亚临界雷诺数二维圆柱绕流边界层流场动态特性的实验研究

本文给出了关于亚临界雷诺数二维圆柱绕流动态特性的实验结果。应用热膜、热线和压力传感器测量了壁面剪切应力脉动、壁面压力脉动和流场的速度脉动,给出了壁面剪切应力脉动频率在驻点附近和分离前后变化的特征,计算了这些脉动量在圆柱面上任意两点间的相关特性。实验结果表明,在亚临界雷诺数二维圆柱绕流的边界层流场中存在着一个频率与涡脱落频率相同的整体同步脉动。     

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.      

EXPERIMENTAL STUDY ON PROPERTIES OF TURBULENT/NON-TURBULENT INTERFACE OVER RIBLETS SURFACES AT LOW REYNOLDS NUMBERS1)

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.     

沟槽方向对湍流边界层流动结构影响的实验研究

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.     

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.     

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.     

对管内湍流边界层结构与流动阻力特性的数值研究

在研究紊流边界层的过程中,本文考虑了分子粘性对紊流产生的作用、雷诺数以及壁面附近脉动动能的耗散不是各向同性对紊流产生的影响,采用Jones-Launder模型对管内紊流流动边界层厚度、边界层内的脉动动能K,动能耗散ε,管壁切应力τ0以及由此可得的管内流动摩擦阻力系数λ进行了数值计算,计算结果与实验值、理论计算值得具有较好的一致性。     

激励小尺度模式在湍流圆管射流中的应用

采用非涡黏性的激励小尺度（Stimulated Small Scale)模式对空间发展的轴对称湍流圆管射流进行了大涡模拟。以雷诺数为10000的流动为例,考证了激励小尺度模式在自由剪切流模拟中的可行性,描述了湍流强度、雷诺应力和湍流耗散量的变化,同时与标准的Smagorinsky涡黏性模式的计算结果进行了比较。数值结果显示,激励小尺度模式能够更为合理地描述湍流的耗散特性和能量传输特性,从而较为准确地展示出空间发展射流中由于流动不稳定而出现的旋涡产生、发展、破碎及合并等过程。     

粗糙颗粒动理学及稠密气固两相流动的数值模拟

考虑颗粒碰撞过程中摩擦作用, 给出了粗糙颗粒碰撞动力学. 引入颗粒相拟总温来表征颗粒平动和转动脉动能量的特征. 基于气体分子运动论, 建立颗粒碰撞中平动和旋转共同作用的粗糙颗粒动理学, 给出了颗粒相压力和黏度等输运参数计算模型. 运用基于颗粒动理学的欧拉-欧拉气固两相流模型, 数值模拟了流化床内气体颗粒两相流动特性, 分析了颗粒旋转流动对颗粒碰撞能量交换和耗散的影响. 模拟得到的流化床内径向颗粒浓度和提升管内颗粒轴向速度与他人实验结果相吻合. 模拟结果表明随着颗粒浓度的增加, 颗粒相压力和能量耗散逐渐增加, 而颗粒拟总温先增加后下降. 随着颗粒粗糙度系数的增加, 床内平均颗粒相拟总温和能量耗散增加, 表明颗粒旋转产生的摩擦将导致颗粒旋转脉动能量的改变, 影响床内气体-颗粒两相宏观流动特性.     

明渠中跌坎后突扩分离流数值研究

利用大涡模拟技术，对明渠中跌坎后的二维突扩分离流结构进行了数值模拟，探讨了这类典型分离流各特征区的流动性质，分析了再附区壁面上不同测点处脉动压力的统计特征，并与试验结果进行了比较．在数值模拟中，采用了弱压缩流的控制方程和非均匀网格系统．     

基于LBM的壁湍流跨尺度能量传递结构统计

壁湍流不同尺度间能量传输特性存在着明显的各向异性, 了解能量不同尺度间传递的空间分布是进一步构造高保真各向异性大涡模拟亚格子模式的前提. 基于格子Boltzmann数值(lattice Boltzmann method, LBM)模拟方法对雷诺数$Re_{\tau } =180$的槽道湍流进行直接数值模拟. 结果与公开的槽道湍流数据库进行对比, 平均速度剖面、雷诺应力和脉动速度均方根均取得了较好的一致性, 验证了LBM方法模拟槽道湍流的可靠性. 对模拟后的数据采用空间滤波方法得到不同尺度间能量交换量的空间分布场, 结合结构识别捕捉方法——聚类分析法对不同尺度间能量传输结构的空间分布特性进行了分析. 结果表明尺度间能量传输结构在全流场物理空间中主要为小尺寸结构, 结构的体积概率密度呈现出$-$4/5幂律, 按结构距壁面最小距离以结构距壁面距离又可将结构划分为附着结构和分离结构, 其中附着结构以较小的数量占比达到了较高的体积占比, 表明附着结构多为大尺寸结构, 进一步的对附着结构的统计表明结构在尺寸上存在着一定的幂律关系, 表明不同尺度间能量输运结构也具有Townsend提出的附面涡的自相似性, 最后对能量正反传附着结构的成对特性研究发现, 能量正传$\!-\!$反传结构对倾向于沿展向并排排列.      

Effects of dimensional wall temperature on Reynolds stress budgets in a supersonic turbulent channel flow with thermally perfect gas

ABSTRACT

Direct numerical simulations of temporally evolving supersonic turbulent channel flow of thermally perfect gas are conducted at Mach number 3.0 and Reynolds number 4800, combined with constant dimensional wall temperatures from 149.075 to 1788.90?K to study the influence of dimensional wall temperature on the characteristics of Reynolds stress budgets. It is found that, as the dimensional wall temperature increases, the production, diffusion, pressure–velocity gradient correlation and dissipation terms increase, whereas the compressibility-related term decreases. This is mainly due to variations in mean flow properties. The mechanism of inter-component transfer (ICT) is insensitive to the dimensional wall temperature. The ICT relating to the pressure–velocity gradient correlation term can be divided into inner and outer regions, and the critical position separating these regions is at the semi-local scaling of approximately 16 irrespective of the different dimensional wall temperature.     

AN EXTENDED k-ε MODEL FOR NUMERICAL SIMULATION OF WIND FLOW AROUND BUILDINGS

It is assumed in this paper that for a high Reynolds number nearly homogeneouswind flow, the Reynolds stresses are uniquely related to the mean velocity gradientsand the two independent turbulent scaling parameters k and E. By applying dimensionalanalysis and owing to the Cayley-Hamilton theorem for tensors, a new turbulenceenclosure model so-called the axtended k-ε model has been developed. The coefficientsof the model expression were detemined by the wind tunnel experimental data ofhomogeneous shear turbulent flow. The model was compared with the standard k-εmodel in in composition and the prediction of the Reynold’s normal Stresses. Using thenew model the numerical simulation of wind flow around a square cross-section tallbuilding was performed. The results show that the extended k-ε model improves theprediction of wind velocities around the building the building and wind pressures on the buildingenvelope.     

Three-dimensional particle-tracking velocimetry measurement of turbulence statistics and energy budget in a backward-facing step flow

Using a three-dimensional (3-D) particle-tracking velocimeter, detailed turbulent flow measurements were made in a plane channel with a one-sided 50% abrupt expansion, which acted as a backward-facing step. The turbulent channel flow reached a fully developed state well upstream of the step. The Reynolds number based on the upstream centerline velocity and the step height H was 5540. With the mean reattachment point located at 6.51H downstream of the step, the measurement region ranged from −2H upstream to 12H downstream of the step. Various turbulent statistics and the energy budget were calculated from numerous instantaneous vector distributions. As in previous experimental investigations, the Reynolds normal and shear stresses had maximum values upstream of the reattachment. The stress anisotropy tensor revealed a peculiar phenomenon near the reattachment wall, wherein the spanwise normal stress was the largest among the three normal stresses. The triple velocity correlations indicated large values in the separating shear layer, and hence the turbulent diffusion was a major term in the energy budget. Comparison was made between the present results and those of the direct numerical simulation (DNS) of Le et al. (1993), and it was found that the mean and fluctuating velocities, the Reynolds shear stress, and the turbulent energy budget were in excellent agreement, although there was a considerable difference in the inflow conditions.     

Laminar flow and mass transfer in channels with a porous bottom wall and with fins attached to the top wall

 The steady incompressible, viscous, two- dimensional flow of a solution in a channel was considered. The bottom wall was porous and the fins were attached to the top wall. Employing control volume approach, a computer program based on SIMPLE algorithm was developed. Computations were carried out to investigate the effects of the inlet Reynolds number, the fin length, the suction Reynolds number and the slip coefficient on the flow structure and the concentration distribution. It was observed that the thickness of concentration boundary layer increases in the flow direction. The concentration on the porous wall and the concentration boundary layer thickness decrease with increasing fin length, the slip coefficient and the inlet Reynolds number. These results show that fins attached to the upper wall of the channel can be utilized to reduce the concentration polarization and hence improve the effectiveness of the separation process. Received on 24 February 1999     

Wall shear stress measurements in the plane of symmetry of a turbulent spot

Wall shear stress measurements were made in the plane of symmetry of turbulent spots. When represented in terms of the skin friction coefficient versus momentum thickness Reynold's number, the spot's wall shear distribution is composed of four linear segments each having a different slope. The spot's zone averaged wall shear stress is approximately equal to the zone averaged Reynolds's stress distribution in a spot while the ensemble mean wall shear is 10%–15% lower than that found in a turbulent boundary layer.     

Roughness effects on turbulent plane wall jets in an open channel

This paper reports the effects of surface roughness on the mean flow characteristics for a turbulent plane wall jet created in an open channel. The velocity measurements were obtained using a laser Doppler anemometer over smooth and transitionally rough surfaces. The power law proposed by George et al. (2000) was used to determine the friction velocity. Both conventional scaling and the momentum–viscosity scaling proposed by Narasimha et al. (1973) were used to analyze the streamwise evolution of the flow. The results show that surface roughness increases the skin friction coefficient and the inner layer thickness, but the jet half-width is nearly independent of surface roughness.     

SCALING EXPONENTS IN FULLY DEVELOPED ASYMMETRIC CHANNEL FLOW

IntroductionDuetotheimportanceofglobalandlocalscalingexponents,theyattractmanyturbulentresearchersduringthepasttwentiesyears.Butresultsobtaineduptonowaremainlyonisotropicturbulence ,bothinexperimentsandnumericsimulations[1].Itisfoundthatξpisnonlinearfunc…     

Dominant parameters for maximum velocity induced by body-force models for plasma actuators

This study investigates the relationship between body-force fields and maximum velocity induced in quiescent air for development of a simple body-force model of a plasma actuator. Numerical simulations are conducted with the body force near a wall. The spatial distribution and temporal variation of the body force are a Gaussian distribution and steady actuation, respectively. The dimensional analysis is performed to derive a reference velocity and Reynolds number based on the body-force distribution. It is found that the derived Reynolds number correlates well with the nondimensional maximum velocity induced in quiescent conditions when the center of the Gaussian distribution is fixed at the wall. Additionally, two flow regimes are identified in terms of the Reynolds number. Considering the variation of the center of gravity of force fields, another Reynolds number is defined by introducing a new reference length. The nondimensional maximum velocity is found to be scaled with the latter Reynolds number, i.e., the maximum induced velocity in quiescent conditions is determined from three key parameters of the force field: the total induced momentum per unit time, the height of the center of gravity, and the standard deviation from it. This scaling turns out to be applicable to existing body-force models of the plasma actuator, despite the force distributions different from the Gaussian distribution. Comparisons of velocity profiles with experimental data validate the results and show that the flow induced by a plasma actuator can be simulated with simple force distributions by adjustment of the key body-force parameters.     

Shear‐improved Smagorinsky modeling of turbulent channel flow using generalized Lattice Boltzmann equation

Generalized Lattice Boltzmann equation (GLBE) was used for computation of turbulent channel flow for which large eddy simulation (LES) was employed as a turbulence model. The subgrid‐scale turbulence effects were simulated through a shear‐improved Smagorinsky model (SISM), which is capable of predicting turbulent near wall region accurately without any wall function. Computations were done for a relatively coarse grid with shear Reynolds number of 180 in a parallelized code. Good numerical stability was observed for this computational framework. The results of mean velocity distribution across the channel showed good correspondence with direct numerical simulation (DNS) data. Negligible discrepancies were observed between the present computations and those reported from DNS for the computed turbulent statistics. Three‐dimensional instantaneous vorticity contours showed complex vortical structures that appeared in such flow geometries. It was concluded that such a framework is capable of predicting accurate results for turbulent channel flow without adding significant complications and the computational cost to the standard Smagorinsky model. As this modeling was entirely local in space it was therefore adapted for parallelization. Copyright © 2010 John Wiley & Sons, Ltd.