纵向涡对近壁湍流结构的影响

在封闭水槽中用氢气泡方法观察了纵向涡对近壁湍流结构的影响，涡的下洗侧出现了展向距离较宽、流动较稳定的快斑区，流向速度快；涡的上洗侧出现了含有慢斑的区域，流动结构复杂，流向速度慢，纵向涡使下洗侧速度较快的流体流向壁面，使上洗侧速度较慢的流体远离壁面。     

逆向涡对超疏水壁面减阻影响的TRPIV实验研究

超疏水壁面由于具有减阻和自清洁功能而成为国内外减阻和海洋防污等研究领域的热点之一,而20世纪湍流中相干结构的发现为湍流的控制指出新的方向,尤其近壁区涡结构对摩擦阻力贡献很大.利用高时间分辨率粒子图像测速技术,研究了超疏水壁面(SH)以及亲水壁面(PH)湍流边界层中正负展向涡的空间分布特征,研究逆向涡对超疏水壁面近壁区流动结构的影响和超疏水壁面的减阻机理.首先利用空间多尺度局部平均涡量的概念提取壁湍流发卡涡展向涡头(顺向涡)和逆向涡,实现了准确识别涡心并排除小尺度涡的干扰;然后根据检测到的顺向涡和逆向涡流线分布图,发现逆向涡始终处于正向涡的上游和下方,并且对正向涡的进一步发展起抑制作用;最后对两种壁面边界层中逆向涡数量以及出现概率进行对比,发现具有减阻效果的超疏水壁面边界层中出现更多逆向涡.说明逆向涡可抑制上方顺向涡与壁面的强烈剪切,并使靠近壁面的流体加速,从而产生减阻效果;超疏水壁面中涡结构具有更大的β角,使其更好地阻碍了发卡涡头附近强烈的喷射和扫略;超疏水壁面逆向涡出现概率明显大于亲水壁面.这些结果表明:超疏水壁面表现出的减阻特性(Reδ≈13 500,减阻5.8%)与两板产生逆向涡的差异有关.     

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.     

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.     

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.     

固体颗粒对沟槽湍流边界层影响的实验研究

本文采用粒子图像测速技术(particles image velocimetry, PIV)研究固体颗粒对放置在平板湍流边界层中的平壁和沟槽壁面减阻效果的影响. 实验对清水和加入粒径为155 μm聚苯乙烯颗粒的流法向二维速度场信息进行采集, 对不同工况下的平均速度剖面、雷诺应力和湍流度等统计量进行对比, 分析流体在边界层中的行为. 运用空间局部平均结构函数提取了不同工况湍流边界层喷射?扫掠行为的空间拓扑结构并进行比较. 结果发现, 在不同的壁面条件下, 粒子加入后的对数律区中无量纲速度均略大于清水组, 雷诺切应力有所降低, 湍流度有所减弱. 对于不同流场速度下的沟槽而言, 颗粒的加入均降低了壁面附近的阻力, 而颗粒单独作用于光滑壁面的减阻效果并不明显. 加入粒子后的相干结构数目有所增加, 法向脉动速度下降. 沟槽壁面附近的相干结构数目有所增加, 法向脉动速度在自由来流速度较大时有所上升, 在速度较小时有所下降. 这表明不同减阻状况下的沟槽均能将大涡破碎成更多的涡, 并且粒子的加入强化了这种破碎作用.      

基于流动仿真大数据应对旋涡-湍流的研究进展

文章以流体科学进入二十一世纪后，在大规模超算、云存储、数据通信和人工智能为支撑的大数据时代背景下，结合目前在复旦大学航空航天系所构建的热流体湍流直接数值仿真数据库，以及复旦大学团队近期与美国德州大学刘超群教授、上海理工大学蔡小舒教授以及国内水动力学杂志编辑部所合作开展的第三代涡识别技术研究，初步概念性地展示旋涡和湍流，特别是针对有工程实际背景和直接应用价值的壁湍流，在这两个流体力学关键基础议题上的最新认知，和基于大数据深度学习的相关湍流工程模拟实践成果．这些成果包括：(1) 基于第三代涡识别技术的尾迹湍流中的涡运动学和动力学探索；(2) 流-热统一完整的类-1、类-2湍流边界层壁面律构建；(3) 基于第三代涡识别量对Kolmogorov 幂次律的再认知；(4) 基于DNS统计数据和神经网络深度学习构建新型湍流封闭模型及RANS计算实践．通过这些成果展示，论证解决这两个基础流体科学议题的技术路径，进而促进流体及相关学科研究在现代大数据背景下取得实质性进展和突破，并惠及现代流体、气动、水利、动力和化工等工程领域．     

基于表面设计减阻的水下航行体模型实验与数值分析

针对水下航行体高速航行时提高航速的需要,开展近壁面湍流边界层减阻的研究。进行表面设计使旋涡流动加强导致局部空化从而产生壁面附着的微气泡,以降低流体粘性系数,伴随近壁面湍流结构的改变来实现一定幅度的减阻。设计并进行轴对称体模型在高压循环水洞中的空化现象观测和阻力测量并做了相应条件下的数值计算。。实验和数值结果分析的基本结论比较一致:某种特征表面形貌在湍流中有一定幅度的减阻,局部空化效应在中低流速下牺牲压差阻力而导致航行总阻力升高,在高流速条件下则降低总阻力。     

柔性壁面湍流边界层相干结构控制的实验研究

本文利用热膜测速技术对刚性壁面和柔性壁面湍流边界层的流向速度分量进行了实验测量,首先研究了柔性壁面对平均速度分布和湍流度分布的影响,结果表明:柔性壁面的边界层速度分布在对数律层向上有所平移,缓冲层加厚,具有一般的壁面减阻特征;而柔性壁的湍流度比刚性壁的湍流度要低,分布也更为平坦。然后综合运用自相关法和条件采样技术研究了湍流近壁区的相干结构,结果表明:刚性壁自相关曲线的第二峰值出现的时间比柔性壁的短,柔性壁的猝发频率比刚性壁的低许多。实验结果表明柔性壁面具有一定的减阻作用。     

基于PIV技术分析颗粒在湍流边界层中的行为

采用粒子图像测速技术(particle image velocimetry, PIV)在平板湍流边界层内开展实验研究,对比颗粒相及单相液体的平均速度剖面、湍流强度、雷诺应力等湍流统计量,分析颗粒在湍流边界层中的行为.利用空间多尺度局部平均涡量的概念提取壁湍流发卡涡展向涡头(顺向涡)并统计其数量规律,得到不同法向位置处顺向涡周围流向脉动速度及流线的空间拓扑结构,比较分析顺向涡发展程度及周围的湍流相干结构.结果发现:与清水工况相比,颗粒相湍流边界层的缓冲层变薄、对数律区下移,湍流强度得到增强,雷诺应力在对数律区有所增大;颗粒的流向脉动速度在展向涡周围的分布与清水工况不同,颗粒能够被流体展向涡周围的猝发过程有效传递;颗粒相的顺向涡涡核较大,且随着法向位置的升高逐渐发展完整,涡和条带在流向上拉伸得更长;同时发现在两种工况下,顺向涡的左下方始终存在一个逆向涡,颗粒相逆向涡的形成弱于单相流体;两种工况下的顺向涡数量均随着法向位置的升高而减少,最后逐渐趋于稳定.     

Temporal development of turbulent boundary layers with embedded streamwise vortices

The interaction of streamwise vortices with turbulent boundary layer has been investigated using large-eddy simulation. The initial conditions are a pair of counterrotating Oseen vortices with flow between them directed toward the wall (common-flow-down), superimposed on various instantaneous realizations of a turbulent boundary layer. The time development of the vortices and their interaction with the boundary layer are studied by integrating the filtered Navier-Stokes equations in time. The most important effects of the vortices on the boundary layer are the thinning of the boundary layer between vortices (downwash region) and the thickening of the boundary layer in the upwash region. The vortices first move toward the wall as a result of the self-induced velocity, and then apart from each other because of the image vortices due to the solid wall. The Reynolds stress profiles highlight the highly three-dimensional structure of the turbulent boundary layer modified by the vortices. The presence of significant turbulent activity near the vortex center and in the upwash region suggests that localized instability mechanisms in addition to the convection of turbulent energy by the secondary flow are responsible for this effect. High levels of turbulent kinetic energy and secondary stresses in the vicinity of the vortex center are also observed. The numerical results show good agreement with experimental results.This work was supported by the Office of Naval Research under Grant N00014-89-J-1638. Computer time was supplied by the San Diego Supercomputing Center.     

充分发展圆管湍流的实验研究

采用粒子数字图像测速（digital particle image velocimetry,DPIV)和定量流动显示技术（quantitative flow visualization,QFA)对充分发展的圆管湍流进行了研究。测量结果和直接数值模拟（direct numerical simulation,DNS)结果进行了比较，结果表明作者开发的DPIV技术取得了满意的精度。在此基础上对圆管湍流的动力学机理进行了研究，分析了上抛和下扫在湍流生成中的贡献以及流动显示结构内的脉动速度分布，测量结果显示在圆管湍流的近壁区存在横向强脉冲现象和流动显示所能观察到的结构为上抛占主导地位的结构。     

Effect of velocity ratio on the streamwise vortex structures in the wake of a stack

The time-averaged velocity and streamwise vorticity fields within the wake of a stack were investigated in a low-speed wind tunnel using a seven-hole pressure probe. The experiments were conducted at a Reynolds number, based on the stack external diameter, of Re_D=2.3×10⁴. The stack, of aspect ratio AR=9, was mounted normal to a ground plane and was partially immersed in a flat-plate turbulent boundary layer, where the ratio of the boundary layer thickness to the stack height was δ/H≈0.5. The jet-to-cross-flow velocity ratio was varied from R=0 to 3, which covered the downwash, crosswind-dominated and jet-dominated flow regimes. In the downwash and crosswind-dominated flow regimes, two pairs of counter-rotating streamwise vortex structures were identified within the stack wake. The tip vortex pair located close to the free end of the stack, and the base vortex pair located close to the ground plane within the flat-plate boundary layer, were similar to those found in the wake of a finite circular cylinder, and were associated with the upwash and downwash flow fields within the stack wake, respectively. In the jet-dominated flow regime, a third pair of streamwise vortex structures was observed, referred to as the jet-wake vortex pair, which occurred within the jet-wake region above the free end of the stack. The jet-wake vortex pair had the same orientation as the base vortex pair and was associated with the jet rise. The peak vorticity and strength of the streamwise vortex structures were functions of the jet-to-cross-flow velocity ratio. For the tip vortex structures, their peak vorticity and strength reduced as the jet-to-cross-flow velocity ratio increased.     

Direct numerical simulation of particle-laden plane turbulent wall jet and the influence of Stokes number

The distribution and motion of inertial particles in plane turbulent wall jet are investigated using direct numerical simulation, under the assumption of one-way coupling. To our knowledge, this appears to be the first direct numerical simulation of a particle-laden plane turbulent wall jet. It is shown that, in outer part of the wall jet, the behaviour of particles closely resembles that of a free plane jet. Due to the streamwise decay of particle Stokes number, the particle streaks formed in the near wall region of the wall jet are characterized by their intensity variation, which differs significantly from those in the channel flow. The streamwise growth of the particle velocity half-width is approximately equal to that of the fluid velocity half-width and the maximum velocity of particles decays slower than that of fluid due to inertia. The outer scaling can collapse the mean particle velocity in both the inner and outer region for heavier particles. In the buffer region, the particle–fluid velocity difference can be negative or positive depending on the Stokes number since there are two competing effects, namely the memory effect and turbophoresis. In the viscous region, the larger particles are on average faster than fluid and the velocity difference is found to be self-similar depending on outer Stokes number. The near-wall distribution of velocity difference is significantly correlated with the presence of high-momentum particles which are entrained by vortical structures generated in the outer region of the wall jet. These results are useful for environmental and engineering applications.     

A bypass wake induced laminar/turbulent transition

The process of laminar to turbulent transition induced by a von Karman vortex street wake, was studied for the case of a flat plate boundary layer. The boundary layer developed under zero pressure gradient conditions. The vortex street was generated by a cylinder positioned in the free stream. An X-type hot-wire probe located in the boundary layer, measured the streamwise and normal to the wall velocity components. The measurements covered two areas; the region of transition onset and development and the region where the wake and the boundary layer merged producing a turbulent flow. The evolution of Reynolds stresses and rms-values of velocity fluctuations along the transition region are presented and discussed. From the profiles of the Reynolds stress and the mean velocity profile, a ‘negative' energy production region along the transition region, was identified. A quadrant splitting analysis was applied to the instantaneous Reynolds stress signals. The contributions of the elementary coherent structures to the total Reynolds stress were evaluated, for several x-positions of the near wall region. Distinct regions in the streamwise and normal to the wall directions were identified during the transition.     

Large Eddy Simulation of boundary layer transition over an isolated ramp-type micro roughness element

Boundary layer transition over an isolated surface roughness element is investigated by means of numerical simulation. Large Eddy Simulation (LES) flow-modeling approach is employed to study flow characteristics and transition phenomenon past a roughness element immersed within an incoming developing boundary layer, at a height-based Reynolds number of 1170. LES numerical results are compared to experimental data from literature showing the time-averaged velocity distribution, the velocity fluctuation statistics and the instantaneous flow topology.Despite slight difference in the intensity of streamwise velocity fluctuations, the present LES results and experimental data show very good agreement. The mean flow visualization shows streamwise counter-rotating vortices pairs formation downstream of the obstacle. The primary pair induces an upwash motion and a momentum deficit that creates a Kelvin-Helmholtz type flow instability. The instantaneous flow topology reveals the formation of coherent K-H vortices downstream that produce turbulent fluctuations in the wake of the roughness element. These vortices are streched and lifted up when moving downstream. The velocity fluctuations results show that the onset of the turbulence is dominated by the energy transfer of large-scale vortices.     

The stability and development of tip and root vortices behind a model wind turbine

An inclined turbulent jet discharging a passive scalar into a turbulent crossflow is investigated under conditions of favorable, zero and adverse streamwise pressure gradient. Experiments are conducted in water by means of magnetic resonance velocimetry and magnetic resonance concentration measurements. The velocity ratio and density ratio are equal to one for all cases. The flow configuration is relevant to film cooling technology, the molecular properties of the fluid being immaterial in the fully turbulent regime. Under favorable pressure gradient (FPG), the streamwise acceleration tilts the jet trajectory toward the wall, which would be beneficial for the film cooling performance. However, the counter-rotating vortex pair is strengthened in the accelerating flow by streamwise stretching. Also, the crossflow boundary layer is significantly thickened by increasingly adverse pressure gradient, which affects the mass transfer from the jet. Overall, the more intense counter-rotating vortices and the thinner boundary layer associated with increasingly FPG enhance the scalar dispersion into the main flow, hampering the film cooling performance in terms of surface effectiveness.     

Multisensor Hot Wire Vorticity Probe Measurements of the Formation Field of Two Corotating Vortices

Experimental evidence is reported, regarding the formation of a pair of co-rotating tip vortices by a split wing configuration, consisting of two half wings at equal and opposite angles of attack. Simultaneous measurements of the three-dimensional vector fields of velocity and vorticity were conducted on a cross plane at a downstream distance corresponding to 0.3 cord lengths (near wake), using an in-house constructed 12-sensor hot wire anemometry vorticity probe. The probe consists of three closely separated orthogonal 4-wire velocity sensor arrays, measuring simultaneously the three-dimensional velocity vector at three closely spaced locations on a cross plane of the flow filed. This configuration makes possible the estimation of spatial velocity derivatives by means of a forward difference scheme of first order accuracy. Velocity measurements obtained with an X-wire are also presented for comparison. In this near wake location, the flow field is dictated by the pressure distribution established by the flow around the wings, mobilizing large masses of air and leading to the roll up of fluid sheets. Fluid streams penetrating between the wings collide, creating on the cross plane flow a stagnation point and an “impermeable” line joining the two vortex centres. Along this line fluid is directed towards the two vortices, expanding their cores and increasing their separation distance. This feeding process generates a dipole of opposite sign streamwise mean vorticity within each vortex. The rotational flow within the vortices obligates an adverse streamwise pressure gradient leading to a significant streamwise velocity deficit characterizing the vortices. The turbulent flow field is the result of temporal changes in the intensity of the vortex formation and changes in the position of the cores (wandering).