INFLUENCE OF YOUNG'S MODULUS ON DRAG-REDUCTION IN TURBULENT FLOW USING FLEXIBLE TUBES

In order to develop technologies of friction drag reduction with a flexible tube to be used for water transport, experimental studies were carried out on the influence of Young's modulus on the turbulent drag reduction. The friction coefficients of flexible tubes with different Young's modulus were examined by using a sleeve-tube structure. The fluctuating vibration of the outer wall and the fluctuating pressure on the inner wall of the tubes were measured with a laser displacement sensor and a pressure sensor. The results are as follows. The smaller the Young's modulus of a flexible tube and the larger the Reynolds number, the larger the turbulent drag reduction rates become. The transition from laminar flow to turbulent flow can be delayed with an appropriate Young's modulus. Non-dimensional amplitude of fluctuating vibration on the outer wall is smaller than that of a viscous sub-layer thickness, and is positively correlated with the friction drag reduction.     

柔管与紊流耦合减阻效果的初步研究

为探讨柔管与紊流耦合减阻的初步机理,采用双重管结构及激光传感测位仪,在通过实验确认柔性管与刚体管相比确有紊流减阻效果的基础上,对柔管的紊动振动特性及相应的紊流减阻效果以及二者的相关性进行了实验研究。结果表明：柔管的壁厚越小,管外壁振动的脉动位移越大,无量纲的猝发周期也越大,紊流减阻效果越好;柔管壁外为压力平衡空气且雷诺数约为17 500时,壁厚分别为2,3,4 mm柔管的无量纲猝发周期依次为141,126,105,这将为设计高效的流体输送技术提供了一种实验技术上的选择。     

DRAG REDUCTION EFFECT OF COUPLING FLEXIBLE TUBES WITH TURBULENT FLOW

To analyze the mechanism of drag reducing effect by coupling flexible tubes with turbulent flow, based on experimental examination of more obvious turbulent drag reduction effect in flexible tubes than in rigid tubes, experimental investigation was performed on the effect of turbulent drag reduction, fluctuating vibration characteristics of flexible tube and the correlations by using a double-tube system and laser displacement sensor. The results are as follows： with the decrease of the thickness of the flexible tubes, the root mean square of fluctuating amplitude of the outer wall of the tubes increases, and the non-dimensional burst period increases, resulting in the increase of the reduction rate of drag coefficient by coupling flexible tubes with turbulent flow. At applied pressure-balanced air on the outer wall and the Reynolds number of about 1.75×10^4, the non-dimensional burst periods of the flexible tubes with the thickness of 2 mm, 3 mm, 4 mm are 141, 126, 105, respectively.     

弹性管湍流输水减阻的效果评价和应用前景分析

为开发弹性管减阻技术,通过双重管结构,在对弹性管外壁充入几乎与管内压力相平衡的压力空气的条件下,实验考察了弹性管湍流输水的减阻效果。研究结果表明:基于双重管的弹性管减阻特性的评价是可行的;弹性管的壁厚越小,减阻率越大,雷诺数越高,减阻效果越好。使用激光位移传感器对弹性管外壁的脉动振动特性测试表明,弹性管外壁的脉动振动幅度与弹性管减阻率之间存在正相关关系。认为弹性管湍流输水减阻在国内的应用潜力巨大。     

圆管流掺气减阻理论研究

本文研究了圆管中气液分层层流、分层紊流及掺气混合紊流情况下的阻力减小规律。理论研究表明，圆管中的气液分层层流、分层紊流，即在近壁层内充以空气，可以在相同的水流量下，使阻力下降，实现减阻。而掺气混合紊流时，只有当壁面附近的含气浓度大于圆管中心处的含气浓度时，才能在相同水流量下，使阻力下降，实现减阻     

AN INVESTIGATION OF FLOW CHARACTERISTIC OF AERATED DRAG REDUCTION IN TUBE

Under the aerated conditions of wall and top intube, the turbulent flow in the tube was measured by usingLDA. The turbulent structure of the flow field and the mech-anism of aerating drag reduction in the tube were discussed. It is shown that the energy dissipations of turbulence flow andmean flow will reduce and the flow velocity (or flow rate) willincrease by injecting mini-bubbles to the wall or top of tube,namely the effect of aerating drag reduction is attained.     

Advances of drag-reducing surface technologies in turbulence based on boundary layer control

Our living environment is surrounded by turbulence, which is also a concern of the global energy consumption and the greenhouse gas emission, and the viscous force on the solid-liquid/solid-gas interface is an important part of the turbulence. Reducing friction force in turbulence to the greatest extent is becoming an urgent issue to be resolved at present. In this paper, the various state-of-the-art approaches of drag-reducing and energy-saving technologies based on the boundary layer control are reviewed, focusing on the polymer drag reduction additives, the micro-morphology, the super-hydrophobic surface, the micro air bubbles, the heating wall, the vibrant flexible wall and the composite drag reduction methods. In addition, the mechanisms of different drag reductions based on the boundary layer control and the potential applications in fluid engineering are discussed. This paper aims not only to contribute to a better understanding of drag reduction mechanisms, but also to offer new perspectives to improve the current drag-reducing and energy saving technologies.     

Analysis of shear rate effects on drag reduction in turbulent channel flow with superhydrophobic wall

We examined the shear rate effect on drag reduction of superhydrophobic surfaces with different slip lengths. For this purpose, turbulent channel flow was considered at the friction Reynolds numbers of Reτ = 180, 395, 500. By using Navier＇s slip condition it is shown that increasing shear rate leads to the greater reduction in drag force and also more reduction occurs in larger slip length. Based on the results, more than 25% drag reduction happens at a friction Reynolds number of Reτ= 500 for slip length of 1 ×10 5 m. The simulation results suggest that reduction in drag force occurs because slip condition reduces the Reynolds stresses, also weakens vorticity filed and the near-wall coherent structures, and therefore turbulence production is decreased.     

DRAG REDUCTION IN A TURBULENT CHANNEL FLOW WITH HYDRO- PHOBIC WALL

This paper investigates a theoretical prediction of friction drag reduction in turbulent channel flow which is achieved by using superhydrophobic surfaces. The effect of the hydrophobic surface is considered to be a slip boundary condition on the wall, and this new boundary condition is added to Large Eddy Simulation (LES) equations. The predicted drag reduction at Reτ=180 is approximately 30%, which concurs with results obtained from Direct Numerical Simulation (DNS). An important implication of the present finding is that the near-wall turbulence structures are modified with streamwise slip velocity. In addition, a noticeable effect on the turbulence structure occurs when the slip length is greater than a certain value.     

肋条减阻

随着世界上能源消耗的不断上升，使人们不得不认真考虑如何有效的保护有限的能源，探求节约能源的新技术和新方法。湍流减阻就是在这种应用背景下提出在新课。经过二十多年的努力，特别是湍流理论的发展，使得湍流减阻理论和应用得到了突破性的进展。就减阻技术讲，有肋条减阻、聚合物减阻、大涡硫碎减阻、吹气和吸气减阻、微气泡减阻等，这些减阻技术一个共贩考虑，就是要控制边界层内的湍流结构，特别是拟序结构，减少湍能的耗损以     

弹性管的紊流减阻效果与流激振动特性的实验研究

在实验确认弹性管确有紊流减阻效果的基础上,对其流激振动特性及紊流减阻效果进行了实验研究。结果表明:管的壁厚越小,管外壁的流激振动的脉动位移有效值越大,紊流减阻效果越好;在雷诺数Re约为2×104时,壁厚为2mm、3mm及4mm弹性管的流激振动减阻率依次约为12%、10%、8%。     

近壁圆柱绕流的壁面积沙现象机理分析

针对近壁圆柱绕流过程中出现的在平板壁面积沙现象用数值模拟方法进行了机理分析。应用标准k-ε湍流模型和流函数-涡量法分析了不同间隙比（圆柱与壁面间的距离对圆柱直径之比）下的尾流特性，得到圆柱表面升阻力和平板壁面压强脉动。结合流动显示的染色粒子图方法，进行了详细的分析并提出近壁圆柱绕流过程中在圆柱下游平板壁面形成沙粒线的可能机理。     

EXPERIMENTAL INVESTIGATION ON THE DRAG REDUCTION CHARACTERISTICS OF TRAVELING WAVY WALL AT HIGH REYNOLDS NUMBER IN WIND TUNNEL

Drag reduction experiments of the traveling wavy wall at high Reynolds number, ranging from 1.46×10⁶ to 5.83×10⁶ based on the free-stream velocity and the model length, were conducted. A suit of traveling wavy wall device was developed and its characteristics of drag reduction at high Reynolds number were investigated. The drag forces of the traveling wavy wall with various wave speeds (c) were measured at different wind speeds (U) in the FL-8 low-speed wind tunnel and compared with the drag force of the flat plate. The results show that the mean drag force of the traveling wavy wall decreases as the value of c/U increases, at different wind velocities, the values of c/U corresponding to minimal drag force of the traveling wavy wall are different, when the values of c/U are larger than 0.6, the mean drag forces of the traveling wavy wall are smaller than those of the flat plate, and the drag reduction can be up to 60%. The drag reduction effectiveness of traveling wavy wall is thus achieved. Furthermore, as the value of c/U increases, the traveling wavy wall can restrain the separation and improve the quality of flow field.     

壁面微结构流动控制技术的减阻机理研究

为了研究壁面微结构流动控制技术的减阻效应及其产生的原因,利用循环管路系统的方形管道进行了压降测定试验,并利用粒子成像测速仪测量了边界层内部结构和对应的参数。试验采用了沟槽和肋条两种不同类型的微结构壁面,每种形状的微结构各有3种不同的结构尺寸。试验研究结果表明:在一定的无量纲宽度s+范围内,6种不同的微结构壁面都具有减阻效果;减阻率随着s+的增大,呈现先增大后减小的趋势,其中沟槽壁面2的减阻效果最好,最大减阻率为9.90%;壁面微结构通过影响流场内部的涡结构、湍流脉动、雷诺切应力和平均流速等使得不同壁面微结构具有减阻效果。     

明渠流掺气增流减阻规律的理论研究

本文研究了明渠中气液分层层流、分层紊流情况下的水流量增加或阻力减小规律。研究表明,明渠中的气液分层层流、分层紊流,即在近壁层内充以空气,可以在相同条件下明显地增大明渠的水流量,实现有效的减阻。     

基于大主应力迹线的无黏性土挡墙土压力分析

墙土接触面的摩擦效应是挡墙土压力分析中需考虑的重要影响因素,首先,通过研究土体应力的分布规律,提出了墙土摩擦效应下墙后土体主应力迹线的确定方法;然后,根据大主应力迹线进行曲线型薄层单元分层,在探究土体微元小主应力变化规律的基础上,结合曲线单元体的静力平衡方程,建立考虑墙土摩擦效应的挡墙主动土压力分析新方法;最后,将本文方法与试验数据和其他方法进行验证和对比分析,对主动土压力系数的影响因素进行敏感性分析。研究表明:考虑墙土摩擦引起应力偏转的分析方法更能准确反映土体应力分布规律,验证了本文方法的合理性与可行性;主动土压力系数随填土摩擦角φ的增大而减小,随墙土间摩擦角δ的增大而增大。     

条纹薄膜减小湍流阻力的试验研究

本文从湍流边界层的相关结构出发，探讨了条纹薄膜减少湍流边界层阻力的机理，通过平板模型的水洞试验，研究了条纹薄膜的减阻作用。试验结果表明，条纹薄膜具有明显的减阻效果，最大降阻量大约为１０％。     

挡土结构墙后黏土主动土压力改进计算

经典的朗肯土压力理论忽略了墙土内摩擦角与挡墙倾角对土压力的影响,计算模型简便,而库伦土压力理论虽考虑因素较多,但没有考虑黏聚力对土压力的影响。为更好地反映挡墙墙后黏性土土压力的变化规律,在库伦土压力理论基础上,推导了一种能考虑墙土摩擦角、墙顶张拉裂缝高度、墙体位移状态及折线滑移面的黏性土主动土压力计算方法,采用其他常用理论与模型实测值对比验证,证明了该方法对计算墙后黏性土主动土压力计算的有效性。与其他类似方法相比,该方法计算的墙后黏性主动土压力与模型实测值更为接近,且计算过程更为简便;当黏聚力为0时,该方法可直接化简为库伦主动土压力公式。因此,该法对实际支挡工程具有一定的理论指导意义。     

DIRECT NUMERICAL SIMULATION OF TURBULENT FLOW IN A STRAIGHT SQUARE DUCT AT REYNOLDS NUMBER 600

This article presents the direct numerical simulation results of the turbulent flow in a straight square duct at a Reynolds number of 600, based on the duct width and the mean wall-shear velocity. The turbulence statistics along the wall bisector is examined with the turbulent flow field properties given by streamwise velocity and vorticity fields in the duct cross section. It was found that the solutions of the turbulent duct flow obtained in a spatial resolution with 1.2×10⁶ grid points are satisfactory as compared to the existing numerical and experimental results. The results indicate that it is reasonable to neglect the sub-grid scale models in this spatial resolution level for the duct flow at the particular friction Reynolds number.     

NUMERICAL STUDY OF FLOW PAST AN AXISYMMETRIC BODY WITH A RING WING AT VARIOUS ATTACK ANGLES

1 .　INTRODUCTIONFortraditionalbody fittedgridsystemusedtocomputetheflow pasttheaxisymmetricbody ,thesingularityonthelongitudinalaxisisalongstand ingobstacle .Rationalsolutionishardtobeachievedevenwithfinemeshesgeneratedwithinthevicinityofthelongitudinalaxis .Hence ,appro priategridsystemisreguiredtoavoidthesingulari ty .Extensivenumericalexperimentsareconductedtoobtainthedesired gridsysteminthis paper .BasedontheideaofDDM[1] ,theoriginalflowfieldofthebodyisdividedinto 8sub domainsac cor…