Riblet Drag Reduction Research Articles

Shark Skin—An Inspiration for the Development of a Novel and Simple Biomimetic Turbulent Drag Reduction Topology

In this study, a novel but simple biomimetic turbulent drag reduction topology is proposed, inspired by the special structure of shark skin. Two effective, shark skin-inspired, ribletted surfaces were designed, their topologies were optimized, and their excellent drag reduction performances were verified by large eddy simulation. The designed riblets showed higher turbulent drag reduction behavior, e.g., 21.45% at Re = 40,459, compared with other experimental and simulated reports. The effects of the riblets on the behavior of the fluid flow in pipes are discussed, as well as the mechanisms of fluid drag in turbulent flow and riblet drag reduction. Riblets of various dimensions were analyzed and the nature of fluid flow over the effective shark skin surface is illustrated. By setting up the effective ribletted surface on structure’s surface, the shark skin-inspired, biomimetic, ribletted surface effectively reduced friction resistance without external energy support. This method is therefore regarded as the most promising drag reduction technique.

Numerical Study of Effect of Sawtooth Riblets on Low-Reynolds-Number Airfoil Flow Characteristic and Aerodynamic Performance

Riblets with an appropriate size can effectively restrain turbulent boundary layer thickness and reduce viscous drag, but the effects of riblets strongly depend on the appearance of the fabric that is to be applied and its operating conditions. In this study, in order to improve the aerodynamic performance of a low-pressure fan by using riblet technology, sawtooth riblets on NACA4412 airfoil are examined at the low Reynolds number of 1 × 105, and the airfoil is operated at angles of attack (AOAs) ranging from approximately 0° to 12°. The numerical simulation is carried out by employing the SST k–ω turbulence model through the Ansys Fluent, and the effects of the riblets’ length and height on aerodynamic performance and flow characteristics of the airfoil are investigated. The results indicate that the amount of drag reduction varies greatly with riblet length and height and the AOA of airfoil flow. By contrast, the riblets are detrimental to the airfoil in some cases. The most effective riblet length is found to be a length of 0.8 chord, which increases the lift and reduces the drag under whole AOA conditions, and the maximum improvements in both are 17.46% and 15.04%, respectively. The most effective height for the riblet with the length of 0.5 chord is 0.6 mm. This also improves the aerodynamic performance and achieves a change rate of 12.67% and 14.8% in the lift and drag coefficients, respectively. In addition, the riblets facilitate a greater improvement in airfoil at larger AOAs. The flow fields demonstrate that the riblets with a drag reduction effect form “the antifriction-bearing” structure near the airfoil surface and effectively restrain the trailing separation vortex. The ultimate cause of the riblet drag reduction effect is the velocity gradient at the bottom of the boundary layers being increased by the riblets, which results in a decrease in boundary thickness and energy loss.

Study of riblet drag reduction for an infinite span wing with different sweep angles

This paper studies the riblet drag reduction effect for an infinite swept wing under a low Reynolds number using a large-eddy simulation. The results show that the drag reduction ratio is not linear under different sweep angles. The maximum drag reduction ratio in this study is 9.5% for a wing with a 45° sweep angle. The local surface streamline angle and turbulence quantities are calculated to analyze the drag reduction mechanism. The results demonstrate that the riblets considerably suppress the Reynolds stresses above the wing upper surface, while the turbulence kinetic energy in the near wake is increased. A possible relaminarization phenomenon is observed at the middle part of the wing. Quasi-two-dimensional flow structures are observed near the wall, and a peak frequency is considered as the dominant frequency of the region.

The Influence of Boundary Layer Caused by Riblets on the Aircraft Surface

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.

Study on Riblet Drag Reduction Considering the Effect of Sweep Angle

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.

Numerical study of an airfoil with riblets installed based on large eddy simulation

The implicit large eddy simulation method is applied in a riblet drag reduction study of a low-speed airfoil. The numerical method is tested by a channel flow with streamwise riblets installed on one side. The mean velocity profile, root mean square of velocity and Reynolds shear stress match well with reference direct numerical simulation data. Next, numerical investigation is conducted on an airfoil called Eppler E374 at free stream Mach number 0.2, angle of attack 3° and Reynolds number 2.0 × 105. The simulated flow around the airfoil without or with a numerical trip is in good agreement with the experimental data. When isosceles triangle riblets are installed on the airfoil along the streamwise direction, the lift coefficient is increased, and the friction drag is decreased. The pressure distribution on the airfoil is slightly changed, corresponding to the increased pressure drag. The Reynolds stresses are greatly reduced by the riblets at locations of strong pressure gradient. The result of the power spectrum density of pressure shows that the high-frequency fluctuations are suppressed when the riblet film is installed on the airfoil, and the vortex structures in the boundary layer are also reduced.

Study on Drag Reduction Performance of Antifouling Ribbed Surfaces

Drag reduction by ribbed surfaces is a potentially effective strategy for reducing the energy consumption of ships. However, complicated by possible marine biofouling, it is meaningless to consider the isolated effect of drag reduction of riblets. There is a strong correlation between drag reduction and antifouling, and thus it is crucial to study their synergistic behaviors. In this study, shell is taken as a biomimetic object, which has ribbed surfaces and antifouling properties. On the premise of antifouling, the microstructures of the shell surface are simplified as ribbed surfaces with an interval w between neighboring riblets. The impact of the riblet interval w on drag reduction performance is studied both numerically and experimentally. The numerical simulation adopts the Shear Stress Transport k — ω turbulence model, and the characteristics of smooth and ribbed surfaces are analyzed in the same computational domain. Numerical results show that the riblet interval w has a complex nonmonotonic effect on drag reduction. The corresponding experimental measurement shows similar trend to the numerical results. 1. Introduction Antifouling and drag reduction of ships are two critical issues in the shipping industry. As the consumption of fossil fuel is increasing in line with economic growth, severe energy crisis, and environmental pollution have become intractable social problems in recent years. Under this circumstance the concept of "green ship" which has both antifouling and drag reduction features has emerged. It is known that surface friction drag accounts for a great portion of the total fluid drag (more than 60% for a cargo ship). Furthermore, microstructures on a moving surface influence the flow primarily in a small region near the surface, but this influence may have a profound impact on the drag (Elfriede et al. 2010). For instance, microriblets are formed over the whole body of a shark with aligned tiny placoid scales, leading to superior drag reduction, the so-called "shark skin effect" (Chen et al. 2013). Under such inspiration, Walsh and coworkers at the NASA Langley Research Center investigated several different types of ribbed surfaces and reported an 8% drag reduction for symmetric V-groove riblets (Walsh 1983; Walsh & Lindemann 1984). Choi and Orchard (1997) found that such drag reduction occurs only in turbulent flow regimes, whereas the drag is actually increasing in laminar flow regimes due to the presence of riblets on the surface. Bechert et al. (1997) investigated a number of configurations of riblets including rectangular, scalloped, and shark-skin-shape riblets. The effects of various parameters such as the riblet height and length have been reported in the literature (Henn & Sykes 1999; Choi & Suzuki 2005; Vlachogiannis & Hanratty 2004). Tests conducted in a wind tunnel and a water channel also showed a drag reduction of 7–8% by shark skin and ribbed surface (Elfriede et al. 2010; Meng et al. 2011).

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.

The effects of rectangular riblets on rectangular micro air vehicles for drag reduction

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.

リブレットの抵抗低減効果を模擬する乱流モデルの構築

For practical CFD-aided design of riblets on aircrafts, we are aiming to develop a Reynolds-Averaged Navier-Stokes (RANS) turbulence model which can simulate drag reduction effects without resolving fine-scale flows near the riblets. Wilcox's rough wall boundary condition for Menter's SST model is modified to reproduce velocity shift in logarithmic region corresponding to riblet's drag reduction effects. Two basic relations for this model are derived from available experimental data and parametric analysis, and are validated in zero-pressure gradient boundary layers with two different-geometry riblets. Drag reduction rates corresponding to experimental results can be obtained with these relations. The new model, however, underestimates the effects of riblet mounted on NACA0012, which suggests that further improvement of the model is needed to account for influences of pressure gradient.

Riblet drag reduction in mild adverse pressure gradients: A numerical investigation

Riblet films are a passive method of turbulent boundary layer control that can reduce viscous drag. They have been studied with great detail for over 30years. Although common riblet applications include flows with Adverse Pressure Gradients (APG), nearly all research thus far has been performed in channel flows. Recent research has provided motivation to study riblets in more complicated turbulent flows with claims that riblet drag reduction can double in mild APG common to airfoils at moderate angles of attack. Therefore, in this study, we compare drag reduction by scalloped riblet films between riblets in a zero pressure gradient and those in a mild APG using high-resolution large eddy simulations. In order to gain a fundamental understanding of the relationship between drag reduction and pressure gradient, we simulated several different riblet sizes that encompassed a broad range of s+ (riblet width in wall units), similarly to many previously published experimental studies. We found that there was only a slight improvement in drag reduction for riblets in the mild APG. We also observed that peak values of streamwise turbulence intensity, turbulent kinetic energy, and streamwise vorticity scale with riblet width. Primary Reynolds shear stresses and turbulence kinetic energy production however scale with the ability of the riblet to reduce skin-friction.

SHARK-SKIN SURFACES FOR FLUID-DRAG REDUCTION IN TURBULENT FLOW: A REVIEW

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.

The effect of riblets in rectangular duct flow

Much is known about the benefits of surface structures which mimic the riblets found on the skin of fast swimming sharks. Structures have been fabricated for study and application which replicate and improve upon the natural shape of the shark skin riblets, providing a maximum drag reduction of nearly 10% in external turbulent fluid flow. Mechanisms of fluid drag in turbulent flow and riblet-drag reduction theories from experiment and simulation are discussed. A brief review of riblet performance studies is given, and optimal riblet geometries are defined for external flow. A discussion of the structure of internal turbulent fluid flow is provided, and existing data for riblet drag reduction benefit is presented. A flow cell for studying riblet effects in internal rectangular duct flow is discussed, and data collected using several riblet surfaces fabricated for the flow cell is presented and analyzed. A discussion of the effects of the riblets on fluid flow is given, and conclusions are drawn about the possible benefits of riblets in internal fluid flow.

Shark-skin surfaces for fluid-drag reduction in turbulent flow: a review

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.

Similarity-Law Analysis and Turbulence Modeling for Riblet Drag Reduction

A complete similarity-law analysis is developed for riblet drag reduction which results in a flat-plate drag formula. As an example, a design chart is developed for a particular riblet configuration from its empirical drag characterization. The riblet drag characterization is applied to turbulence modeling such as an eddy-viscosity formula and the k-ε method.

Effect of detailed surface geometry on riblet drag reduction performance

A comparison is made of the effect of small changes in v-groove geometry, for several riblet films applicable for drag reduction to commercial transport aircraft, whose nominal v-groove dimension is of the order of 0.002 inch. The films were tested in a water towing-tank facility. The results obtained indicate that small riblet peak geometry variations can result in a deterioration of riblet drag-reduction efficacy of as much as 40 percent, while interriblet valley curvature was found not to be critical to riblet performance.