粗糙边界上浸润现象的数学分析

粗糙界面上浸润现象在工业生产和日常生活中有很多应用.刻画粗糙界面上宏观接触角大小的经典Wenzel和Cassie公式被广泛使用,但关于其正确性有很多争议.本文主要介绍作者近几年对该问题所做的一些数学分析.从数学上讲,粗糙界面上浸润现象是一个具有多尺度边条件的自由界面问题.通过对该问题的不同模型做均匀化,本文显示经典公式在考虑系统全局极小时是成立的,而考虑局部极小点时,宏观接触角应由新的公式描述.本文还分析了实际应用中比较感兴趣的接触角滞后现象,推导出某些条件下接触角变化的方程.     

由平均曲率和外力场之差支配的超曲面的发展

考虑由平均曲率和外力场之差支配的超曲面的发展运动. 证明了如果初始曲面的平均曲率大于某一个仅依赖于外力场导数的常数, 则这样的流将在有限时间爆破. 对于线性外力, 可以证明在运动过程中超曲面的凸性是保持的, 并且如果初始曲率小于某一与外力场有关的常数, 则流将光滑的存在于任意有限时间, 且曲面将扩张到无穷.     

由仿射平均曲率支配的超曲面的发展

研究由仿射平均曲率支配的严格凸超曲面的发展运动.在假定仿射平均曲率流存在并且曲面保持严格凸的条件下,通过对曲面支撑函数的计算,给出了高斯曲率的发展方程.     

考虑磁通流动效应的超导薄膜基底结构界面断裂行为研究北大核心CSCD

超导薄膜是一种采用化学涂层制备而成的多层薄膜结构,作为性能优越的导电功能结构材料,其载流能力与结构完整性直接相关.在超导薄膜制备过程中,超导层与金属基底之间的界面裂纹很难避免.因此,在载流运行过程中,由于外磁场的存在,这类界面裂纹的强度问题成为关键.为此,该文针对超导薄膜结构,以磁通量子穿透薄膜理论和线弹性断裂理论为基础,建立了研究超导层与基底界面裂纹强度问题的解析模型.深入分析了外加磁场作用下界面裂纹强度问题,得到了超导磁通流动对裂纹尖端应力场和能量释放率的影响.结果表明:磁通流动速度越大,界面裂纹尖端处应力越大且能量释放率越大,这将导致界面更容易发生裂纹破坏.该文所得结果有助于分析相关的界面裂纹问题.     

一种未血管化肿瘤生长模型整体解的证明

研究了一种未血管化肿瘤生长模型的自由边界问题,模型与此类其它模型有着明显的不同,它引入新的运动项来描述肿瘤内细胞的运动,反映了肿瘤内细胞运动的"接触抑制"性质.运用Banach不动点理论和抛物型方程的L~P理论,证明了模型存在唯一整体解.     

自由表面与粘性尾迹的相互作用

解析地研究了无限深不可压粘性流体中运动物体产生层流尾迹与自由表面波的相互作用．以定常的Oseen方程模拟受扰流动,对于小振幅自由表面波则采用线性化的运动学和动力学边界条件．在数学描述上,运动物体以Oseen极子模拟,受扰流场分解成表述粘性尾迹的无界奇异Oseen流和描述自由面效应的有界正则Oseen流之和．通过积分变换法,得到自由表面波的精确解．借助Lighthill的两步格式,导出了自由面波高带有附加校正项的渐近解．所得对称解显示了波动的振幅因粘性和潜深的存在而呈指数衰减．     

数值计算与计算机应用2019年第40卷第4期摘要

张庆海·多相流界面追踪问题的理论框架及高阶数值方法[J].数值计算与计算机应用,2019,40(3):161-187.摘要:界面追踪是多相流最基本最重要的子问题之一.现有方法的思路是把其中的几何和拓扑问题转化为求解数值偏微分方程,从而避免处理这些复杂的几何和拓扑结构.与此形成鲜明对比的是,我们提出的mars理论和高阶数值方法试图运用几何和拓扑的工具来解决几何和拓扑的问题.这篇综述性文章将简明扼要的介绍MARS理论和其衍生方法的核心内容,包括殷空间(连续介质流相的数学模型)、殷空间上的布尔代数及其算法实现、流相拓扑变化的同调分析、捐献区间(标量守恒率下相空间中的粒子分类和通量计算解析解)、VOF方法的收敛阶证明、一个四阶精度的界面追踪方法cubic MARS、以及一个四阶及以上精度的曲率估计算法HFES.经典数值测试的结果表明cubic MARS和HFES无论在效率上还是精度上相对于现有方法都具有很大优势.     

喷涂于运动边界上液体薄膜射流的流变效应

采用非Newton流体的二阶流体模型分析了相对高温的液体熔体薄膜由模口喷出并涂于运动的固体膜上. 讨论了由自由面上温度梯度驱动的非Newton液体薄膜的热毛细流动, 考虑热毛细流动的流变效应. 分析是基于润滑理论近似和摄动理论近似. 得到了液体高度方程和非Newton液体薄膜的热流体力学过程描述, 具体求解了弱流变流体效应的情况.     

有限温度下线性谐振子晶格的分子动力学模拟

基于双向界面条件和声子热浴,提出了一种新的热流输入方法,该方法未引入任何耗散因子或经验参数,能在局域的空间和时间上实现有限温度下的原子模拟.对于一维线性谐振子晶格,采用双向界面条件作为系统的边界,目的是为了让热流能从外界输入系统,同时允许内部的波动自由地传出,从而实现系统中能量的动态平衡.通过数值计算发现,双向界面条件能让正方向的波完整地输入,同时还能抑制反方向的波的输入,因此,边界条件可以起到行波的二极管的作用.声子热浴的正则模态能很好地描述原子的热振动,通过推导可将正则模态分解为正方向和反方向的输入波,取正方向的波来构造热源项.数值算例表明,热流输入方法对于线性谐振子链非常有效,系统能快速地达到预期的温度,并且能够维持在稳定的状态,同时,还能很好地处理有限温度下的非热运动.     

H~∞最优控制器的不唯一性及稳定最优控制器的求取

利用求解模型匹配问题的方法求得使(1.9)达到最小的参数 Q~*∈RH~∞,代入(1.5),(1.6)得最优控制器 K~*.上述 H~∞最优控制器在某些情况下是不唯一的,这种不唯一的最优控制器可以表示成具有自由参数的控制器参数化公式.而这部分自由参数可以用来满足一些其它设计要     

An Energy-Stable Parametric Finite Element Method for Simulating Solid-State Dewetting Problems in Three Dimensions

We propose an accurate and energy-stable parametric finite element method for solving the sharp-interface continuum model of solid-state dewetting in three-dimensional space. The model describes the motion of the film\slash vapor interface with contact line migration and is governed by the surface diffusion equation with proper boundary conditions at the contact line. We present a weak formulation for the problem, in which the contact angle condition is weakly enforced. By using piecewise linear elements in space and backward Euler method in time, we then discretize the formulation to obtain a parametric finite element approximation, where the interface and its contact line are evolved simultaneously. The resulting numerical method is shown to be well-posed and unconditionally energy-stable. Furthermore, the numerical method is generalized to the case of anisotropic surface energies in the Riemannian metric form. Numerical results are reported to show the convergence and efficiency of the proposed numerical method as well as the anisotropic effects on the morphological evolution of thin films in solid-state dewetting.     

A priori error analysis of the mortar finite element method for variational inequalities

The mortar finite element method is a special domain decomposition method, which can handle the situation where meshes on different subdomains need not align across the interface. In this article, we will apply the mortar element method to general variational inequalities of free boundary type, such as free seepage flow, which may show different behaviors in different regions. We prove that if the solution of the original variational inequality belongs to H²(D), then the mortar element solution can achieve the same order error estimate as the conforming P₁ finite element solution. Application of the mortar element method to a free surface seepage problem and an obstacle problem verifies not only its convergence property but also its great computational efficiency. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2008     

An interface-fitted adaptive mesh method for elliptic problems and its application in free interface problems with surface tension

This work presents a novel two-dimensional interface-fitted adaptive mesh method to solve elliptic problems of jump conditions across the interface, and its application in free interface problems with surface tension. The interface-fitted mesh is achieved by two operations: (i) the projection of mesh nodes onto the interface and (ii) the insertion of mesh nodes right on the interface. The interface-fitting technique is combined with an existing adaptive mesh approach which uses addition/subtraction and displacement of mesh nodes. We develop a simple piecewise linear finite element method built on this interface-fitted mesh and prove its almost optimal convergence for elliptic problems with jump conditions across the interface. Applications to two free interface problems, a sheared drop in Stokes flow and the growth of a solid tumor, are presented. In these applications, the interface surface tension serves as the jump condition or the Dirichlet boundary condition of the pressure, and the pressure is solved with the interface-fitted finite element method developed in this work. In this study, a level-set function is used to capture the evolution of the interface and provide the interface location for the interface fitting.     

Some dynamic properties of volume preserving curvature driven flows

This paper is concerned with the following three types of geometric evolution equations: the volume preserving mean curvature flow, the intermediate surface diffusion flow, and the surface diffusion flow. Important common properties of these flows are the preservation of volume and the decrease of perimeter. It is shown in this paper that the intermediate surface diffusion flow can lose convexity. Hence the volume preserving mean curvature flow is the only flow among the evolution equations under consideration which preserves convexity, cf. [11, 16, 14, 17]. Moreover, several sufficient conditions are presented, which illustrate that each of the above mentioned flows can move smooth initial configurations into singularities in finite time.     

Anisotropic curve shortening flow in higher codimension

We consider the evolution of parametric curves by anisotropic mean curvature flow in ?ⁿ for an arbitrary n?2. After the introduction of a spatial discretization, we prove convergence estimates for the proposed finite‐element model. Numerical tests and simulations based on a fully discrete semi‐implicit stable algorithm are presented. Copyright © 2007 John Wiley & Sons, Ltd.     

边界元方法的一些研究进展

本文旨在综述我们小组近二十年来在边界元方法这一领域的一些研究成果,在简要介绍边界元方法的基本思想后,主要介绍了一类非线性界面问题的有限元-边界元耦合方法、求解电磁散射问题的有限元-边界元耦合方法和超奇异积分的一类计算方法.     

A novel algorithm based on parameterization method for calculation of curvature of the free surface flows

In this paper, a new approach based on parameterization method is presented for calculation of curvature on the free surface flows. In some phenomena such as droplet and bubble, surface tension is prominent. Therefore in these cases, accurate estimation of the curvature is vital. Volume of fluid (VOF) is a surface capturing method for free surface modeling. In this method, free surface curvature is calculated based on gradient of scalar transport parameter which is regarded as original method in this paper. However, calculation of curvature for a circle and other known geometries based on this method is not accurate. For instance, in practice curvature of a circle in interface cells is constant, while this method predicts different curvatures for it. In this research a novel algorithm based on parameterization method for improvement of the curvature calculation is presented. To show the application of parameterization method, two methods are employed. In the first approach denoted by, three line method, a curve is fitted to the free surface so that the distance between curve and linear interface approximation is minimized. In the second approach namely four point method, a curve is fitted to intersect points with grid lines for central and two neighboring cells. These approaches are treated as calculus of variation problems. Then, using the parameterization method, these cases are converted into the sequences of time-varying nonlinear programming problems. With some treatments a conventional equivalent model is obtained. It is finally proved that the solution of these sequences in the models tends to the solution of the calculus of variation problems. For verification of the presented methods, curvature of some geometrical shapes such as circle, elliptic and sinusoidal profile is calculated and compared with original method used in VOF process and analytical solutions. Finally, as a more practical problem, spurious currents are studied. The results showed that more accurate curve prediction is obtained by these approaches than the original method in VOF approach.     

Some remarks on the flux-free finite element method for immiscible two-fluid flows

We give some theoretical considerations on the the flux-free finite element method for the generalized Stokes interface problem arising from the immiscible two-fluid flow problems. In the flux-free finite element method, the flux constraint is posed as another Lagrange multiplier to keep the zero-flux on the interface. As a result, the mass of each fluid is expected to be preserved at every time step. We first study the effect of discontinuous coefficients (viscosity and density) on the error of the standard finite element approximations very carefully. Then, the analysis is extended to the flux-free finite element method.     

A structure-preserving finite element approximation of surface diffusion for curve networks and surface clusters

We consider the evolution of curve networks in two dimensions (2d) and surface clusters in three dimensions (3d). The motion of the interfaces is described by surface diffusion, with boundary conditions at the triple junction points lines, where three interfaces meet, and at the boundary points lines, where an interface meets a fixed planar boundary. We propose a parametric finite element method based on a suitable variational formulation. The constructed method is semi-implicit and can be shown to satisfy the volume conservation of each enclosed bubble and the unconditional energy-stability, thus preserving the two fundamental geometric structures of the flow. Besides, the method has very good properties with respect to the distribution of mesh points, thus no mesh smoothing or regularization technique is required. A generalization of the introduced scheme to the case of anisotropic surface energies and non-neutral external boundaries is also considered. Numerical results are presented for the evolution of two-dimensional curve networks and three-dimensional surface clusters in the cases of both isotropic and anisotropic surface energies.     

Coupled wetting meniscus model for the mechanism of spontaneous capillary action

Capillarity plays a significant role in many natural and artificial processes, but the mechanism responsible for its dynamics is not completely understood. In this study, we consider capillary flow characteristics and propose a coupled wetting meniscus model for the mechanism of spontaneous capillary action. In this model, capillary action is considered as the dynamic coupling of two interfacial forces, i.e., the wall wetting force at the contact line and the meniscus restoring force on the free interface. The wetting force promotes the motion of the contact line directed toward an equilibrium contact angle, whereas the meniscus restoring force promotes a reduction in the interface curvature, which is more consistent with a 90° contact angle. The competing interaction between these two forces is coupled together via the evolution of the interface shape. The model is then incorporated into a finite volume method for a two-fluid flow with an interface. Capillary flow experiments were performed, including vertical and horizontal flows. Phenomena analysis and data comparisons were conducted to verify the proposed model. According to the results of our study, the model can explain the capillary flow process well and it can be also used to accurately guide capillary flow calculations.