A HIGH RESOLUTION CELL-CENTERED ALE METHOD FOR LARGE DEFORMATION MULTIMATERIAL INTERFACE

针对基于积分形式的Euler方程组耦合质量组份模型方程而发展的多介质整体ALE方法耗散大的问题,采用基于微分形式的Euler方程组所发展的高分辨率界面反耗散的思想来控制界面处的数值耗散,发展了一种二维平面中积分意义下的耦合质量组份方程和体积组份方程的界面反耗散的高分辨率多介质中心型ALE方法,从而高分辨率地模拟大变形物质界面。     

基于WENO重构的高阶移动网格动理学格式

提出一种基于WENO重构的高阶(至少三阶)移动网格动理学格式.利用流体力学方程的积分形式得到移动网格上离散格式,再利用自适应移动网格方法移动网格,进而得到网格速度,利用WENO重构得到高阶插值多项式,最后使用时间方向上精确的动理学数值方法构造数值通量,得到移动网格单元上新的物理量.数值实验表明这种格式同时具有高精度、高分辨率的特点.     

Shape Optimization of Body Located in Incompressible Viscous Flow Based on Optimal Control Theory

This paper presents a numerical method of shape optimization of a body located in an incompressible viscous flow described by the Stokes and Oseen equations. The purpose of this study is to find the optimal shape that minimizes the fluid forces subjected to the body. The formulation of the shape optimization is based on the optimal control theory. The first thing that should be carried out in the optimal control theory is to define a performance function, which expresses the optimal shape. In this study, the fluid forces minimization problem is treated, i.e. fluid forces are directly used in the performance function. The performance function must be minimized subject to the basic equation. The optimal shape, which minimizes the fluid force, is pursued in this paper. This problem can be transformed into the minimization problem without constraint conditions by the Lagrange multiplier. As a numerical example, drag force minimization problems of a body located in low Reynolds number flows are carried out.     

A time-dependent numerical analysis of flow in a mechanical heart valve: Comparison with experimental results

There is a great need to fabricate heart valves that have similar haemodynamic properties with the natural ones. Towards this goal, we examine the dynamics of fluid flow in a mechanical heart valve with one leaflet. The fluid is incompressible and Newtonian and the leaflet is a neo-Hookean material. The Arbitrary Lagrangian Eulerian method is used to model the fluid-leaflet interaction, and the system of equations is solved using the Finite Element method. The pseudo solid approach along with a set of algebraic equations are used to deform the mesh, while care is taken to avoid remeshing of the domain, at the moment of valve closure. The computational results are compared against the experimental results, and we find an excellent agreement for the time period of valve closure, the time the valve is fully opened, and the value of the maximum valve opening angle. This study indicates that the present model is capable of describing the valve dynamics in physiological geometries.     

Hyperviscosity for unstructured ALE meshes

An artificial viscosity, originally designed for Eulerian schemes, is adapted for use in arbitrary Lagrangian–Eulerian simulations. Changes to the Eulerian model (dubbed ‘hyperviscosity’) are discussed, which enable it to work within a Lagrangian framework. New features include a velocity-weighted grid scale and a generalised filtering procedure, applicable to either structured or unstructured grids. The model employs an artificial shear viscosity for treating small-scale vorticity and an artificial bulk viscosity for shock capturing. The model is based on the Navier–Stokes form of the viscous stress tensor, including the diagonal rate-of-expansion tensor. A second-order version of the model is presented, in which Laplacian operators act on the velocity divergence and the grid-weighted strain-rate magnitude to ensure that the velocity field remains smooth at the grid scale. Unlike sound-speed-based artificial viscosities, the hyperviscosity model is compatible with the low Mach number limit. The new model outperforms a commonly used Lagrangian artificial viscosity on a variety of test problems.     

Free vibration of membrane/bounded incompressible fluid

Vibration of a circular membrane in contact with a fluid has extensive applications in industry.The natural vibration frequencies for the asymmetric free vibration of a circular membrane in contact with a bounded incompressible fluid are derived in this paper.Considering small oscillations induced by the membrane vibration in an incompressible and inviscid fluid,the velocity potential function is used to describe the fluid field.Two approaches are used to derive the free vibration frequencies of the system,which include a variational formulation and an approximate solution employing the Rayleigh quotient method.A good correlation is found between free vibration frequencies evaluated by these methods.Finally,the effects of the fluid depth,the mass density,and the radial tension on the free vibration frequencies of the coupled system are investigated.     

Fluid formulae for damping changeability conceptual design of railway semi-active hydraulic dampers

Damping changeability design and evaluation is the most fundamental issue at the beginning of any new railway semi-active hydraulic damper development. Therefore, physical fluid mechanics for the calculation of basic structure and resistance parameters of the damper should be carefully studied in the conceptual phase. Fluid formulae for changeable damping performance evaluation of two commercial railway semi-active hydraulic dampers are established. Simulation results show that the damper switched by high-speed solenoid valves obtains a wide range of changeable damping coefficients, which guarantees the absorption of a wide spectrum of vibrations; however, a different low cost damper regulated with an inversely proportional relief valve, whose Force-velocity characteristics share the same rising curve, is relatively limited in damping ability. In order to overcome the drawback of the latter one with no obvious cost increase, a new semi-active hydraulic damper which is regulated by a simple proportional throttle valve is proposed. Continued fluid formulation and simulation suggests that the damper can change its damping force rising curves or “effective” damping coefficients continuously, within a considerably wide range. Thus, fluid formulae explicitly established in this study are of significance in the damping changeability conceptual design, further refinement and control design for the three semi-active hydraulic dampers. The proposed new damper, which has both a simple configuration and an easy-to-control ability, might be feasible for industry applications.     

Selfdual spin 2 theory in a 2+1 dimensional (A)dS space–time

The lagrangian constraint analysis of the selfdual massive spin 2 theory in a 2+1 dimensional flat space–time and its extension to a curved one were performed. Demanding consistency of the degrees of freedom and causality in the model with gravitational interaction, gives rise to physical restrictions on non minimal coupling terms and background. Finally, a constant curvature scenario was explored, showing the existence of classes of forbidden mass values. Notorious aspects related with the construction of the reduced action and the one-particle exchange amplitude were mentioned.     

计算含大位移动边界非定常流动的无网格算法

研究了可以计算处理包含大位移动边界非定常流动的无网格算法.创建了无网格方法中由于大位移边界运动造成的不合要求点的判断标准,采用高效的填充方法实现点云重构,运用线性插值方法得到新点参数,实现了局部重构处理大位移动点.流场计算方面,在计算域自动布点基础上,采用曲线逼近计算导数及HLLC格式计算数值通量,发展了求解基于无网格的ALE方程组的算法.最后,对活塞问题进行了模拟,结果与解析解相吻合,验证了算法的准确性;另外,计算比较了气流流过静止圆柱以及圆柱在静止流场运动流场,结果表明方法是成功的.