微尺度流道中流体流动前沿的喷泉流动仿真

采用经典G a lerk in有限元法和N ew ton-R aphson迭代法,配合适当的边界条件实现了微流道中牛顿流体和剪切变稀流体前沿喷泉流动的数值仿真。结果表明:两种流体在喷泉流动区域的速度压力分布规律相同,在微尺度条件下流体的本构特性对喷泉流动的形式影响较小,仅是剪切变稀流体的喷泉区域略大于牛顿流体。在微尺度条件下,喷泉效应仍然是一种纯流体动力学现象。表面张力对微流体前沿喷泉流动区域的速度压力分布影响很小,但其在流体前沿自由表面上产生的切向应力使流体前沿的形状产生较大变形,与理想的半圆形相差较远。     

Droplet Deformation and 2-D/3-D Marangoni Flow Phenomena in Droplets Levitated by Electric Fields

An integrated numerical model is presented for free surface phenomena and Marangoni fluid flows in electrically levitated droplets under both terrestrial and microgravity conditions. The model development is based on the boundary element solution of the Maxwell equations simplified for electrostatic levitation applications and the free surface deformation that is primarily caused from the surface Maxwell stresses resulting from the applied electric fields. The electric and free surface model is further integrated with a finite element model for the surface-tension-induced fluid flows in the levitated droplets. Both 2-D and 3-D fluid flow structures may be developed in the electrically levitated droplets depending on the applied laser heating sources. The integrated model is applied to study the electric field distribution, free surface deformation, and 2-D and 3-D internal fluid flow structures in normal and microgravity for single, symmetric two-beam, four-beam, and six-beam laser heating arrangements. Among these arrangements, the six-beam arrangement with equal heating intensity gives the smallest temperature difference and the smallest maximum velocity.     

Experimental study of surface deformation and flow pattern on buoyant-thermocapillary convection

Free surface deformation is one of the most important physical phenomena in fluids with free surface. In the present paper, convection and surface deformation caused by thermocapillary effect in a rectangular cavity were investigated. In ground experiments, the convection was also affected by gravity. The cavity has a horizontal cross section of 52mm×42mm and the thikkness of the liquid layer is 4mm. Temperature difference between two sides of the liquid layer was increased gradually, and the flow in liquid layer will develop from steady to unstable convection. An optical diagnostic system consisting of a revised Michelson interferometer with image processor was developed to study fluid surface deformation in convection, and the displacements of free surface oscillation were determined. PIV technique was adopted to observe the evolution of flow pattern, and the velocity fields were obtained quantitatively. The present experiments demonstrate that surface deformation is quite distinct in buoyant-thermocapillary convection. in order to understand the mechanism of buoyant-thermocapillary convection, not only the hydrothermal wave instability but also the surface wave instability should be discussed.     

Thermal imaging of hazardous organic fluids in concrete

Thermal imaging is a simple method by which it is possible to visualize volatile fluids in concrete specimens. For this purpose test specimens are split after being exposed to a volatile fluid, so that the fluid distribution at the split surface can be determined. This is possible because the region of the split surface which is saturated with the volatile fluid has a lower temperature than the dry region. The decrease in temperature in the fluid saturated region is caused by the evaporation of the fluid. A theoretical explanation of this phenomenon is given, and it may be used to calculate the fluid distribution provided that the temperature distribution at the split surface is known. It is also possible to predict the decrease in temperature, provided that the physical parameters of the concrete and fluid are known. The temperature distribution at the split surface is nearly constant with time. Therefore the fluid distribution can be observed over a certain period of time with high accuracy. The experimental investigations were carried out using water and different organic fluids in concrete with different moisture content.     

Interfacial capillary–gravity waves due to a fundamental singularity in a system of two semi-infinite fluids

The interfacial capillary–gravity waves due to a transient fundamental singularity immersed in a system of two semi-infinite immiscible fluids of different densities are investigated analytically for two- and three- dimensional cases. The two-fluid system, which consists of an inviscid fluid overlying a viscous fluid, is assumed to be incompressible and initially quiescent. The two fluids are each homogeneous, and separated by a sharp and stable interface. The Laplace equation is taken as the governing equation for the inviscid flow, while the linearized unsteady Navier–Stokes equations are used for the viscous flow. With surface tension taken into consideration, the kinematic and dynamic conditions on the interface are linearized for small-amplitude waves. The singularity is modeled as a simple mass source when immersed in the inviscid fluid above the interface, or as a vertical point force when immersed in the viscous fluid beneath the interface. Based on the integral solutions for the interfacial waves, the asymptotic wave profiles are derived for large times with a fixed distance-to-time ratio by means of the generalized method of stationary phase. It is found that there exists a minimum group velocity, and the wave system observed will depend on the moving speed of the observer. Two schemes of expansion of the phase function are proposed for the two cases when the moving speed of an observer is larger than, or close to the minimum group velocity. Explicit analytical solutions are presented for the long gravity-dominant and the short capillary-dominant wave systems, incorporating the effects of density ratio, surface tension, viscosity and immersion depth of the singularity.     

Structuralization of Magnetic Nanoparticles Induced by Laser Heating in Magnetic Fluids

The structuralization of magnetic particles in magnetic fluids due to the thermodiffusion induced by laser light illumination was experimentally observed in two types of magnetic fluids: one based on a mineral oil with magnetite particles covered by a monolayer of oleic acid as a surfactant and the other a kerosene-based magnetic fluid sterically stabilized by a double layer consisting of oleic acid and dodecylbenzenesulphonic acid (DBS). Forced Rayleigh scattering (FRS) showed different behaviors of magnetic particle structuralization in the observed magnetic fluids. While for the case of mineral oil-based magnetic fluids, there was observed a positive thermodiffusion (S > 0), an indication of negative thermodiffusion (S < 0) was observed in magnetic fluids based on kerosene. This was also confirmed by the time-dependent decay of a grating of magnetic particles. Numerical simulation of aggregation for the case of negative thermodiffusion was confirmed by the observed aggregation after laser illumination in kerosene-based magnetic fluids and enabled an estimated value of the negative Soret constant in the magnetic fluid studied (S ≈ −10⁻² K ⁻¹).     

切削液扰动对BTA深孔加工系统横向振动频率的影响

为探究切削液扰动下BTA深孔镗削系统横向振动频率的影响,通过建立BTA深孔镗杆系统计及内、外切削液流固耦合及其自由液面效应的横向振动模型,解析系统不同情况下的横向振动的一、二阶频率表达式,并以不同情况下,深孔镗杆内、外切削液的横截面面积及其对深孔镗杆的附加质量来表征对应的自由液面变化,通过计算,明确了BTA深孔镗杆的横向振动频率对切削液流速、轴向力的敏感性及其运动转换趋势。BTA深孔镗杆横向振动频率对轴向力的敏感性规律为:在共振脊区域,在内、外切削液都无自由液面时最大;内、外切削液都有自由液面时最小;在共振翅区域,只内切削液有自由液面时最大,内、外切削液都无自由液面时最小。BTA深孔镗杆横向振动频率对切削液流速的敏感性规律为:在共振脊区域,只内切削液有自由液面时最大,内、外切削液都无自由液面时最小;在共振翅区域,内、外切削液都无自由液面时最大,只内切削液有自由液面时最小。系统在切削液流速、轴向力达到临界等效值时,发生弯曲或屈曲;在静力失稳后,系统将会在更高的切削液流速值以混阶模态形式发生耦合颤振等复杂运动。该研究结果可为BTA深孔镗削加工的生产实践提供一定理论指导。     

基于有限元计算的磁流变液特性分析及其与基于磁偶极子模型结果的比较

采用ANSYS有限元软件,计算了磁流变液单链在剪切过程中的力学特性,并以此为基准分别与基于简化偶极子模型和基于不简化偶极子模型的结果进行了比较。结果表明,简化偶极子模型在描述磁流变液力学特性方面具有较高的精度,但由其得到的剪切屈服应力略低于有限元计算结果。这对于改进偶极子模型以建立磁流变液性能更精确快捷的多层次描述方法,进而设计高性能磁流变液具有重要的意义。     

Diode laser bending of tongues from slotted steel tubes

A high power diode laser was used to bend slotted tubes. Two stainless steel tubes, different in diameter but with the same thickness, were cut to generate a rectangular tongue on their cylindrical surface. Subsequently these tongues were formed by diode laser in similar process conditions (laser power, rim speed and focalization condition). Two finite element models were built to predict the laser forming processes for the two different tubes and a good agreement between numerical and experimental results was obtained. Even if the tubes behave in a similar way under laser exposure, a larger deformation of the tongue is present for the biggest tube. Moreover, the deformation history during the laser scan differs. However, in both cases, the single tongue deformed in analogy with a flat sheet metal, despite of the cylindrical surface.     

On-line surveillance of a dynamic process by a moving system based on pulsed digital holographic interferometry

A method based on pulsed digital holographic interferometry for the measurement of dynamic deformations of a surface by using a moving system is presented. The measuring system may move with a speed of several meters per minute and can measure deformation of the surface with an accuracy of better than 50 nm. The deformation is obtained by comparison of the wavefronts recorded at different times with different laser pulses produced by a Nd:YAG laser. The effect due to the movement of the measuring system is compensated for by digital processing of the different holograms. The system is well suited for on-line surveillance of a dynamic process such as laser welding and friction stir welding. Experimental results are presented, and the advantages of the method are discussed.     

Diffusion of a fluid through a spherical elastic solid undergoing large deformations

The diffusion of a fluid through a spherical elastic solid undergoing large deformation is described in this paper. The constitutive model used is the single-constituent model for diffusion of fluids in nonlinear elastic solids, originally presented by Baek and Srinivasa [S. Baek, A.R. Srinivasa, Int. J. Nonlinear Mech. 39 (2004) 201-218] and based on a variational method and on the assumption of continuity of chemical potential across the solid-fluid interface. The balance laws for a single continuum with mass diffusion are cast in spherical coordinates, and suitable boundary conditions are posed to describe the radial diffusion of fluid through an elastic spherical shell with finite thickness. Its inner surface is adjacent to a rigid wall, either impermeable or permeable, while the outside surface is in contact with the fluid that swells the solid, diffuses through it, and exerts a hydrostatic pressure on its surface.     

弹性圆柱薄壳在流体中的变形与内力分析

应用薄壳弹性理论及流体力学基本方程,给出了圆柱壳在流体中流固耦合问题的基本关系式。阐述并应用相容拉格朗日-欧拉方法推导出流体绕壳横向流动时弹性圆柱薄壳的小弯曲变形问题的方程,并求解了流场的速度势和壳体的变形及内力分布。通过具体算例,绘出了流场分布图,讨论了有关参数变化对圆柱壳变形和表面压力系数的影响。     

Dispensing and surface-induced crystallization of zeptolitre liquid metal-alloy drops

The controlled delivery of fluids is a key process in nature and in many areas of science and technology, where pipettes or related devices are used for dispensing well-defined fluid volumes. Existing pipettes are capable of delivering fluids with attolitre (10-18 l) accuracy at best. Studies on phase transformations of nanoscale objects would benefit from the controlled dispensing and manipulation of much smaller droplets. In contrast to nanoparticle melting whose fundamental pathway was studied extensively, experiments on crystallization, testing classical nucleation theory, are hindered by the influence of support interfaces. Experiments on free-standing fluid drops are extremely challenging. Here, we demonstrate the operation of a pipette, which, observed by transmission electron microscopy, delivers a metal-alloy melt with zeptolitre (10-21 l) resolution. We use this exquisite control to produce nearly free-standing Au72Ge28 drops suspended by an atomic-scale meniscus at the pipette tip, and to image their phase transformations with near-atomic resolution. Our observations of the liquid-solid transition challenge classical nucleation theory by providing experimental evidence for an intrinsic crystallization pathway of nanometre-sized fluid drops that avoids nucleation in the interior, but instead proceeds via liquid-state surface faceting as a precursor to surface-induced crystallization.     

Inverse method of identification for three-dimensional subsurface cracks in a half-space

A procedure is presented which is well suited for three-dimensional subsurface crack identification in a half-space through the inversion of measured surface displacements. The investigation began with the linear, forward problem of generating contour maps of surface deformation produced by a fracture of known geometry and loading which is embedded in a finite medium. The fundamental solutions for tensile and shear multipoles in a half-space provided an efficient mathematical representation of the three-dimensional fracture. The inverse problem of crack identification centers on the development of a hybrid of the Marquardt–Levenberg algorithm. Initial guesses for the constrained set of search variables were determined heuristically from the correspondences between crack geometry and loading and the resulting uplift at the free surface. Physical measurements of surface deformation were taken for a cube of transparent acrylic polyester in which a fracture was hydraulically pressurized. Displacements induced at the surface of the specimen, which were measured by laser interferometry, had a strong correlation with predictions of the computational model (coupled with a finite element discretization). Numerical tests demonstrate the robustness of the inverse methodology even in the presence of the random and systematic errors corresponding to the experimental interferometric measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Engineering the surface texture and shape of channels in ceramic substrates

Ceramic/ceramic joining can transform surface channels on a ceramic subcomponent into an interior channel. Interior channels have a number of potential uses in advanced ceramic applications, including application as conduits of a variety of fluids, such as fuel, medicine, or cooling fluids. For cooling fluids in particular, the surface texture and shape of the channel can greatly affect its heat transfer and fluid flow characteristics. This paper explores means of “engineering” the surface textures and shapes of such channels, including the fabrication of channels with circular and non-circular cross-sections as well as channels with smooth or textured walls (and channels with a combination of smooth and textured regions). The microstructure of the channels is analyzed using laser scanning confocal microscopy (LSCM). Although LSCM has been used extensively in biological studies, this study is one of the first studies to apply LSCM as a tool to characterize three-dimensional shapes and textures in ceramic materials.     

脉冲激光作用Al2O3的表面形貌演变过程

激光表面加工是一种能够有效降低陶瓷材料表面粗糙度的方法,为了研究脉冲激光与陶瓷材料作用后表面形貌演变过程及激光工艺参数对表面粗糙度的影响,本文通过有限元法建立了瞬态二维轴对称数值模型,研究了单脉冲、多脉冲激光作用于氧化陶瓷表面形貌的演变过程及激光工艺参数对表面粗糙度的影响。结果表明：单脉冲激光对材料表面具有显著的平滑效果;多脉冲激光加工在脉冲2～3次时具有最好的平滑效果,继续增加脉冲数量,熔池内的表面轮廓开始向下凹陷,表面平滑效果减弱;采用不同工艺参数加工时,过大的频率、脉宽、激光功率都会导致材料表面粗糙度增加。当表面温度处于熔化温度与蒸发温度之间,熔池内流体以毛细力为主要驱动力,促使表面逐渐平滑;当表面温度超过蒸发温度时,熔池内流体受反冲压力挤压,促使表面粗糙。实验与模拟结果对比表明,该模型具有较高的准确性。     

Control of bio-MEMS surface chemical properties in micro fluidic devices for biological applications

Surface chemistry of silicon/glass based bio-MEMS was controlled by depositing plasma polymerized acrylic acid (ppAc) films at two different electrode positions in a two-stage plasma reactor. AFM and XPS were used to characterize the surface roughness and surface chemistry of the films, respectively. The surface of bio-MEMS was highly functionalized with carboxylic/ester functionalities with a very good surface uniformity. The proportion of carbon atoms as C-OX, C(==O)OX functionalities was decreased and an increase in C==O functionalities was observed when the electrode position was increased from the mesh. These functionalized bio-MEMS devices have advantages in fabrication of reusable micro fluidic devices and the variation of fluid velocity by changing the surface properties may be used to develop a micro-mixing system to control the mixing ratio of different fluids for different biological and chemical applications.     

The manipulation of carbon nanotubes on a polymer surface using a laser beam

Controlled noninvasive manipulation of porphyrin-doped single-walled carbon nanotubes (SWCNT) by laser beam is described. SWCNT/porphyrin complexes have been deposited on a polymer surface and irradiated by a scanning beam of laser light with the wavelength of 405 nm. Laser energy was absorbed by the porphyrin and converted into heat through an energy transfer within the complexes. This led to periodical deformation of the initially flat polymer surface. As a result of the surface deformation the SWCNTs or SWCNT bundle move in the direction given by the laser scanning. It was proved that SWCNTs can be moved to a desired position using the focused laser beam.     

Modal frequency degeneracy in thermally loaded optical resonators

We observe power coupling from the fundamental mode to frequency-degenerate higher-order spatial modes in optical resonators illuminated with a 30 W laser. Thermally-induced modal frequency degeneracy facilitates power transfer from the fundamental mode to higher-order modes, reduces power coupling into the cavity, and triggers power fluctuations. Modeling thermoelastic deformation of a mirror's surface shows predicted modal frequency degeneracy to be in reasonable agreement with experimental observations. Predictions for the Laser Interferometer Gravitational-wave Observatory (LIGO) show that the circulating fundamental-mode power necessary for gravitational-wave detection is compromised at coating absorptions of 3.8 and 0.44 ppm for Enhanced and Advanced LIGO Fabry-Pérot cavities, respectively.     

Transport of organic fluids through concrete

The absorption of fluids in concrete is influenced by various parameters such as the surface tension and dynamic viscosity of the fluid, and also the porosity, pore size distribution and the interconnection of the pores in concrete. In addition, chemical reactions between the penetrating fluid and concrete may occur, which affect the absorption behaviour. Chemical reactions can cause deviations from the square-root-of-time relation and a reduction of sorptivity. Both are observed if using water or ethylene glycol as testing fluids. The deviation from the square-root-of-time relation is caused by the dissolving of Ca(OH)₂ from the cement matrix by the absorbed fluid. In the case of water, the reduction of the sorptivity is due to rehydration and wetting expansion. If a fluid is being absorbed in concrete, the effective porosity is smaller than the available porosity. The greater the surface tension of the fluid, the greater the measured effective porosity. This can be explained by trapped air inside the pores, which cannot get out and is being compressed by the capillary pressure, either in dead-end pores or due to fingering of the penetrating fluid. Experimental results on sequential absorption of two different fluids in concrete can only be explained if a trapped pore volume is assumed.