Channel flow of a liquid crystal polymer around an obstacle: Weld line structure and strain field

 We have studied by in situ microscopy the flow of a lyotropic liquid crystal polymer, hydroxypropylcellulose (HPC) in water, around an obstacle placed in a rectangular flow channel. The obstacle separates the flow into two parts which rejoin downstream of the obstacle, resulting in the formation of a `weld-line'. Measuring the velocity field in the vicinity of the weld-line beyond the obstacle, we find as expected a positive elongational strain (acceleration) along the weld (parallel to the flow direction). For an anisotropic (concentrated) HPC solution we observe in addition a significant shear strain in the weld-line region, there being an important velocity gradient perpendicular to the plane of the weld line. Isotropic (lower concentration) solutions of the same polymer demonstrate no visible weld line, a larger elongational strain rate near the obstacle, and no shear component of strain downstream of the obstacle. These results are similar to observations reported for fluids reinforced by macroscopic fibres. Polarised light observations of the anisotropic solution show that the strain field generates a generally increased degree of orientation of the liquid crytalline polymer near the weld (generally reduced crossed-polariser transmitted intensity when the polariser is parallel to the flow direction), however there is also a striking fine birefringent colour variation in the weld-line region, reminiscent of the structure observed at the channel side walls in rectangular channel flow (Haw and Navard 2000). The results show that the simple concept of weld-line structure as confined to an enhanced alignment along the weld due to elongational strain is incomplete; the two-dimensional shear strain field must also be taken into account for the anisotropic fluid. Received: 22 December 1999/Accepted: 4 January 2000     

Influence of viscosity on the flow in the circulation region in front of a flat obstacle perpendicular to the axis of a supersonic underexpanded jet

The results are given of a series of experiments to investigate the flow in front of an obstacle by high-sensitivity anemoscope probes. In order to study directly the influence of viscosity on the interaction process, the characteristic Reynolds number of the jet was varied in a wide range by changing the stagnation pressure, while the degree of underexpansion of the jet and the distance of the obstacle from the nozzle were kept unchanged. These parameters were selected, in accordance with earlier recommendations [3], to guarantee the existence of a circulation region in front of the obstacle in high-density jets.     

Neutron Diffraction Measurement of Stress Redistribution in Parallel Seven-Wire Strands after Local Fracture

We report results from neutron diffraction experiments where partitioning of applied tensile load between the inner and outer wires of seven-wire parallel and quasi-parallel wire strands were measured while 1-all wires were undergoing elastic deformation, 2-where one wire within the bundle was undergoing plastic flow and, 3-when one or more wires fractured under load. The results indicate that mechanical interference and friction mechanisms have similar contributions to the load transferred to fractured wires, and both mechanisms should be included in analytical or numerical formulations of strain partitioning in quasi-parallel wire cables.     

Numerical simulation and design optimization of an electrohydrodynamic pump for dielectric liquids

With an ever increasing demand for more effective heat sinks, liquid based electronic cooling has become a new prospect in the field. The present study introduces an electrohydrodynamic (EHD) pump with a simple design for dielectric liquids which have potential applications for electronic cooling. The pump consists of an eccentrically sandwiched wire electrode placed at the horizontal centerline between two parallel flat-plate electrodes. The EHD flow of dielectric liquid induced by the space charge generated due to the Onsager effect was obtained by the numerical solution of the Poisson–Nernst–Planck equations for ion transport and the Navier–Stokes equations for fluid flow. Good agreement obtained in the comparison of the numerical and the experimental results of velocity for the centrally sandwiched wire electrode case confirmed the validity of the numerical results. For a fixed voltage, the pump flow rate depends on the eccentricity of the wire electrode with respect to the plate electrodes and also with the electrode dimensions. By using the Taguchi method an optimum design for the EHD pump is obtained considering the wire electrode diameter, the flat plate electrode length and the eccentricity (the horizontal distance between the centers of wire and flat-plate electrodes) as the design parameters for fixed channel dimensions.     

Transverse and parallel injection of hydrogen with supersonic combustion in a shock tunnel

An experimental comparison has been made of the combustion induced pressure rise in a constant area duct when hydrogen is injected transverse to the flow by using a surface orifice, and when it is injected parallel to the flow by using a central injection strut. The experiments were conducted in a shock tunnel at a flow Mach number of 4.2 and stagnation enthalpies of 5.6, 6.5 and 8.9 MJ kg. Both room temperature and heated hydrogen were injected, and a method of heating the hydrogen by compression in a gun tunnel which was slaved to the shock tunnel is described. It was found that, for both unheated and heated hydrogen, the combustion pressure rise was not measurably dependent on the method of introducing the hydrogen, not withstanding the complicated shock related flow pattern arising from transverse injection. Received August 14, 1995 / Accepted February 14, 1996     

Turbulent flow around a surface-mounted obstacle using 2D-3C DPIV

The mean turbulent flow structure around a cube mounted on the surface of an open-surface water channel was studied using a two-dimensional implementation of digital particle image velocimetry (DPIV). The out-of-plane velocity component was obtained by the use of the concept of continuity applied to two-dimensional velocity fields recorded in parallel planes. Various methods were used for the identification and localization of large-scale vortical structures in the three-dimensional flow around the surface-mounted obstacle. The results show the feasibility of its application to three-dimensional PIV data and the superior performance of recent identification techniques (namely swirling strength and normalized angular momentum), over the classical vorticity-based criterion.     

Estimation of velocity eigenfunction and vorticity distributions from the timeline visualization technique

For the case of quasi-periodic flow, it is demonstrated that use of the hydrogen bubble timeline method leads to reasonable estimates of the eigenfunction of the streamwise velocity fluctuation. Both amplitude and phase distributions across an unstable wake flow are well-approximated. It is shown that the vorticity extrema, as well as the degree of concentration of vorticity, are in good agreement with those calculated from linear stability theory. A critical assessment is given of the possible uncertainties associated with this technique: the existence of a finite, but unknown cross-stream velocity component; bubble rise due to buoyancy effects; wake defect created downstream of the bubble wire; and resolution of the digitized image. Furthermore, the uncertainty in the streamwise velocity, arising from existence of a finite cross-stream velocity component, is actually less than that corresponding to a single-element hot film probe over certain regimes of operation.     

Supersonic flow of a rarefied gas around a wire grid

The direct simulation Monte Carlo method is used to study a plane-parallel supersonic gas flow through a grid formed by a number of infinite parallel wires. Characteristic features of the shock disturbance formation of in the interaction of a supersonic flow with a permeable grid are revealed. Particular attention is paid to studying the influence of geometrical parameters of the wire target on the number of particles colliding with the surface of the wires.     

A pulsed-wire technique for velocity and temperature measurements in natural convection flows

A pulsed-wire probe based on the use of one or two parallel wires, capable of measuring the velocity and the temperature in natural convection flows is described. These measurements are based on the analysis of the relaxation response of a pulsing wire, submitted to a very short electrical pulse. The analysis of the temperature variation on an optional second receiver wire, gives information about the velocity direction. The implementation simplicity of this probe, its good spatial precision, the lack of thermal contamination of the flow, as well as the possibility of obtaining simultaneous velocity and temperature measurements, allow the integration of the device in a multi-point measurement network, capable to deliver thermal and dynamic cartographies of unsteady convection flows.     

Class of solutions for flow near a stagnation point

This paper considers a class of solutions for flow of a perfect gas near the stagnation point on a two-dimensional obstacle, where the flow is rotational far upstream from the obstacle. It is shown that the potential flow near the stagnation point is a special case of this class of solutions. Solutions accounting for the rotationality of the outer flow are obtained for flow in the mixing layer with an obstacle, and these solutions differ appreciably from the analogous Jimenez solution for potential flow near the stagnation point on a two-dimensional obstacle.     

Synthesis of diamond structures from the jet of the H<Subscript>2</Subscript> + CH<Subscript>4</Subscript> mixture in a cocurrent axisymmetric hydrogen flow

The flow of a hydrogen–methane mixture through heated coaxial cylindrical tungsten channels with a built-in tungsten wire is studied by the Direct Simulation Monte Carlo method. The purpose of the study is further development of the gas-phase method of deposition of diamond structures. The axial distributions of the concentrations of the components of the hydrogen–carbon mixture are calculated by means of solving a system of chemical kinetics equations. A series of experiments on deposition of diamond structures from various flows of the hydrogen–methane mixture is performed. The calculated results are compared with the experimental data. Based on these comparisons, it is concluded that numerical optimization of operation modes of gas-dynamic reactors can be used for deposition of diamond structures.     

A single helicopter’s flying model in the low airspace with a parallel electrical wire network

In this paper, we use car-following theory to propose a model with consideration of multi-telegraph poles and electrical wires to describe a single helicopter’s flying behavior in the low airspace with a parallel electrical wire network. The numerical results show that the proposed model can describe the qualitative effects of the parallel electrical wire network on the evolutions of a helicopter’s speed, safe distances for the telegraph poles and electrical wires and safe sphere. These results can help us to better understand the helicopter’s flying behavior in the low airspace with a parallel electrical wire network.     

Dissipative Processes in a One-Dimensional Supersonic Gas Flow Across a Permeable Obstacle

One-dimensional supersonic gas flow across an infinite permeable surface (obstacle of zero thickness) is investigated by the direct statistical simulation (Monte-Carlo) method. Both the characteristic features of the formation of a shock perturbation in the interaction between the plane supersonic flow and the permeable obstacle and the effect of the perturbation on the flow parameters behind the obstacle are found. It is concluded that the accommodation coefficient can be determined from the data on the equilibrium flow macroparameters behind the obstacle.     

并联机器人刚度与静力学研究现状与进展

并联机器人的刚度与静力学分析, 对于机构力学性能研究具有重要的理论价值和意义. 本文围绕杆支撑、绳牵引和钢带传动 3 种结构形式, 详细阐述了国内外并联机构刚度和静力学分析的研究现状. 着重从有限元、解析模型和性能分析 3 方面分析了杆支撑并联机构的刚度研究进展. 讨论了有关绳牵引并联机构中绳拉力、动载荷频率、绳牵引预紧力与刚度、静力学之间关系的研究成果. 根据钢带并联机器人结构的特殊性, 对钢带并联机构的刚度与静力学分析中可能遇到的失稳与振动问题进行了探讨. 最后, 对并联机器人技术发展情况进行总结与展望, 指出随着刚度分析与静力学分析的不断深入, 并联机器人的力学理论将会日趋成熟和完善, 为并联机器人机构优化设计提供更深入、系统的理论依据.      

Experimental study of flow and heat transfer in rib-roughened rectangular channels

Secondary flow patterns, pressure drop and heat transfer in rib-roughened rectangular channels have been investigated. The aspect ratio of the channels is 1–8, and ribs are attached to the wide channel walls in order to set up swirling motions. The geometries tested consist of channels having cross ribs, parallel ribs, cross V-ribs, parallel V-ribs, and multiple V-ribs (Swirl Flow Tube). The flow patterns were investigated using smoke wire visualization and LDV measurements. The smoke wire experiments have been performed at Re=1100 and the LDV measurements at Re=3000 at periodic fully developed conditions. The heat transfer and pressure drop are described by j and f factors for Reynolds numbers from 500 to 15 000. The distributions of axial mean velocity and turbulent fluctuations are strongly influenced by the secondary flows. Large mean velocities and small fluctuations are found in regions where the secondary flow is directed towards a surface, while small mean velocities and large fluctuations are found in regions where the secondary flow is directed away from a surface. The Swirl Flow Tube provides a significant increase in the j factor at Reynolds numbers from 1000 to 2000, but unfortunately also an increase in the f factor. At higher Reynolds numbers, the j and f factors of the Swirl Flow Tube are of the same order of magnitude as for the other rib-roughened channels. It is found that the flow direction in a channel with parallel V-ribs has important influence on the j/f ratio. At Reynolds numbers above 4000, this channel provides the highest j/f ratio if the V-ribs are pointing upstream; while it provides the lowest j/f ratio of all rib configurations, if the V-ribs are pointing downstream.     

Simulation of flow around a thin,flexible obstruction by means of a deforming grid overlapping a fixed grid

This paper presents a numerical method for simulation of coupled flows, in which the fluid interacts with a thin deformable solid, such as flows in cardiovascular valves. The proposed method employs an arbitrary Lagrangian–Eulerian (ALE) method for flow near the solid, embodied in the outflow domain in which the mesh is fixed. The method was tested by modelling a two‐dimensional channel flow with a neo‐Hookean obstacle, an idealization of the coupled flow near a cardiovascular valve. The effects of the Reynolds number and the dimensionless elastic modulus of the material on the wall shear stress, the size of the downstream reverse flows, and the velocity and pressure profiles were investigated. The deformation of the obstacle, the pressure drop across the obstacle, and the size of the top reverse flow increased as the Reynolds number increased. Conversely, increasing the elastic modulus of the obstacle decreased the deformation of the obstacle and the size of the top reverse flows, but did not affect the pressure drop across the obstacle over the range studied. Copyright © 2007 John Wiley & Sons, Ltd.     

Computational analysis and flow structure of a transitional separated-reattached flow over a surface mounted obstacle and a forward-facing step

Large-eddy simulation (LES) of transitional separating-reattaching flow on a two-dimensional square surface mounted obstacle and a forward facing step has been performed using a dynamic sub-grid scale model. The Reynolds number based on the uniform inlet velocity and the obstacle/step height is 4.5 × 10³. The mean LES results for both the obstacle and step flow compare reasonably well with the available experimental and DNS data.

The flow structures upstream of the surface-mounted obstacle (referred to hereafter as obstacle) and the forward-facing step (referred to hereafter as FFS) consist of unstable two-dimensional structures and coherent rib-shaped structures. These structures with the aid of 3D streamline visualisation strongly indicate that the upstream separation bubble is a closed one rather than an open one in the sense that there is little evidence to suggest that there is fluid injection from the upstream separation region into the downstream separated region for the two geometries. The spectra and time history for the velocities and pressure fields at locations immediately upstream of the obstacle and FFS (including the recirculation region) were analysed using both the Fourier and wavelet transforms and revealed the unsteady nature of the recirculation region upstream of the obstacle and FFS.

The transition process has been elucidated using both 2D and 3D flow visualisation of the flow. In both geometries (obstacle and FFS), the separated boundary layer downstream of the leading edge shows 2D nature and roll-up shortly downstream of the separation line leading to 2D K-H rolls to be shed from the leading edge. Coherent structures such as the λ-shaped and rib-like vortices commonly associated with a flat plate boundary layer and also found in the separated-reattached flow of a blunt leading edge plate aligned horizontally to a flow are not common in the separated-reattached flow over the obstacle and FFS.     

基于沿程坐标积分模式颗粒流与结构物阵列相互作用的数值模拟

近年来, 由颗粒物质流动主导的泥石流、滑坡等自然灾害评估及其防护工作越来越受到人们的关注. 本文基于沿程坐标积分模式建立了陡峭地形条件结构物作用下颗粒流运动的数值模型, 可以较为准确地表征陡峭地形情形结构物影响下颗粒流的流态特征和运动过程, 尤其是相互作用过程中激波结构的形成与演化, 颗粒流的反射、绕射和爬升等动力效应. 通过数值模拟研究了颗粒流与不同分布密度四面体结构物阵列相互作用时的流态演化与堆积形貌, 提出了新型偏转效率无量纲指标, 结合流通效率, 定量评估了四面体结构物阵列对颗粒流流通距离和侧向铺展特征的影响. 结果表明, 单个四面体结构物对颗粒流的作用包括耗散作用和偏转作用两种模式, 其中偏转作用尤为显著; 结构物阵列对颗粒流产生综合的耗散和偏转作用, 通过多级作用形成系列的弓形激波耗散颗粒流能量, 通过偏转作用分隔和改变颗粒流路径, 增强耗散作用, 调控堆积形态, 可望对下游地区产生显著的防护效果.      

多孔障碍物对预混火焰传播的影响

以甲烷为代表性气体，研究了半封闭管道中设置多孔障碍物对可燃气体爆炸火焰传播的影响，基于大涡模拟对实验进行了重现，对比了实验与模拟中火焰传播过程的形状、位置及速度，分析了模拟结果中火焰穿过障碍物前后的流场和表面积变化，给出了衡量火焰褶皱程度的指标及算法。结果表明:大涡模拟结果与实验结果有较好的一致性；火焰在存在障碍物的管道内传播，经历层流快速膨胀、受阻回流、湍流快速发展和脉动减速4个阶段，各阶段火焰依次分别呈现加速、减速、二次加速、二次减速的波动变化；当可燃气体在开口与点火位置同端的管道内爆炸，火焰在接近障碍物时，受管道封闭端和障碍物约束显著，而出现脉动回流现象；火焰穿过多孔障碍物后，传播速度骤升至峰值，较未穿过障碍物前的最大速度可增加58.7%；障碍物是导致火焰面破碎以及面积褶皱率增大的直接原因，火焰褶皱率最大可达44.8%，比未穿过障碍物前的最大褶皱率增大39.27%。     

Implementation and efficiency analysis of parallel computation using OpenACC: a case study using flow field simulations

The Open Accelerator (OpenACC) application programming interface is a relatively new parallel computing standard. In this paper, particle-based flow field simulations are examined as a case study of OpenACC parallel computation. The parallel conversion process of the OpenACC standard is explained, and further, the performance of the flow field parallel model is analysed using different directive configurations and grid schemes. With careful implementation and optimisation of the data transportation in the parallel algorithm, a speedup factor of 18.26× is possible. In contrast, a speedup factor of just 11.77× was achieved with the conventional Open Multi-Processing (OpenMP) parallel mode on a 20-kernel computer. These results demonstrate that optimised feature settings greatly influence the degree of speedup, and models involving larger numbers of calculations exhibit greater efficiency and higher speedup factors. In addition, the OpenACC parallel mode is found to have good portability, making it easy to implement parallel computation from the original serial model.