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.     

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.      

泡沫铝材料动态本构参数的实验确定

基于泡沫材料的动态刚性-线性硬化塑性-刚性卸载(D-R-LHP-R)模型,结合连续性方程,动量守恒方程及刚体的运动方程,得到了激波在泡沫材料中的量纲一消失位置X_s/L₀和动态屈服应力Y_i、激波波速c_p、冲击初始应变ε_i之间的如下关系式: $\frac{X_{\mathrm{s}}}{L_{0}}=\exp \left(-\frac{\rho_{0} c_{\mathrm{p}} v_{\mathrm{i}}}{Y}\right)=\exp \left(1-\frac{\sigma_{\mathrm{i}}}{Y}\right)=\exp \left(-\frac{\rho_{0} c_{\mathrm{p}}^{2} \varepsilon_{\mathrm{i}}}{Y}\right)$ 采用Taylor-Hopkinson装置进行实验,当直接测得泡沫铝试样密度ρ₀、边界初始应力σ_i、初始打击速度v_i、泡沫铝杆原长L₀及激波在泡沫铝杆中消失长度X_s后,利用方程式(a)可反演求得D-R-LHP-R模型下的泡沫铝动态应力应变曲线。最后通过与泡沫铝准静态实验数据对比,表明该泡沫铝是应变率敏感性材料。     

分段吸气高层建筑减阻性能的数值研究

为减小高层建筑的风致阻力, 采用CFD方法研究了主动吸气控制下高层建筑模型的风载荷减阻性能, 分析了竖向开孔位置、吸气孔高度和吸气速度等参数对减阻性能的影响, 并详细展示流场,讨论吸气控制机理. 结果表明: 保持流量系数不变, 增加吸气孔高度(或减小吸气速度)使得模型各表面的风压折减效 率$\eta_{\rm{PR}}$, 阻 力折减效率$\eta_{\rm{DR}}$和基底弯矩折减效率$\eta_{\rm{MR}}$增大, 且只有$\eta_{\rm{MR}}$在较大吸气孔高度时超过1.0. 拟合了$\eta_{\rm{DR}}$ 和$\eta_{\rm{MR}}$关于吸气孔中心高度、吸气孔高度和吸气速度的经验公式, 为分段吸气控制的应用提供参考. 基于最大风压折减效率和最小吸气功率,比较了 各分段吸气模型和全高吸气模型的减阻性能, 发现全高吸气模型的减阻性能优于分段吸气模型. 可在高层建筑中上部设置吸气装置来减小基底弯矩或改善其局部风压特性.     

垂直壁面附近上升单气泡的弹跳动力学研究

采用高速摄影技术结合阴影法,对静止水中垂直壁面附近上升单气泡运动进行实验研究,对比气泡尺度及气泡喷嘴与壁面之间的初始无量纲距离 ($S^{\ast}$)对气泡上升运动特性的影响,分析气泡与壁面碰撞前后,壁面效应与气泡动力学机制及能量变化规律.结果表明,对于雷诺数$Re \approx 580 \sim 1100$,无量纲距离$S^{\ast } <2 \sim3$时,气泡与壁面碰撞且气泡轨迹由无约束条件下的三维螺旋转变成二维之字形周期运动;当$S^{\ast } >2 \sim3$时,壁面效应减弱,有壁面约束的气泡运动与无约束气泡运动特性趋于一致.气泡与壁面碰撞前后,壁面效应导致横向速度峰值下降为原峰值的70%,垂直速度下降50%;气泡与壁面碰撞前,通过气泡中心与壁面距离($x/R$)和修正的斯托克斯数相关式可预测垂直速度的变化规律.上升气泡与壁面碰撞过程中,气泡表面变形能量单向传输给气泡横向动能,使得可变形气泡能够保持相对恒定的弹跳运动.提出了气泡在与壁面反复弹跳时的平均阻力系数的预测模型,能够很好地描述实验数据反映出的对雷诺数${Re}$、韦伯数${We}$和奥特沃斯数${Eo}$等各无量纲参数的标度规律.      

金属射流失稳断裂的理论分析

基于Hamilton原理,提出一个包含射流强度、剪切、应变率效应、动力学黏性、表面张力和速度梯度等多因素耦合的金属射流拉伸运动方程,具体分析了各种失稳因素,并由数值解定量给出其影响大小,以及最不稳定波长与初始应变率乘积\lambda_{m}\dot{\varepsilon}_{0} 值的变化范围;给出了射流断裂的时间判据和近似理论公式,计算得到的 t_{b}-\dot{\varepsilon}_{0}曲线与射流实验点、Chou\&Carleone拟合公式三者符合较好.      

超声速化学反应流动中燃料预喷研究述评

研制基于超声速燃烧的高效吸气式推进装置(运行Mach数在3.5以上)需要寻求改善混合效率的有效机制, 这对于采用常规碳氢燃料(特别是可以增加密度的液体燃料)的装置尤为重要. 延长混合时间的一种途径是在飞行器燃料室的上游喷入部分燃料. 壁面喷射一直是超声速气动力学最具挑战性的课题, 这里包括使比冲损失最小、改善燃料-空气的混合、减少入口段/燃烧室的相互作用以及增进火焰稳定性等. 综述了超声速入口段或燃烧室的隔离器中液体燃料(个别情况下为气体燃料)喷射的研究进展. 在这些研究中, 燃料都是从后掠型细支架尾迹中的壁面处喷射出来的, 动压比很低($q_{\rm {jet}}/q_{\rm {air}}$=0.6$\sim$1.5). 它们涉及入口段和燃烧室的隔离器中单个支架/喷射器的几何结构及其组合方式、各种各样的喷射条件、不同的引射剂, 并且评估了这些因素对于燃料羽流喷散、比冲损失以及混合效率的影响. 述评引用了46篇参考文献.     

壁面对串列双圆柱尾迹影响的实验研究

为研究壁面对近壁等直径串列双圆柱尾迹特性的影响,用PIV和压力传感器测量尾迹湍流的涡结构及频谱.实验在循环水槽内进行,基于圆柱直径D的雷诺数为1696,壁面边界层厚度为6.6D.影响尾迹流场结构的两个重要的特征参数是T/D和G/D(T为两圆柱中心间的距离,G为圆柱下表面与壁面间的距离),文中主要考察G/D的影响.实验中...     

湍流冲击射流流动与传热的数值研究进展

湍流冲击射流由于其冲击表面时具有很高的局部传热率和冲击力,被广泛应用于如表面的加热、电子元件的冷却、纸张的干燥和材料的切割等工程应用和工业过程中.由于其流动的复杂性,也常被作为一种理想的测试实例来评价湍流模型的性能.此外,湍升力射流与地面之间的空气动力作用对V/STOL (垂直或短距离起落)飞机的性能具有很大的影响.长期以来,人们从理论分析、实验测量和数值模拟方面对冲击射流进行了广泛而系统的研究,积累了丰富的资料.本文在分析了湍流冲击射流的数值研究现状的基础上,对近年来有关湍流冲击射流流动与传热的数值研究方面的文献有选择地进行了综述,重点评述了不同湍流模型对冲击射流流动与传热的预测能力,讨论了存在的问题并对该领域今后的研究方向进行了展望.      

各向同性湍流内颗粒碰撞率的直接模拟研究

对 Re_{\lambda } 约为51均匀各向同性湍流内 St_{k}(=\tau_{p}/\tau_{k}) 为 0 ~10.0 的 有限惯性颗粒的碰撞行为进行了直接数值模拟，以研究湍流对有限惯性 颗粒碰撞的影响. 结果表明，具有一定惯性颗粒的湍流碰撞率完全不同于零惯性的轻颗粒 (St_{k}=0) 和可忽略湍流作用的重颗粒 (St{k} \to \infty) , 其变化趋势极其复杂： 在Stk为 0~1.0 之间，颗粒的碰撞率随 St 的增加而近乎线性地剧烈增长，在 Stk≈1.0 3.0(对应的StE=τp/Te≈0.5)附近，颗粒碰撞率出现两个峰值，在Stk>3.0以后，颗粒的碰撞率随惯性增 大而逐渐趋向于重颗粒极限；在峰值处，有限惯性颗粒的平均碰撞率的峰值较轻颗粒增强了 30倍左右. 为进一步分析湍流作用下颗粒碰撞率的影响因素，分别使用可能发生碰撞 的颗粒对的径向分布函数和径向相对速度来量化颗粒的局部富集效应和湍流掺混效应，表明 St_{k} \approx 1.0 时局部富集效应最为强烈，使得颗粒的碰撞率出现第1个峰值； 湍流掺混效应则随着颗粒Stk的增大而渐近增大；局部富集和湍流掺混联合作用的结果， 使得颗粒碰撞率在 St_{k} \approx 3.0 附近出现另一个峰值.     

Mean flow and turbulence measurements of the impingement wall jet on a semi-circular convex surface

The detailed mean flow and turbulence measurements of a turbulent air slot jet impinging on two different semi-circular convex surfaces were investigated in both free jet and impingement wall jet regions at a jet Reynolds number Re_w=12,000, using a hot-wire X-probe anemometer. The parametric effects of dimensionless circumferential distance, S/W=2.79-7.74, slot jet-to-impingement surface distance Y/W=1-13, and surface curvature D/W=10.7 and 16 on the impingement wall jet flow development along a semi-circular convex surface were examined. The results show that the effect of surface curvature D/W increases with increasing S/W. Compared with transverse Reynolds normal stress, [`(v<sup>2</sup> )] /U<sub>m</sub><sup>2</sup> \overline {v^2 } /U_{\rm m}^2 , the streamwise Reynolds normal stress, [`(u² )] /U_m² \overline {u^2 } /U_{\rm m}^2 , is strongly affected by the examined dimensionless parameters of D/W, Y/W and S/W in the near-wall region. It is also evidenced that the Reynolds shear stress, -[`(uv)] /U_m² - \overline {uv} /U_{\rm m}^2 is much more sensitive to surface curvature, D/W.     

Wall pressure and shear stress measurements beneath an impinging jet

This paper presents comprehensive measurements of wall pressure and surface shear stress beneath a plane, two-dimensional, turbulent jet impinging normally onto a flat surface. The results cover a wider range of Reynolds number and ratio of impingement height (H) to nozzle gap (D) than do previous studies. The pressure distributions are nearly Gaussian, independent of Reynolds number, and closely balance the momentum flux from the jet nozzle as H/D varies. Particular attention was paid to probe size in measuring the wall shear stress because this has a significant effect on the results. A range of Preston tubes and Stanton probes were tested from which it was found that a 0.05-mm-high Stanton probe—the smallest that we could make—appeared to give accurate results. As expected, the shape of the wall shear stress distributions depended both on H/D and on Reynolds number. Furthermore, the relation between wall pressure and shear stress from Hiemenz's theoretical solution for stagnation flow is not in agreement with the results. It is postulated that the discrepancy is due to the relatively high free-stream turbulence level in the jet. Future papers will document the mean flow field and turbulence and the time dependence of the surface pressure.     

The development of an axisymmetric curved turbulent wall jet

An experimental study has been carried out of the low speed Coanda wall jet with both streamwise and axisymmetric curvature. A single component laser Doppler technique was used, and by taking several orientations at a given point, values of the three mean velocities and five of the six Reynolds stresses were obtained. The lateral divergence and convex streamwise curvature both enhanced the turbulence in the outer part of the jet compared with a plane two-dimensional wall jet. The inner layer exhibited a large separation of the positions of maximum velocity and zero shear stress. It was found that the streamwise mean velocity profile became established very rapidly downstream of the slot exit. The profile appeared fairly similar at later downstream positions, but the mean radial velocity and turbulence parameters showed the expected nonself preservation of the flow. Removal of the streamwise curvature resulted in a general return of the jet conditions toward those expected of a plane wall jet. The range and accuracy of the data may be used for developing turbulence models and computational techniques for this type of flow.     

Layered structure of turbulent plane wall jet

This paper investigates the layered structure of a turbulent plane wall jet at a distance from the nozzle exit. Based on the force balances in the mean momentum equation, the turbulent plane wall jet is divided into three regions: a boundary layer-like region (BLR) adjacent to the wall, a half free jet-like region (HJR) away from the wall, and a plug flow-like region (PFR) in between. In the PFR, the mean streamwise velocity is essentially the maximum velocity, and the simplified mean continuity and mean momentum equations result in a linear variation of the mean wall-normal velocity and Reynolds shear stress. In the HJR, as in a turbulent free jet, a proper scale for the mean wall-normal flow is the mean wall-normal velocity far from the wall and a proper scale for the Reynolds shear stress is the product of the maximum mean streamwise velocity and the velocity scale for the mean wall-normal flow. The BLR region can be divided into four sub-layers, similar to those in a canonical pressure-driven turbulent channel flow or shear-driven turbulent boundary layer flow. Building on the log-law for the mean streamwise velocity in the BLR, a new skin friction law is proposed for a turbulent wall jet. The new prediction agrees well with the correlation of Bradshaw and Gee (1960) over moderate Reynolds numbers, but gives larger skin frictions at higher Reynolds numbers.     

Turbulent boundary layer on a mildly curved convex surface

Extensive single point turbulence measurements made in the boundary layer on a mildly curved heated convex wall show that the turbulence heat fluxes and Stanton number are more sensitive to a change in wall curvature than the Reynolds stresses and skinfriction coefficient, and that downstream, as the flow adjusts to new curved conditions, the St/c _f ratio of Reynolds analogy is appreciably lower than in plane wall flow for the same conditions. Details of the turbulence structure in unheated flow have been documented in an earlier paper; temperature field measurements now described comprise mean temperature distributions, the streamwise variation of wall heat flux, profiles of the temperature variance, transverse and streamwise heat fluxes, and triple correlations. Turbulent diffusion of heat flux is drastically reduced even by mild curvature; changes in the heat fluxes are of the same order as changes in the shear stress, that is, an order of magnitude greater than the ratio of boundary layer thickness to wall radius of curvature. The data include plane flow measurements taken in a developed boundary layer upstream of a change in wall curvature.     

高频吹气扰动影响近壁区拟序结构统计特性的实验研究

利用恒温热线风速仪测量了零压力梯度平板上施加由合成射流激发的狭缝周期吹气扰动前后不同流向位置湍流边界层的速度信号, 展开高频吹气扰动影响近壁区湍流结构的统计特性研究. 研究结果表明:高频周期吹气扰动在狭缝下游产生明显的减阻效果. 扰动强度在湍流边界层内的发展沿流向呈衰减趋势, 其与湍流结构的相互作用也相应衰减. 然而, 因高频扰动产生运动的展向涡结构与猝发引起的结构变化尺度相当, 直接影响了近壁区拟序结构产生与发展的统计, 从而使得猝发检测方法VITA 表现出与低频或定常吹气减阻机理相异的现象.      

Low Reynolds number axisymmetric turbulent boundary layer on a cylinder

In the present study, an axisymmetric turbulent boundary layer growing on a cylinder is investigated experimentally using hot wire anemometry. The combined effects of transverse curvature as well as low Reynolds number on the mean and turbulent flow quantities are studied. The measurements include the mean velocity, turbulence intensity, skewness and flatness factors in addition to wall shear stress. The results are presented separately for the near wall region and the outer region using dimensionless parameters suitable for each case. They are also compared with the results available in the open literature.The present investigation revealed that the mean velocity in near wall region is similar to other simple turbulent flows (flat plate boundary layer, pipe and channel flows); but it differs in the logarithmic and outer regions. Further, for dimensionless moments of higher orders, such as skewness and flatness factors, the main effects of the low Reynolds number and the transverse curvature are present in the near wall region as well as the outer region.     

Numerical simulation of two‐dimensional laminar slot‐jet impingement flows confined by a parallel wall

Two‐dimensional laminar incompressible impinging slot‐jet is simulated numerically to gain insight into flow characteristics.Computations are done for vertically downward‐directed slot‐jets impinging on a plate at the bottom and confined by a parallel surface on top. The behaviour of the jet with respect to aspect ratio (AR) and Reynolds number (Re) are described in detail. The computed flow patterns for various AR (2–5) and for a range of jet‐exit Reynolds numbers (100–500) are analysed to understand the flow characteristics. The transient development of the flow is also simulated for AR = 4 and Re = 300. It is found that the reattachment length is dependent on both AR and Reynolds number for the range considered. The correlation for reattachment length is suggested. The maximum resultant velocity V_rmax and its trajectory is reported. A detailed study of horizontal velocity profile at different downstream locations is reported. It is found that the effect of Reynolds number and AR is significant to the bottom wall vorticity in the impingement and wall jet regions. Copyright © 2007 John Wiley & Sons, Ltd.     

PIV study of large-scale flow organisation in slot jets

The paper reports on particle image velocimetry (PIV) measurements in turbulent slot jets bounded by two solid walls with the separation distance smaller than the jet width (5–40%). In the far-field such jets are known to manifest features of quasi-two dimensional, two component turbulence. Stereoscopic and tomographic PIV systems were used to analyse local flows. Proper orthogonal decomposition (POD) was applied to extract coherent modes of the velocity fluctuations. The measurements were performed both in the initial region close to the nozzle exit and in the far fields of the developed turbulent slot jets for Re ⩾ 10,000. A POD analysis in the initial region indicates a correlation between quasi-2D vortices rolled-up in the shear layer and local flows in cross-stream planes. While the near-field turbulence shows full 3D features, the wall-normal velocity fluctuations day out gradually due to strong wall-damping resulting in an almost two-component turbulence. On the other hand, the longitudinal vortex rolls take over to act as the main agents in wall-normal and spanwise mixing and momentum transfer. The quantitative analysis indicates that the jet meandering amplitude was aperiodically modulated when arrangement of the large-scale quasi-2D vortices changed between asymmetric and symmetric pattern relatively to the jet axis. The paper shows that the dynamics of turbulent slot jets are more complex than those of 2D, plane and rectangular 3D jets. In particular, the detected secondary longitudinal vortex filaments and meandering modulation is expected to be important for turbulent transport and mixing in slot jets. This issue requires further investigations.     

冲击高度对自由冲击射流影响的实验研究

采用热线风速仪测量了雷诺数为 ２３ ０００时四种冲击高度下率流自由冲击射流流场，并给出详细的结果．表明壁面的“阻尼”影响主要集中在近壁面０．５Ｄ以内．小冲击高度时径向速度下降得比大冲击高度时明显要快，量值也较小；在ｒ／Ｄ≤１．５处，小冲击高度时紊动能的数值大小和分布趋势与大冲击高度时不同，特别是在喷管出口距冲击板高度Ｚ与喷管直径Ｄ之比Ｚ／Ｄ为８时分布特殊，在其它测点处，紊动能的分布趋势基本一致，只是大冲击高度下的值较大；流动结构在Ｚ／Ｄ为６～８时发生了较大的变化，这种变化与势流核心区有关，在势流核心区的顶端以及下游的一段距离内紊流度都很高．