基于SPH与FEM的结构入水分析方法

建立基于光滑粒子动力学（smoothed particle hydrodynamics, SPH）、有限元法（finite element method, FEM）和无反射边界耦合的结构入水分析方法,将无限水域利用无反射边界条件截断成有限水域,将有限水域分为流体变形大的SPH区域、流体变形小的FEM区域和声学流体FEM区域,结构用FEM离散。采用通用接触算法模拟SPH与FEM的耦合,采用声固耦合方法处理FEM区域之间的耦合,建立流固耦合的SPH FEM分析方法。该方法结合SPH模拟大变形的优点和FEM的高效性,可实现含自由液面变形、液体飞溅和无限水域等特点的流固耦合问题的模拟,为结构入水分析缩小离散区域、降低自由度和SPH粒子数等提供一种有效的分析方法。     

水下爆炸冲击载荷作用下舵的动态特性

利用Arbitrary Lagrangian-Eulerian(ALE)有限元方法求解爆炸冲击过程中的流固耦合问题:采用ALE算法描述流体和炸药模型,采用Lagrangain方法描述舵结构模型,不同介质间的界面采用接触罚函数耦合算法。应用有限元软件ANSYS/LS-DYNA仿真模拟舵遭受TNT炸药爆炸冲击作用的全过程,得到舵的应力云图、位移云图,典型位置压力时间历程,加速度时间曲线等冲击响应。计算结果表明:利用A L E方法可以预估舵在水下爆炸冲击载荷作用下的损伤情况,为舵的抗冲击设计提供依据。     

固体入水的边界压力处理及表面粒子位置的修正

基于传统光滑粒子流体动力学(SPH)方法的边界力法、虚粒子法或耦合力法处理固体入水时,流 体与固体交互界面的粒子密度不连续、压力不稳定、固体边界处会发生部分流体粒子穿透或分离等现象,而流 体表面因为受到力的作用,表面破碎后,液面较粗糙。针对上述问题,结合边界力和虚粒子的优点,对耦合力 法进行改进,处理运动固体边界,阻止流体粒子穿透固体边界;改进交互界面的压力计算方法,提高计算精度, 稳定交互界面压力场;对流体表面的粒子位置进行校正,提升流体表面自由流动液面边界的模拟效果。通过经 典的二维固体入水实验,对该方法进行了验证,实验结果表明,本文方法在流体粒子与固体粒子交互后,交互 界面压力稳定,界面分离清晰无穿透,表面流体粒子分布均匀,流场的运动真实自然。     

随机最优切换模型的建立与数值求解

利用动态规划原理,建立了由生产(经营)模式一向模式二切换的随机最优切换模型,得到了一个关于期权价值的偏微分方程互补问题,进而利用罚函数方法求解此问题而得到了一个非线性偏微分方程.在空间离散上采用拟合有限体积法求解此非线性偏微分方程,获得了最优切换模型的最优执行边界.最后进行了数值模拟,并讨论了参数关于最优执行边界的灵敏度.     

基于ANSYS/LS-DYNA的返回舱海面撞水动力研究

在理论分析刚形体垂直撞水动力特性的基础上,分别推导出基于von Karman理论和通用Wagner理论的圆球底返回舱撞水冲击过载公式.然后借助ANSYS/LS-DYNA动力显式程序中ALE算法的优势,进行返回舱海向垂直撞水动力数值仿真;针对数值仿真结果和理论分析预报结果的比较分析来验证数值仿真的可行性,同时给出数值计算弹性体模型垂直撞水的撞水冲击过载;结果表明:采用ANSYS/LS-DYNA的ALE算法可以有效地数值模拟返回舱撞水动力特性,为进一步控制返回舱海面回收以及数值分析撞水动力特性提供有力的技术支持,且大大节约了试验经费.     

基于LS-DYNA的爆炸流场荷载的数值模拟研究

随着微型计算机计算能力的提高以及数值方法和商业软件的推动,基于有限元方法的数值模拟在科学和工程研究中得到广泛应用,一些复杂的物理问题可以容易地进行数值模拟研究。介绍了在微型机上基于LS-DYNA软件分别使用普通ALE方法和ALE映射方法模拟限制空间内爆炸波流场荷载的方法,并对结果进行了比较分析。研究表明,爆炸波传播及荷载可以在微型机上进行有效模拟;ALE网格映射方法对于普通ALE方法是一种改进,可以降低结果对网格的敏感性。     

具有散射效果的室内外光束实时绘制

依据空气散射的物理原理,提出一种高效的能够实时模拟动态自然光束的真实感绘制方法.首先推导出散射计算的解析表达式以取代传统的数值积分方法;然后基于 GPU 特点设计了一种有效的光束体表示,作为光束体和阴影体统一形式共同参与 GPU 着色器的运算,从而生成具有阴影效果的光束.该方法不仅可以模拟单个光源形成的光束,还可以实时模拟动态太阳光源产生的多个自然光束.由于采用了 GPU 加速计算,可以满足实时应用需求.     

汽车换热器风室试验台的CFD分析

为给汽车前端和发动机舱内气流数值计算提供参考依据,基于FLUENT对某汽车换热器风室试验台进行建模和数值模拟;分析风室内部空气流动状况,针对流动特征,给出风室计算流体力学（Computational Fluid Dynamics,CFD）校核的评价．网格采用四面体结构,模型中采用三维不可压的雷诺平均N．S方程,速度压力耦舍采用SIMPLE方法．空间离散格式为2阶迎风格式,时间离散格式为2阶隐式．选用realizable k-ε占模型模拟风室内部空气的湍流流动．固体壁面采用无滑移边界条件和非平衡壁面函数边界条件．模型进口采用速度入口来给定风量,出口采用压力出口．比较计算结果与试验设计标准,喷嘴压差的相对偏差范围在5％以内,基本达到对设备的精度要求,对风室设计有一定指导意义．     

真实软组织特性的肝脏组织物理建模及受力分析优化

为解决人软组织系统物理建模过程中存在的精确性与实时性之间的矛盾,选择典型的人体软组织——肝脏为实验对象,建立了一套满足实时性的复杂软组织物理建模方法.首先在基于表面模型的质点-弹簧系统基础上引入了体弹簧,采用能够满足体行为模拟的质点-弹簧模型;在质点质量分配方面,采用体离散化的方法完成了质点质量的确定,同时提出了基于层次包围盒的重心修正方法;在弹簧参数设定方面,通过真实软组织生物力学实验和参数分析,提出分段线性化弹簧参数设定方法;最后提出了作用力影响范围的受力优化分析方法,以提升变形仿真的速度.实验结果表明,该方法能够更好地模拟软组织的形变,并提高模拟仿真的实时性.     

交错网格结构的涡粒子烟模拟

目的 流体模拟方法中的基于旋度的模拟方法相比于基于速度的模拟方法,能提供较多细节,但是通常难以处理不同的边界条件,比如固体边界和自由表面,而且通常难以保证模拟的稳定性。本文的目的就是为了解决基于旋度的模拟方法的边界问题和稳定性问题。方法 提出一种交错网格结构,在这种网格结构下,旋度分量被错开放置在每个网格的棱的中心点。利用这种网格离散格式,提出了几种修改求解速度场方程组的策略,以应对不同的边界条件。结果 给出多种场景下的流体模拟结果图,以及几种场景下的总动能变化图和时间效率表。结果显示,本文方法能够处理好不同边界条件,并保持模拟的稳定性。结论 本文提出了一种新的涡粒子流体模拟方法,该方法利用一种交错网格结构辅助模拟,在这种新的网格离散格式下,该方法解决了基于旋度的模拟方法的边界问题和稳定性问题。     

Euler–Lagrange coupling with damping effects: Application to slamming problems

During a high velocity impact of a structure on a nearly incompressible fluid, impulse loads with high-pressure peaks occur. This physical phenomenon called ‘slamming’ is a concern in shipbuilding industry because of the possibility of hull damage. Shipbuilding companies have carried out several studies on slamming modeling using FEM software with added mass techniques to represent fluid effects. In the added mass method inertia effects of the fluid are not taken into account and are only valid when the deadrise angle is small. This paper presents the prediction of the local high pressure load on a rigid wedge impacting a free surface, where the fluid is represented by solving Navier–Stokes equations with an Eulerian or ALE formulation. The fluid–structure interaction is simulated using a coupling algorithm; the fluid is treated on a fixed or moving mesh using an ALE formulation and the structure on a deformable mesh using a Lagrangian formulation.A new coupling algorithm is developed in the paper. The coupling algorithm computes the coupling forces at the fluid–structure interface. These forces are added to the fluid and structure nodal forces, where fluid and structure are solved using an explicit finite element formulation. Predicting the local pressure peak on the structure requires an accurate fluid–structure interaction algorithm. The Euler–Lagrange coupling algorithm presented in this paper uses a penalty based formulation similar to penalty contact in Lagrangian analyses. Both penalty coupling and penalty contact can generate high frequency oscillations due to the nearly incompressible nature of the fluid. In this paper, a damping force based on the relative velocity of the fluid and the structure is introduced to smooth out non-physical high frequency oscillations induced by the penalty springs in the coupling algorithm.     

磨刀门水道垂向盐水楔及其流动的二维精细模拟

磨刀门水道观测结果发现,在盐水上溯过程中盐水楔存在与流动相关的典型内部结构.模拟盐水楔结构对盐水运动的作用,针对盐水楔结构及其运动和变化特点,提出河口盐水运动的精细模拟计算方法.盐度垂向分布计算结果与实测结果较为接近,速度垂向分布计算误差较大.在连续24h大潮中盐水楔垂向二维运动的模拟计算结果显示,涨潮阶段盐水楔间断结构保持稳定并向上游运动,最大流速出现在河道中部间断面附近;落潮阶段盐水楔的间断面结构溃灭并向下游移动,垂向最大流速出现在水道表层.     

A Parallel Implementation of ALE Moving Mesh Technique for FSI Problems using OpenMP

This paper investigates a high performance implementation of an Arbitrary Lagrangian Eulerian moving mesh technique on shared memory systems using OpenMP environment. Moving mesh techniques are considered an integral part of a wider class of fluid mechanics problems that involve moving and deforming spatial domains, namely, free-surface flows and Fluid Structure Interaction (FSI). The moving mesh technique adopted in this work is based on the notion of nodes relocation, subjected to a certain evolution as well as constraint conditions. A conjugate gradient method augmented with preconditioning is employed for solution of the resulting system of equations. The proposed algorithm, initially, reorders the mesh using an efficient divide and conquer approach and then parallelizes the ALE moving mesh scheme. Numerical simulations are conducted on the multicore AMD Opteron and Intel Xeon processors, and unstructured triangular and tetrahedral meshes are used for the 2D and 3D problems. The quality of generated meshes is checked by comparing the element Jacobians in the reference and current meshes, and by keeping track of the change in the interior angles in triangles and tetrahedrons. Overall, 51 and 72% efficiencies in terms of speedup are achieved for both the parallel mesh reordering and ALE moving mesh algorithms, respectively.     

An approach to modelling high pressure waterjet oil barriers

A numerical model is employed to study flow characteristics of high pressure waterjet barriers. The high pressure waterjet barrier has been developed as a means of containing and deflecting oil spills that float on water. An array of waterjets directed above the contaminated surface will generate an air flow that moves the oil. A two-dimensional model of the turbulent entrained air flow is done using the spectral element method. The moving boundaries of the air flow are modeled analytically and implemented in the computer code. The upper boundary, represented by the high pressure waterjet, is assumed to be moving with the centerline velocity of the jet. The lower boundary is the interface of the air-driven wavy liquid surface for which interfacial shear and pressure drop are directly related to waviness characteristics and mobility of the interface. This boundary is modeled as a solid-like wavy surface. The interfacial speed is neglected at this stage since it is much smaller than the air speed. An algebraic turbulence closure model is used for estimating shear stress at the lower boundary. A case study is presented where the numerical model has been applied to a base case for which experimental data are available. The predicted air flow velocities of the model compare relatively well with experimental values. Results of the model have also shown that the air flow maintains significant momentum over a long distance beyond the range when the waterjet itself has ceased.     

A global arbitrary Lagrangian–Eulerian method for stratified Richtmyer–Meshkov instability

Richtmyer–Meshkov (RM) instability arises when a material interface is accelerated impulsively by shock waves. In this work, an arbitrary Lagrangian–Eulerian method, global ALE method, was proposed for the simulation of stratified RM instability. In the global ALE method, an Eulerian diffusion interface model was implemented based on mass fraction function. Thus all the meshes can be remeshed arbitrarily no matter whether they are material interface or not. Some benchmark problems, such as shock tube problem with different specific ratio, RM instability with small initial perturbation, were computed with the global ALE method, and the numerical results agree well with exact solution or theoretical model. Also, we proposed some stratified RM instability model problems with two or more material interfaces in planar, cylindrical and spherical geometries. Then the stratified RM instabilities were simulated with global ALE method. The interface evolution process was studied and compared in different geometry cases based on simulation results. To overcome the spurious mesh distortion, a sub-zonal Riemann solver method was proposed in appendix part of the paper based on the analysis of the error source of 2D Lagrangian computation due to non-uniform multi-dimensional mesh.     

Implicit Lagrangian-Eulerian Tvd Method For Solving Two-Dimensional Hydrodynamic Equations On Unstructured Meshes

An ALE method for solving hydrodynamic equations on unstructured meshes is presented. It is based on an implicit finite-volume scheme derived in Godunov’s approach. The basic quantities (the density, temperature, and velocity) are defined at cell centers. For relations between pressure and velocities at the centers and their analogs at the nodes, we use the relations proposed by P.H. Maire et al. A piecewise-linear TVD reconstruction of the pressure and velocity in the cell is used to achieve the second-order approximation of smooth solutions, preserving their monotonicity. Mesh rezonings are implemented during the calculation. To recalculate the values, the old mesh is covered onto the new one so that a bounded piecewise-linear representation is used for the values in the cells of the old mesh, while the interface in the mixed cells is reconstructed by the VOF method. The mass, momentum, and total energy are conserved under the recalculation.     

FGMRES preconditioning by symmetric/skew-symmetric decomposition of generalized stokes problems

The generalized Stokes problem is solved for non-standard boundary conditions. This problem arises after time semi-discretization by ALE method of the Navier–Stokes system, which describes the flow of two immiscible fluids with similar densities but different viscosities in a horizontal pipe, when modeling heavy crude oil transportation. We discretized the generalized Stokes problem in space using the “Mini” finite element. The inf-sup condition is proved when the interface between the two fluids and its discretization match exactly. The linear system obtained after discretization is solved using different iterative Krylov methods with and without preconditioning. Numerical experiments with different meshes are presented as well as comparisons between the methods considered. The results suggest that FGMRES and a preconditioning technique based on symmetric/skew-symmetric decomposition is a promising candidate for solving large scale generalized Stokes problem.     

插值边界的四边网格离散极小曲面建模方法

如何实现极小曲面的快速三维建模,是几何设计与计算领域中的难点和热点问题.给定一条封闭的边界离散折线,本文研究如何构造以其为边界的四边网格离散极小曲面.首先从曲面的内蕴微分几何度量出发,给出了离散四边网格极小曲面的数学定义;然后利用保长度边界投影、四边网格生成、径向基函数插值映射和非线性优化技术,提出了由给定边界离散折线快速构造离散四边网格极小曲面的一般技术框架.最后通过若干建模实例验证了本文方法的有效性.该方法可实现四边网格极小曲面的高质量建模,在建筑几何领域具有一定的应用价值.