不同排列小间距双方柱涡激振动数值模拟研究

为研究不同排列下小间距双方柱涡激振动特性及其振动机理,在雷诺数为100时,对间距比为2、质量比为3的串列、错列和并列排列双方柱涡激振动进行数值模拟研究,分析了双柱在折合流速U_r=1～30下的响应振幅及频率特性,并得到振动柱体的升阻力系数,以了解其气动力系数的变化情况,探讨了不同排列下双柱尾流结构的变化情况。结果表明：串列及错列排列中,下游柱涡激振动振幅远大于单柱,上游柱振动受到抑制(θ=60°的双方柱排列情况除外)。并列排列中,双柱振动曲线几乎一致,其涡激振动振幅比单柱稍大。各排列双柱均发生尾流驰振现象,使其在U_r超出共振区时仍保持较高的振幅。不同排列的双柱阻力系数C_D(θ=30°的双柱排列下游柱除外)均在共振区内突增,在共振区外保持不变。在共振区内,双柱C_Lrms随U_r的变化情况与A^*的大小有关;在其余U_r范围内,双柱C_Lrms基本不变。串列双柱存在3种尾流模式,θ=30°双柱排列的尾流模式与串列双柱相似,但在...     

不同雷诺数下方柱绕流的数值模拟

在不同雷诺数条件下,对流体绕经方形柱体的流动进行了数值模拟,计算雷诺数分别为100,1×103,1×104和2.2×104.当Re=100时,直接采用N-S方程进行计算;当Re=1×103,1×104和2.2×104时,则引入k-ε湍流模型进行计算.应用Galerkin有限元法对控制方程进行离散和求解,利用分离时间步长法处理控制方程中的非线性项.模拟计算得出了在不同雷诺数下的卡门涡街脱落形态.方柱后尾涡的形态会随雷诺数的变化而产生一定的变化.当雷诺数较低时,尾涡会拖得比较长,随着雷诺数的增加,尾涡长度会随之缩短.计算得到了方柱的受力系数和Strouhal数.将计算结果与文献上的实验和计算结果进行了比较,两者吻合较好.     

Numerical simulation of turbulent convection of air in a square cavity heated on the side

A set of three-dimensional unsteady-state equations is used, with the Rayleigh number of 1.58×10⁹, to calculate the turbulent, statistically two-dimensional convection of air in a closed cavity with side walls heated to different temperatures. The turbulence is simulated using the equation of transfer of kinetic energy of turbulent motion, with the dissipation rate calculated differently in the vicinity of and away from the walls. The calculation results are compared with available data of experimental and numerical investigations of other authors. The possibilities are discussed of using this approach to predict averaged characteristics of free convection.     

Numerical simulation of unsteady MHD natural convection of CNT-water nanofluid in square cavity heated sinusoidally from below

ABSTRACT

Numerical unsteady natural convection flow in a square cavity in the presence of uniform magnetic field is investigated. The cavity is filled with CNT-water nanofluid and heated from below with sinusoidal temperature distribution. This study has been conducted for the certain pertinent parameters of Rayleigh number Ra, Hartmann number Ha and the CNT volume fraction from Φ?=?0 to 0.12. The results indicate, that for large values of Ha, increasing Φ results in an increase of the normalized average Nusselt number. The rate corresponding to CNT- volume fraction nanoparticles of 10% ensures an enhancement about 50% of the heat transfer rate compared to the standard case. The flow undergoes a period bifurcation at a Rayleigh number beyond the value 10⁶. A critical Rayleigh number Ra_c of 1.077?×?10⁶ is then computed. Finally, a correlation giving the average Nusselt number as a function of the frequency of the periodic regime is established.     

小管径椭圆管开缝翅片换热器的数值模拟

对小管径椭圆管开缝翅片换热器空气侧的流动与传热特性进行数值模拟,对影响其换热性能的2个主要参数椭圆管偏心率和开缝翅片开缝错列高度分布进行优化,与传统管翅式换热器换热性能进行比较。模拟结果表明:当椭圆管两轴之比Rx:Ry=2:3(偏心率),开缝高度分布为0.8 mm,0.6 mm和0.4 mm时,换热效果最好。与传统管翅式换热器相比,小管径椭圆管开缝翅片换热器换热系数提高10%~20%,而压降几乎相等,总体换热性能提高。     

Numerical simulation of free convection of MHD non-Newtonian nanofluid within a square wavy enclosure using Meshfree method

Abstract

In this article, flow and heat transfer of nanofluids inside a wavy square enclosure filled with non-Newtonian (shear thinning) nanofluid under magnetic field has been simulated numerically. Single-phase model is used. The governing equations have been solved numerically using element free Galerkin method. The results are obtained for isotherms, streamlines, and average Nusselt number for various values of Hartmann number, Rayleigh number, nanoparticles volume fraction, and power-law index. Here, the main objective is to explore the effect of power-law index, Rayleigh number, Hartmann number, and volume fraction on average Nusselt number. It is found that in the absence of magnetic field, Nusselt number drops on increasing the value of power-law index whereas in the presence of magnetic field, heat transfer rate increases with increase in power-law index. With the increase in Rayleigh number and volume fraction of nanoparticles, the heat transfer rate increase in all cases. This type of problem has a direct application in the coolant systems, solar collector where the structure used is wavy in order to increase the rate of heat flow. Here, EFGM is efficiently applied for simulation due to irregular domain, which creates a novelty in the work.     

构件跌落碰撞的数值模拟研究

针对构件跌落碰撞问题,将Hertz-damp模型与有限元方法结合,给出了结构碰撞位移突加约束条件,建立了碰撞结构体系的动力方程。该方法能考虑碰撞体质量分布对碰撞过程的影响,同时可描述结构碰撞过程中的刚度非线性和能量损失。建立了构件跌落碰撞模型,初步分析了跌落构件质量、质量分布以及跌落高度等参数对碰撞过程的影响。研究表明:结构碰撞模拟中碰撞体质量沿接触面法线轴向不对称时应考虑其质量分布,否则会高估碰撞反应;碰撞荷载峰值和单次碰撞持续时间与跌落构件质量和跌落高度均成指数关系,碰撞荷载峰值随跌落构件质量和跌落高度增大而增大,而单次碰撞持续时间则随跌落构件质量增大而增大,随跌落高度增大而减小。这些规律能为减轻结构碰撞破坏研究提供参考。     

Numerical modelling of dynamically loaded metal foam-filled square columns

The crushing behaviour of dynamically loaded metal foam-filled square columns has been investigated using an extended version of the existing self-similar pressure dependent constitutive model for metal foams. The model has been implemented in ABAQUS/Explicit and analyses have been conducted using different approaches to model the uniaxial and hydrostatic hardening behaviour of metal foams. A practical and reliable procedure to approximate the observed anisotropic behaviour within the computational framework of isotropic plasticity is introduced. The comparison between the available experimental and newly generated numerical results is presented in order to illustrate the accuracy and efficiency of the implemented model in predicting the crashworthiness of filled columns.     

Dynamics of cyclones

The dynamics of the cyclone over a short period of time is worked out. The two-arms structure of the cyclone is explained through the mathematical solution. A possible way to track a cyclone is concluded.     

Numerical micromagnetics of small particles

The results of applying the authors' micromagnetic code to a homogeneous spherical particle that is large enough to support inhomogeneous magnetization are given. For smaller particles, magnetization reversal is by coherent rotation. Larger particles initially exhibit curling as the applied magnetic field is reduced from a saturating value. Then one of two new behaviors is observed. For weak crystalline anisotropy, the axis of the curling state rotates and bends, and the magnetization reversal process is reversible, or nearly so. For strong crystalline anisotropy, a sudden discontinuous transition occurs to a vortex state with axis perpendicular to the anisotropy axis, and the vortex moves across the particle as reversal of the applied magnetic field continues. The formation and disappearance of the vortex are irreversible, but all other aspects of the process are reversible     

Numerical simulation of multi-species diffusion

A numerical model has been developed to simulate the transport of several ionic species across a saturated concrete or mortar sample. The chloride binding as well as the electrical coupling between the different ionic fluxes are included in the model by using the Nernst-Planck system of equations. This model highlights which parameters affect substantially chloride penetration into reinforced concrete structures and then shows that the use of Fick’s first law in a predictive model for chloride penetration is strongly challenged. The simulations are in good agreement with diffusion-cell experiments and membrane potential measurements.

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Résumé Un modèle numérique a été développé afin de simuler le transport de plusieurs espèces ioniques dans une éprouvette de béton ou de mortier saturée. La fixation des ions chlorures ainsi que le champ électrique créé par les divers flux ioniques sont modélisés à l’aide d’un système d’équations de Nernst-Planck. Le modèle permet, premièrement, de mettre en évidence les paramètres qui influencent le plus la pénétration des ions chlorures et, deuxièmement, il montre que l’utilisation de la 1^e loi de Fick dans un modèle de prédiction de la pénétration des ions chlorures est fortement remise en cause. Les résultats sont en accord avec les expériences de diffusion et les mesures expérimentales du potentiel de membrane.

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Editorial Note Prof. Jean-Pierre Ollivier is a RILEM Senior Member. He works at the LMDC, a RILEM Titular Member and participates in RILEM TCs 170-CSH (The structure of C-S-H) and 178-TMC (Testing and modelling chloride penetration in concrete). Prof. Lars-Olof Nilsson is a RILEm Senior Member.     

Numerical simulation of monsoon circulations

Short period variations in monsoon rainfall are caused by the westward passage of low pressure systems (depressions) from the northern sector of the Bay of Bengal. A primitive equation model was used to predict the movement of one such depression (20 August 1977). Four research ships from the USSR, which formed a part of the recently concluded Monsoon Experiment, provided additional meteorological data within the field of this depression. This enabled us to fix the depression’s initial position with greater accuracy than would have been possible without the ships’ data. The model used a co-ordinate system in which the lower boundary coincided with the earth’s surface, while the upper boundary was placed at 200 mb. It resembled a three-dimensional channel with side walls at 0° and 140°E, and northern and southern boundaries at 60°N and the equator. The grid spacing was 250 km. Numerical integration was performed upto 5 days of model time. Gravity waves and other forms of ‘noise’ were filtered out every 24 hr by a process of adjustment referred to as ‘initialisation’. Initialisation after every 24 hr was necessary because the boundary conditions in this regional model did not permit sufficiently rapid dispersal of gravity waves. In the first experiment only orographic features were included, but the second experiment considered the main features of atmospheric radiation in addition to orography. The paper presents a statement of deviations between the predicted and actual movement of the depression, and discusses reasons for such deviations.     

钢坯加热的数值模拟

借助CFD仿真手段,模拟蓄热式加热炉内钢坯加热的实际状况,研究了钢坯长度、宽度方向上下表面和中心温度的温度分布,提出钢坯存在上下温差、四角边缘温度高和靠近出钢口钢坯温度低等问题。同时,研究了炉内不同厚度钢坯与加热时间的基本关系,比较了仿真计算结果与实际炉内加热时间,通过数值拟合,得出钢坯厚度与加热时间的拟合公式,可在生产实际中使用。上述研究结果可为加热炉钢坯加热制度的制定和优化提供依据。     

Numerical simulation of electroosmotic flow

We have developed a numerical scheme to simulate electroosmotic flows in complicated geometries. We studied the electroosmotic injection characteristics of a cross-channel device for capillary electrophoresis. We found that the desired rectangular shape of the sample plug at the intersection of the cross-channel can be obtained when the injection is carried out at high electric field intensities. The shape of the sample plug can also be controlled by applying an electric potential or a pressure at the side reservoirs. Flow induced from the side channels into the injection channel squeezes the streamlines at the intersection, thus giving a less distorted sample plug. Results of our simulations agree qualitatively with experimental observations.     

Numerical simulation of electrodeposition problems

The mathematical modelling of the electrodeposition phenomenon leads to a linear elliptic partial differential equation subject to non-linear mixed boundary contitions. This is complicated further by the fact that the domain under consideration is evolving with time. Owing to the nature of the mathematical model, the boundary element method is a very efficient numerical technique for the solution of such problems. The authors report their numerical simulation of the above problem which also has applications in the cathodic protection systems for corrosion prevention.     

Numerical simulation of void formation in LCM

The existence of void type defects in composite laminates manufactured using the liquid composite molding process alters the mechanical characteristics of the final product. The object of this paper is to present a procedure to simulate mold filling and to incorporate void formation. The model is composed of a unique combination of robust and accurate numerical algorithms for solving the transport equation. The saturation ratio is a macroscopic entity yet it is clearly the consequence of microscopic phenomena and especially of air entrapment within tows, hence the presence of micropores. In the model, the source term is dependent on the capillary number and is related to the micro–macro scale effects with the dual-porosity. The unsaturated flow and transport model was applied to a problem and was found to produce accurate, mass conserving solutions when compared to experimental results in void content.     

Numerical simulation of quasibrittle fracture in concrete

A triangular unit, constructed from constant strain triangles with nodes along its sides and not at the vertex, is developed for the simulation of fracture in quasibrittle materials. Fracture is modelled through a constitutive softening-fracture law at the interface nodes, with the material within the triangular unit remaining linear elastic. The inelastic displacement at an interface node represents the crack opening, which is related to the conjugate inter-nodal force by the appropriate softening relationship. The path-dependent softening behaviour is solved in nonholonomic rate form within a quasiprescribed displacement formulation. At each event in the loading history, all equilibrium solutions for the prescribed mesh can be established and the critical equilibrium path with the minimum increment of external work adopted. The crack profile or trajectory is restricted in that it can only follow the interface boundaries of the defined mesh. No remeshing is carried out. Solutions to the nonholonomic rate formulation are obtained using a mathematical programming procedure based on the solution of an LCP. Several examples are given and compared, where possible, with published results. The advantage of this formulation is that branching and interacting cracks can be tracked subject to the limitations of the prescribed mesh.     

两并列方柱绕流相互干扰的数值研究

为了研究并列双方柱在不同间距比时气动力特性的干扰效应,采用刚性模型测压风洞试验的方法,通过改变两方柱之间的距离,得到了不同间距比下并列双方柱的风压系数、升力系数和阻力系数。结果表明：当间距比1.2≤L/D<2.5时,并列双方柱平均风压系数的干扰效应明显,且主要表现在内侧面,平均阻力系数和脉动升力系数的干扰效应表现为减小效应;当间距比L/D≥2.5时,并列双方柱的平均风压系数、平均阻力系数和脉动升力系数的干扰效应均不明显。     

Numerical simulation of precipitation kinetics in multicomponent alloys

A universal numerical model based on the particle size distribution(PSD) approach has been developed for the simulation of precipitation kinetics in multicomponent alloys during isothermal ageing. Nucleation was implemented utilizing the classical nucleation theory(CNT). Growth and coarsening were modeled by a single growth kinetics equation, which is constructed based on the interfacial diffusion flux balance and the capillarity effect. Only partial off-diagonal terms in the diffusion matrix(di...