添加网格对增强表面活性剂减阻流体传热性能的试验研究

本文应用热电偶测温系统和PIV（Particle Image Velocimetry）,激光测量系统对二维流道内添加网格后CTAC（Cetyltrimethyl Ammonium Chloride）表面活性剂减阻流体的传热性能和流动结构进行试验研究。研究表明：在完全减阻区内,减阻流体的传热性能大幅度降低;当雷诺数超过临界雷诺数后,传热性能呈指数级快速增长;网格的添加破坏了表面活性剂的胶束网状结构,在一定程度上恢复了流场的湍流强度,从而使传热效果显著增强。     

基于小波分析的表面活性剂减阻流体二维流场流动特性研究

该文采用粒子图像测速仪对矩形槽道内减阻流体二维流场进行测量,采用小波分析方法将流场分解为平均速度场与7个不同尺度的脉动场.结果显示减阻流体横向速度脉动几乎被完全抑制,流向速度脉动略低于水,但从波数为2开始小尺度脉动被明显削弱.减阻流体流向速度脉动呈条带分布,同流向基本平行,并且具有较大尺度,统计结果曲线也在相应位置出现局部大值,显示高脉动层存在的普遍性.     

表面活性剂减阻流体传热与阻力关系试验研究

针对在水中添加表面活性剂可以出现阻力系数减少而同时传热性能发生变化的现象，采用测压差装置和热电偶测温系统分别对二维流道内不同的减阻流体的减阻性能、传热性能进行了试验，研究了浓度、配比、雷诺数和热流密度对减阻流体传热与阻力系数之比值的影响。结果表明，表面活性剂减阻流体的传热系数考尔朋（Colburn）因子与阻力系数之比和水的情况有较大的差异；当减阻流体的减阻率稳定时，随着雷诺数的增大，考尔朋恩因子与阻力系数比值缓慢下降；达到临界雷诺数后，考尔朋因子与阻力系数之比值波动较大，并且比值迅速增大到接近没有添加减阻剂的水平。     

计算湍流附壁射流的一个简单模型

在许多含有回流的复杂湍流剪切流中,在回流区域内的断面平均流速分布与湍流脉动强度分布具有某种共同的性质,根据这一性质,作者提出回流副层概念以及把滞点视为粗糙壁的作用,利用这些概念建立了一个附壁射流的简单模型,提出其相似的流速分布式,采用流带积分法进行数值计算得到速度比尺,长度比尺等变化,数值计算结果与实验结果进行了比较。     

推移质运动对清水和宾汉体泥浆湍流结构的影响

通过试验研究了清水湍流和泥浆湍流中加入推移质后时均流速分布、脉动强度、概率密度分布、相关系数、频谱密度分布等的变化.试验结果证明,清水和泥浆湍流中加入推移质后时均流速减小,时均流速梯度增大,脉动加强.泥浆过渡区湍流上部为层流,有流核,加入推移质后,上部层流转变为湍流,流核消失.分析证明,这是由于推移质运动增大了雷诺应力和时均流速梯度,因而增大了时均流动向脉动传递的能量的缘故,清水湍流中加入推移质后脉速概率密度分布偏离正态,偏态系数加大.泥浆湍流中加入推移质后相关系数降低,脉动的随机性增大.泥浆湍流的脉动能量集中于低频脉动.由于推移质也可以吸收低频脉动能来支持其运动,所以加入推移质后泥浆湍流的低频脉动能量降低,高频脉动能量增大.     

沟槽张角对流体减阻机理的数值模拟研究

为了研究边界层涡结构对沟槽壁面摩擦阻力的影响,该文利用FLUENT软件对湍流状态下沟槽壁面进行数值模拟计算,分析了90°、120°和150°三种不同张角下沟槽壁面的减阻效果,获得了不同张角的沟槽结构对减阻效果的影响规律。结果表明:沟槽内形成的旋涡结构可以有效减小沟槽布置区域法向速度梯度,减弱流层间剪切作用,增加条带结构稳定性,湍流猝发频率降低,从而使壁面摩擦阻力降低。当来流速度v=25 m/s,沟槽张角β=90°时,可取得最佳减阻效果。在沟槽结构布置区域,流向涡的涡头上扬角度变大,上扬和下扫强度减弱,有效减小了摩擦阻力;同时,沟槽结构改变近壁区速度分布,使流向涡密度减小,从而减小壁面摩擦阻力。     

水平旋转内消能泄洪洞流速分布及紊动特性试验研究

该文通过模型试验,研究了在自由流、吸吮流和淹没流三种基本流态下,水平旋转内消能泄洞的流速分布与脉动特性。激光测速的结果表明,水平旋流洞的切向流速沿径向的分布可以看成是组合涡分布,轴向流速沿径向的分布可看成是最大值靠近壁面一侧的二次曲线分布。在流速的变化趋势上各种流态是相同的,但具体分布参数因流态不同,有较大的差异。流态不同时,轴向脉动流速与切向有较大差异,湍流强度变化规律也不同,自由流与吸吮流流态湍流强度总体上的变化规律为Twθ和Twz沿程均有所增大;沿径向先减小,在R/R0大于一定值之后基本维持一常量或略有增大。     

展向周期振动的平板近壁湍流结构研究

通过直接数值模拟(DNS)对壁面作展向周期运动的槽道湍流进行研究,建立了槽道湍流数据库.通过改变振幅大小和振动周期,可以使壁面阻力明显减少.根据计算结果分析了减阻率和振动参数的关系.利用减阻后近壁湍流拟序结构的变化,研究了壁面展向周期振动减阻的内在机理,揭示了三种减阻过程中涡结构对近壁流体的上抛、下扫运动以及条带结构的影响规律.     

高分子聚合物减阻流的紊流结构

减阻紊流刘许多科学技术领域都具有重要意义。然而由于问题复杂,目前对其时均流速分布的研究,基本上局限于试验数据的经验分析,而对脉动流速的研究,还处于对试验资料的定性描述阶段。本文从粘弹性流体的基本方程式出发,导出了粘弹性流体的剪力关系式;并在此基础上应用作者提出的紊流随机理论,导出了光滑明渠和管道中减阻紊流的时均流速分布公式和脉动流速强度分布公式;最后根据聚合物的化学物理性质,给出了确定上述公式中减阻参数的公式。导出的时均流速分布公式和脉动流速分布公式,不仅适用于紊流核心区,而且也适用于近壁的粘性区和过渡区,从而使原来只有用三个公式才能近似表述的时均流速分布规律,有了一个统一公式,并使脉动流速问题得到了定量分析和概括。文中引用各家的减阻流试验数据对导得各公式进行了验证,得到了几乎完全一致的结果。本文为研究减阻机理和阻力等问题提供了理论基础,然而对这些问题的研究结果,将专文加以论述,不在此文之内。     

流速梯度对悬浮颗粒脉动强度的影响

从简化的颗粒运动方程出发，分析了在剪切流场中颗粒脉动强度和流体脉动强度之间的关系。结果表明，由于纵向时均流速的垂线分布梯度的作用，颗粒在两个方向上的脉动强度均可能超过相应的流体脉动强度。     

热分层湍流直接模拟的高精度算法和数值分析

该文发展了高阶精度有限差分格式的直接数值模拟方法,并将其成功地应用于带有自由面的热分层槽道湍流问题的计算,与低阶精度格式相比,高精度格式有着更宽的低耗散波段,对中高波数分量有较好的模拟能力。该文直接在非均匀网格上采用Lagrange多项式插值构造迎风偏置格式,克服了传统二阶格式在网格变化剧烈时精度损失的缺点,提高了在近壁区的计算精度。对三维槽道湍流以及带有自由面的热分层湍流的流场和温度场的统计特征、脉动特征以及能谱进行了分析,进一步验证了高精度差分方法对热分层剪切湍流问题直接模拟的有效性。总体上讲,该格式精度高,对高频非物理振荡有抑制作用,可以适应变化的空间网格,有效地模拟复杂湍流流动结构,并正确地揭示其流动机理.     

AN INVESTIGATION OF TURBULENT HEAT TRANSFER IN CHANNEL FLOWS BY LARGE EDDY SIMULATION

Large Eddy Simulation (LES) of fully developed turbulent channel flow with heat transfer was performed to investigate the effects of the Reynolds number on the turbulence behavior. In the present study, the bottom wall of the channel was cooled and the top wall was heated. The Reynolds numbers, based on the central mean-velocity and the half-width of the channel, were chosen as 4000, 6000, 104 and 2×104, and the Prandtl number as 1.0. To validate our calculations, the present results were compared with available data obtained by Direct Numerical Simulation (DNS), which proves to be in good agreement with each other. To reveal the effects of the Reynolds number, some typical quantities, including the velocity fluctuations, temperature fluctuation, heat fluxes and turbulent Prandtl number, were studied.     

WALL PRESSURE FLUCTUATIONS OF TURBULENT FLOW OVER BACKWARD-FACING STEP WITH AND WITHOUT ENTRAINMENT： MICROPHONE ARRAY MEASUREMENT

1. INTRODUCTION Turbulent boundary layer flows over surface irregularity are of significant importance from both fundamental and practical essences. Among others, turbulent flows over backward-facing step (hereinafter referred to BFS) received widespread …     

同轴套管间涡旋流动及强化传热数值模拟

相对旋转两同轴套管间的涡旋流动,能够带来二次流强化传热传质作用,在航空、水处理、生态保护、生物工程和膜分离等领域都具有广泛的应用价值。本文使用Fluent软件,对同轴套管间涡旋流动及传热特性进行数值模拟,考察了内管转速、内外管壁面温差等操作参数变化对同轴套管间流体传热性能的影响,分析了涡旋流动与传热效率之间的关联关系。模拟结果表明:内管转速增加在流场中形成泰勒涡,涡流扰动增大了高温壁面与流体间的热流密度,增强了流体传热效率。增大内外管壁面温差,也可加强流体传热性能,但其强化作用不及内管转速的强化作用显著。受流场中泰勒涡影响,流体速度、温度及热流密度沿轴向的分布都呈正弦状周期性波动,在相邻两涡交界面处,流体传热性能最好,在涡中心处的传热性能最差。     

SIMULTANEOUS FLOW VISUALIZATION AND WALL-PRESSURE MEASUREMENT OF THE TURBULENT SEPARATED AND REATTACHING FLOW OVER A BACKWARD-FACING STEP

Unsteady behaviors of the large-scale vortical structure superimposed in the turbulent separated and reattaching flow over a backward-facing step were convincingly delineated by performing simultaneous measurements of the wall pressure fluctuations and visualizations of the flow. Toward this end,a synchronized instrumentation system integrated with the microphone array and the high-speed camera was established. The smoke-wire technique was employed to visualize the unsteady events. A thorough analysis based on the wall pressure fluctuations disclosed that the large-scale vortical structure shedding at the frequency of fH /U 0= 0.064 gave a primary contribution to the wall pressure fluctuations,and consequently dominated unsteady behaviors of the turbulent shear layer. The convection velocity of the large-scale vortical structure was determined as U C=0.55 U 0. The instantaneous flow visualizations and wall pressure were compared in a straightforward manner. Below the separation bubble and the reattachment zone,the negative peak of the time-varying wall pressure was in phase with passage of the local large-scale vortical structure. In the redeveloping turbulent boundary layer,the decaying large-scale vortical structure was clearly revealed.     

Fully rough submerged plane wall-jets

This study presents the turbulent flow field in submerged plane wall-jets on horizontal fully rough walls detected by a Vectrino velocimeter. For the comparison between the fully rough and smooth submerged wall-jets, the smooth submerged wall-jet case was also revisited. The two-dimensional Reynolds averaged boundary layer equations of a steady turbulent flow are analyzed to determine the velocity and Reynolds shear stress profiles in the fully developed zone of smooth and fully rough submerged wall-jets. The response of the turbulent flow characteristics in submerged wall-jets to wall roughness is examined from the point of view of similarity characteristics, growth of the length scale, and decay of the velocity and turbulence characteristics scales; and compared with the response of those to smooth and transitionally rough walls. The significant observation is that with an appropriate scaling, the velocity, Reynolds shear stress and turbulence intensities in the fully developed zone of fully rough submerged wall-jets are reasonably similar. The rate of decay of jet-velocity on fully rough walls is greater than that on smooth wall, but it is less than that on transitionally rough walls due to the presence of roughness sub-layer on the fully rough walls. Analysis of the third-order moments of velocity fluctuations reveals that the jet-layer is associated with the arrival of low-speed fluid parcel causing an effect of retardation; while the inner-layer of circulatory flow is associated with the arrival of high-speed fluid parcel causing an effect of acceleration. Wall roughness influences the peak values (both positive and negative) of the third-order moments increasing them significantly. Thus, on fully rough walls, the arrival processes of low- and high-speed fluid parcels become stronger in the jet-layer and the inner-layer of circulatory flow, respectively. Also, the peak values of streamwise and vertical flux of turbulent kinetic energy increase significantly in the presence of fully rough walls.     

PIV and PTV measurements in hydro-sciences with focus on turbulent open-channel flows

PIV is one of the most popular measurement techniques in hydraulic engineering as well as in fluid sciences. It has been applied to study various turbulent phenomena in laboratory experiments related to natural rivers, e.g., bursting phenomena near the bed, mixing layers observed at confluences, wake turbulence around dikes and piers, and so on. In these studies, PIV plays important roles in revealing the space-time structure of velocity fluctuations and coherent vortices. This review article focuses particularly on the applications of PIV to turbulent open-channel flows, which have been conducted for the past decade in Hydraulics Laboratory of Kyoto University. In Section 2, we introduce our experimental setup and PIV/PTV algorithm. In Section 3, we apply the PIV measurements to reveal turbulence characteristics and coherent structures in open-channel flows as well as in vegetated canopy flows. For complex flow situations, various applications of PIV to compound open-channel flows and wind-induced water waves are considered to reveal coherent vortices. In Section 4, we discuss some advanced PIV measurements in open-channel flows. The free-surface-elevation fluctuations and velocity components were measured simultaneously with two sets of cameras to examine phase-averaged parameters of turbulence. A multi-layer scanning PIV was developed to reveal 3D turbulence structure in compound open-channel flows. Our discriminator PIV/PTV was applied successfully to sediment-laden open-channel flows and revealed the fluid/particle interaction and the relationship between coherent structures and sediment concentration. Finally, we conducted simultaneous measurements of velocity and dye concentration with a combination of PIV and LIF in vegetated open-channel flow, which enables us to examine turbulent scalar flux of a passive contaminant.