一维对流扩散反应方程的高精度数值方法

通过将原方程变换为对流扩散方程,将所得方程的对流项采用四阶组合紧致迎风格式离散,扩散项采用四阶对称紧致格式离散之后,对得到的空间半离散格式采用四阶龙格库塔方法进行时间推进,得到了一种求解非定常对流扩散反应问题的高精度方法,其收敛阶为O(h4+τ4).经数值实验并与文献结果进行对比,表明该格式适用于对流占优问题的数值模拟,验证了格式的良好性能.     

一维非定常对流扩散方程的高阶组合紧致迎风格式

通过将对流项采用四五阶组合迎风紧致格式离散,扩散项采用四阶对称紧致格式离散之后,对得到的半离散格式在时间方向采用四阶龙格库塔方法求解,从而得到了一种求解非定常对流扩散方程问题的高精度组合紧致有限差分格式,其收敛阶为O(h~4+τ~4).经Fourier精度分析和数值验证,证实了格式的良好性能.三个数值算例包括线性常系数问题,矩形波问题和非线性问题,数值结果表明:该格式具有很高的分辨率,且适用于对高雷诺数问题的数值模拟.     

二维非线性反应扩散方程的局部间断Galerkin谱元法

本文提出了二维非线性反应扩散方程的局部间断Galerkin谱元法.在空间方向上采用了Legendre-Galerkin Chebyshev谱配置法,即在每个子区域上,该格式按Legendre-G alerkin谱方法形成,子区域交界面处的跳跃项利用数值流量进行处理,非线性项采用在Chebyshev-GaussLobatto点上的插值进行计算.时间方向上采用四阶低存储Runge-Kutta方法.文中给出了半离散格式下的稳定性和收敛性分析,以及单区域和多区域算法的数值算例,并与间断Galerkin有限元方法进行比较.     

基于特征线法的群体平衡系统的数值模拟

本文利用有限元方法和特征线离散技术,对工业结晶过程中化学反应沉降过程进行数值模拟.该化学反应沉降过程被称作群体平衡系统,它由五个耦合的偏微分方程描述,分别是描述溶液流动的不可压Navier-Stokes方程,描述两个反应物在溶质中进行化学反应的非线性对流扩散反应方程,描述一个生成物在溶质中生成与成核的非线性对流扩散反应方程,描述沉淀颗粒性态的颗粒尺寸分布方程(也称粒数衡算方程).除了时间和空间坐标外,颗粒尺寸分布方程还包含一个描述颗粒大小的内部坐标.我们利用特征线方法,将这个高维方程的求解转化成一系列可以并行进行的低维问题的求解.基于对在方腔内的碳酸钙沉降过程的数值模拟,本文定性地给出反应物入流口位置和生成物出流口位置的相对关系对沉淀物颗粒大小的影响.     

分数阶对流-弥散方程的移动网格有限元方法

相比经典的对流-弥散方程,分数微分算子的非局部性质导致分数阶对流-弥散方程(FADE)的有限元方法在每个单元上的计算都联系一个带弱奇异核的数值积分.当弥散项分数阶μ接近1时,穿透曲线出现重度拖尾,数值解产生振荡.研究表明:时间半离散后的FADE在特殊的变分形式下,有限元刚度矩阵有直接计算公式;以De Boor算法为基础的移动网格方法能很好地消除数值振荡.     

对流扩散方程的间断Galerkin自适应方法

本文给出求解二维非线性对流扩散方程的局部间断Galerkin有限元方法在非协调三角网格上的自适应算法实现.数值算例表明这种方法可以高效追踪真解的剧烈变化.     

不同密度与粘性的多相流移动接触线问题的自适应有限元方法

本文中,对于具有不同密度与粘性差的多相流移动接触线问题,我们提出了一种自适应有限元方法.我们所使用的模型为Cahn-Hilliard-Navier-Stokes模型,以及其广义Navier边界条件.在时间上,我们使用分裂方法来求解此系统:对于Cahn-Hilliard方程,使用一种基于凸分解的半隐式方法求解;对于Navier-Stokes方程,采用了压力稳定化方法求解.这种方法在满足某些条件下,是能量稳定的,而且对于处理大密度差问题特别有效.在空间上,我们采用了满足LBB条件的有限元方法进行离散,而且基于后验误差估计理论,我们给出了其网格的自适应加密/放粗方法.数值试验证明了我们算法的正确性与有效性.     

微重环境下Cassini贮液腔中液体晃动特性研究

针对我国某一型号大型卫星液体燃料Cassini贮箱（腰为圆柱,两底为半球）,应用有限元方法研究了微重环境下液体的小幅晃动问题和横向受迫晃动问题,采用Galerkin方法得到了系统的有限元离散方程；得到了晃动固有频率和等效力学模型参数.针对周期脉冲激励,推导了液体作用于贮箱壁的晃动力和晃动力矩计算公式并给出了数值计算结果和分析结论.     

求解复杂交通流模型的低耗散中心迎风格式

针对非均匀道路上的多车种LWR交通流模型,提出一种低耗散中心迎风格式。以4阶中心加权基本无震荡重构和低耗散中心迎风数值通量为基础,通过构造不同形式的全局光滑因子及增大非光滑模板对应的非线性权重优化数值格式的耗散特性,并采用Runge-Kutta方法对半离散数值格式在时间方向上进行离散使其保持4阶精度。对非均匀道路上多车种LWR交通流模型的车道数变化和交通信号灯控制问题进行数值模拟,结果表明该格式具有4阶求解精度,且分辨率高。     

广义空间分数阶Burgers方程的Legendre Galerkin-Chebyshev配置方法逼近

本文采用Legendre Galerkin-Chebyshev配置方法求解广义空间分数阶Burgers方程.该方法基于Legendre Galerkin变分形式,但是非线性项与右端源项采用Chebyshev-Gauss插值逼近.首先,通过在空间方向采用Legendre Galerkin-Chebyshev配置方法离散,时间方向采用leap-frog/Crank-Nicolson格式离散,得到了方程的全离散格式,其中非线性项能够显式计算.接着,给出了稳定性分析及L~2-范数下的误差估计.数值算例显示该方法的稳定性,高效性及易实现性.     

Development of a second order approximation for the Navier-Stokes equations

The purpose of this paper is the development of a 2nd order finite difference approximation to the steady state Navier-Stokes equations governing flow of an incompressible fluid in a closed cavity. The approximation leads to a system of equations that has proved to be very stable. In fact, numerical convergence was obtained for Reynolds numbers up to 20,000. However, it is shown that extremely small mesh sizes are needed for excellent accuracy with a Reynolds number of this magnitude. The method uses a nine point finite difference approximation to the convection term of the vorticity equation. At the same time it is capable of avoiding values at corner nodes where discontinuities in the boundary conditions occur. Figures include level curves of the stream and vorticity functions for an assortment of grid sizes and Reynolds numbers.     

On the solution of the unsteady Navier- Stokes equations including multicomponent finite rate chemistry

An explicit-implicit time-dependent finite difference technique is presented which has been successfully implemented for the solution of the unsteady Navier-Stokes equations including multicomponent finite rate chemistry. Stability restrictions encountered in explicit schemes due to the variable relaxation times associated with the finite rate chemical reactions are eliminated. As a result, the stability of the coupled Navier-Stokes and species conservatiion finite difference equations is shown to be governed only by the hydrodynamic stability cri- terion. The viscous nonequilibrium flow about a blunt axisymmetric nosetip at high Mach number and low Reynolds number, where the viscous shock layer and shock transition zone are merged, was computed using this time-dependent numerical scheme. Solutions along the stagnation streamline are compared with corresponding solutions obtained by Dellinger using the thin layer approximation of the Navier-Stokes equations.     

Computation of taylor vortex flow by a transient implicit method

A finite difference method is presented for the computation of steady axisymmetric solutions of Navier-Stokes equations using the time dependent stream function, vorticity, and tangential velocity formulation. The scheme involves implicit fractional steps and fast Fourier transforms. Upwind differencing for convective terms is used in order to increase the stability for high values of the Reynolds number. The method is applied to the flow in an annulus of rectangular cross section with rotating walls. Attention is focused upon the problem of centrifugal instabilities, non-uniqueness of the steady state solution, and selection of wavelengths in the supercritical range.     

Flow around the impulsively started elliptic cylinder at various angles of attack

The flow around an impulsively started elliptic cylinder at 0, 30, 45 and 90° incidence is investigated. The fluid is viscous, incompressible and its flow is governed by the Navier-Stokes equations. Semi-analytical solutions are calculated by solving numerically the system of coupled partial differential equations which are obtained by substituting the expanded finite Fourier Series of the stream and vorticity functions in the Navier-Stokes equations. The symmetrical solutions are presented for Reynolds number 200 and eccentricity 0.809 and 0.943 in terms of patterns of streamlines, lines of constant vorticity, pressure and vorticity distributions around the surface, drag coefficient and wake length at 0 and 90° and compared with the experimental results. A comparison of the calculations has been made for Reynolds number 100 and eccentricity 0.648 with different number of terms at 90°. A Kármán vortex street develops for Reynolds numbers 200 and 60 at 30 and 45° incidence and the solutions are presented in terms of various characteristics including Strouhal number. The vanishing of wall-shear does not denote separation in any meaningful sense in various cases.     

A finite element analysis of laminar flows through planar and axisymmetric abrupt expansions

Laminar flow of a Newtonian fluid in planar and axisymmetric abrupt expansions is studied by solving the Navier-Stokes equations using the finite element method. The results consolidate information provided in the literature and provide a broader picture of how the expansion ratio and Reynolds number influence the reattachment length, downstream location of the eddy and the relative eddy intensity in both co-ordinate systems.     

A fine grid finite element computation of two-dimensional high Reynolds number flows

A velocity—pressure integrated, mixed interpolation, Galerkin finite element computation of the Navier-Stokes equations using fine grids, is presented. In the method, the velocity variables were interpolated using complete quadratic shape functions: and the pressure was interpolated using linear shape functions defined on a triangular element, which is contained inside the quadratic element for velocity variables. Comprehensive computational results for a cavity flow for Reynolds number of 400 through 10,000 and a laminar backward-facing step flow for Reynolds number of 100 through 900 are presented in this paper. Many high Reynolds number flows involve convection dominated motion as well as diffusion dominated motion (such as the fluid motion inside the subtle pressure driven recirculation zones where the local Reynolds number may become vanishingly small) in the flow domain. The computational results for both of the fluid motions compared favorably with the high accuracy finite difference computational results and/or experimental data available.     

Time-dependent solutions of the viscous incompressible flow past a circular cylinder by the method of series truncation

Semi-analytic solutions of the Navier-Stokes equations are calculated for two-dimensional, symmetrical, viscous incompressible flow past a circular cylinder. The stream and vorticity functions are expanded in the finite Fourier series and then substituted in the Navier-Stokes equations. This led to a system of coupled parabolic partial differential equations which are solved numerically. More terms of the series are required as Reynolds number increases and the present calculations were terminated at Reynolds number 600 with 60 terms of Fourier series. The results are compared with similar calculations and experimental data for Reynolds numbers 60, 100, 200, 500, 550 and 600. At the termination of the calculations for Reynolds numbers 60 and 100, the separation angle, the wake length, the drag coefficient, and the vorticity distributions around the surface were very close to their steady-state values. A secondary vortex appeared on the surface of the cylinder in the case of Reynolds numbers 500, 550 and 600. The wake length, the drag coefficient and the separation angle differ slightly at a given instant in the case of Reynolds numbers 500, 550 and 600.     

Numerical solutions of 2-D steady incompressible flow over a backward-facing step, Part I: High Reynolds number solutions

Numerical solutions of 2-D laminar flow over a backward-facing step at high Reynolds numbers are presented. The governing 2-D steady incompressible Navier-Stokes equations are solved with a very efficient finite difference numerical method which proved to be highly stable even at very high Reynolds numbers. Present solutions of the laminar flow over a backward-facing step are compared with experimental and numerical results found in the literature.     

Numerical studies of the flow around a circular cylinder by a finite element method

Numerical solutions of the steady, incompressible, viscous flow past a circular cylinder are presented for Reynolds numbers R ranging from 1 to 100. The governing Navier-Stokes equations in the form of a single, fourth order differential equation for stream function and the boundary conditions are replaced by an equivalent variational principle. The numerical method is based on a finite element approximation of this principle. The resulting non-linear system is solved by the Newton-Raphson process. The pressure field is obtained from a finite element solution of the Poisson equation once the stream function is known. The results are compared with those determined by other numerical techniques and experiments. In particular, the discussion is concerned with the development of the closed wake with Reynolds number, and the tendency of R ≥ 40 flow toward instability.     

Numerical calculations on viscous flow fields through cylinder arrays

The incompressible time independent Navier-Stokes equations for two-dimensional laminar flow through arrays of parallel circular cylinders are solved numerically. A square mesh in the physical field is used to effect a finite difference iterative procedure for solving the elliptic type partial differential equations. Results which exhibit the main characteristics of this type of fluid flow are given for certain regular arrays at cylinder Reynolds numbers of 1 and 20.