长江三峡巴东县城区三道沟滑坡成因研究

构造了三维混合层流场用离散涡丝描述的计算模型并进行了数值模拟．给出了流场的涡结构以及不同剪切率、不同扰劝强度下涡结构的特征，得到了流场中的平均速度、动量厚度．由计算被动粒子显示了流场螺旋涡结构的图像．做了相应的流动显示实验研究，所得结果与计算结果非常相近．本研究为了解该流场并由此进行流场控制提供了依据．     

超声速平面混合层小激波的形成与演变

为了揭示超声速混合层中小激波形成机理及其与涡相互作用的演变过程,本文基于大涡模拟（LES）方法,结合五阶精度混合TCD／WENO格式,对超声速平面混合层在对流马赫数为Mc=0．65条件下的流场结构进行了数值模拟,数值结果详细描述了超声速混合层中小激波的形成过程。研究了小激波形成后,随涡运动而产生的变形、脱落及发展过程。同时,对混合层双涡合并过程中,小激波与相邻涡相互作用所产生的变形与演变过程进行了讨论。     

刚杆状聚合物和混合层中拟序结构的相互影响

用港方法（即Galerkin方法）求解多球刚杆模型位型空间分布函数的扩散方程，用拟谱方法求解物理空间的N－S方程，同时得到了加入刚杆状聚合物分子的二维混合层中拟序结构的演变过程和聚合物分子的最可几取向．计算结果较为合理地描述了聚合物分子和拟序结构的相互影响．||关键词##4多球刚杆分子模型;;混合层;;拟序结构;;谱方法     

带展向曲率的超音速混合层中扰动演化的研究

超音速混合层的流动不稳定性较之亚音速或不可压的混合层大大减弱，为了提高混合效率， 通过数值模拟的方法分别研究了展向曲率、展向速度、来流马赫数等因素对混合效率所 起的作用. 计算结果表明：在给定展向速度的情况下，带有展向曲率的三维混合层，曲率越 大三维扰动增长率越大，而且法向的卷起范围也越大. 当展向曲率不为零时，展向速度的增 大也能有效地增强混合能力. 由流场中的高频扰动波产生的涡，在向下游发展过程中会有破 碎、拉伸，低频扰动波没有发现这一现象. 对于有展向曲率和展向速度的混合层，提高来流 马赫数时，流场中最不稳定扰动的增长率仍很大. 因此，这是一种提高混合层混合效率的新 途径.     

流场非均匀性对非平面激波诱导的Richtmyer-Meshkov不稳定性影响的数值研究

基于Navier-Stokes方程组,采用可压缩多介质黏性流动和湍流大涡模拟程序MVFT (multi-viscousflow and turbulence),模拟了均匀流场与初始密度呈现高斯函数分布的非均匀流场中马赫数为1.25的非平面激波加载初始扰动air/SF6界面的Richtmyer-Meshkov (RM)不稳定性现象。数值模拟结果表明,初始流场非均匀性将会影响非平面激波诱导的RM不稳定性演化过程。反射激波加载前,非平面激波导致的界面扰动振幅随着流场非均匀性增强而增大;反射激波加载后,非均匀流场与均匀流场条件下的界面扰动振幅差异有所减小。进一步,定量分析流场中环量分布及脉动速度统计量揭示了前述规律的原因。此外,还与平面激波诱导的RM不稳定性进行了简单对比,发现由于非平面激波波阵面区域的涡量与激波冲击界面时产生的涡量的共同作用,使得非平面激波与平面激波诱导的界面失稳过程存在差异。     

混合层流动拟序结构的大涡模拟

采用大涡模拟方法对空间发展的二维平面混合层进行了数值模拟 ,动量方程采用分步投影法求解 ,亚格子项采用标准Smagorinsky亚格子模式模拟 ,压力泊松方程采用修正的循环消去法快速求解 ,同时求解了标志物输运方程以实现数值流场显示。模拟结果给出了混合层流动的瞬态发展过程以及流动中拟序结构的发展演变过程 ,成功地模拟了混合层发展中的各种瞬态细节过程 ,如涡的卷起、增长 ,涡与涡之间的配对、合并过程 ,以及大涡破碎为小涡的级联过程 ,为各种以混合层流动为原型流动的射流、尾流等工业流动的控制和优化提供了理论基础。     

气固两相混合层固粒分布特性的分数维方法描述

本文对气固两相混合层中固粒的运动进行了数值模拟，得到了不同Ｓｔ数下，涡卷起和配对过程中固粒的分布特性，计算了不同时刻固粒分布的相关维数，说明分数维是一个可用来定量描述固粒分布均匀程度的重要参数，Ｓｔ数为１０的数量级时，对应最小的相关维数，固粒分布最不均匀，研究结果对控制固粒的均匀程度具有指导意义。     

熔喷双槽形喷嘴气体射流流场初探

在熔喷非织造布加工中,气体射流作为工作介质使聚合物熔体实现拉伸,气体射流流场的研究对熔喷气流拉抻数学模型研究非常重要。熔喷双槽形喷嘴形成的流场可以看作两股平面射流的合成。从单个点涡的性质出发,研究了涡偶的性质和涡偶代替射流的可行性。研究表明,在喷丝孔轴线附近,涡偶和射流的速度分布趋势相同,且有比较相近的速度分布,从而说明以涡偶代替射流是可行的。在此基础上,用两个涡偶分别代替两股射流,然后进行合成,推导出两股射流合成后速度分布的理论公式,该公式的计算结果与实验结果吻合较好。将该公式引入熔喷气流拉伸数学模型,预测出的纤维直径与采用经验公式时的预测结果几乎完全相同。结果表明,应用涡偶代替射流推导出的气流速度分布公式能够较好地描述熔喷双槽形喷嘴气体射流流场,可以用于完善熔喷气流拉伸数学模型。     

结晶型高分子材料的平面取向演化与塑性响应

建立了描述结晶型高分子材料的平面取向演化及其塑性响应的解析框架。文中针对材料中分子链不可伸长的特点，修正了Ｔａｙｌｏｒ假定。引入连续的取向分布函数，并将它展开成不可约的张量形式表示，通过建立并求解展开系数的演化方程，最终获得问题的解。文中模拟了单轴拉伸和简单剪切时链轴朝拉伸方向偏转的过程，描述了应力的上扬硬化现象．     

气固两相混合层二维涡配对的数值研究

采用双向耦合模型中的速度耦合模型,数值模拟了气固两相混合层中涡的配对、合并过程,文中采用拟谱方法对流场进行了直接数值模拟,用Lagrange模型跟踪固粒,结果发现,在双向耦合过程中Stokes数仍然是重要的参数,但不是唯一影响流场的参数,流场的发展还与固粒的体积浓度、固粒的相对密度以及固粒大小等因素有关。固粒抑制流场的发展,阻碍涡的配对、合并,加快了涡量的扩散;小St数的固粒仍然跟随流体运动,大St数的固粒趋向于低涡量区的趋势减弱。     

Rheology and flow-birefringence from viscoelastic polymer-clay solutions

 The shear orientation of viscoelastic clay-polymer solutions was investigated by means of rheology and flow birefringence (Δn). The polymer chains are in dynamic adsorption/desorption equilibrium with the clay particles to form a “network”. The elastic behavior of the network was characterized by constant stress, oscillatory shear, and stress relaxation experiments. Constant stress experiments indicated a yield stress upon which shear flow started and no strain recovery could be observed. Oscillatory shear experiments showed a broad elastic region followed by flow when a critical strain was reached. Stress relaxation experiments showed several relaxation times when the same critical strain was reached. Experiments under steady flow characterized the transient behavior of the network. With increasing steady shear rate a pronounced minimum in birefringence was observed at a critical shear rate. The shear rate dependent viscosity showed near power law behavior and no corresponding critical feature. While birefringence detects orientational effects on a microscopic length scale, rheology averages over macroscopic changes in the sample. The same degree of orientation could be achieved under constant shear rate or constant stress conditions. Received: 25 January 2001 Accepted: 22 May 2001     

Three dimensional microstructural effects on plane strain ductile crack growth

Ductile crack growth under mode I, plane strain, small scale yielding conditions is analyzed. Overall plane strain loading is prescribed, but a full 3D analysis is carried out to model three dimensional microstructural effects. An elastic-viscoplastic constitutive relation for a porous plastic solid is used to model the material. Two populations of second-phase particles are represented, large inclusions with low strength, which result in large voids near the crack tip at an early stage, and small second-phase particles, which require large strains before cavities nucleate. The larger inclusions are represented discretely and the effects of different three dimensional distributions on the crack path and on the overall crack growth rate are analyzed. For comparison purposes, a two dimensional distribution of cylindrical inclusions is analyzed. Crack growth occurs off the initial crack plane in all 3D computations, whereas straight ahead crack growth occurs with the two dimensional cylindrical inclusions. As a consequence, the three dimensional distributions of spherical inclusions exhibit an increased crack growth resistance as compared to the two dimensional distribution of cylindrical inclusions.     

Variation of fiber orientation in turbulent flow inside a planar contraction with different shapes

In this study, we have investigated the influence of shape of planar contractions on the orientation distribution of stiff fibers suspended in turbulent flow. To do this, we have employed a model for the orientational diffusion coefficient based on the data obtained by high-speed imaging of suspension flow at the centerline of a contraction with flat walls. This orientational diffusion coefficient depends only on the contraction ratio and turbulence intensity. Our measurements show that the turbulence intensity decays exponentially independent of the contraction angle. This implies that the turbulence variation in the contraction is independent of the shape, consistent with the results by the rapid distortion theory and the experimental results of axisymmetric contractions. In order to determine the orientation anisotropy, we have solved a Fokker–Planck type equation governing the orientation distribution of fibers in turbulent flow. Although the turbulence variation and the orientational diffusion are independent of the contraction shape, the results show that the variation of the orientation anisotropy is dependent on shape. This can be explained by the variation of the rotational Péclet number, Pe_r, inside the contractions. This quantity is a measure of the importance of the mean rate of the strain relative to the orientational diffusion. We have shown that when Pe_r < 10 turbulence can significantly influence the evolution of the orientation anisotropy. Since in contractions with identical inlet conditions the streamwise position where Pe_r = 10 depends on the shape, the orientation anisotropy is dependent on the variation of rate of strain in a given contraction. We demonstrate the shape effect by considering contraction with flat walls as well as three contractions with different mean rate of strain variation.     

Experimental study on mixing enhancement in two dimensional supersonic flow

 Studies on mixing enhancement with two dimensional (2D) lobed nozzle have been conducted in a dual stream supersonic flow facility. The distributions of momentum flux, stagnation pressure and stagnation temperature across a plane at different axial distances from the nozzle exit were considered as a measure of mixing. The results indicated an enormous enhancement in mixing when 2D lobed nozzle was employed in comparison with conventional plain 2D nozzle. The enhanced mixing performance could be attributed to the large scale axial vortices observed in the flow-field of subsonic lobed nozzles by earlier investigators. Received: 27 December 1996 / Accepted: 21 August 1997     

Coherent motion of turbulent thermal plume in stably stratified fluid

The purpose of this study is to clarify the existence of an ordered and large scale coherent motion in a turbulent plane thermal plume in a thermally-stable stratified fluid inside a comparatively large enclosure. First, the upper part of the thermal plume was carefully observed by a flow visualization. Secondly, a wave form of plume temperature variation was measured. Thirdly, a spectrum analysis was carried out on time series data of the thermal plume. Finally, physical characteristics were investigated on vortices in the thermal plume based on results of the wave form and the spectrum analysis of the plume temperature. As a result, the main conclusions are obtained as follows. (1) An existence of vortices near the upper part of the thermal plume was firstly found by careful flow visualization. (2) From the wave form of temperature variation and the spectrum analysis of the thermal plume, it was clarified that the vortices are generated in the transition state and are transported to the turbulent state. (3) The vortices are ordered and they behave as a large scale coherent motion in the turbulent thermal plume.     

Three-component velocity field measurements of propeller wake using a stereoscopic PIV technique

A stereoscopic PIV (Particle Image Velocimetry) technique was used to measure the three-dimensional flow structure of the turbulent wake behind a marine propeller with five blades. The out-of-plane velocity component was determined using two CCD cameras with an angular displacement configuration. Four hundred instantaneous velocity fields were measured for each of four different blade phases, and ensemble averaged in order to find the spatial evolution of the propeller wake in the region from the trailing edge up to one propeller diameter (D) downstream. The influence of propeller loading conditions on the wake structure was also investigated by measuring the velocity fields at three advance ratios (J=0.59, 0.72 and 0.88). The phase-averaged velocity fields revealed that a viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were generated periodically and the slipstream contracted in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. As the flow moved downstream, the turbulence intensity, the strength of the tip vortices, and the magnitude of the out-of-plane velocity component at trailing vortices all decreased due to effects such as viscous dissipation, turbulence diffusion, and blade-to-blade interaction.     

Cycle resolved multi-planar flow measurements in a four-valve combustion engine

The non-reacting flow in a one-cylinder four-valve combustion engine is measured via cycle resolved two-component/two-dimensional (2C/2D) particle-image velocimetry (PIV). The three-dimensional structure of the velocity field is analyzed based on the flow field measured in eight planar planes within the cylinder for several crank angles during the intake and compression phase. Using the mean and statistical values of the single planes quasi three-dimensional flow fields are reconstructed for crank angles of 80°, 160°, and 240° atdc. This enables the detailed analysis of the spatial distribution of the large and small scale flow structures, e.g., by visualizing large vortical structures and the distribution of the turbulent kinetic energy. It was found that two ring vortices evolving beneath the inlet valves are the dominant large scale structures that seem to be of major concern for the mixing process in the cylinder of a four-valve combustion engine operated at 1500 rpm. Furthermore, the temporal evolution of the flow field within the symmetry plane of the cylinder, measured for crank angles between 40° and 320° atdc in steps of 20°, is discussed. The results give new insight into the complex three-dimensional flow in the combustion chamber of a one-cylinder four-valve combustion engine. That is, the tumble vortex only seems to be of secondary importance for the flow concerning the mixing process at 1500 rpm. This is an essential result for future work considering the fluid mechanics of fuel-air-interaction processes and mixing principles in combustion engines.     

Reynolds stress and vorticity in turbulent wall flows

Experimental measurements in a boundary layer and a large-eddy simulation of plane channel flow have been used to study the dynamics of vorticity and mass transport in the nearwall region. It was found that Reynolds stress generation occurs in the vicinity of quasi-streamwise vortices, and that smoke particles tend to be ejected from the wall near these vortical structures.     

Mixing Intensification on the Edge of a Jet by Means of Longitudinal Vortices

The action of an artificially generated spanwise flow in the form of periodical longitudinal vortices on a plane turbulent mixing layer is investigated. It is shown that the disturbances result in a significant increase in the thickness of the mixing region. For two kinds of spanwise flow, namely, vortices whose centers lie in the plane separating the streams and vortices located above this plane, the dependence of the mixing layer thickness on the vortex amplitude and vertical dimension and on the longitudinal coordinate is found.