用胶囊式液晶粒子同时测量流场的温度和速度

胆甾相液晶在不同温度下显示不同的颜色，这一特性已被广泛地应用于固体表面或流体的温度测量．胶囊式胆甾相液晶具有粒子形态和温度－颜色反应特性，可以用来同时测量流体的速度和温度场．作者改进了粒子测速的测试精度；还标定了色度－温度关系，获得了自然对流流场中的速度和温度场     

A high-accuracy calibration technique for thermochromic liquid crystal temperature measurements

There are a variety of phenomena which may impact the accuracy of wide-band thermochromic liquid crystal temperature measurements, including: irregularities in liquid crystal and black paint layers, reflective components from light sources, and variations in the lighting/viewing angle across the surface. A wide-band calibration technique has been developed which inherently accounts for these and other sources of uncertainty by employing a point-wise calibration of the entire test surface. Both on and off-axis lighting arrangements are assessed for ease of implementation and accuracy of color displayed under uniform temperature conditions. The technique employs a series of uniform-temperature images to construct calibration curves relating the local hue component to temperature in a point-wise manner for the entire test surface. An off-axis lighting/viewing arrangement is found to be most practical for typical experimental setups. Hysteresis effects are quantified for excursions beyond both the lower and upper clearing point of the liquid crystals. Finally, the total uncertainty of the measured temperature is determined to vary from ±1.2% to ±7.2% across the bandwidth of the liquid crystals. Received: 21 October 1998/Accepted: 7 August 1999     

Temperature sensing with thermochromic liquid crystals

A review of the most recent developments in the application of thermochromic liquid crystals to fluid flow temperature measurement is presented. The experimental aspects including application, illumination, recording, and calibration of liquid crystals on solid surfaces, as well as in fluid suspensions, are discussed. Because of the anisotropic optical properties of liquid crystals, on-axis lighting/viewing arrangements, combined with in-situ calibration techniques, generally provide the most accurate temperature assessments. However, where on-axis viewing is not possible, calibration techniques can be employed, which reduce the uncertainty associated with off-axis viewing and lighting arrangements. It has been determined that the use of hue definitions that display a linear trend across the color spectrum yield the most accurate correlation with temperature. The uncertainty of both wide-band and narrow-band thermochromic liquid crystal calibration techniques can be increased due to hysteresis effects, which occur when the temperature of the liquid crystals exceeds their maximum activation temperature. Although liquid crystals are commonly used to provide time-mean temperature measurements, techniques are available which allow the monitoring of temporal changes. Selected examples illustrating the use of thermochromic liquid crystals are shown, and a survey of reported temperature measurement uncertainties is presented. Received: 3 February 1999/Accepted: 30 March 2000     

Flow visualization of a recirculating flow by rheoscopic liquid and liquid crystal techniques

Experiments were conducted in a three-dimensional lid-driven cavity flow to study the behavior of longitudinal Taylor-Görtler-like vortices. Flow visualization was accomplished by use of a rheoscopic liquid and of liquid crystals, together with laser-light and white-light sheets, respectively. Photographs of the lighted planes in the flow confirmed the existence of the vortices for a wide range of Reynolds numbers and for stable, neutrally-buoyant and buoyant global flow conditions. As usual the flow visualization revealed flow patterns not deducible by in situ measurements; the liquid crystal photographs give both flow pathlines and temperature distribution on any lighted plane.     

Color-based image processing to measure local temperature distributions by wide-band liquid crystal thermography

This study presents a color-image-processing procedure for non-intrusive local temperature measurements by thermochromic liquid crystals (TLCs). The image evaluation software is completely independent of the color detection and acquisition hardware. This allows to use a wide variety of hardware solutions. An easy reproducible calibration of camera and light source is presented. The dependence of the detected hue values on intensity is investigated and further the hueversus temperature relation is studied.Sprayable TLC formulations and TLC-coated polyester sheets are studied and compared with regard to their signal-to-noise ratio and the dependence of their hue values on illumination and viewing angle. Furthermore, a method to investigate the hue resolution is presented. The relation between the resolution of hue values and the illumination intensity and its influence on signal noise is discussed for the first time for TLC applications. Different techniques of signal noise reduction are implemented in the image processing system. Their effects on the signal noise level are discussed. As an example the two dimensional temperature distribution caused by wing-type vortex generators in a channel flow is given.     

Angular effects on thermochromic liquid crystal thermography

This paper directly discusses the effects of lighting and viewing angles on liquid crystal thermography. This is because although thermochromic liquid crystals (TLCs) are a widely-used and accepted tool in heat transfer research, little effort has been directed to analytically describing these effects. Such insight is invaluable for the development of effective mitigation strategies. Using analytical relationships that describe the perceived color shift, a systematic manner of improving the performance of a TLC system is presented. This is particularly relevant for applications where significant variations in lighting and/or viewing angles are expected (such as a highly curved surface). This discussion includes an examination of the importance of the definition of the hue angle used to calibrate the color of a TLC-painted surface. The theoretical basis of the validated high-accuracy calibration approach reported by Kodzwa et al. (Exp Fluids s00348-007-0310-6, 2007) is presented. This work was funded by the General Electric Aircraft Engines University Strategic Alliance Program.     

Digital particle image thermometry: The method and implementation

A computerized flow visualization technique capable of automatically quantifying the temperature field in a two-dimensional cross section of a flow field is described. The temperature sensors used are fast-response temperature-sensitive micro-encapsulated liquid crystal particles. Illuminating the flow by a thin sheet of white light, the reflected colors from the liquid-crystal particles were captured through a 3-chip video color camera and stored onto a videotape for subsequent data processing. The temperature field was obtained through an automatic color-temperature calibration scheme in HSI rather than RGB space, thus allowing for data processing of approximately one-third the time of RGB processing. The technique is finally applied to the study of a heated vortex-ring and some preliminary results are discussed.     

Influence of the optical configuration on temperature measurements with fluid-dispersed TLCs

An accurate temperature calibration of fluid-dispersed thermochromic liquid crystal (TLC) particles is an important prerequisite for quantitative liquid crystal thermometry (LCT) measurements in flows. Encapsulated TLCs are subjected to uniform and linear temperature fields and are illuminated with a sheet of white light. A digital camera records the color distribution reflected by the particles. For the first time, a telecentric objective is used to eliminate the angular dependence of the color within the image plane. The paper systematically assesses how the temperature calibration is affected by the angle between the camera axis and the light-sheet plane, and by the properties of the working fluid. The obtained results provide design criteria for quantitative LCT measurements in situations where small spatial variations of the fluid temperature need to be resolved, namely for turbulent heat transfer problems in wall-bounded flows. Received: 22 January 2001/Accepted: 16 October 2001     

Abnormal rotation of a deformed liquid crystal droplet immersed in an isotropic fluid after transient flow

The morphology evolution of liquid crystal droplets immersed in an isotropic fluid in flow field is found to be different from flexible polymer droplets. In this paper, we investigated the retraction of a liquid crystal droplet after transient flow. It is found that the liquid crystal droplet will rotate during the shape recovery, which has never been observed for an isotropic droplet. The factors that influence the rotational angle of a single liquid crystal droplet during retraction progress were studied, including the temperature, the dimension of the droplets, the time of shear flow, the shear rate, the flow type, and the properties of liquid crystal molecules. The rotation of liquid crystal droplet during shape recovery is ascribed to both the bulk elasticity of liquid crystal droplets and the anisotropic properties of the interface between liquid crystal and isotropic fluid.     

Topological structure bifurcation of temperature field of deformable drop in Marangoni migration

The unsteady processes of the Marangoni migration of deformable liquid drops are simulated numerically in a wider range of Marangoni number (up to Ma = 500) in the present work. A steady terminal state can always be reached, and the scaled terminal velocity is a monotonic function decreasing with increasing Marangoni number, which is generally in agreement with corresponding experimental data. The topological structure of flow field in the steady terminal state does not change as the Marangoni number increases, while bifurcation of the topological structure of temperature field occurs twice at two corresponding critical Marangoni numbers. A third critical value of Marangoni number also exists, beyond which the coldest point jumps from the rear stagnation to inside the drop though the topological structure of the temperature field does not change. It is found that the inner and outer thermal boundary layers may exist along the interface both inside and outside the drop if Ma > 70. But the thickness decreases with increasing Marangoni number more slowly than the prediction of potential flow at large Marangoni and Reynolds numbers.     

Measurement of surface shear stress vector distribution using shear-sensitive liquid crystal coatings

The global wall shear stress measurement technique using shear-sensitive liquid crystal (SSLC) is extended to wind tunnel measurements. Simple and common everyday equipment is used in the measurement; in particular a tungsten-halogen light bulb provides illumination and a saturation of SSLC coating color change with time is found. Spatial wall shear stress distributions of several typical flows are obtained using this technique, including wall-jet flow, vortex flow generated by a delta wing and junction flow behind a thin cylinder, although the magnitudes are not fully calibrated. The results demonstrate that SSLC technique can be extended to wind tunnel measurements with no complicated facilities used.     

Nonsteady-state nonisothermal flow of a magnetic liquid in a plane channel

Magnetic liquids are finding wider and wider use in various fields of technology [1]. Such liquids can be used as heat exchange fluids in equipment which generates a magnetic field under conditions of weightlessness [2] and in a number of other applications. The efficiency of heat exchange equipment is determined to a significant degree by the temperature of the magnetic liquid. In connection with this fact, it is of interest to examine nonisothermal flows at a temperature near the Curie point, where the dependence of volume magnetization M on temperature is expressed most clearly. In this case the character of the liquid flow will be affected not only by the dependence of saturation volume magnetization on temperature, but also by temperature inhomogeneity caused by development of external heat sources and sinks produced by the magnetocaloric effect. We note that although this is a weak effect [3], the temperature redistribution over channel section which it produces may be significant. With a high gradient in the external magnetic field H even a small change in temperature can significantly change the force acting on a magnetic liquid element. The unique features of magnetic liquid flow at a temperature close to the Curie point can be investigated by simultaneously solving the equations of motion and thermal conductivity.Translated from Zhurnal Prikladnoi Mekhaniki i Technicheskoi Fiziki, No. 1, pp. 93–96, January–February, 1984.The author expresses his gratitude to the participants in K. B. Pavlov's scientific seminar for their evaluation of the study.     

Viscosity of some clay-based coating colors at high shear rates

The shear viscosity of clay-based coating colors containing latex and carboxymethyl cellulose (CMC) has been measured over a relatively large shearrate region. In the shear-rate range of 50–1500 s^–1 the measurements were performed using a rotational viscometer and, at higher shear rates extending into the region 10⁵ – 10⁶ s^–1, a high pressure capillary viscometer was employed. The viscosity of the clay colors increased with increasing CMC-concentration, but the influence of the CMC-content was less pronounced at higher shear rates. The apparent shear-thinning behavior of the investigated colors could, in part, be attributed to the shear-thinning of the corresponding polymer (CMC) solution constituting the liquid phase of the color, but the influence of another factor was also indicated. At low shear rates, the interaction between the color components can produce relatively high viscosity levels, but in the high shear rate region these interactions appear to be less important for the viscosity level. It is also of interest to note that the viscosity dependence on the solids content in the high shear-rate region could be described with reasonable accuracy using an empirical equation neglecting interactions between the color components.     

白光散斑照相术的部分相干理论及其应用

本文从光的相干性的角度出发,用部分相干理论分析了白光散斑照相过程。讨论了影响象面散斑对比度的诸因素,并对我们在此理论指导下完成的部分实验工作作了简介。     

The effects of forced boundary conditions on flow within a cubic cavity using digital particle image thermometry and velocimetry (DPITV)

Buoyancy-driven convection within a cavity, whose sidewalls are heated and cooled, is a problem of great interest, because it has applications in heat transfer and mixing. Most studies to date have studied one of two cases: the steady-state case or the development of the transient flow as it approaches steady state. Our main concern was to study the response of the cavity to time-varying thermal boundary conditions. We therefore decided to observe the flow phenomena within a convection cavity under sinusoidal thermal forcing of the sidewalls. To map the flow properly, it is necessary to have simultaneous kinematic and thermal information. Therefore, the digital particle image thermometry and velocimetry (DPITV) is used to acquire data. Implementing this technique requires seeding the flow with encapsulated liquid crystal particles and illuminating a cross section of the flow with a sheet of white light. Extraction of the thermal and kinematic content is in two parts. For the first, the liquid crystals will reflect different colors of the visible spectrum, depending on the temperatures to which they are subjected. Therefore, calibrating their color reflection with temperature allows for the extraction of the thermal content. For the second part, the kinematic information is obtained through the use of a digital cross-correlation particle image velocimetry technique. With the use of DPITV, the flow within a convection cavity is mapped and studied under steady forcing and sinusoidally forced boundary conditions at the Brunt-Väisälä frequency. For the sinusoidally forced case, three cases are studied. In the first, the heating between the two walls is in phase. In the second, the heating between the two walls is 180° out of phase. In the third, the heating between the two walls is 90° out of phase. For steady forcing, the thermal plots show that the flow develops a linearly stratified profile within the center of the cell. At the sidewalls, however, owing to forcing, hot/cold thermal boundary layers develop at the left/right walls. These hot/cold thermal boundary layers then turn around the upper-left/lower-right corners and develop into intrusion layers that extend across the top and bottom walls. The vorticity and streamlines show that the bulk of the fluid motion is concentrated around the walls, whereas the fluid within the center of the cell remains stationary. For the sinusoidally forced cases, the thermal plots show the existence of many thermal “islands,” or pockets of fluid where the temperature is different with respect to its surroundings. The vorticity plots show that the center of the cell is mostly devoid of vorticity and that the vorticity is mainly confined to the sidewalls, with some vorticity at the top and bottom walls. For the 0° forcing, the streamlines show the development of two counterrotating rollers. For the 180° forcing, the streamlines show the development of only one roller. Finally, for the 90° forcing, the streamlines show the development of both a two-roller and a one-roller system, depending on the position within the forcing cycle.     

Liquid crystal techniques of visualization in rotating annulus experiments

To the well-known rotating annulus experiments we applied liquid crystal techniques of visualization in order to obtain clear video-pictures of internal flow and temperature in the fluid. Then we developed the idea of simultaneously injecting several types of liquid crystals of different temperature ranges to observe the fluid with a wide temperature range. It was shown that with this idea it was possible to take clear video-pictures throughout the whole interior of the fluid. This revealed that the pattern of the bottom flow does not have the characteristics of the Eady type baroclinic waves. Furthermore, the typcial meridional gradient of temperature of the baroclinic wave was directly observed from isothermal lines appearing in the fluid as colour band lines.     

On rod-like polymers

Recent work on suspensions of rigid rod particles and molecules has prompted a search for some water soluble rigid-rod polymers and for a quick test of rod likeness.The existence of a liquid crystal phase is a good indication of the rod likeness of a polymer. A convenient, fast test for a liquid crystal phase involves drying of a small dilute sample of the polymer solution which is sandwiched between a glass slide and coverslip. The resulting sample is then examined by polarised light microscopy.The chemical structure of gums has lead to an examination of several samples of plant gums. A cheap, safe gum derived from seaweed (-carrageenan) was found to be rod-like in aqueous solution; -carrageenan exhibits a liquid crystal phase at a concentration of 12.5 g/100 ml. A liquid crystal phase is not observed in gums with linear backbones which have even short branches, leading to the conclusion that these can not be regarded as being rod-like in solution.     

The effect of liquid crystal structure on the rheological properties of nitrocellulose-dimethylacetamide solutions

Solutions of Pyro grade nitrocellulose (NC) in dimethyl acetamide (DMA), containing between 42.5% and 60% NC (w/w), have been studied by differential scanning calorimetry and polarised light microscopy. The results showed that NC forms a lyotropic liquid crystal structure in DMA. A transition from the liquid crystal phase to an isotropic phase occurred over the temperature range 27 °C to 67 °C, and the enthalpy of transition increased with NC concentration. Rheological properties were determined using an extrusion rheometer with a slit die. The solutions were shown to have a yield stress for flow which increased with increasing NC concentration. The solutions were also found to be thixotropic.     

一种岩石断面形貌间接描述的新方法研究

基于图像图形学的YUV颜色模型理论,本文提出了一种间接描述岩石断面形貌的新方法,即YUV维数法。选定一张有m个像素点的彩色数码照片,以每个像素点的Y(灰度)、U、V(色彩和色饱和度)分量分别构造欧氏空间中对应点的z、x、y坐标分量。在每个像素点处均采用上述构建方法,可在欧氏空间中确定m个点(与像素个数相同),连接构造的m个点,即可构建粗糙的YUV表面。采用分形布朗运动理论,导出了n-1~n维分形体的自仿射维数计算方法,并特例化至2~3维。求解YUV表面的自仿射维数,该维数即为YUV维数。为验证YUV维数方法的可行性,分别完成了同一岩石断面的YUV维数数值试验、激光表面仪扫描试验、扫描电子显微镜扫描试验,并采用YUV维数、普通维数和灰度维数分析同一试件的表面形貌特征。对比分析显示,相同试件的YUV维数与普通维数基本相同,且与灰度维数总体趋势近似一致。这证实了YUV维数方法的可行性。另外,计算YUV维数过程中,由于采集初始数据(彩色照片)的设备是数码相机,所以它克服了普通维数的缺陷。同时,YUV模型表征的真彩图片颜色包含了颜色的灰度、色彩和色饱度,而灰度图仅保存了灰度信息。所以,YUV维数比灰度维数更全面充分揭示了图像的分形几何特征。总之,YUV维数有着普通维数和灰度图维数无法比拟的优越性,是一种间接描述表面形貌描述的新方法。     

Mechanics of mechanochemically responsive elastomers

Mechanochemically responsive (MCR) polymers have been synthesized by incorporating mechanophores – molecules whose chemical reactions are triggered by mechanical force – into conventional polymer networks. Deformation of the MCR polymers applies force on the mechanophores and triggers their reactions, which manifest as phenomena such as changing colors, varying fluorescence and releasing molecules. While the activation of most existing MCR polymers requires irreversible plastic deformation or fracture of the polymers, we covalently coupled mechanophores into the backbone chains of elastomer networks, achieving MCR elastomers that can be repeatedly activated over multiple cycles of large and reversible deformations. This paper reports a microphysical model of MCR elastomers, which quantitatively captures the interplay between the macroscopic deformation of the MCR elastomers and the reversible activation of mechanophores on polymer chains with non-uniform lengths. Our model consistently predicts both the stress–strain behaviors and the color or fluorescence variation of the MCR elastomers under large deformations. We quantitatively explain that MCR elastomers with time-independent stress–strain behaviors can give time-dependent variation of color or fluorescence due to the kinetics of mechanophore activation and that MCR elastomers with different chain-length distributions can exhibit similar stress–strain behaviors but very different colors or fluorescence. Implementing the model into ABAQUS subroutine further demonstrates our model's capability in guiding the design of MCR elastomeric devices for applications such as large-strain imaging and color and fluorescence displays.