Distribution of eddy viscosity and mixing length in a non-Newtonian flow of a solid particle-gas suspension

The present work concerns the study of the radial distribution of eddy viscosity and mixing length and their dependence upon the Reynolds number and the concentration of the solid phase in a non-Newtonian flow of a suspension of solid particles in a gas. The investigated systems have a pseudoplastic character and the deviation from Newtonian behaviour increases with an increase in the concentration of the dispersed phase. Relations are presented for the eddy viscosity and mixing length in the flow of pseudoplastic fluids. From the analysis of results it follows that the mixing length and eddy viscosity increases with an increase in the Reynolds number. In contrast, an increase in the concentration of the solid phase and consequently of the pseudoplasticity causes a decrease in the investigated quantities. The radial distribution of the mixing length and the eddy viscosity is characterized by a maximum, after which the investigated quantities vary only slightly. This enables the area of the core of the turbulent flow to be defined. E Non-dimensional eddy viscosity - K fluid consistency defined by Ostwald-De Waele's formula (power law) - K fluid consistency, eq. (12) - L mixing length - L _t non-dimensional mixing length - N ⁺ position parameter, eq. (3) - n power-law index - n ⁺ Reichardt's position parameter, eq. (4) - n slope of the dependence ln _w = f[ln (8w/D)] - R pipe radius - r radial distance from the pipe axis - Re=D W /µ Reynolds number - U ⁺ non-dimensional local mean axial velocity, eq. (2) - u ^* = w (/8)^0.5 friction velocity - non-dimensional local mean axial velocity - local mean axial velocity - u turbulence velocity component - mean axial velocity at pipe axis - w average velocity over cross-section of pipe - loading ratio of solid to air, i.e. ratio of mass flow rates of solids to air - Y _m ⁺ =Rⁿ u^2–n /K 8^n–1 non-dimensional distance of pipe centre from the wall - y distance from pipe wall - y ⁺ non-dimensional distance from pipe wall, eq. (5) - friction factor - µ _L laminar part of viscosity - µ _t eddy viscosity - density - shear stress - _w shear stress on the wall     

Fluid-Damping Controlled Instability in Tube Bundles Subjected to Air Cross-Flow

There are different excitation mechanisms that cause fatal damages due to undesirable vibrations in heat exchanger tube bundles subjected to cross-flow. One of them is the fluid-damping-controlled instability (galloping) that is characterised by a sudden appearance of large amplitudes of the tubes exclusively in cross-flow direction. This paper reports on investigations using an experimental set-up in a wind tunnel where the galloping mechanism in a tube bundle can be observed as an isolated phenomenon. The apparatus allows to realise several tube bundle configurations and geometry's of real heat exchangers. The position of a flexible test tube with a linear iso-viscoelastic mounting inside the tube array is variable. The test tube is equipped with dynamical pressure sensors which are placed directly under pressure holes inside the tube. For the investigation of the acting fluid forces the non-stationary pressure distribution is measured simultaneously at 30 points on the circumference in mid plane and at 15 points in line along the tube together with the tube motion. The acting fluid forces are determined by integration of the whole pressure field process. The study gives insights into the effect of the fluid-damping-controlled instability that is still not fully understood. Moreover, a flow visualization gives an impression of the mechanism at relevant Reynolds-numbers. The results show that in case of instability due to galloping the correlation length of the forces acting along the tube axis increases suddenly to large values. The fluid forces are correlated well for the whole tube when galloping is dominant. The exciting fluid forces show harmonic character and lead to a classical resonance behaviour. Instead of a simple free vibration test in vacuum or still air, which is done mostly for fluid excited structures, the damping coefficient of the oscillating system is determined under operating conditions on the basis of the measured fluid forces. A comparison of the results with those of a free vibration test in still air is shown. This revised version was published online in July 2006 with corrections to the Cover Date.     

磁探针速度传感器研究

应用Hopkinson杆装置,对磁探针速度计进行了实验测定。发现对待测自由面速度阶跃,磁探针有两种类型稳定的输出信号,分别对应于不同的速度范围。对某个特定磁探针来说,其初始信号峰值与速度成正比的条件是速度大于某个阈值。实验还测定了间距大小对磁探针输出信号的影响,给出了磁探针对阶跃实验的响应波形。基于实验条件,本文探讨了磁探针原理、测量电路特性以及靶板涡流的穿透深度等理论问题,得到的结果可以定性解释磁探针的两种特征信号以及它们间的转化机理。本文工作表明,磁探针可以用来定量测量自由面速度的初始阶跃幅度,准确测定全时域的速度阶跃时刻,但不足以确定整个速度历史。为使这种传感器更加成熟,其结构和线路有待改进。     

立交桥多跨连续梁复位监测

某立交桥在长期使用中,弯桥处多跨连续梁发生横向滑移近40cm,为此对该梁进行顶升和横移的复位施工。使用位移传感器对多跨连续梁的顶升和降落进行监测和控制,同时使用应变片监测多跨连续梁的应力。监测结果表明,相邻桥墩处的连续梁最大竖向位移差在3.40～7.89mm之间,连续梁产生的最大附加应力在-2.06～1.78MPa之间,连续梁没有裂缝产生,从而确保了多跨连续梁在复位施工中的安全。同时对钢支架的应力进行监测,得出钢管混凝土柱始终处于严重偏心受压状态的结论。在长时间监测中,采取分段测试然后测试数据累加的方法,减小了应变仪连续使用中的漂移问题。该桥的监测方法也可以用于其它桥梁的施工中。     

PVDF在动态应变测量中的应用

研究了用PVDF压电薄膜进行动态应变测量的原理和方法,在单向应变块上对两种PVDF应变片的应变电荷常数进行了标定,并对一维应力杆中的应变波形进行了测量。实验给出了较好的测量精度。研究结果表明,PVDF应变片在动态应变测量中具有很好的应用潜力。     

A Variational Principle for the Revised Goodman–Cowin Theory with Internal Length

In the present study a variational principle is proposed for the revised Goodman–Cowin theory with internal length for cohesionless granular materials (Fang et al. in Continuum Mech Thermodyn in press). The balance equations of the internal variables employed in the theory in equilibrium states, the equilibrium expressions of the constitutive variables and the corresponding natural boundary conditions are derived by use of the proposed variational principle for both cases of compressible and incompressible grains. It is demonstrated that the derived results coincide with those obtained by use of the thermodynamic analysis. The current work serves as a supplementary variational verification of the constitutive theory proposed in Fang et al. (in Continuum Mech Thermodyn in press).     

冠状毛细血管长度非线性变化及渗透作用下的毛细血管血液流动

建立冠状毛细血管内血流的血液动力学模型，研究毛细血管长度沿轴向的非线性变化及冠状毛细血管与周围组织的渗透和物质交换对血液流动的影响。考虑单根长弹性并具有通透性的冠状毛细血管，毛细血管长度非线性变化，毛细血管内血液为粘性不可压缩牛顿流体，跨壁滤过遵循Starling定律，血浆胶体渗透压和血浆蛋白浓度成线性关系。建立血管半径的无量纲方程，数值求解不同生理参数情况下，左心室壁不同层中毛细血管沿长度平均的无量纲半径和不同层毛细血管的平均流量。结果显示，冠状毛细血管与周围组织的渗透和物质交换及毛细血管长度沿轴向的非线性变化对平均无量纲半径和平均出口流量几乎没有影响；而对弹性模量较小的毛细血管，则对心脏收缩期时的平均进口流量有不同程度的影响。     

Dynamics of polydisperse hard-spheres under strong confinement

We use molecular simulation to probe the connection between local structure and the unusual re-entrant dynamics observed for polydisperse hard-sphere liquids confined in thin slit pores. The local structure is characterised by calculating 2-D bond-orientational order parameters associated with square and hexatic order for particles in the layer adjacent to the confining walls. When the wall separation is commensurate with the average particle size, the particles primarily exhibit local hexatic order, whereas local square order increases in prevalence for incommensurate geometries. The relaxation time extracted from the ensemble-averaged mean-square displacement increases exponentially with the static correlation length associated with hexatic local order in strongly confined commensurate geometries, in agreement with theoretical predictions for dynamical slowing. Square order, by contrast, is not associated with a growing length scale for either commensurate or incommensurate geometries, indicating that it is strongly geometrically frustrated. Our results suggest that the influence of bond-orientational order on dynamical slowing may be altered by changing the extent of confinement.     

Polymer physics inspired approaches for the study of the mechanical properties of amyloid fibrils

Amyloid structures constitute a class of highly ordered nanomaterials formed by insoluble protein aggregates. These aggregates are characterized by a cross‐β structural motif in which β‐sheets are oriented perpendicular to the fibril axis and bound together by a dense hydrogen bonding network. Although they have been associated with several neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases, amyloid fibrils have also been found in many physiologically beneficial roles, for instance in adhesives and hormone storage. Inspired by this natural occurrence of functional amyloid, the hierarchal self‐assembly of these structures has recently been used to develop artificial biomaterials for applications in medicine and nanotechnology. In order to realize the full potential of amyloids as functional materials, it is important to understand their fundamental mechanical properties. This review explores a range of experimental strategies to determine the mechanical properties of amyloid fibrils and discusses the results in the context of polymer physics concepts. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 281–292     

Effects of graft length and density of well‐defined graft polymers on the thermoresponsive behavior and self‐assembly morphology

A series of well‐defined thermoresponsive graft polymers with different lengths and graft densities, poly(glycidyl methacrylate)‐graft‐poly(N‐isopropylacrylate) (PGMA‐g‐PNIPAM), were successfully prepared by combination of controlled/living free radical polymerization and click chemistry. Effects of grafting length and density on the thermoresponsive behavior, aggregating mean diameter, and self‐assembly morphology are systematically investigated. The thermosensitive characteristics of graft polymers in aqueous solution prove that the length and graft density had positive co‐relationship with the lower critical solution temperature value and mean diameter of micelles as well as the size distribution, while the effect of graft length of polymers is more significant than that of density. Transmission electron microscopy analysis shows that the conformations of PGMA₄₅‐g‐PNIPAM₂₀ and PGMA₄₅‐g‐PNIPAM₄₆ with longer length and bigger grafting density in aqueous solutions are spherical nanoparticles with the increasing trend of the diameters, while that of PGMA₄₅‐g‐PNIPAM_{(73, 50%)} shows a spherical‐like morphology, which indicates that the graft length and density have a significant effect on the mean diameter of micelle but not on the self‐assembly morphology. These results reveal that to obtain desired thermoresponsive behavior and self‐assembly morphology of functional polymers, it is essential to design and fabricate the structure of graft polymers with proper length and graft density. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2442–2453