wall units), while the number of riblets (NRib in the range 1–56) along the periodic spanwise direction is varied to find the optimum. The swirls are generated by the spanwise opposed wall-jet forcing (SOJF) in the Navier–Stokes equation, whose controlling parameters follow the optimal ones as for the smooth wall. In total, 12 cases of combined SOJF and riblets are performed to investigate the coupling effects between the two methods. We find a range of NRib=7–14 (with the spanwise width z+≈140−280) yields the largest drag reduction (up to 20%) for Reτ=180, much higher than riblets control only (about 3%). Compared to SOJF control only, riblets suppress the secondary swirls of SOJF hence decreasing drag, while the lateral and down washing motions of SOJF impinging on riblets would increase drag—the opposite two effects thus giving rise to an optimal. Through examinations on coherent structures, we elucidate that the attenuation of both large-scale coherent motions and small-scale random fluctuations leads to the net drag reduction. We conclude that large-scale control is a robust approach in the cases of rough surfaces, and the parameters can be selected for maximum drag reduction in each particular situation.

Numerical study on influence of protrusion heights on Reynolds stress and viscous stress variations in turbulent vortical structures

Analysing the influence mechanism of the riblet protrusion height on turbulent drag components is more beneficial in organising the vortical structure over the riblet surface. Therefore, the Large Eddy Simulation (LES) is used to investigate the vortex structure over the riblet surface with different protrusion heights. Then, the variations of Reynolds stress and viscous shear stress in a turbulent channel are analysed. As a result, the drag reduction rate increases from 3.4% when the riblets are completely submerged in the turbulent boundary layer to 7.9% when the protrusion height is 11.2. Further analysis shows that the protrusion height affects the streamwise vortices and the normal diffusivity of spanwise and normal vortices, thus driving the variation of Reynolds stress. Compared with the smooth surface, the vorticity strength and the number of streamwise vortices are weakened near the wall but increase in the logarithmic layer with increased protrusion height. Meanwhile, the normal diffusivity of spanwise vorticity decreases with the increase of protrusion height, and the normal diffusivity of normal vorticity is the smallest when the protrusion height is 11.2. Moreover, the protrusion height affects the velocity gradient of the riblet tip and riblet valley, thus driving the variation of viscous shear stress. With the increase of protrusion height, the velocity gradient of the riblet tip increases dramatically but decreases in the riblet valley.

Drag reduction of the turbulent boundary layer over divergent sawtooth riblets surface with a superhydrophobic coat

Energy conservation and environmental protection have become pivotal components of the green economy. In recent years, underwater drag reduction technology has garnered significant interest. This study discusses a novel composite surface that combines divergent riblets with a superhydrophobic coating (D-rib&SHS) to enhance the drag reduction rate. Alongside this new surface, a riblet surface with a superhydrophobic coating (rib&SHS, without yaw angles) and a smooth surface are used as comparison groups. The turbulent boundary layer flow of these three surfaces is measured using a two-dimensional particle image velocimetry system. The results indicated that the maximum drag reduction rate of D-rib&SHS is approximately 27% higher than that of rib&SHS, and the drag reduction range is increased to Reθ≈4100 compared to rib&SHS (Reθ≈2200). Using correlation algorithms, it observed that the spacing between low-speed streaks over D-rib&SHS is larger than that of rib&SHS and the smooth surface. This finding suggested that the spacing between the hairpin vortex legs of D-rib&SHS is wider. The increased spacing between the hairpin vortex legs reduces the likelihood of vortex head formation between the two quasi-streamwise vortices, ultimately suppressing the auto-generation of hairpin vortices. Consequently, the development of hairpin vortex packets over D-rib&SHS is also inhibited. These phenomena observed over D-rib&SHS can be attributed to the combined effects of velocity slip on the superhydrophobic coating and the secondary flow over the divergent riblets near the wall. In addition, unlike divergent riblets that are not covered with a superhydrophobic coating, where the drag reduction effect is more pronounced with a yaw angle of 30° compared to 10°, D-rib&SHS with a 10° yaw angle demonstrated a superior drag reduction effect compared to D-rib&SHS with a 30° yaw angle. This innovative composite surface enhances the drag reduction effect and expands the drag reduction Reynolds number range, offering a new approach to mitigating drag in turbulent boundary layer flows.

Drag reduction capacity of multi‐scale and multi‐level riblet in turbulent flow

AbstractFor high‐speed moving objects, drag reduction has been a prolonged major challenge. To address this problem, passive and negative strategies have been proposed in the preceding decades. The integration of creatures and nature has been continuously perfected during biological evolution. Unique structure characteristics, material properties, and special functions of marine organisms can provide inexhaustible inspirations to solve this intractable problem of drag reduction. Therefore, a simple and low‐cost laser ablation method was proposed. A multi‐scale and multi‐level riblet (MSLR) surface inspired by the denticles of the sharkskin was fabricated by controlling the density of the laser path and ablation times. The morphology and topographic features were characterised using an electron microscope and a scanning white‐light interfering profilometer. Then, the drag reduction capacity of the bionic riblet surface was measured in a circulating water tunnel. Finally, the mechanism of drag reduction was analysed by the computational fluid dynamics (CFD) method. The results show that the MSLR surface has a stable drag reduction capacity with an increase in Reynold (Re) number which was contributed by high‐low velocity stripes formed on the MSLR surface. This study can provide a reference for fabricating spatial riblets with efficient drag reduction at different values of Re and improving marine antifouling.

Bayesian-Optimized Riblet Surface Design for Turbulent Drag Reduction via Design-by-Morphing With Large Eddy Simulation

Abstract A computational approach is presented for optimizing new riblet surface designs in turbulent channel flow for drag reduction, utilizing design-by-morphing (DbM), large Eddy simulation (LES), and Bayesian optimization (BO). The design space is generated using DbM to include a variety of novel riblet surface designs, which are then evaluated using LES to determine their drag-reducing capabilities. The riblet surface geometry and configuration are optimized for maximum drag reduction using the mixed-variable Bayesian optimization (MixMOBO) algorithm. A total of 125 optimization epochs are carried out, resulting in the identification of three optimal riblet surface designs that are comparable to or better than the reference drag reduction rate of 8%. The Bayesian-optimized designs commonly suggest riblet sizes of around 15 wall units, relatively large spacing compared to conventional designs, and spiky tips with notches for the riblets. Our overall optimization process is conducted within a reasonable physical time frame with up to 12-core parallel computing and can be practical for fluid engineering optimization problems that require high-fidelity computational design before materialization.

A viscous vortex model for predicting the drag reduction of riblet surfaces

This paper introduces a viscous vortex model for predicting the optimal drag reduction of riblet surfaces, eliminating the need for expensive direct numerical simulations (DNSs) or experiments. The footprint of a typical quasi-streamwise vortex, in terms of the spanwise and wall-normal velocities, is extracted from smooth-wall DNS flow fields in close proximity to the surface. The extracted velocities are then averaged and used as boundary conditions in a Stokes-flow problem, wherein riblets with various cross-sectional shapes are embedded. Here, the same smooth-wall-based boundary conditions can be used for riblets, as we observe from the DNSs that the quasi-streamwise vortices remain unmodified apart from an offset. In particular, the position of these vortices remain unpinned above small riblets. The present approach is compared with the protrusion-height model of Luchini et al. (J. Fluid Mech., vol. 228, 1991, pp. 87–109), which is also based on a Stokes calculation, but represents the vortex with only a uniform spanwise velocity boundary condition. The key novelty of the present model is the introduction of a wall-normal velocity component into the boundary condition, thus inducing transpiration at the riblet crests, which becomes relevant as the riblet size increases. Consequently, the present model allows for the drag-reduction prediction of riblets up to the optimal size. The present approach does not rely on the scale separation formally required by homogenisation techniques, which are only applicable for vanishingly small riblets.

Numerical investigation of the linear evolution of Tollmien–Schlichting waves over longitudinal riblet surface

The effect of longitudinal riblets on the spatially developing Tollmien–Schlichting (T–S) waves within the boundary layer is numerically investigated by direct numerical simulation. The riblets, designed to reduce turbulent drag and featuring a blade-like shape with zero thickness, are the primary focus. Part of the flat plate is replaced by riblet surface, and T–S waves with varying frequencies are introduced prior to the onset of the riblets. Moreover, the influence of riblet size is further discussed, and the underlying mechanism by which riblets affect T–S waves is identified based on the analysis of disturbance energy equation. The results demonstrate that the presence of riblets significantly enhances the growth of T–S waves. The modulation of base flow by riblets results in the emergence of an inflection point in the velocity profile within the boundary layer, thereby enhancing the flow instability. The growth rate of T–S waves and the unstable region on the riblet surface are observed to be considerably amplified, and an increase in riblet's lateral spacing and height to spacing ratio intensifies this amplification. From the perspective of disturbance energy, it is shown that although riblets cause additional energy dissipation in their vicinity, their modification of the mean velocity gradient and the phase difference between streamwise and wall-normal velocity fluctuations contribute to a significant increase in the production term, which consequently accelerates the growth of T–S waves.

Bio-inspired Surface Texture Fluid Drag Reduction using Large Eddy Simulation

Skin friction drag can be reduced through the application of bio-inspired riblet surfaces. Numerical simulations were performed using Large Eddy Simulation (LES) to investigate the effect of using riblets on reducing skin friction drag. In this study, three different riblet configurations were used; scalloped, sawtooth and a new design, hybrid, riblet. To validate the effect of using the proposed hybrid riblet design compared with other riblets used in the literature; before applying to complex geometries, they were initially applied to a flat plate in parallel arrangement. Results showed skin friction coefficient reduction of 14% using the proposed hybrid riblet. This reduction was 9.2 times and 1.2 times more compared to sawtooth and scalloped configurations, respectively. The hybrid riblet was then applied partially and fully to NACA 0012 airfoil. Skin friction coefficient reduction of 34.5% was obtained when the hybrid riblet fully applied on the airfoil surface. Furthermore, the Convergent-Divergent (C-D) arrangement was studied, where the riblets were placed fully on the NACA 0012 and aligned with a yaw angle with respect to the flow direction. The convergent lines are inspired by the sensory part of the shark skin, whereas the divergent lines or herringbone are found on the bird feather. The two different riblet configurations, sawtooth and hybrid were modeled with the C-D arrangement and the hybrid riblet with C-D arrangement contributed to higher skin friction coefficient reduction, 34.5%, than the sawtooth riblet shape, 26.75%. Moreover, the C-D arrangement was compared to the parallel arrangement and shown that the C-D arrangement increased the lift coefficient (cl) of the airfoil, the flow separation was delayed and the overall performance of the airfoil was enhanced.

Drag Reduction Performance of Triangular (V-groove) Riblets with Different Adjacent Height Ratios

Surface structure is used to interfere with the turbulent boundary layer in the groove drag reduction, which is important to the endurance and stability of high-speed and ultrahigh-speed aircraft. The size of the groove structure directly affects the flow in the turbulent boundary layer and changes the drag reduction effect. The drag reduction characteristics of bionic triangular (V-groove) riblets were studied through Particle Image Velocimetry (PIV) experiment and Finite Volume Method (FVM) simulation. Triangular riblets with adjacent height ratios (AHR) of 1.00, 1.74, and 3.02 were considered in this research, and the influence of these groove structures on the flow characteristics of turbulence near the wall is compared with those of the smooth plates. The distribution of time-averaged velocity, turbulence intensity, and coherent structures of turbulent boundary layer on the riblet surface is analyzed to document the effects of the geometric parameters of various groove structures on drag reduction rates. Results showed that the best drag reduction is obtained using the V-groove riblets with adjacent height ratio of 1:1 under the low free-stream velocity. The results can be used as a reference for further optimization of drag reduction structures with surface grooves.

The Roles of Riblet and Superhydrophobic Surfaces in Energy Saving Using a Spatial Correlation Analysis.

Riblet and superhydrophobic surfaces are two typical passive control technologies used to save energy. In this study, three microstructured samples-a micro-riblet surface (RS), a superhydrophobic surface (SHS), and a novel composite surface of micro-riblets with superhydrophobicity (RSHS)-were designed to improve the drag reduction rate of water flows. Aspects of the flow fields of microstructured samples, including the average velocity, turbulence intensity, and coherent structures of water flows, were investigated via particle image velocimetry (PIV) technology. A two-point spatial correlation analysis was used to explore the influence of the microstructured surfaces on coherent structures of water flows. Our results showed that the velocity on microstructured surface samples was higher than that on the smooth surface (SS) samples, and the turbulence intensity of water on the microstructured surface samples decreased compared with that on the SS samples. The coherent structures of the water flow on microstructured samples were restricted by length and structural angles. The drag reduction rates of the SHS, RS, and RSHS samples were -8.37 %, -9.67 %, and -17.39 %, respectively. The novel established RSHS demonstrated a superior drag reduction effect and could improve the drag reduction rate of water flows.

Drag Reduction Technology of Water Flow on Microstructured Surfaces: A Novel Perspective from Vortex Distributions and Densities.

Revealing the turbulent drag reduction mechanism of water flow on microstructured surfaces is beneficial to controlling and using this technology to reduce turbulence losses and save energy during water transportation. Two microstructured samples, including a superhydrophobic and a riblet surface, were fabricated near which the water flow velocity, and the Reynolds shear stress and vortex distribution were investigated using a particle image velocimetry. The dimensionless velocity was introduced to simplify the Ω vortex method. The definition of vortex density in water flow was proposed to quantify the distribution of different strength vortices. Results showed that the velocity of the superhydrophobic surface (SHS) was higher compared with the riblet surface (RS), while the Reynolds shear stress was small. The vortices on microstructured surfaces were weakened within 0.2 times that of water depth when identified by the improved ΩM method. Meanwhile, the vortex density of weak vortices on microstructured surfaces increased, while the vortex density of strong vortices decreased, proving that the reduction mechanism of turbulence resistance on microstructured surfaces was to suppress the development of vortices. When the Reynolds number ranged from 85,900 to 137,440, the drag reduction impact of the superhydrophobic surface was the best, and the drag reduction rate was 9.48%. The reduction mechanism of turbulence resistance on microstructured surfaces was revealed from a novel perspective of vortex distributions and densities. Research on the structure of water flow near the microstructured surface can promote the drag reduction application in the water field.

A Feasibility Study of Riblet for Aeroacoustics Applications

Although the use of finlet, serration or porous surface has been shown a good level of success in reducing aerofoil trailing-edge noise, they are largely incompatible to the otherwise streamlined aerofoil body. This paper is a feasibility study to investigate the riblet, which has so far been quite successful as a drag-reducing device, for its potential to reduce the turbulent pressure sources that are important for aerofoil self-noise radiation. The completed results show that the riblet used in the current study can reduce the skin-friction coefficient, as well as the turbulence-intensity in the boundary-layer profiles. In addition, the turbulence structures in the convective field can be dissipated more rapidly when crossing the riblet surface. It is also found that (1) the riblet produces a slight reduction of the wall-pressure power spectral density level at the low and high frequency ranges, but experiences an increase at the mid frequency, (2) the riblet can reduce the lateral turbulence coherence length-scale across a large frequency range. The product of these two hydrodynamic sources represents an important mechanism for the radiation of the trailing-edge self-noise, whose low and high frequency ranges are found to be sensitive to the riblet effect where reduction can occur.

Implementation of flow loss minimization incorporating the riblet technique on cascade profile and experimental validation

For the purpose of reducing the flow loss in the compressor cascade, the aerodynamic design of the blade geometry is essential. The traditional aerodynamic design focuses on curve modification, and the surface structure design is rarely considered in the optimization framework. Recent studies have revealed that micro riblet surface provides an excellent application prospect in drag reduction, which offers a new idea for the design of compressor cascade. In this paper, a concept of “geometry-flow loss” control is proposed, and an innovative optimization method combining profile and riblet design is developed further to improve the aerodynamic performances of the compressor cascade. In the design process of multiple iterations, the multi-scale simulations describing the whole flow field with a massive amount of grids would cause unaffordable costs. Therefore, the blade-riblets simplified simulation strategy is employed to evaluate the aerodynamic performances of the cascade profile with riblets and make the profile with riblet design practicable. The flow loss characteristics of the original cascade and two optimal cascades are assessed and verified by wind tunnel tests. The simulation and experimental results both indicate that the profile with the riblet design significantly further reduces the total pressure loss of the compressor cascade compared to the traditional profile design. The proposed concept in this paper increases the dimensionality of the geometry design and further exploits the design potential of the compressor cascade.

Numerical simulation of turbulent channel flow with particle-adhered riblet surfaces

Numerical simulation of turbulent channel flow with particle-adhered riblet surfaces

Drag reduction of turbulent boundary layer over sawtooth riblet surface with superhydrophobic coat

The application of drag reduction tech holds great significance to energy saving. To achieve better drag reduction, we investigated the flow characteristics of the turbulent boundary layer (TBL) over a composite surface made of sawtooth riblets with superhydrophobic coat (rib&SHS), a superhydrophobic surface (SHS), and a smooth surface using particle image velocimetry. The results showed that the drag reduction rate of the composite surface was higher than that of the superhydrophobic surface at the same Reynolds number. When the Reynolds number reached 2015, the drag reduction effect of SHS was almost ineffective (drag reduction was only 1.2%), whereas rib&SHS maintained satisfactory results (drag reduction was 20.2%). By proper orthogonal decomposition (POD), the second-order POD mode showed the tilt angles of the interface of Q2 and Q4 events inside the TBL over rib&SHS, and SHS were reduced compared with the smooth surface in the drag reduction cases. With drag reduction of rib&SHS and SHS, the hairpin vortexes were lifted away from the wall and the distances of vortexes within hairpin vortex packets decreased. Compared with SHS, rib&SHS had a greater effect on hairpin vortexes and hairpin vortex packets because the riblets made the Q2 events of rib&SHS weaker than that of SHS. So, the rib&SHS has a higher drag reduction rate and a larger drag reduction Reynolds number range than the SHS. It can be used to guide the drag reduction design of underwater vehicles.

Aerodynamic Drag Reduction on Speed Skating Helmet by Surface Structures

The aerodynamic drag for speed skating helmets with surface structures was investigated in this work by using numerical and experimental methods. Computational fluid dynamic (CFD) research was performed to analyze the detail of the flow field around the helmets. The simplified helmet models, with riblet and bump surface structures, were analyzed using the CFD simulations. The pressure distribution and velocity field around the helmets were obtained through the CFD analysis. The CFD results showed that the boundary layer separation position was obviously delayed, and the pressure changed to a higher value at the back area for structured helmets. Therefore, the aerodynamic drag for the structured helmet was lower than that of the original model. According to the CFD results, three types of helmets, with the of riblet and bump surface structure printed on the helmets by using flexible film, were tested in a wind tunnel. A full-scaled skater mannequin of half a body was used in the experiment to simulate the actual skating process. Compared with the original helmet, a drag reduction rate of 7% was achieved for the helmet with the bump at the middle region in the wind tunnel experiment, at the average speed in competitions for skaters.

Effects of the Yaw Angle on Air Drag Reduction for Various Riblet Surfaces.

Biomimetic riblet surfaces, such as blade, wavy, sinusoidal, and herringbone riblet surfaces, have widespread applications for drag reduction in the energy, transportation, and biomedicine industries. The drag reduction ability of a blade riblet surface is sensitive to the yaw angle, which is the angle between the design direction of the riblet surface and the average flow direction. In practical applications, the average flow direction is often misaligned with the design direction of riblet surfaces with different morphologies and arrangements. However, previous studies have not reported on the drag reduction characteristics and regularities related to the yaw angle for surfaces with complex riblet microstructures. For the first time, we systematically investigated the aerodynamic drag reduction characteristics of blade, wavy, sinusoidal, and herringbone riblet surfaces affected by different yaw angles. A precisely adjustable yaw angle measurement method was proposed based on a closed air channel. Our results revealed the aerodynamic behavior regularities of various riblet surfaces as affected by yaw angles and Reynolds numbers. Riblet surfaces with optimal air drag reduction were obtained in yaw angles ranging from 0 to 60° and Reynolds numbers ranging from 4000 to 7000. To evaluate the effects of the yaw angle, we proposed a criterion based on the actual spanwise spacing (d+) of microstructure surfaces with the same phase in a near-wall airflow field. Finally, we established conceptual models of aerodynamic behaviors for different riblet surfaces in response to changes in the airflow direction. Our research lays a foundation for practical various riblet surface applications influenced by yaw angles to reduce air drag.