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%。     

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。     

柔性管自激振动减阻效果的实验研究

在实验确认柔性管紊流减阻效果的基础上,采用双重管结构,对柔性管的自激振动减阻效果进行了实验研究。结果表明:管的壁厚越小自激振动的减阻率越大;在雷诺数约为2.2×104时,壁厚为2 mm,3 mm及4 mm的柔性管自激振动减阻率依次约为13%,10%,8%。     

Drag reduction of wall bounded incompressible turbulent flow based on active dimples/pimples

The control of turbulence by dimples/pimples has drawn more and more attention. The objective of this paper is to investigate the effectiveness of the active dimples/pimples for the drag reduction in the incompressible turbulent flow. Firstly, the drag reduction by the opposition control based on active dimples/pimples at the lower wall is studied via the direct numerical simulation of the turbulent channel flow. It is found that large active dimples/pimples can not suppress the streamwise vortices significantly and thus almost no drag reduction is achieved. Small active dimples and pimples with the diameter of one fourth of the streak width can both reduce the friction drag, but pimples will induce a larger pressure drag than dimples. Then the suboptimal control scheme is examined based on small active dimples using the spanwise wall shear information only. It is shown that the friction drag decreases by about 4.5% but the total drag is only reduced by about 2.7% abated by the pressure drag. Compared with the actuation of the all-point blowing/suction or the all-point wall movement, the effectiveness of the turbulent drag reduction based on active shallow dimples is much smaller.     

柔性管紊流区域减阻效果机理实验研究

为探讨柔性管的紊流区域减阻效果的产生机理,采用高精度高频率响应特性的应变仪对柔性管外径随雷诺数的频率变化特性进行了研究.研究结果表明:随着雷诺数的增加,全频带的外径脉动的均方根值加大,但高频带外径脉动的均方根值减小,说明柔性管管壁脉动确实改变了原刚性管的脉动流场,大尺度涡的运动加剧,低尺度涡运动减弱,从而使得能量损失减少,减阻效果增大.     

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.     

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.     

STUDY ON TURBULENCE FEATURES NEAR AN OSCILLATING CURVED WALL

In pursuit of possibly true turbulent characters and for exploring a change in turbulence structures near an oscillating flexible wall-curved surface, a sinusoidal oscillation mode was forced to a curved wall, whose vibrations disturbed the flow with an interacting effort between the fluid and the structure. The methodology used was the Large Eddy Simulation (LES) with fluid-structure interaction. The oscillating configuration was on a Fourier sinusoidal mode from the measurements of a Francis hydro turbine blade vibration. The effects of the vibration on the skin friction coefficient, vortices, turbulent coherent structures, and other statistical quantities were studied. The results showed that the streamwise velocity gradient along the normal direction and the normal velocity gradient along the spanwise direction were considerably increased within the viscous sublayer because of the oscillating wall, which additionally caused the low speed streaks to stay away from the wall and the high-momentum flows to be toward the wall. As a result, the streamwise vortices were much more elongated along the downstream to get an energy balance, and the wall skin friction coefficient or the wall friction velocity rose up.     

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.     

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。     

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.     

Numerical analysis of shell-side flow-induced vibration of elastic tube bundle in heat exchanger

The responses of the flow-induced vibration of an elastic tube bundle subjected to the shell-side cross flow are investigated in this paper. The weak coupling method and the fluid solid interface are used to solve the fluid-structure interaction problem with consideration of the geometry and physical natures. The effects of the shell-side fluid flow velocity and the structural parameters on the flow-induced vibration are discussed. Numerical results demonstrate that the vibration frequency and amplitude at the monitor points increase with the increase of the shell-side water inlet velocity in all directions. The wall thickness and the external diameter of the elastic tube bundle have significant effects on the responses of the flow-induced vibration. The structural parameters affect the vibration frequency and amplitude, and the vibration equilibrium position in the water flow direction. The vibration frequency decreases with the increase of the tube external diameter. In addition, the vibration in the water flow direction has a lower equilibrium position when the elastic tube bundle has a larger wall thickness or smaller external diameter.     

仿生非光滑沟槽形状对减阻效果的影响

利用有限体积法对三角形、扇贝形和刀刃形三种仿生非光滑沟槽表面流场进行了数值计算.近壁面区采用B-L两层模型,远离壁面区采用雷诺应力模型.分析了三种沟槽非光滑表面的流场特性,对计算域中心 Z＝3mm平面的速度场和湍流统计量进行了研究,分析了不同沟槽形状对减阻效果的影响,为最佳减阻沟槽设计提供了理论依据.三种沟槽具有相同的特征尺寸,顶点间距 s＝0.1mm,沟槽尖顶到谷底高度 h=0.05mm,与光滑表面相比减阻效果分别为 3.2%、9.1%、9.7%,数值计算结果和风洞与油槽实验结果有较好的吻合.     

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.     

明渠渠底糙率突变对紊流摩阻流速及壁面切应力影响的试验分析

通过激光测速仪,对壁面糙率突变时的明渠紊流进行试验,测量了糙率突变不同水深情况下明渠紊流的纵向时均流速和脉动流速。通过实测资料分析不同糙率突变和不同水深的紊流摩阻流速变化情况和水流的壁面切应力的特点。     

淤泥质床面上减阻流动的紊动特征

本文对不同密度的淤泥质床面上水流运动的流速分布、减阻特征、紊流度、紊动猝发周期和能谱结构进行了系统的实验研究，并与刚性床面的结果进行了比较分析，结果表明：淤泥质床面具有减阻、增速、抑制紊流猝发产生和减小紊动耗散率的作用。     

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     

基坑开挖位移及土体水平抗力比例系数的反分析

基坑设计土体参数的正确选取直接影响支护结构的位移和内力计算结果,特别是土体水平抗力比例系数m值对支护结构位移的影响最为显著。结合基坑开挖工程实例,通过材料性状与基坑开挖施工过程的平面有限差分法模拟,模拟计算基坑分布开挖过程结构的水平位移;通过分析诸多土体参数对基坑支护结构水平位移的影响程度,确定杨氏模量作为位移反分析所需获取的参数,构建实测位移与模拟计算位移之间的目标函数,反分析给出各层土的杨氏模量;基于弹性地基梁法推导的m值与杨氏模量的关系,给出反映实际侧向变形特性各层土的m值。研究结果为基坑工程设计参数的合理确定提供了一定理论参考依据,对有效控制基坑位移与变形具有重要的实践意义。