低耗散非结构网格多维限制器

基于极值原理分析非结构网格多维限制器的构造准则，通过引入约束位置和约束模板的定义，将非结构网格多维限制器构造方法统一化，并以此为基础提出一种新型非结构网格多维限制器.通过典型算例计算与已有限制器进行比较.结果表明：对于包含激波、接触间断等复杂波系干扰流动，新型多维限制器在保证数值稳定的条件下具有更低的数值耗散，对于复杂波系结构具有更高的流动分辨率.     

非结构网格边粘性公式

利用Kuropatenko提出的人为粘性形式和Christiansen建议的TVD(Total Variation Diminishing)输运限制子构造非结构网格边粘性,用最大角原则选择邻边构造限制子,研制包括边粘性的相容流体程序,给出边粘性与体粘性数值模拟比较,对Saltzman活塞问题、Noh问题进行数值实验.数值实验结果比较说明,非结构网格边粘性准确检出强速度梯度不存在的情形,减弱人为粘性力的作用,有效捕捉流体的激波间断,提高模拟精度.     

一种隐式预处理方法及其在定常和非定常流动数值模拟中的应用

将Choi-Merkle矩阵预处理方法与LU-SGS隐式方法、双时间法以及多重网格方法结合,发展适用于绕飞行器定常和非定常粘性流动的高效隐式预处理计算方法和程序.介绍一种针对定常和非定常流动的LU-SGS隐式预处理方法的统一表述方法.在不改变流动解的前提下,对Navier-Stokes方程的伪时间导数项实施Choi-Merkle矩阵预处理,从而改善可压缩控制方程在低速情况下的系统刚性,使基于LU-SGS时间推进格式的数值模拟方法同时适用于从极低马赫数到可压缩范围内的数值模拟.对Jameson中心格式的人工粘性进行相应的修改,以提高低速流动的计算精度.翼型、机翼以及翼身组合体绕流的数值模拟研究表明,隐式预处理方法获得了很高的计算效率,可使马赫数0.1左右的低速流动计算时间减少50%以上;通过对现有可压缩计算程序进行小量改动,便可使其均匀覆盖整个低速流动范围,提高CFD程序在飞行器绕流数值模拟中的实用性.     

一种非结构/结构多层混合网格方法及其应用

对于粘性绕流的数值模拟,在自适应直角网格基础上,结合三角形非结构网格和结构化网格,利用其各自的优势和特点,提出一种生成混合杂交网格的思路和方法.在物面附近生成适合粘性流计算的大长宽比结构化网格,在远场分布自适应直角网格,快速离散计算空间.对于复杂的多体问题,采用三角形网格来连接各体网格,并运用网格合并的方法,保证各网格之间的光滑过渡与连接,提高网格质量.针对一些二维、三维外形的绕流问题,在上述网格基础上,采用B-L代数湍流模型和中心有限体积法,完成Navier-Stokes和Euler方程数值模拟的对比计算,结果表明网格生成和流场计算是正确的.     

凹型钝头体超音速粘性流场的数值模拟

本文首先从单个方程出发,根据TVD思想,比较自然地给出了二阶带通量限制器的CSCM^[1]差分方法,然后推广到方程组情形。为了验证方法和程序,将球头绕流计算结果同文献[2]作了比较,然后运用此方法模拟了轴对称凹型钝体的粘性超音速分离绕流流场,对一阶与二阶、粘性与无粘性数值结果作了比较。计算网格采用了简单、快速易行的拟正交代数生成方法。     

基于非结构网格间断有限元方法在可压缩流体中的应用

本文开发了一套基于非结构网格的间断有限元方法(DG)程序,并对与单元形状无关的斜率限制器进行了研究。此程序支持多种网格类型,能够方便应用于具有混合单元的非结构网格,具有处理复杂几何结构的能力,为研究叶轮机械内部复杂流动现象提供了有效的研究工具。本文利用该程序对若干典型无黏和黏性问题进行数值模拟,结果表明,该程序具有较高的可信度,能够处理具有混合单元的非结构网格,并给出良好的数值模拟结果。     

类Apollo返回舱外形肩部热流的数值影响因素分析

预测类Apollo返回舱外形在高焓来流下的气动热特性,研究网格Reynolds数、壁面温度、多种化学反应模型以及限制器对预测热流的影响.采用ESI-CFD-FASTRAN软件作为数值模拟平台,使用基于温度梯度及分子扩散效应的热流模型;空间离散采用Roe-FDS格式,时间推进采用点隐式;采用等温壁面条件.数值计算表明:（1）热流在返回舱头部驻点处达到一个极值,沿着壁面热流不断下降,经过返回舱肩部热流有突越上升;（2）满足网格Reynolds数小于10的网格获得的热流较为准确;（3）使用Gupta模型计算得到的热流与Park85模型得到的类似,但是获得的热流分布类似;（4）采用湍流模型获得的头部肩部热流结果与层流结果相同;（5）二阶minmod限制器实现了高阶格式,其计算得到的热流结果在肩部略高,但是整体分布略低于不带限制器的格式.因此,在计算中采用满足网格Reynolds数壁面网格,采用带限制器的高阶格式计算获得的热流分布更加准确;由于头部热流主要贡献并非来源于湍流,因此对于肩部热流采用层流模型足够准确.      

非结构网格上大涡模拟数值方法研究

随着计算能力的飞速发展,在非结构网格上进行大涡模拟是研究叶轮机械真实几何结构下复杂流动问题的有效途径。本文针对非结构网格上数值格式人工黏性过大的缺点,利用湍流中动能级联特性,发展了适用于大涡模拟的低耗散数值格式,并结合亚格子模型,考察了其在基础湍流算例中的有效性。通过与传统迎风格式的对比,说明本文发展的低耗散格式可以显著提高非结构网格上大涡模拟的精度和可靠性。     

非结构网格上大涡模拟数值方法研究EI北大核心CSCD

随着计算能力的飞速发展,在非结构网格上进行大涡模拟是研究叶轮机械真实几何结构下复杂流动问题的有效途径。本文针对非结构网格上数值格式人工黏性过大的缺点,利用湍流中动能级联特性,发展了适用于大涡模拟的低耗散数值格式,并结合亚格子模型,考察了其在基础湍流算例中的有效性。通过与传统迎风格式的对比,说明本文发展的低耗散格式可以显著提高非结构网格上大涡模拟的精度和可靠性。     

二维非结构网格有限体积格式的壁热现象和人工热流方法

基于Noh人工热流方法的思想,推导出任意多边形网格人工热流的格式,将Noh的结构网格人工热流方法推广到任意多边形非结构网格的拉氏有限体积方法中,抑制或消除了壁热现象.几个数值实验,包括Noh激波反射问题和两块钢金属碰撞问题,验证了方法的可行性.     

Suitability of artificial bulk viscosity for large-eddy simulation of turbulent flows with shocks

The artificial bulk viscosity method to numerically capture shocks is investigated for large-eddy simulation (LES). Different variations of this method are tested on a turbulent flow over a cylinder at Reynolds number of 10,000 and free-stream Mach number of 0.85. The artificial bulk viscosity model by Cook and Cabot, which is parameterized by the strain rate magnitude, is found to provide unnecessary bulk viscosity in turbulent regions away from shocks. While developed turbulent structures are found unaffected, this extra bulk viscosity is shown to significantly damp the sound field. An alternative formulation of the model which is parameterized by the rate of dilatation is proposed. This formulation is shown to avoid the unnecessary bulk viscosity and enhance the sound-prediction capability of the model. It was found that standard LES combined with artificial bulk viscosity is a promising approach for simulation of turbulent flows with shocks. The formulation of the model on curvilinear coordinates is presented in the appendix.     

A tensor artificial viscosity using a finite element approach

We derive a tensor artificial viscosity suitable for use in a 2D or 3D unstructured arbitrary Lagrangian–Eulerian (ALE) hydrodynamics code. This work is similar in nature to that of Campbell and Shashkov [1]; however, our approach is based on a finite element discretization that is fundamentally different from the mimetic finite difference framework. The finite element point of view leads to novel insights as well as improved numerical results. We begin with a generalized tensor version of the Von Neumann–Richtmyer artificial viscosity, then convert it to a variational formulation and apply a Galerkin discretization process using high order Gaussian quadrature to obtain a generalized nodal force term and corresponding zonal heating (or shock entropy) term. This technique is modular and is therefore suitable for coupling to a traditional staggered grid discretization of the momentum and energy conservation laws; however, we motivate the use of such finite element approaches for discretizing each term in the Euler equations. We review the key properties that any artificial viscosity must possess and use these to formulate specific constraints on the total artificial viscosity force term as well as the artificial viscosity coefficient. We also show, that under certain simplifying assumptions, the two-dimensional scheme from [1] can be viewed as an under-integrated version of our finite element method. This equivalence holds on general distorted quadrilateral grids. Finally, we present computational results on some standard shock hydro test problems, as well as some more challenging problems, indicating the advantages of the new approach with respect to symmetry preservation for shock wave propagation over general grids.     

The structure of chaotic magnetic field lines in a tokamak withexternal nonsymmetric magnetic perturbations

We consider the effects of external nonsymmetric magnetostatic perturbations caused by resonant helical windings and a chaotic magnetic limiter on the plasma confined in a tokamak. The main purpose of both types of perturbation is to create a region in which field lines are chaotic in the Lagrangian sense: two initially nearby field lines diverge exponentially through many turns around the tokamak. The equilibrium field is obtained from the equations of magneto-hydrodynamic equilibrium written down in a polar toroidal coordinate system. The magnetic fields generated by the resonant helical windings and the chaotic magnetic limiter are obtained through an analytical solution of Laplace equation. The magnetic field line equations are integrated to give a Hamiltonian mapping of field lines that we use to characterize the structure of chaotic field lines. In the case of resonant windings, we obtained the map by both numerical integration and a Hamiltonian formulation. For a chaotic limiter, we analytically derived a symplectic map by using a Hamiltonian formulation     

高功率PIN限幅器设计及测试方案

 介绍了利用PIN二极管进行高功率无源限幅器的设计方法,分析了影响限幅器功率容量、限幅电平、尖峰泄露、响应速度及恢复时间等的因素。利用平面微带电路的形式,提出了无源检波式及主动式PIN限幅器设计,仿真结果表明:该无源检波式限幅器起限电平约为-3 dBm,脉冲功率容量60 dBm,限幅电平15 dBm左右。针对设计的限幅器,提出了用于测试限幅器性能的高功率实验平台。该平台采用双路双频耦合测量,具有很高的大小信号隔离度,能够进行高功率脉冲、连续波测试,在准确测量限幅器的功率容量、限幅电平、响应速度及恢复时间等指标方面具有很高的可靠性。     

Method of adaptive artificial viscosity

A new finite-difference method for the numerical solution of gas dynamics equations is proposed. This method is a uniform monotonous finite-difference scheme of second-order approximation on time and space outside of domains of shock and compression waves. This method is based on inputting adaptive artificial viscosity (AAV) into gas dynamics equations. In this paper, this method is analyzed for 2D geometry. The testing computations of the movement of contact discontinuities and shock waves and the breakup of discontinuities are demonstrated.     

Anisotropic viscous models of large-deformation Mohr–Coulomb failure

We have developed a way to represent Mohr–Coulomb failure within a mantle-convection fluid dynamics code. We use a viscous model of deformation with an orthotropic viscoplasticity (a different viscosity is used for pure shear to that used for simple shear) to define a prefered plane for slip to occur given the local stress field. The simple-shear viscosity and the deformation can then be iterated to ensure that the yield criterion is always satisfied. We again assume the Boussinesq approximation, neglecting any effect of dilatancy on the stress field. An additional criterion is required to ensure that deformation occurs along the plane aligned with maximum shear strain-rate rather than the perpendicular plane, which is formally equivalent in any symmetric formulation. We also allow for strain-weakening of the material. The material can remember both the accumulated failure history and the direction of failure. We have included this capacity in a Lagrangian-integration-point finite element code and show a number of examples of extension and compression of a crustal block with a Mohr–Coulomb failure criterion. The formulation itself is general and applies to 2- and 3-dimensional problems.     

Analysis of Suzuki, Shanker, and Kumar formulations under strong compressions

A critical analysis of the Suzuki, Shanker, and Kumar formulations is presented by studying different classes of materials under high pressure. A similar trend for all the materials studied in the present work, demonstrates that Suzuki formulation is not capable to yield compression behaviour of solids. The Shanker formulation improves the results obtained by the Suzuki formulation in small compression range (0.9<V/V₀<1). For further compressions Shanker formulation also fails. On the other hand, the Kumar formulation is found to work well for the entire range of pressure. The reasons for the failure of Suzuki and Shanker formulations are discussed.