Drag measurements on planar riblet surfaces at high subsonic speeds

This paper presents the results of an investigation of riblet performance at high subsonic Mach numbers, and Reynolds numbers of about 20 000 based on the momentum thickness, in both zero and adverse pressure gradient boundary layers. The maximum length Reynolds number of the ribbed section was 3.4×10⁷ so the results were directly relevant to flight applications on the engine nacelles of civil airliners. Seven different sizes of riblets with heights h (equal to spacing s) ranging from 0.0007 (0.0178mm) to 0.006 (0.1524 mm) have been studied, covering a range of h+, s+ from 10 to 106. The maximum percentage skin friction reduction, as deduced from velocity profiles measured at the downstream end of the riblet surfaces, under nominally zero pressure gradient conditions was 5.5±1; rather less than that recorded in low speed studies, but consistent with a recent theoretical analysis of the effect of Reynolds number. The values of h+ required for maximum and zero skin friction reduction agreed closely with other data. In addition subsequent floating element drag balance measurements revealed little effect of yaw angles up to 15°, again in line with other findings, and also suggested that the extent of the initial development length on, and recovery length behind, the riblets was approximately 5. The adverse pressure gradient studies indicated that riblet performance was essentially unaffected by mild gradients (=0.25), but diminished to zero in a more severe gradient (=0.5).     

FLOW STRUCTURE IN THE TURBULENT BOUNDARY LAYER OVER DIRECTIONAL RIBLETS SURFACES

本文采用时间解析的二维粒子图像测速技术,对零压力梯度光滑以及汇聚和发散沟槽表面平板湍流边界层统计特性和流动结构进行了研究.结果表明在垂直于汇聚和发散沟槽表面的对称平面内,相对于光滑壁面,发散沟槽壁面使当地边界层厚度、壁面摩擦阻力、湍流脉动、雷诺应力等明显减小;而汇聚沟槽壁面对湍流边界层特性和流动结构的影响正好相反,汇聚沟槽使壁面流体有远离壁面向上运动的趋势,因而导致边界层厚度增加了约43%;同时,在汇聚沟槽表面情况下流向大尺度相干结构更容易形成,这对减阻是不利的.此外,顺向涡数量在湍流边界层的对数区均存在一个极大值,发散沟槽表面所对应的极大值位置更靠近沟槽壁面,而在汇聚沟槽表面则有远离壁面的趋势,由顺向涡诱导产生的较强的喷射和扫掠运动会在湍流边界层中产生较强的剪切作用,顺向涡数量的减少是发散沟槽壁面当地摩擦阻力降低的主要原因.     

Tomographic PIV investigation on coherent vortex structures over shark-skin-inspired drag-reducing riblets

Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-inspired riblets on coherent vortex structures in a turbulent boundary layer(TBL) is investigated.This is done by means of tomographic particle image velocimetry(TPIV) measurements in channel fl ws over an acrylic plate of drag-reducing riblets at a friction Reynolds number of 190.The turbulent fl ws over drag-reducing riblets are verifie by a planar time-resolved particle image velocimetry(TRPIV) system initially,and then the TPIV measurements are performed.Two-dimensional(2D) experimental results with a dragreduction rate of around 4.81% are clearly visible over triangle riblets with a peak-to-peak spacing s+of 14,indicating from the drag-reducing performance that the buffer layer within the TBL has thickened;the logarithmic law region has shifted upward and the Reynolds shear stress decreased.A comparison of the spatial topological distributions of the spanwise vorticity of coherent vortex structures extracted at different wall-normal heights through the improved quadrant splitting method shows that riblets weaken the amplitudesof the spanwise vorticity when ejection(Q2) and sweep(Q4) events occur at the near wall,having the greatest effect on Q4 events in particular.The so-called quadrupole statistical model for coherent structures in the whole TBL is verified Meanwhile,their spatial conditional-averaged topological shapes and the spatial scales of quadrupole coherent vortex structures as a whole in the overlying turbulent fl w over riblets are changed,suggesting that the riblets dampen the momentum and energy exchange between the regions of near-wall and outer portion of the TBL by depressing the bursting events(Q2 and Q4),thereby reducing the skin friction drag.     

On the relationship between drag and vertical velocity fluctuations in flow over riblets and liquid infused surfaces

Direct numerical simulations (DNS) of flow over triangular and rectangular riblets in a wide range of size and Reynolds number have been carried out. The flow within the grooves is directly resolved by exploiting the immersed-boundary method. It is found that the drag reduction property is primarily associated with the capability of inhibiting vertical velocity fluctuations at the plane of the crests, as in liquid-infused surfaces (LIS) devices. This is mimicked in DNS through artificial suppression of the vertical velocity component, which yields large drag decrease, proportionate to the riblets size. A parametrization of the drag reduction effect in terms of the vertical velocity variance is found to be quite successful in accounting for variation of the controlling parameters. A Moody-like friction diagram is thus introduced which incorporates the effect of slip velocity and a single, geometry-dependent parameter. Reduced drag-reduction efficiency of LIS-like riblets is found as compared to cases with artificially imposed slip velocity. Last, we find that simple wall models of riblets and LIS-like devices are unlikely to provide accurate prediction of the flow phenomenon, and direct resolution of flow within the grooves in necessary.     

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 30 years. 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.     

Prediction of the drag reduction effect of pulsating pipe flow based on machine learning

Prediction of drag reduction effect caused by pulsating pipe flows is examined using machine learning. First, a large set of flow field data is obtained experimentally by measuring turbulent pipe flows with various pulsation patterns. Consequently, more than 7000 waveforms are applied, obtaining a maximum drag reduction rate and maximum energy saving rate of 38.6% and 31.4%, respectively. The results indicate that the pulsating flow effect can be characterized by the pulsation period and pressure gradient during acceleration and deceleration. Subsequently, two machine learning models are tested to predict the drag reduction rate. The results confirm that the machine learning model developed for predicting the time variation of the flow velocity and differential pressure with respect to the pump voltage can accurately predict the nonlinearity of pressure gradients. Therefore, using this model, the drag reduction effect can be estimated with high accuracy.     

Rotating cylinder drag balance with application to riblets

Experimental results are reported and discussed for a rotating cylinder drag balance designed to predict drag reduction by surfaces like riblets. The apparatus functions by measuring the torque applied to the inner cylinder by a fluid, such as water, that is set in motion by the controlled rotation of the outer cylinder. The instrument was validated by calibration for laminar flow and comparison of turbulent flow results to the those of G. I. Taylor. The ability to predict drag reduction was demonstrated by testing 114 m symmetric sawtooth riblets, which gave a maximum reduction of about 5% and an overall drag reduction range of 5<S ⁺<20, both of which are in excellent comparison to results reported in literature. The most suitable conditions for testing riblets are to apply the riblets only to the inner cylinder surface and to use cylinders for which the curvature of the flow is minimized. Received: 2 February 1999/Accepted: 1 October 1999     

Drag reduction and improvement of material transport in creeping films

It is widely accepted that for bodies in turbulent flows a reduction of skin friction can be reached if the surface of the body is provided with small ridges aligned in the local flow direction. This surprising and counterintuitive phenomenon is called the shark-skin effect, motivated from the dermal surface morphology of sharks. In the present article we examine the possibility of resistance reduction due to a rippled surface topography in Stokes flow. We especially analyse the influence of wall riblets perpendicular to the flow direction on the mean transport velocity in gravity-driven creeping film flows following the idea that eddies generated in the valleys of the riblets act like fluid roller bearings and hence may reduce drag. Using a theoretical treatment of the Stokes equations with complex function theory, parameter studies with varying flow rate, bottom amplitude and bottom shape are presented. For the given bottom shapes the maximum enhancement of transport velocity is found by optimising the film thickness.     

沟槽面湍流边界层结构实验研究

应用激光测速技术和氢气泡流动显示技术对沟槽面湍流边界层特性及近壁区拟序结构特征进行了精细的测量和观察。实验结果表明：与光滑面湍流边界层相比,沟槽面端流边界层的黏性底层厚度、过渡层厚度及流速分布对数公式中的积分常数C均有所增大,说明采用的沟槽面具有减阻特性。此外,无量纲低速带条间距明显减小,最多减小20%,说明无量钢低速带条平均间距的缩短与湍流减阻密切联系。     

Polymer drag reduction with surface roughness in flat-plate turbulent boundary layer flow

Experimental results from a study of surface roughness effects on polymer drag reduction in a zero-pressure gradient flat-plate turbulent boundary layer are presented. Both slot-injected polymer and homogeneous polymer ocean cases were considered over a range of flow conditions and surface roughness. Balance measurements of skin friction drag reduction are presented. Drag reductions over 60% were measured for both the injected and homogeneous polymer cases even with fully rough surfaces. As the roughness increased, higher polymer concentration was required to achieve a given level of drag reduction for the homogeneous case. With polymer injection, increasing surface roughness caused the drag reduction to decrease to low levels more quickly when the polymer expenditure was decreased or the freestream velocity was increased. However, the percent drag reductions on the rough surfaces with polymer injection were often substantially larger than on the smooth surface. Remarkably, in some cases, the skin friction drag force on a rough surface with polymer injection was less than the drag force observed on a smooth surface at comparable conditions. An erratum to this article can be found at     

湍流流动中鲨鱼皮表面流体减阻研究进展

快速游动的鲨鱼, 其皮肤表面沿流动方向有序地排列着沟槽状结构, 人们认为这种结构能在湍流流动 中减小表面摩擦阻力. 人们仿真这种生物结构来进行实验研究和应用, 通过复制和改善鲨鱼皮肤表面沟槽状 结构, 使得摩擦阻力最大减小了近10%. 在实验和模拟仿真中, 人们不断讨论和研究湍流流动阻力的形成机制 和沟槽减阻的理论特性. 本文综述了沟槽减阻理论特性的一些研究方法, 并且归纳定义了沟槽减阻最优几何 形状及其尺寸; 详细考虑流体流动的特点, 给出了一种用来选择最优沟槽形状及其尺寸的方法; 综述了目前的 沟槽加工制造技术. 由于鲨鱼皮肤表面存在少量黏液, 从仿生学的角度, 文章最后综述并展望了通过局部应用 疏水性材料来改变沟槽附近流场属性, 从而达到更大程度上减小阻力的目标.      

The influence of the type of gas on the reduction of skin friction drag by microbubble injection

The influence of the type of gas on the performance of microbubble skin friction reduction was investigated on an axisymmetric body. Gases were selected which covered a wide range of densities and solubilities. Integrated skin friction measurements, which span a range of velocities (U _∞) from roughly 10 to 20 m/s and tunnel pressures from 1 to 2.6 atm, are presented as a function of gas flow rate. All gases show qualitatively similar behavior. The gas volume flowrate, referenced to injector ambient conditions (tunnel temperature and pressure), is shown to correlate the drag reducing behavior of all the gases at one velocity, independent of pressure. A normalization based on the volume flowrate through the turbulent boundary layer is shown to nearly collapse all the results independent of velocity or pressure. The results indicate that high ambient pressures may degrade the drag reducing capabilities of highly soluble gases.     

脊状表面航行器模型减阻特性的风洞实验研究

针对航行器提高航程和航速的需要,开展脊状表面湍流边界层减阻的实验和数值仿真研究。在航行器模型的外表面加工具有特定形状、尺寸的脊状结构,导致湍流边界层的流动稳定性增强,壁面摩擦阻力降低。在风洞中对具有光滑表面和脊状表面的航行器模型在不同风速和攻角下进行阻力测试,得到其减阻特性曲线。实验结果表明,具有横向脊状表面的航行器模型在一定来流速度范围内具有很好的减阻效果,实验获得的最大减阻量为23.5%。数值仿真结果则发现,在脊状结构内形成了稳定的"二次涡",边界层内湍动能和湍流猝发强度降低,很好地揭示了减阻机理。     

LES of the adverse-pressure gradient turbulent boundary layer

We describe large-eddy simulations (LES) of the flat-plate turbulent boundary layer in the presence of an adverse pressure gradient. The stretched-vortex subgrid-scale model is used in the domain of the flow coupled to a wall model that explicitly accounts for the presence of a finite pressure gradient. The LES are designed to match recent experiments conducted at the University of Melbourne wind tunnel where a plate section with zero pressure gradient is followed by section with constant adverse pressure gradient. First, LES are described at Reynolds numbers based on the local free-stream velocity and the local momentum thickness in the range 6560–13,900 chosen to match the experimental conditions. This is followed by a discussion of further LES at Reynolds numbers at approximately 10 times and 100 times these values, which are well out of range of present day direct numerical simulation and wall-resolved LES. For the lower Reynolds number runs, mean velocity profiles, one-point turbulent statistics of the velocity fluctuations, skin friction and the Clauser and acceleration parameters along the streamwise, adverse pressure-gradient domain are compared to the experimental measurements. For the full range of LES, the relationship of the skin-friction coefficient, in the form of the ratio of the local free-stream velocity to the local friction velocity, to both Reynolds number and the Clauser parameter is explored. At large Reynolds numbers, a region of collapse is found that is well described by a simple log-like empirical relationship over two orders of magnitude. This is expected to be useful for constant adverse-pressure gradient flows. It is concluded that the present adverse pressure gradient boundary layers are far from an equilibrium state.     

Drag measurements on V-grooved surfaces on a body of revolution in axial flow

In water flows with velocities of up to 9 m/s the friction drag of a body of revolution in axial flow was investigated for dependence on the body surface structure. This was done for different types of riblet film fixed on the surface with the riblet direction aligned with the flow. The lateral spacing between the triangular shaped riblets varied between 0.033 mm and 0.152 mm. In all cases the riblet spacing was equal to the riblet height. For comparison a smooth reference film was used.Depending on the Reynolds number and the non-dimensional riblet spacings ⁺, a turbulent drag reduction of up to 9% could be achieved with riblets in comparison with the flow over a smooth surface.In the region of transition to turbulent flow and with non-dimensional riblet spacings ofs ⁺10–15 drag reductions of up to 13% were obtained. It is therefore conjectured, that in addition to hampering the near wall momentum exchange, the riblets can delay the development of initial turbulent structures in time and space.     

TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

采用高时间分辨率粒子图像测速技术对沟槽壁面平板湍流边界层速度矢量场的时间序列及其统计量进行了实验测量,讨论了在同一来流速度下沟槽壁面对平均速度剖面﹑雷诺切应力及湍流强度的影响. 用流向速度分量的多尺度空间局部平均结构函数辨识壁湍流多尺度相干结构,用条件采样和相位平均技术提取壁湍流多尺度相干结构喷射和扫掠事件的脉动速度、展向涡量的二维空间拓扑形态. 结果表明,与同材料光滑壁面对比,沟槽壁面实现了10.73%的摩阻减小量;沟槽壁面湍流边界层湍流强度及雷诺切应力皆比光滑平板湍流边界层对应统计量小,说明沟槽壁面有效降低了湍流边界层内流体的脉动. 通过比较壁湍流相干结构猝发事件各脉动速度分量与展向涡量的空间分布特征,肯定了沟槽壁面的减阻效果,发现沟槽壁面通过抑制相干结构猝发事件实现减阻.     

Combined effect of longitudinal riblets and LEBU-devices on turbulent friction on a plate

The possibility of reducing turbulent friction with the help of large-eddy-breakup devices (LEBUs) and riblets is studied experimentally. The tests were conducted in a low-turbulence wind tunnel on a flat plate for 2·10⁶ Re 7·10⁶. The local friction coefficient was measured using internal strain-gauge balances, and the total drag was estimated by the momentum-transfer method. It is shown that a combination of LEBUs and riblets makes it possible to reduce the total turbulent friction drag of a flat plate 1800 mm long by 16%. The effects of the length of a ribbed surface on the efficiency of friction reduction and of LEBUs and riblets on the structure of a turbulent boundary layer are analyzed.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 39–46, May–June, 1995.     

Drag characteristics of longitudinal and transverse riblets at low dimensionless spacings

A surface grooved with microscopic riblets aligned parallel to the flow is an effective means to reduce the turbulent skin friction up to 10% compared to a smooth surface. The maximum drag reduction is found for a dimensionless rib spacing s ⁺ in the range of 15–17. For s ⁺ < 10, a linear behaviour of the drag reduction curve is predicted by viscous theory. This linear slope of the drag reduction curve is in contradiction to Schlichting’s postulation of a hydraulically smooth behaviour of small-scale roughness in a turbulent flow. This regime of evanescent dimensionless rib spacings is investigated experimentally by direct wall shear stress measurements in a fully developed channel flow. Additionally, a numerical calculation of the viscous flow over riblets was carried out to predict the drag reducing behaviour. The experimental results show a linear drag reducing behaviour down to s ⁺ = 0.3, which is in good agreement with the numerical results of the viscous simulation. The postulation of Schlichting’s hydraulically smooth regime of a rough surface was not confirmed, neither for a riblet surface nor for a surface geometry with grooves oriented perpendicular to the flow. In the latter case, the drag increases as a quadratic function of the roughness height.     

Application of the Dorodnitsyn finite element method to swirling boundary layer flow

The Dorodnitsyn finite element method for turbulent boundary layer flow with surface mass transfer is extended to include axisymmetric swirling internal boundary layer flow. Turbulence effects are represented by the two-layer eddy viscosity model of Cebeci and Smith¹ with extensions to allow for the effect of swirl. The method is applied to duct entry flow and a 10 degree included-angle conical diffuser, and produces results in close agreement with experimental measurements with only 11 grid points across the boundary layer. The introduction of swirl (w_e/u_e = 0.4) is found to have little effect on the axial skin friction in either a slightly favourable or adverse pressure gradient, but does cause an increase in the displacement area for an adverse pressure gradient. Surface mass transfer (blowing or suction) causes a substantial reduction (blowing) in axial skin friction and an increase in the displacement area. Both suction and the adverse pressure gradient have little influence on the circumferential velocity and shear stress components. Consequently in an adverse pressure gradient the flow direction adjacent to the wall is expected to approach the circumferential direction at some downstream location.     

低雷诺数沟槽表面湍流/非湍流界面特性的实验研究

湍流/非湍流界面是流动中湍流和无旋流的边界,其相关研究在加深对湍流与无旋流之间的物质、动量和能量交换的理解有重要意义.本文采用时间解析的二维粒子图像测速技术,分别对零压梯度光滑、顺流向锯齿形沟槽表面平板在不同雷诺数下对湍流/非湍流界面的几何特征及动力学特性进行了实验研究.实验雷诺数为$Re_{\tau } =400\sim1000$.本文采用了湍动能准则对湍流/非湍流界面进行了识别,并分析界面高度分布、分形特征及界面附近的条件平均速度和涡量.结果表明在不同雷诺数下, 无论是光滑壁面还是沟槽壁面,界面平均高度在0.8 $\sim$ 0.9$\delta_{99} $附近. 对于沟槽壁面而言,减阻时对应的界面高度的概率密度分布与光滑壁面基本一致, 均遵循正态分布,而当阻力增大时, 界面高度分布偏离正态分布出现正的偏度. 在本实验情况下,界面分形维度、跨界面速度跳变均会随着雷诺数增大而增大. 此外,不同壁面情况下无量纲条件平均涡量在界面附近的分布相近,而界面附近无量纲速度梯度最大值近似为常数.