Riblet Shape Research Articles

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.

Riblet-generated flow mechanisms that lead to local breaking of Reynolds analogy

We investigate the Reynolds analogy over riblets, namely the analogy between the fractional increase in Stanton number $C_h$ and the fractional increase in the skin-friction coefficient $C_f$ , relative to a smooth surface. We investigate the direct numerical simulation data of Endrikat et al. (Flow Turbul. Combust., vol. 107, 2021, pp. 1–29). The riblet groove shapes are isosceles triangles with tip angles $\alpha = {30}^{\circ }, {60}^{\circ }, {90}^{\circ }$ , a trapezoid, a rectangle and a right triangle. The viscous-scaled riblet spacing varies between $s^+ \approx 10$ to $60$ . The global Reynolds analogy is primarily influenced by Kelvin–Helmholtz rollers and secondary flows. Kelvin–Helmholtz rollers locally break the Reynolds analogy favourably, i.e. cause a locally larger fractional increase in $C_h$ than in $C_f$ . These rollers induce negative wall shear stress patches which have no analogue in wall heat fluxes. Secondary flows at the riblets’ crests are associated with local unfavourable breaking of the Reynolds analogy, i.e. locally larger fractional increase in $C_f$ than in $C_h$ . Only the triangular riblets with $\alpha = {30}^{\circ }$ trigger strong Kelvin–Helmholtz rollers without appreciable secondary flows. This riblet shape globally preserves the Reynolds analogy from $s^+ = 21$ to $33$ . However, the other riblet shapes have weak or non-existent Kelvin–Helmholtz rollers, yet persistent secondary flows. These riblet shapes behave similarly to rough surfaces. They unfavourably break the global Reynolds analogy, and do so to a greater extent as $s^+$ increases.

Profile Loss Reduction of High-Lift Turbine Blades With Rough and Ribbed Surfaces

Abstract Transitional boundary layers on low-pressure turbines (LPTs) are prone to separation on the suction surface of the blade under strong local adverse pressure gradients. Intermittent freestream turbulence, periodic wakes shed by the upstream blades, and surface roughness due to in-service degradation of the blades are shown to suppress the separation. Although this generally leads to a profile loss reduction, some of the benefits are offset by a loss increase associated with an increased turbulent wetted area. In this work, we explore a strategy where the losses in both the transitional and turbulent boundary layers can be reduced. In particular, we employ surface roughness in the transitional regime to reduce the separation bubble-related losses and riblets in the turbulent regime to further reduce the losses due to the turbulent wetted area. The efficacy of this ‘rough-riblet blade surface’ is studied using high-fidelity eddy resolving simulations on the configuration of a flat surface subjected to streamwise varying pressure gradients. Two riblet shapes, sawtooth and scalloped, are considered. When compared to the roughness alone configuration, scalloped riblets reduced the skin friction drag by ≈10% and are much more effective than the sawtooth riblets. Through the streamwise evolution of the boundary layer parameters such as trailing edge momentum thickness, maximum turbulent kinetic energy, and Reynolds stresses, the additional losses incurred at the junction between the smooth wall and riblet leading edge are highlighted.

Influence on drag reduction effect of riblet surfaces by particle adhesion in turbulent channel flow

Numerical simulations of fully developed turbulent channel flows with riblet surfaces were carried out at the friction Reynolds number of 180. The influence of particles adhering to the rectangular riblets on the drag reduction effect is investigated. The one-way coupling method is employed: the particles are affected by the fluid, but the fluid is not affected by the particles. The simulation consists of three steps. First, we calculate the detailed particles motion and the adhesion position of the particles to riblets in the turbulent channel flow. Second, the riblet shape is modified based on particle adhesion distribution. Here, the condition that the particle adhesion distribution is uniform in the streamwise direction and independent of the number of particles adhered is given. Third, we calculate the flow over the riblet with the adhered particles. As a result, most of the particles adhered to the top wall and the upper half of the side wall surfaces of the riblets. Due to the particle adhesion on the riblets, the drag increases as compared with the case of the initial riblet of the riblet.

전산 소음 해석 기법을 이용한 잠수함 표면 처리에 따른 소음 효과 관찰

Submarine has been performed for national defense activities representatively as a multi-purpose underwater vehicle. Underwater Radiated Noise (URN) which emitted during underwater vehicle operation is directly related to the survivability of the underwater vehicle. This study focused on examining the effects of surface treatment on noise aspects in whole submarine body for the first time. Prior to the acoustic analysis on the submarine model, the acoustic analysis procedure using commercial CFD software was verified using a 2D cylinder model. Acoustic analysis on submarine model was conducted for two cases: smooth surface as a reference case, and a few micrometers of V-type applied riblet case. To solve the incompressible fluid flow passing the submarine, the Reynolds-averaged Navier-Stokes (RANS) equation coupled with the k-ω SST turbulence model was used. For acoustic analysis, the Ffowcs Williams and Hawkings acoustic analogy, which calculates flow-induced noise by designating a physical wall as a noise source, was used. The noise effects according to the surface treatment on submarine shape were examined by comparing overall sound pressure level (OASPL) between smooth and riblet case and the results indicate that noise around submarine shape was reduced about 4.3 dB in maximum on surface-treated area. However, when calculating OASPL excluding noise below 20 Hz, which is a frequency range where noise does not propagate, the noise tends to increased due to riblet shape. This study provides established procedure in acoustic analysis on whole submarine body and a new perspective that underwater radiation noise should be considered when designing surface treatment on submarine.

Influence of riblet shapes on the occurrence of Kelvin–Helmholtz rollers

Abstract

Direct Numerical Simulations of Turbulent Flow Over Various Riblet Shapes in Minimal-Span Channels

Riblets reduce skin-friction drag until their viscous-scaled size becomes large enough for turbulence to approach the wall, leading to the breakdown of drag-reduction. In order to investigate inertial-flow mechanisms that are responsible for the breakdown, we employ the minimal-span channel concept for cost-efficient direct numerical simulation (DNS) of rough-wall flows (MacDonald et al. in J Fluid Mech 816: 5–42, 2017). This allows us to investigate six different riblet shapes and various viscous-scaled sizes for a total of 21 configurations. We verify that the small numerical domains capture all relevant physics by varying the box size and by comparing to reference data from full-span channel flow. Specifically, we find that, close to the wall in the spectral region occupied by drag-increasing Kelvin–Helmholtz rollers (Garcia-Mayoral and Jimenez in J Fluid Mech 678: 317–347, 2011), the energy-difference relative to smooth-wall flow is not affected by the narrow domain, even though these structures have large spanwise extents. This allows us to evaluate the influence of the Kelvin–Helmholtz instability by comparing fluctuations of wall-normal and streamwise velocity, pressure and a passive scalar over riblets of different shapes and viscous-scaled sizes to those over a smooth wall. We observe that triangular riblets with a tip angle $$\alpha =30^{\circ }$$ and blades appear to support the instability, whereas triangular riblets with $$\alpha =60^{\circ }$$ – $$90^{\circ }$$ and trapezoidal riblets with $$\alpha =30^{\circ }$$ show little to no evidence of Kelvin–Helmholtz rollers.

Resolvent Analysis for Turbulent Channel Flow with Riblets

This paper extends the resolvent formalism for wall turbulence to account for the effect of streamwise-constant riblets. Under the resolvent formulation, the Navier–Stokes equations are interpreted as a forcing-response system: the nonlinear convective term is interpreted as a feedback forcing on the remaining linear terms, which generates a velocity and pressure response. A gain-based decomposition of the linear forcing-response transfer function—the resolvent operator—yields highly amplified velocity and pressure modes, which can be considered key building blocks of the turbulent flow field. Previous work has shown that these high-gain modes provide substantial insight into turbulence statistics, structure, and control of smooth-walled flows. To introduce the effect of riblets within this framework, a linear spatially varying body force is added to the governing equations. In other words, volume penalization is used to approximate the surface features. Predictions for spanwise-periodic and streamwise-constant riblets show that specific high-gain modes identified from the modified governing equations reproduce observations made in prior direct numerical simulations with limited computation. The deterioration in performance with increasing riblet size is predicted well and so is the emergence of spanwise rollers resembling Kelvin–Helmholtz vortices. This new modeling framework is also used to pursue limited riblet shape optimization.

Effects of Riblet Shape of Non-smooth Surface on Aerodynamic Characteristics of Vehicle

To reduce the aerodynamic drag of the vehicle, three kinds of riblet shape non-smooth surfaces were arranged in the tail of the Motor Industry Research Association (MIRA) stepped-back vehicle mode. The shapes of the non-smooth riblet elements are triangular, circular and rectangular. Computational fluid dynamics (CFD) method was used for numerical analysis. To verify the effectiveness of the analysis method, the analysis result of the standard MIRA stepped-back vehicle mode was compared with wind tunnel test data. The tail-flow, pressure and turbulent kinetic energy of the MIRA stepped back vehicle modes, which were arranged with riblet elements, were analyzed. The effects of riblet shape of non-smooth surfaces on the reduction of vehicle’ aerodynamic drag were compared with each other. The simulation results show that the three kinds of non-smooth surfaces have a good effect on drag reduction by reducing the negative pressure in the tail zone of the model, reduce the turbulent kinetic energy and reduce energy dissipation. The circular non-smooth surface is better than the other, with drag reduction effect of 5.53%.

Homogenization of a Large Eddy Simulation Model for Turbulent Fluid Motion Near a Rough Wall

In this paper, we are interested in the numerical simulation of a turbulent viscous flow in the vicinity of a periodic rough wall. To this aim, we consider a large eddy simulation (LES) model, and perform its asymptotic analysis, letting the period $$\varepsilon $$ of the roughness tend to zero. Relying on an homogenization method closely related to the two-scale convergence method (Allaire in SIAM J Math Anal 23(6):1482–1518, 1992; Nguetseng in SIAM J Math Anal 20(3):608–623, 1989), we exhibit a critical scaling associated with parameter $$\lambda :=\lim _{\varepsilon \rightarrow 0}{a_\varepsilon }/{\varepsilon ^{5/3}}$$ , where $$a_\varepsilon $$ stands for the amplitude of the roughness. In the critical regime $$0<\lambda <\infty $$ , the roughness effect is described by an additional, nonlinear friction term, that depends locally on the solution to a nonlinear auxiliary problem of boundary layer type. In the particular case of riblets, which is of major practical interest, we show that the directional invariance of the geometry results in a simplified expression of this extra term, and we propose a numerical method to simulate the homogenized LES model, based on that observation. Finally, we study the influence of parameter $$\lambda $$ on the flow approximated by the homogenized model, using different commonly used riblet shapes. Our simulations demonstrate that the extra friction term can be handled numerically, and may have an important influence on the results of LES in presence of roughness.

Flow Simulation of Various Riblet Shapes and a Uniform Model for Boundary Layer

Flow Simulation of Various Riblet Shapes and a Uniform Model for Boundary Layer

ELUCIDATION OF LONGITUDINALLY GROOVED-RIBLETS DRAG REDUCTION PERFORMANCE USING PRESSURE DROP MEASUREMENTS

The need to determine affordable and environmentally friendly methods of reducing skin friction can be identified as one of the reasons contributing towards the study of the effectiveness of riblet shapes. Water tank experiments were carried out to optimize the shape and dimensions of microstructure grooves over a flat plate. The use of organized microstructures on channel walls is proposed to obtain lower values of pressure losses on smooth walls. Three shapes of microstructure grooves were investigated, with same groove height (600 μm) and five spacing dimensions (600, 750, 1000, 1500 μm), in water flows with velocities of up to 0.4 m/s. This was done for all selected types of riblet, which are fixed with the direction aligned with the flow. The experimental results showed that the size and shape of the riblets can massively incubate some of the turbulent structures formed on the surface and that will lead to a more controllable flow environment, which can result in drag reduction.

Optical Riblet Sensor: Beam Parameter Requirements for the Probing Laser Source.

Beam parameters of a probing laser source in an optical riblet sensor are studied by considering the high demands on a sensors’ precision and reliability for the determination of deviations of the geometrical shape of a riblet. Mandatory requirements, such as minimum intensity and light polarization, are obtained by means of detailed inspection of the optical response of the riblet using ray and wave optics; the impact of wavelength is studied. Novel measures for analyzing the riblet shape without the necessity of a measurement with a reference sample are derived; reference values for an ideal riblet structure obtained with the optical riblet sensor are given. The application of a low-cost, frequency-doubled Nd:YVO laser pointer sufficient to serve as a reliable laser source in an appropriate optical riblet sensor is discussed.

High-Speed Magnetic Microrobot Actuation in a Microfluidic Chip by a Fine V-Groove Surface

Microrobots are promising tools for the treatment of biological cells because of their lack of skill dependence, high throughput, and high repeatability. Integration of a microfluidic chip and robotics based on microelectromechanical systems technology is a key challenge for biomedical innovations. In addition to the advantage of environmental control by a microfluidic chip, robots enable physical operations on the cell with high throughput. This paper presents high-speed microrobotic actuation driven by permanent magnets in a microfluidic chip. The developed microrobot has a milli-Newton-level output force from a permanent magnet, micrometer-level positioning accuracy, and drive speed of over 280 mm/s. The riblet surface, which is a regularly arrayed V-groove, reduces fluid friction and enables high-speed actuation. Ni and Si composite fabrication was employed to form the optimum riblet shape on the microrobot's surface by wet and dry etching. The evaluation experiments show that the microrobot can be actuated at a rate of up to 90 Hz, which is more than ten times higher than that of the microrobot without a riblet. Two distinguish applications of the developed microrobots were demonstrated in a microfluidic chip.

微細V溝パターンによる磁気駆動マイクロロボットのオンチップ高速駆動

This paper presents the high-speed microrobot driven by permanent magnet in a microfluidic chip. The comprehensive analysis of fluid force, the optimum design and its fabrication was conducted to reduce the fluid force on the magnetically driven microrobot by developing fine V groove pattern on the microrobot. Riblet surface, which is regularly arrayed V grooves, were employed to reduce fluid friction on the microrobot. The upward force is generated by the wedge effect and it generates lubricant film thickness between the microrobot and the glass substrates. Optimization process was conducted to design riblet surface on the microrobot in order to maximize lubricant film thickness. The Ni and Si composite fabrication was employed to form the optimum riblet shape by anisotropic Silicon (Si) wet etching and deep reactive ion etching. The evaluation experiments show the microrobot can be actuated up to 90 Hz, which is about 10 times higher than the original microrobot.

Effect of Riblet Shape on Turbulent Drag Reduction.

This paper presents an effect of riblet shape on turbulent drag reduction. The experiment was carried out in an N.P.L.blow-down-type wind tunnel with a working section of 500mm×500mm×2000mm. In order to investigate the details of drag reduction for riblet shape, rib models of scaling up riblets were used in this experiment. The parameters were 5 kinds for the rib model and 7 kinds for the riblet model. The time-mean velocity and skin friction were measured by the Pitot and static pressure tubes and Preston tube respectively. The Reynolds stress was obtained using the data processing system connected to a hot wire anemometer. Consequently it was found that(i)the Reynolds stress and skin friction become smallest near the surface for the rib with the section of an equilateral triangle, and(ii)the drag reduction attains maximum for the riblet of V groove with nearly equilateral triangular section.

Passive and active control of near-wall turbulence

Wall skin friction reduction is the primary focus of this paper. Turbulent boundary layer structure and near-wall turbulence structure are considered in particular along with how passive and active modifications to the structure can lead to reductions in drag. Direct numerical simulation of low Reynolds number turbulent flow over streamwise aligned micro-grooved striations or riblets, particularly V-groove riblets and the application of a near-wall viscous model to the flow over more complicated riblet shapes (V and U groove, thin element, compound and spanwise non-continuous V-groove) are assessed. This assessment provides a qualitative basis for identifying features that may limit riblets to skin friction reduction of about 10%. This leads to consideration of three dimensional riblets or “humplets”, use of oscillations (both fluid and structural), selective interference with existing turbulence and selective introduction of new scales of turbulence with the aim always towards reducing skin friction.

On the boundary layer/riblets interaction mechanisms and the prediction of turbulent drag reduction

Turbulent drag reduction experienced by ribletted surfaces is the result of both (1) the interaction between riblet peaks and the coherent structures that characterize turbulent near-wall flows, and (2) the laminar sublayer flow modifications caused by the riblet shape, which can balance, under appropriate conditions, the drag penalty due to the increased wetted surface. The latter “viscous” mechanism is investigated by means of an analytical model of the laminar sublayer, which removes geometrical restrictions and allows us to take into account “real” shapes of riblet contours, affected by manufacturing inaccuracies, and to compute even for such cases a parameter, called protrusion height, related to the longitudinal mean flow. By considering real geometries, riblet effectiveness is clearly shown to be related to the difference between the longitudinal and the transversal protrusion heights. A simple method for the prediction of the performances of ribletted surfaces is then devised. The predicted and measured drag reduction data, for different riblet geometries and flow characteristics, are in close agreement with each other. The soundness of the physical interpretation underlying this prediction method is consequently confirmed.

Application of V Shape Riblets to Pipe Flows

Pipes with V shape riblets were tested at Reynolds numbers between 5×103 and 4×104. All riblet pipes indicated some drag reduction. The model with h = 0.55 mm and h/S = 0.483 showed the maximum drag reduction of 8 percent and the widest range of Reynolds number over which the riblet reduces drag. The riblet shape desirable for drag reduction in pipe flows was almost the same as that in flat plate boundary layers, but the value of S+ which provided the maximum drag reduction was quite different; S+ = 23 for pipe flows and S+ = 12 for flat plate boundary layers.

On three-dimensional bodies of minimum drag at high supersonic speed

Wall skin friction reduction is the primary focus of this paper. Turbulent boundary layer structure and near-wall turbulence structure are considered in particular along with how passive and active modifications to the structure can lead to reductions in drag. Direct numerical simulation of low Reynolds number turbulent flow over streamwise aligned micro-grooved striations or riblets, particularly V-groove riblets and the application of a near-wall viscous model to the flow over more complicated riblet shapes (V and U groove, thin element, compound and spanwise non-continuous V-groove) are assessed. This assessment provides a qualitative basis for identifying features that may limit riblets to skin friction reduction of about 10%. This leads to consideration of three dimensional riblets or “humplets”, use of oscillations (both fluid and structural), selective interference with existing turbulence and selective introduction of new scales of turbulence with the aim always towards reducing skin friction.