非正交网格控制容积法分析复杂形状池内的流动

介绍一种基于非正交网格控制容积法的数学模型，及其在圆形沉沙池流动研究中的应用．该模型求解轴对称流动的连续方程及时均Ｎ－Ｓ方程，并采用标准ｋ－ε紊流模型，模拟圆形池内的流动．由于采用非正交网格，此计算模型可精确模拟几何形状较复杂的沉沙池内的流动，利用上述模型对某实际沉沙池进行了流场计算，计算所得流场与模型试验实测值符合良好．     

非正交网格控制容积法分析复杂形状池内的流动

介绍一种基于非正交网格控制容积法的数学模型，及其在圆形沉沙池流动研究中的应用．该模型求解轴对称流动的连续方程及时均Ｎ－Ｓ方程，并采用标准ｋ－ε紊流模型，模拟圆形池内的流动．由于采用非正交网格，此计算模型可精确模拟几何形状较复杂的沉沙池内的流动，利用上述模型对某实际沉沙池进行了流场计算，计算所得流场与模型试验实测值符合良好．     

脑循环脉动流的集中参数模型

脑血管疾病和脑循环动力学异常改变密切相关．脑循环系统和体循环系统具有不完全相同的血液动力学特性．因此，临床上迫切需要一种既能反映脑循环的基本特性，又容易从中分析脑血管动力学参数的力学模型．本文在Ｗｉｌｉｓ环定常流模型的基础上，建立了描述脑循环血液脉动特性的集中参数模型，归结出了模型控制方程及求解方法．通过实例计算发现：理论计算结果和实验检测数据相当吻合，说明模型是符合生理实际的．这将为脑循环研究提供一个较理想的血液动力学模型     

非均匀介质散射问题的体积分方程数值解法

将非均匀介质视为某一均匀背景介质中的扰动，可建立用均匀背景介质格林函数作基本解的体积分方程．给出了配置法求解体积分方程的数值方法，首先解得扰动域内各点以速度扰动为权的波场函数，然后回代计算得到观测面上各接收点的散射波场．与边界元法和Ｂｏｒｎ近似法计算结果比较表明该方法具有很高的精度，可得到穿过非     

基于结构化网格的导管螺旋桨敞水性能计算和艇桨干扰分析

本文采用结构化网格建模求解RANS方程,建立了导管桨和艇桨组合体的水动力性能数值计算方法.计算中以JD75简易导管与Ka4-70螺旋桨组合的导管螺旋桨及其与SUBOFF艇体模型的组合体为研究对象,利用RNG k-ε湍流模型封闭RANS方程得到控制方程.通过滑移网格方法计算了导管桨敞水性能,然后将该桨置于全附体SUBOFF潜艇模型后,计算螺旋桨与主艇体间的干扰特性,文中对流场等结果进行了分析比较.     

复合材料层合板冲击损伤近场动力学模型与分析

近场动力学(简称PD)理论通过域内积分建立物质基本运动方程。不同于传统理论中通过微分建立运动方程的方法,该理论对场函数没有连续性的要求,因而适合求解各类不连续问题。基于此,本文建立了正交各向异性单层板PD理论模型,进而引入单层板层间作用,发展了正交各向异性层合板PD模型及其损伤模型,并模拟了各向同性与各向异性层合板冲击损伤;通过对比分析,对模型的有效性进行了验证。     

线弹性多材料接头端部应力奇异性特征分析

文章研究多材料接头并计算其应力奇性特征.通过引入接头端部位移场的渐近级数展开式,将应力平衡方程转化为关于奇性指数的特征常微分方程组,由插值矩阵法求解获得多材料接头应力奇性指数以及相应的位移特征函数,与已有的结果对照证实了本文方法的有效性.本文计算结果还表明了多材料接头结构在不同材料粘结界面处位移场特征函数的一阶导数发生了突变,导致应力场在接头粘结界面处不连续.     

注塑成型三维流动的数值模拟

建立了非等温、粘性、不可压缩、非牛顿流体流动的控制方程。为了避免同时求解耦合的压力场、速度场,本文通过修改Galerkin方法的变分方程,导出了关于压力场的拟Poisson方程,用迭代法独立地求解连续性方程、动量方程,并进行速度一粘度迭代求出最终的压力场、速度场。由于直接使用Galerkin方法求解能量方程容易引起温度场的振荡,本文采用隐式格式及“上风”法离散能量方程,用超松驰迭代法求解温度场的代数方程组。比较了模拟结果与等温管道流动的解析解及法兰的实际注射结果,算例表明本文方法可以预测注射成型流动过程中的一些重要特征。与传统Galerkin方法相比,本文方法可以减少内存,提高数值方法的稳定性。     

粘弹流体轴对称流动的格子Boltzmann方法

基于BGK碰撞模型，通过在迁移方程中引入作用力项，建立了粘弹流体的轴对称格子Boltzmann模型．通过Chapman-Enskog展开，获得了准确的柱坐标下轴对称宏观流动方程．采用双分布函数对运动方程和本构方程进行迭代求解，模拟分析了粘弹流体管道流动，获得了流场中的速度和构型张量的分布，通过与解析解进行比较，验证了模型的准确性．研究了作为粘弹流体流动基准问题的收敛流动，对涡旋位置进行了定量分析，将回转长度的计算结果与有限体积法进行了比较，两种数值结果十分吻合．研究结果表明，模型能够准确表征粘弹流体的轴对称流动，具有较广阔的应用前景．     

无约束气云爆燃压力场的计算

利用球形流体力学方程和燃烧反应模型导出了无约束气云弱点火爆燃过程的压力分布场 ,编制了求解压力场的计算程序 ,可计算不同尺寸气云爆炸时各点的爆炸压力值。进行了可燃气云爆燃实验 ,对计算结果进行了考核。与实验结果相比 ,计算结果的偏差小于2 0 %。     

Numerical simulation of pedestrian flow past a circular obstruction

In this paper, a revisiting Hughes' dynamic continuum model is used to investigate and predict the essential macroscopic characteristics of pedestrian flow, such as flow, density and average speed, in a two dimensional continuous walking facility scattered with a circular obstruction. It is assumed that pedestrians prefer to walk a path with the lowest instantaneous travel cost from origin to destination, under the consideration of the current traffic conditions and the tendency to avoid a high-density region and an obstruction. An algorithm for the pedestrian flow model is based on a cellcentered finite volume method for a scalar conservation law equation, a fast sweeping method for an Eikonal-type equation and a second-order TVD Runge-Kutta method for the time integration on unstructured meshes. Numerical results demonstrate the effectiveness of the algorithm. It is verified that density distribution of pedestrian flow is influenced by the position of the obstruction and the path-choice behavior of pedestrians.     

一类平衡大气边界层边界条件研究

基于标准k-ε湍流模型,首先利用湍流粘度方程和剪切应力在整个边界层内恒定的假设,推导出一类耗散率表达式,并根据常用的湍动能入口剖面方程以及平均风速剖面方程,计算获得相应的耗散率方程;然后在输运方程中添加自定义源项,通过已经确定的平均速度方程、湍动能方程、耗散率方程计算得到相应输运方程的自定义源项表达式,并进行空风洞数值模拟,从而得到了一类满足平衡大气边界层的来流边界条件．通过将这种边界条件与由湍流平衡条件得到的边界条件进行比较,表明本方法获得的边界条件更适用．并且,本方法无需考虑修正壁面函数和修正湍流模型常数,因而计算更为简单,可为平衡大气边界层的研究提供一种新的思路．     

Gas-kinetic unified algorithm for plane external force-driven flows covering all flow regimes by modeling of Boltzmann equation

The nonequilibrium steady gas flows under the external forces are essentially associated with some extremely complicated nonlinear dynamics, due to the acceleration or deceleration effects of the external forces on the gas molecules by the velocity distribution function. In this article, the gas-kinetic unified algorithm (GKUA) for rarefied transition to continuum flows under external forces is developed by solving the unified Boltzmann model equation. The computable modeling of the Boltzmann equation with the external force terms is presented at the first time by introducing the gas molecular collision relaxing parameter and the local equilibrium distribution function integrated in the unified expression with the flow state controlling parameter, including the macroscopic flow variables, the gas viscosity transport coefficient, the thermodynamic effect, the molecular power law, and molecular models, covering a full spectrum of flow regimes. The conservative discrete velocity ordinate (DVO) method is utilized to transform the governing equation into the hyperbolic conservation forms at each of the DVO points. The corresponding numerical schemes are constructed, especially the forward-backward MacCormack predictor-corrector method for the convection term in the molecular velocity space, which is unlike the original type. Some typical numerical examples are conducted to test the present new algorithm. The results obtained by the relevant direct simulation Monte Carlo method, Euler/Navier-Stokes solver, unified gas-kinetic scheme, and moment methods are compared with the numerical analysis solutions of the present GKUA, which are in good agreement, demonstrating the high accuracy of the present algorithm. Besides, some anomalous features in these flows are observed and analyzed in detail. The numerical experience indicates that the present GKUA can provide potential applications for the simulations of the nonequilibrium external-force driven flows, such as the gravity, the electric force, and the Lorentz force fields covering all flow regimes.     

Investigation of slow monatomic gas flows past a circular cylinder

Slow low-Knudsen-number monatomic-gas flow past a circular cylinder is numerically investigated on the basis of a model kinetic equation. The gas flow is described by a new kinetic equation, from which the continuum equations for slow nonisothermal gas flows containing temperature stresses follow rigorously. It is shown that a closed convective-flow region arises near a nonuniformly heated cylinder in a slow gas flow if the flow impinges on the hot side of its surface. Using a new model of the Boltzmann equation makes it possible to study gas flows both in continuum and rarefied flow regimes.     

高雷诺数湍流风场大涡模拟的并行直接求解方法

在环境流体力学中,风场是风沙流、风雪流等自然环境特性问题研究的动力源和基础. 通常采用壁湍流模型进行风场大涡模拟(large eddy simulation, LES)计算,但受到计算规模的限制使得 高雷诺数风场的模拟计算难以实现. 并行计算技术是解决大规模高雷诺数风场大涡模拟的关键技术之一. 在不可压湍流风场的LES模拟中,压力泊松方程的并行计算技术是进行规模并行计算的困难点. 根据风场流动模拟计算的特点,采用水平网格等距而垂直于地面网格非等距,在解决规模并行计算中求解压力泊松方程的难点问题时,利用FFT解耦三维泊松方程使其变为垂向的一维三对角方程, 并利用可并行的三对角方程PDD求解技术,可建立三维泊松方程的直接并行求解技术. 结合其它容易并行的动量方程计算,本文建立风场LES模拟的并行直接求解方法(parallel direct method-LES, PDM-LES). 在超级计算机上对新方法进行并行效率测试,并行计算效率达到90${\%}$. 新的方法可用于进行湍流风场大涡模拟的大规模并行计算. 计算结果表明,湍流风场瞬时速度分布近壁面存在条带状的拟序结构,平均场的速度分布符合速度对数律特性,风场湍流特性基本合理.      

Calculating buoy response for a wave energy converter-A comparison of two computational methods and experimental results

When designing a wave power plant, reliable and fast simulation tools are required. Computational fluid dynamics(CFD) software provides high accuracy but with a very high computational cost, and in operational, moderate sea states, linear potential flow theories may be sufficient to model the hydrodynamics. In this paper, a model is built in COMSOL Multiphysics to solve for the hydrodynamic parameters of a point-absorbing wave energy device. The results are compared with a linear model where the hydrodynamical parameters are computed using WAMIT, and to experimental results from the Lysekil research site. The agreement with experimental data is good for both numerical models.     

Subgrid continuum modeling of particle motion in a turbulent flow

An Eulerian continuum approach to modeling the motion of dispersed particles within the framework of the large-eddy simulation method is developed. The approach is based on a kinetic equation for the filtered probability density function for the particle velocity in a turbulent flow. Models for the subgrid turbulent stresses of the dispersed phase are presented.     

SPH-based simulation of granular collapse on an inclined bed

The paper presents a numerical model for simulating a granular flow and its deposition on an inclined bed. A granular material is described as an elastic–plastic continuum and its constitutive law, namely Hooke's law, is discretized on the basis of the Smoothed Particle Hydrodynamics (SPH) method. In the equation of motion, however, the artificial viscosity, which is widely used in SPH, is not applied. The diffusive term derived from Hooke's law is introduced with a diffusion coefficient that varies depending on the stress and strain rate based on the Drucker–Prager yield function. The model is verified and validated through two numerical tests. It is shown that the basic elastic–perfectly plastic characteristics are reproduced with a simple shearing test. The effects of the diffusion coefficient and spatial resolution are investigated to show the validity of the model. In the simulation of the gravitational collapse of a granular column on an inclined bed, the performance of the model from the final deposition profile, the time history of the front position of the granular flow, the maximum runout distance, and the velocity profile are investigated for several cases of basal inclinations. The calculated results show good agreement with the experimental results.     

基于多块对接网格的隐式气体运动论统一算法应用研究

基于玻尔兹曼模型方程的气体运动论统一算法(gas kinetic unified algorithm,GKUA) 给出了一种能模拟从连续流到自由分子流跨流域空气动力学问题的途径. 该算法采用传统计算流体力学技术将分子运动和碰撞解耦处理,若采用显式格式将受格式稳定条件限制,在模拟超声速流动尤其是近连续流和连续流区的流动时计算效率较低. 为了提高计算效率,扩展其工程实用性,采用上下对称高斯-赛德尔(LU-SGS) 方法和有限体积法构造了求解玻尔兹曼模型方程的隐式方法,同时在物理空间采用能处理任意连接关系的多块对接网格技术. 通过模拟近连续过渡区并排圆柱绕流问题,计算结果与直接模拟蒙特卡洛方法模拟值吻合较好,验证了该方法用于跨流域空气动力计算的可靠性与可行性.      

Computation of head–disk interface gap micro flowfields using DSMC and continuum–atomistic hybrid methods

This paper discusses computational modeling of micro flow in the head–disk interface (HDI) gap using the direct simulation Monte Carlo (DSMC) method. Modeling considerations are discussed in detail both for a stand‐alone DSMC computation and for the case of a hybrid continuum–atomistic simulation that couples the Navier–Stokes (NS) equation to a DSMC solver. The impact of the number of particles and number of cells on the accuracy of a DSMC simulation of the HDI gap is investigated both for two‐ and three‐dimensional configurations. An appropriate implicit boundary treatment method for modeling inflow and outflow boundaries is used in this work for a three‐dimensional DSMC micro flow simulation. As the flow outside the slider is in the continuum regime, a hybrid continuum–atomistic method based on the Schwarz alternating method is used to couple the DSMC model in the slider bearing region to the flow outside the slider modeled by NS equation. Schwarz coupling is done in two dimensions by taking overlap regions along two directions and the Chapman–Enskog distribution is employed for imposing the boundary condition from the continuum region to the DSMC region. Converged hybrid flow solutions are obtained in about five iterations and the hybrid DSMC–NS solutions show good agreement with the exact solutions in the entire domain considered. An investigation on the impact of the size of the overlap region on the convergence behavior of the Schwarz method indicates that the hybrid coupling by the Schwarz method is weakly dependent on the size of the overlap region. However, the use of a finite overlap region will facilitate the exchange of boundary conditions as the hybrid solution has been found to diverge in the absence of an overlap region for coupling the two models. Copyright © 2009 John Wiley & Sons, Ltd.