流体双折射三维流场显示测试系统及应用研究

本文简要介绍流体双折射三维流场显示测试系统的研制和控制特点,流体双折射介质特性,光力学特性及对水下运动物体的应用研究.     

应用流体双折射方法计算二维二尖瓣模型的流场

本文运用流体双折射的实验数据与数值差分计算的杂交法对模拟二尖瓣模型的二维稳态流场进行定量计算,得到流场的流函数、速度等流场参量。     

流动双折射实验

流动双折射实验周建和，周兴华，张海荣（天津大学力学系天津３０００７２）流动双折射法，是流动显示中的一种光学方法，主要用于显示低雷诺数下的剪切流动。它能形象直观地显示流场中切应力的分布和切应力的方向。通过对干涉条纹的分析处理，可对流场实现定量测量。尤其...     

流动双折射实验

流动双折射实验周建和，周兴华，张海荣（天津大学力学系天津３０００７２）流动双折射法，是流动显示中的一种光学方法，主要用于显示低雷诺数下的剪切流动。它能形象直观地显示流场中切应力的分布和切应力的方向。通过对干涉条纹的分析处理，可对流场实现定量测量。尤其...     

心血管内流场的高质量模拟实验研究（Ⅲ）：动态流体双折射法典型应用分析

用动态流体双折射法对三种有代表性的心血管模型（模拟大血管、动脉狭窄和人造心脏瓣膜）中的脉动血流间隙测试，得出速度剖面，剪应力分布，压力和流量之间的相位关系。结果说明本文所描述的双折射液和模拟血管是适用于心血管内流场的高质量模拟实验的，动态流体双折射方法及简化的光－力学关系是适用于低频脉动模拟血流的定量分析的。     

双折射材料初应力的定量研究相移等达因技术的应用

文章提出两种用相移等达因技术对存在于双折射材料内部的初始应力进行定量分析的方法，并以两种常用的双折射材料Ｐ６和Ｈｏｍａｌｉｔｅ１００为例进行了内部初应力的实验研究，给出了材料内部初应力的分布。以上研究说明双折射内部初始应力不容忽视，数字相移等达因技术是双折射材料内部初始应力进行定量分析的有力手段     

心血管内流场的高质量模拟实验研究（Ⅱ）——动态流体双折射实验技术…

本文重点介绍用于心血管内流场的高质量模拟的动态流体双折射实验技术和系统。首先制备了两种仿血双折射液和一种弹性透明的硅橡胶模拟血管，有关参数测定表明它们适用于模拟血流和血管。其次，为了消除光线穿过圆管产生的图象畸变，用光线追迹法推导出消畸方程并选用最佳匹配浸渍液消畸。最终设计建立了一台由微机控制的集流动、光学、图象与数据采集处理和同步控制子系统于一体的新型测量系统。     

流体双折射方法及其应用

本文在1923—1989年的工作基础上,论述了流体双折射方法,对它的发展和原理进行了详细讨论,介绍了流体双折射方法的各种应用,尤其是在生物力学,二维三维场显示方面的应用.     

心血管内流场的高质量模拟实验研究（Ⅱ）──动态流体双折射实验技术和实验系统

本文重点介绍用于心血管内流场的高质量模拟的动态流体双折射实验技术和系统。首先制备了两种仿血双折射液和一种弹性透明的硅橡胶模拟血管，有关参数测定表明它们适用于模拟血流和血管。其次，为了消除光线穿过圆管产生的图象畸变，用光线追迹法推导出消畸方程并选用最佳匹配浸渍液消畸。最终设计建立了一台由微机控制的集流动、光学、图象与数据采集处理和同步控制子系统于一体的新型测量系统。     

激波管实验研究非均匀流场R-M不稳定性

利用激波管装置及马赫数为1.27的弱入射激波实验研究了SF6非均匀流场的R-M不稳定性。Air/SF6初始正弦界面由厚度为0.5μm的薄膜相隔得到,由阴影方法记录界面演化过程。实验结果表明:由于不稳定性,重流体(SF6)向轻流体(Air)演化成"尖钉"结构,而轻流体演化为"气泡"结构;由于界面切向速度差的Kelvin-Helmholtz不稳定性,"尖钉"头部翻转成蘑菇头形状;由于流场密度分布不均,低密度区流场扰动增长较快,扰动振幅发展的实验结果与PPM数值计算的结果较吻合。     

Flow birefringence study of sharkskin and stress relaxation in polybutadiene melts

In this paper we report rheo-optical and rheological observations made through a transparent slit die attached to a capillary rheometer. We find that the flow birefringence signal oscillates periodically near the die exit when sharkskin-like extrudate distortion is present. In contrast, steady behavior is observed in the die inland region. Specifically, the flow birefringence varies at the die exit with a period identical to that measured directly from the sharkskin extrudate. We also show that the exit flow instability leading to sharkskin can be observed directly through cross-polarizers in terms of the temporal change of the retardation order. We demonstrate that the same kind of interfacial flow instability can occur at a boundary discontinuity within the die land where the upper portion of a clean die wall meets the lower portion of a polysiloxane-coated die wall. Finally, stress relaxation upon the cessation of the slit die flow of two polybutadiene melts is studied through time-dependent flow birefringence measurements. The stress relaxation is then correlated with sharkskin time scales to describe the role of relaxation in sharkskin ridge formation. Received: 8 February 1999 Accepted: 28 July 1999     

Flow of branched polymer melts in a lubricated cross-slot channel: a combined computational and experimental study

Numerical simulations have been performed to evaluate the accuracy of the multimode Giesekus model in predicting the flow behavior of a rheologically well characterized low-density polyethylene melt in a lubricated cross-slot channel. Specifically, the fidelity of the numerical results is established by detailed comparison with flow-induced birefringence measurements in a new optical rheometer with lubricated side walls that allows the creation of ideal two-dimensional flow kinematics that lead to the elimination of end effects commonly encountered in flow birefringence measurements. Based on these comparisons, the ability of the multimode Giesekus model to capture the flow characteristics with reasonable accuracy in the experimentally available Wi range of 21 to 29 has been established. However, it should be noted that the model predictions are, at best, qualitative in the vicinity of the stagnation point. The discrepancy between numerically predicted and experimentally observed stresses in this region is mainly attributed to the inaccuracy of the experimental data that stem from the occurrence of multiple orders of retardation within the measurement volume. Overall, these studies have paved the way for the development of a hi-fidelity lubricated cross-slot channel rheometer.     

Flow birefringence of disordered block copolymer melts

The birefringence relaxation after a step strain S(t), and the oscillatory flow birefringence S ^* are calculated for disordered block copolymer melts, on the basis of four models for the chain dynamics: the Rouse model, the Doi-Edwards reptation model, the reptation model with constraint release, and the reptation model with orientational coupling. All the calculations are performed in the mechanically-uniform limit, i.e., the average subchain length and friction coefficient are independent of block. The net birefringence is assumed to contain no form contribution, and the approach of Kuhn and Gruen is employed in the computation of the intrinsic birefringence. The most important feature of the results is that the stress-optic relation does not apply in general for block copolymers; therefore, unique information about chain relaxation mechanisms can be obtained from measurements of flow birefringence. It is shown that the phase angle of S ^* can be particularly sensitive to the lengths and chain locations of the various blocks, with the most striking effects occurring when two (or more) blocks have optical anisotropies of opposite sign. In contrast, in the mechanically-uniform limit the viscoelastic properties are independent of block length and location.Dedicated to Professor Arthur S. Lodge on the occasion of his 70th birthday and his retirement from the University of Wisconsin.     

Numerical evaluation of extinction angle and birefringence in various directions as a function of velocity gradient

Despite the limitations of the theory of Peterlin and Stuart it still seems to be the best theory for interpreting quantitative flow birefringence measurements performed on solutions of rigid spheroidal particles. First, we repeat the numerical evaluation of the extinction angleX and the orientation factorf _z corresponding to conventional observations realized in a Couette cell, but the values are now given with a higher degree of accuracy and the range of variation of the parameters has been extended. Furthermore, using the same theory, we extend our calculations to other directions of observation. This should contribute towards avoiding false interpretations of birefringence measurements in these directions (as has sometimes occurred). Finally, we present measurements taken along the lines of flow and along the direction of the velocity gradient, thus including the principal situations met in the various cells used in flow birefringence measurements.     

Experimental study of the induced flow birefringence of a suspension of rigid particles at the transition from Couette flow to Taylor vortex flow

The flow birefringence induced in solutions of rigid particles is studied experimentally in the region of the axisymmetrical Taylor vortex flow which arises once the velocity gradient G in the annular gap of a conventional Couette cell reaches a critical value G _c .The measurements are performed for several values of G > G _c and for 10 radial observation points in the annular gap. Solutions of two types of rigid particles are investigated: the first is a suspension of flattened clay particles like bentonite, while the second contains rod-like particles of tobacco mosaic virus (TMV). The variations of the birefringence intensity n and of the extinction angle measured in the domain of the axisymmetrical flow show a different behavior according to the shape of the particle in solution. This fact is confirmed theoretically with a good agreement for the measurements performed with solutions of flat particles.     

Flow birefringence and rheological measurements on shear induced micellar structures

Aqueous solutions of cationic surfactant systems with strongly binding counterions show the striking phenomenon of shear induced phase transitions. At low shear rates or angular frequencies, the solutions exhibit Newtonian flow. At high rates of shear, however, the rheological properties change dramatically. Above a well defined threshold value of the velocity gradient, a supermolecular structure can be formed from micellar aggregates. This shear induced structure (SIS) behaves like a gel and exhibits strong flow birefringence. The formation of the shear induced structure is very complicated and depends on the specific conditions of the surfactant system. In this paper we discuss new results which have been obtained from rheological measurements and from flow birefringence data. We examine the stability of the shear induced state as a function of temperature, surfactant concentration and salt concentration and we analyse the effect of solubilisation of alcohols and hydrocarbons. The results are interpreted in terms of a kinetic model which accounts for the observed behavior.Dedicated to the 60. birthday of Prof. H. Harnisch, Hoechst AGPartly presented at the 2nd Conference of European Rheologists, Prague, June 17–20, 1986     

Flow birefringence visualization of transitions in circular Couette flow

Flow birefringence technique is an experimental method in fluid dynamics from which we can obtain information about the velocity fields in various devices. This method has been used here to observe transitions which occur in the classical Couette device when the angular velocity of the inner cylinder increases beyond a certain critical value. Although earlier experiments have been reported, the observation of the flow birefringence phenomena in the entirely illuminated annular gap of a classical Couette cell consists in a new access to these problems and may well bring important information on the changes occuring in the hydrodynamical field at the different transitions.     

Large Eddy Simulation of Turbulent Flow Through Submerged Vegetation

Large Eddy Simulations (LES) are performed for an open channel flow through idealized submerged vegetation with a water depth (h) to plant height (h _p) ratio of h/h _p = 1.5 according to the experimental configuration of Liu et al. (J Geophys Res Earth Sci, 2008). They used a 1D laser Doppler velocimeter (LDV) to measure longitudinal and vertical velocities as well as turbulence intensities along several verticals in the flow and the data are used for the validation of the present simulations. The code MGLET is used to solve the filtered Navier–Stokes equations on a Cartesian non-uniform grid. In order to represent solid objects in the flow, the immersed boundary method is employed. The computational domain is idealized with a box containing 16 submerged circular cylinders and periodic boundary conditions are applied in both longitudinal and transverse directions. The predicted streamwise as well as vertical mean velocities are in good agreement with the LDV measurements. Furthermore, fairly good agreement is found between calculated and measured streamwise and vertical turbulence intensities. Large-scale flow structures of different shapes are present in the form of vortex rolls above the vegetation tops as well as locally generated trailing and von- Karman-type vortices due to flow separation at the free end and the sides of the cylinders. In this paper, the flow field is analyzed statistically and evidence is provided for the existence of these structures based on the LES.