The fluid hopkinson bar

Experiments in which pressure pulses are propagated in a column of fluid held in a stiff tube are described. A parameter, η, which characterizes the tube stiffness has been defined and a one-dimensional model of the wave propagation which includes dissipation both in the volume of the fluid and at the wall of the containing tube has been developed. It is found that dissipation at the wall dominates dissipation in the fluid volume for pulse lengths long compared to the tube radius. The experimental results delineate practical limits on the ratio of pulse length to tube radius for which the wave propagation can be characterized as one-dimensional. The validity of a one-dimensional representation of pulse transmission and reflection at a solid-fluid interface is also evaluated with the aid of experimental results. Finally, the dissipation model in combination with the experimental results leads to a simple expression for pressure pulse attenuation in terms of a nondimensional physical parameter, Ξ, and tube radius.     

Fins and turbulence promoters for heat transfer enhancement in latent heat storage systems

One of the serious problems associated with the operation of PCM storage system is the heat transfer in and out of the element containing the PCM. This paper presents the results of an experimental investigation of the effects of radial fins and turbulence promoters on the enhancement of phase change heat transfer external to a horizontal tube submersed in the PCM with the working fluid flowing through it. The experimental measurements were realized on a bare cupper tube and an identical cupper tube fitted with radial fins. The fins investigated are 40, 60, 120 and 180 mm diameters. A turbulence promoter made of stainless steel wire of 1.0 mm diameter coiled in a helical form with a pitch of 25.0 mm was inserted into the cupper tubes. The tests were realized on bare tubes, finned tubes and finned tubes with the turbulence promoter inserted into the finned tubes. The measurements were realized for the working fluid temperatures in the range of −10 °C, to −25 °C and six values of the mass flow rate ranging from 0.013 to 0.031 kg/s. The position of the phase interface was photographed by a high resolution digital camera and scanned to determine the real interface position by comparison with a precision measuring scale. The results of the phase interface position, velocity of the interface, solidified mass fraction and the time for complete solidification are presented in function of the working fluid temperature, the working fluid mass and the tube arrangements. The results are presented and discussed.     

On the Capillary Problem for Compressible Fluids

Classical capillarity theory is based on a hypothesis that virtual motions of fluid particles distinct from those on a surface interface have no effect on the form of the interface. That hypothesis cannot be supported for a compressible fluid. A heuristic reasoning suggests that even small amounts of compressibility could have significant effect on surface behavior. In an earlier work, Finn took a partial account of compressibility, and formulated a variant of the classical capillarity equation for fluid surface height in a vertical capillary tube; he was led to a necessary condition for existence of a solution with prescribed mass in a tube closed at the bottom. For a circular tube, he proved that the condition also suffices, and that solutions are uniquely determined for any contact angle γ. Later Finn took more complete account of compressibility and obtained a new equation of highly nonlinear character but for which the same necessary condition holds. In the present work we consider that equation for circular tubes. We prove that the necessary condition again suffices for existence when 0 ≤ γ < π, and we establish uniqueness when 0 ≤ γ ≤ π/2. Our result is put into relief by the observation that for the unconstrained problem of a tube dipped into an infinite liquid bath, solutions do not in general exist when γ > π/2. Presumably an actual fluid would in that case descend to the bottom of the tube. This kind of singular behavior does not occur for the equation previously considered, nor does it occur in the present case under the presence of a mass constraint.     

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。     

激波冲击下air/SF6界面的Richtmyer-Meshkov不稳定性

实验研究了低马赫数(1.27)激波作用air/SF6界面的RM不稳定性问题.air/SF6初始正弦界面由厚度为1～2 μm的薄膜相隔得到,用阴影法测试界面演化过程.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-Helm-ho...     

Comparison of spectral and finite element methods applied to the study of the core‐annular flow in an undulating tube

A Galerkin/finite element and a pseudo‐spectral method, in conjunction with the primitive (velocity‐pressure) and streamfunction‐vorticity formulations, are tested for solving the two‐phase flow in a tube, which has a periodically varying, circular cross section. Two immiscible, incompressible, Newtonian fluids are arranged so that one of them is around the axis of the tube (core fluid) and the other one surrounds it (annular fluid). The physical and flow parameters are such that the interface between the two fluids remains continuous and single‐valued. This arrangement is usually referred to as Core‐Annular flow. A non‐orthogonal mapping is used to transform the uneven tube shape and the unknown, time dependent interface to fixed, cylindrical surfaces. With both methods and formulations, steady states are calculated first using the Newton–Raphson method. The most dangerous eigenvalues of the related linear stability problem are calculated using the Arnoldi method, and dynamic simulations are carried out using the implicit Euler method. It is shown that with a smooth tube shape the pseudo‐spectral method exhibits exponential convergence, whereas the finite element method exhibits algebraic convergence, albeit of higher order than expected from the relevant theory. Thus the former method, especially when coupled with the streamfunction‐vorticity formulation, is much more efficient. The finite element method becomes more advantageous when the tube shape contains a cusp, in which case the convergence rate of the pseudo‐spectral method deteriorates exhibiting algebraic convergence with the number of the axial spectral modes, whereas the convergence rate of the finite element method remains unaffected. Copyright © 2002 John Wiley & Sons, Ltd.     

On the Equations of Capillarity

Some conceptual ambiguities in the derivation of the equations of capillarity on the basis of the principle of virtual work are addressed, and hypotheses are proposed toward obtaining a physically correct characterization in general circumstances. It is shown that under the hypotheses, the classical equations of capillarity for an interface of an incompressible fluid with a fluid of negligible density can be obtained on the basis of global phenomenological reasoning, without recourse to consideration of intermolecular attractions. More generally, the procedure is applied to derive the specific equations arising from a compressible fluid configuration with idealized pressure-density relationship in a capillary tube, and a general necessary condition for existence of a solution is established. It is shown that for symmetric domains, the condition is also sufficient for existence of a unique symmetric solution.     

Pore-Scale Simulation of Interphase Multicomponent Mass Transfer for Subsurface Flow

A simulation framework is proposed to simulate multicomponent multiphase flow in porous media at the pore scale. It solves equations for the species concentrations in the framework of the volume-of-fluid approach including thermodynamics equilibrium at the fluid/fluid interface. Particular attention is paid to the derivation of the boundary condition for the concentration at the solid walls. The method is validated by comparison with analytical solutions of simple setups. Then, the approach is used to investigate and upscale mass transfer across interfaces in different configurations, including the drainage of water in a tube by a gas carrying a contaminant, mass transfer in thin films, and mass transfer in complex porous structures under various flow conditions.     

Heat transfer in circular microchannels during volumetric heating with magnetic field

Convective heat transfer within circular microchannels in a rectangular solid substrate with heat generation due to imposed magnetic field was studied. A detailed parametric study was performed by varying Reynolds number, magnetic field strength, working fluid, and the diameter of the channel. It was found that the heat transfer coefficient decreases downstream along the channel. Nusselt number increased with Reynolds number. The tube diameter, properties of the working fluid, and magnetic field strength affected the temperature distribution and heat transfer rate at the solid-fluid interface.     

Modelling techniques for fluid–solid coupling dynamics of bundle tubes vibrating and colliding in fluids

Nonlinear vibrations that occur in such bundle structures caused collisions between tubes and cross flows of the surrounding fluid. This paper presents modeling techniques for simulating the FSI dynamics of bundle tubes vibrating and colliding in fluids. A typical configuration of a three-dimensional tube bundles submerged in fluid of a cylindrical container is studied. Coupling conditions of displacement, velocity and forces are considered on the fluid-structure coupling interfaces. Contacts boundary between tubes and topological domain changes of the fluid are also considered on the fluid-structure coupling interface. Modeling techniques and algorithm are then established for flow-induced vibrations of the tubes, and collisions between tubes in fluids. The examples are presented to demonstrate the effectiveness of the proposed techniques. It has confirmed that our code produces the correct physics of the FSI problem, and capable of revealing the complex nonlinear mechanism with solid-solid contacts together with fluid-solid interactions.     

非结构动网格在多介质流体数值模拟中的应用

采用非结构动网格方法对含多介质的流场进行数值模拟.采用改进的弹簧方法来处理由于边界运动而产生的网格变形.采用基于格心的有限体积方法求解守恒型的ALE(Arbitrary Lagrangiall-Eulerian)方程,控制面通量的计算采用HLLC(Hartem,Lax,van Leer,Contact)方法,采用几何构造的方法使空间达到二阶精度,时间离散采用四阶Runge-Kutta方法.物质界面的处理采用虚拟流体方法.本文对含动边界的激波管、水下爆炸等流场进行数值模拟,取得较好的结果,不同时刻界面的位置和整个扩张过程被准确模拟.     

Adaptive Lagrangian–Eulerian computation of propagation and rupture of a liquid plug in a tube

Liquid plug propagation and rupture occurring in lung airways can have a detrimental effect on epithelial cells. In this study, a numerical simulation of a liquid plug in an infinite tube is conducted using an Eulerian–Lagrangian approach and the continuous interface method. A reconstruction scheme is developed to allow topological changes during plug rupture by altering the connectivity information about the interface mesh. Results prior to the rupture are in reasonable agreement with the study of Fujioka et al. in which a Lagrangian method is used. For unity non‐dimensional pressure drop and a Laplace number of 1000, rupture time is shown to be delayed as the initial precursor film thickness increases and rupture is not expected for thicknesses larger than 0.10 of tube radius. During the plug rupture process, a sudden increase of mechanical stresses on the tube wall is recorded, which can cause tissue damage. The peak values of those stresses increase as the initial precursor film thickness is reduced. After rupture, the peaks in mechanical stresses decrease in magnitude as the plug vanishes and the flow approaches a fully developed behavior. Increasing initial pressure drop is shown to linearly increase maximum variations in wall pressure and shear stress. Decreasing the pressure drop and increasing the Laplace number appear to delay rupture because it takes longer for a fluid with large inertial forces to respond to the small pressure drop. Copyright © 2010 John Wiley & Sons, Ltd.     

高温下钢管自密实混凝土界面接触热阻试验研究

钢管混凝土结构的火灾性能研究通常要考虑钢管和混凝土的界面接触热阻问题,本文对钢管自密实混凝土柱的界面接触热阻进行了试验研究。制作了8个钢管自密实混凝土柱,依据Fourier定律和Newton冷却定律得到了钢管混凝土界面接触热阻的求解方法。利用有限元和多项式拟合外推得到界面温度,得出钢管混凝土接触热阻随着钢管界面温度的变化规律。试验结果表明,未受载的钢管自密实混凝土界面接触热阻平均值范围在0.002~0.01m2·K/W,与其他文献结果相比有一定可靠性。     

激波诱导下的气体界面不稳定性实验研究

界面不稳定性, 特别是Richtmyer–Meshkov (RM) 不稳定性, 是流体
力学中一项重要的研究内容, 无论在学术研究领域还是工程应用领域都有着
重要的研究价值和应用背景. RM 不稳定性问题自提出以来, 得到了学术界
广泛的关注, 其研究无论是在实验方法、数值模拟还是在理论分析方面都取
得了很大的进展. 在激波管中开展激波与界面相互作用的实验研究, 即研究
界面初始扰动在激波诱导下的演化规律, 是目前研究RM 不稳定性的重要手
段. RM 不稳定性实验研究包括3 个部分, 分别是激波的产生、界面的形成
以及流场的观测. 综述了RM 不稳定性的实验研究进展, 并针对目前研究的
局限性提出了RM 不稳定性今后实验研究的重点和方向: 汇聚激波作用下界
面不稳定性的发展规律; 激波冲击下多种形状及大振幅界面的演化机理; 三
维界面的RM 不稳定性发展规律; 可压缩湍流的形成与混合机理.      

Richtmyer-Meshkov不稳定性流体混合区发展的实验研究

使用矩形激波管，在马赫数分别为$M=1.5$和1.7的条件下实验研究了气/液界面 上(即Atwood数$A$接近1时)由Richtmyer-Meshkov不稳定性引起的流体混合现象. 得到 了气/液界面上Richtmyer-Meshkov不稳定性后期流体混合区域宽度随时间的发展呈现出线 性关系的结果，即$h \propto t$. 比较了不同马赫数和初始扰动下的发展情况，发现当 马赫数增加时，同一时间混合区域 宽度随之增加，混合区域宽度增长变快；而相比于波长差别不大的弱多模态初始扰动(无人 为干扰界面), 当界面初始扰动获得随机外界干扰时，界面混合区域具有较大的宽度以及增 长速度. 并且增加激波马赫数和初始扰动多模态性，流体混合程度更为剧烈.     

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.     

Three‐dimensional boundary element analysis of drop deformation in confined flow for Newtonian and viscoelastic systems

An adaptive (Lagrangian) boundary element approach is proposed for the general three‐dimensional drop deformation in confined flow. The adaptive method is stable as it includes remeshing capabilities of the deforming interface between drop and suspending fluid, and thus can handle large deformations. Both drop and surrounding fluid are viscous incompressible and can be Newtonian or viscoelastic. A boundary‐only formulation is implemented for fluids obeying the linear Jeffrey's constitutive equation. Similarly to the formulation for two‐dimensional Newtonian fluids (Khayat RE, Luciani A, Utracki LA. Boundary element analysis of planar drop deformation in confined flow. Part I. Newtonian fluids. Engineering Analysis of Boundary Elements 1997; 19 : 279), the method requires the solution of two simultaneous integral equations on the interface between the two fluids and the confining solid boundary. Although the problem is formulated for any confining geometry, the method is illustrated for a deforming drop as it is driven by the ambient flow inside a cylindrical tube. The accuracy of the method is assessed by comparison with the analytical solution for two‐phase radial spherical flow, leading to good agreement. The influence of mesh refinement is examined for a drop in simple shear flow. Copyright © 2000 John Wiley & Sons, Ltd.     

Immiscible Displacement in the Interacting Capillary Bundle Model Part II. Applications of Model and Comparison of Interacting and Non-Interacting Capillary Bundle Models

In the first part of this work (Dong et al., Transport Porous Media, 59, 1–18, 2005), an interacting capillary bundle model was developed for analysing immiscible displacement processes in porous media. In this paper, the second part of the work, the model is applied to analyse the fluid dynamics of immiscible displacements. The analysis includes: (1) free spontaneous imbibition, (2) the effects of injection rate and oil–water viscosity ratio on the displacement interface profile, and (3) the effect of oil–water viscosity ratio on the relative permeability curves. Analysis of a non-interacting tube bundle model is also presented for comparison. Because pressure equilibration between the capillaries is stipulated in the interacting capillary model, it is able to reproduce the behaviour of immiscible displacement observed in porous media which cannot be modelled by using non-interacting tube bundle models.