可压缩全速湍流流动的边界数值处理方法研究

计算流体力学问题的边界条件处理方法关系到数值仿真结果的精确度。为解决算法的精度,提出了三维可压缩湍流流动的边界条件数值处理方法,对所研究的边界类型包括进口边界、出口边界和固体壁面,流动的速度范围涉及亚音速、跨音速和超音速。流场数值仿真采用SIMPLE算法,湍流采用k-ε模型仿真。将边界总结为沟通型和孤立型边界两种类型,对每一控制方程分别阐述特定的数值处理方法。应用提出的边界处理方法对单圆弧凸包通道进行数值仿真获得了合理的结果,跨音速和超音速情形下准确地计算出了流场中存在的激波。     

一类四阶微积分方程的四阶差分格式

针对由吊桥模型而建立的四阶微积分方程,提出了四阶差分格式进行求解．对线性项采用紧格式进行离散,积分项则采用复化辛普森求积公式处理,再结合Newton型迭代法对方程进行求解．给出了差分格式解的存在性和收敛性的证明．数值结果表明格式的精度为O（h4）．     

S弯进气道内流分离数值仿真

在现代战斗机的发动机问题的研究中,S进气道是现代航空战斗机推进系统的一个重要组成部分,进气道内的流场品质会显著影响发动机性能.由于进气道内部流动的复杂性,为进气道内的流动特性优化和改善畸变问题,采用计算流体力学方法,空间离散格式采用二阶精度的高精度数值格式,应用基于Rhie和Chow法则的压力-速度耦合算法来求解雷诺平均的N-S方程,对S进气道内部流场进行了数值仿真,主要对S进气道内部流场总性能和流场发展细节进行了研究.数值计算结果反映出了流场的基本物理现象:出口气体畸变和气流分离的发展.同时也说明了所采用的研究方法是可行的,并为S进气道内部气流分离控制奠定了理论基础.     

A型地铁空调系统及客室内流场数值分析

为给目前国内A型地铁车辆的舒适度设计提供理论参考,针对地铁车辆静压风道结构特点,基于k-ε两方程湍流模型和SIMPLE算法,建立包含空调送风风道和客室的三维计算模型.对计算模型的空气流动和传热状况进行CFD数值计算.计算过程综合考虑车体壁面传热和人体散热等多种传热.分析计算结果得到客室内温度场和速度场的分布规律,并对空调通风设计方案进行量化评估.计算结果表明:客室内人体头部区域温度场分布均匀,平均温度为26.6℃,最大温差为6℃;车厢内有较理想的气流组织形式,速度分布范围为0.50~0.79 m/s,而且客室端部和中部区域人体头部周围速度较大.将计算结果与欧洲EN 14750-1标准进行对比分析,认为乘客的舒适性较好.     

基于Actran某车型后视镜风噪声计算

随着汽车发动机与轮胎路面噪声控制方法的研究与应用,汽车风噪声对车内声环境的影响越来越显著.风噪声对车内的作用分为声壁面脉动压力、湍流壁面脉动压力2种机理.选择一款车型,采用通用CFD软件与Actran相结合的方法,对其车内风噪声影响进行计算,并在后处理中分析2种机理导致的声源对车内驾驶员人耳处的噪声贡献率.     

缸内直喷发动机油气分离器模拟分析及试验验证

为分析3种不同结构迷宫式油气分离器的流动分布和压力损失,使用CFD仿真分析软件对某缸内直喷发动机油气分离器内气液两相流场进行数值模拟．采用离散模型模拟油滴粒子喷射,假定油滴粒子与壁面碰撞后即被捕捉,计算得到不同直径油滴的油气分离效率,并设计简单而有效的试验方法对油气分离器分离效率进行间接验证．结果表明,采用CFD软件模拟计算方法能够计算油气分离器油气分离效率,反映流动的本质．根据给定的油气分离效率可优化设计油气分离结构,改进生产方案．     

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.     

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。     

一类非线性延迟抛物偏微分方程的Crank-Nicolson型差分格式

对一类带有Dirichlet边界条件的延迟非线性抛物偏微分方程的初边值问题建立了一个Crank-Nicolson型的线性化差分格式,用离散能量法证明了该差分格式在L_∞范数下是无条件收敛的且是稳定的,其收敛阶为O(r~2+h~2).最后,用数值算例验证了理论结果.     

圆柱绕流的二维数值模拟和尾迹分析

为指导机械设计中参数和布局的选择,研究固定在水流中的圆柱结构件的受力情况和流场分布．利用FLUENT中的三种湍流模型对雷诺数为3900的圆柱绕流进行二维数值模拟并进行对比,得到升力因数、阻力因数、分离角、斯特劳哈尔数和涡街尺寸等参数的模拟结果,与参考文献中的实验结果对比验证二维模拟的预测精度．RKE（Realizable k-ε）和雷诺应力模型（Reynolds Stress Model,RSM）均能在此雷诺数下得出接近实验结果的流场,RSM模型使用POWER LAW离散格式的结果优于QUICK格式．与三维模拟的对比表明二维模拟适合在设计初期的快速估算,能够快速得到合适精度的模拟结果．     

Theoretical evaluation of the influence of geometric parameters and materials on the behavior of the airflow in a solar chimney

An analytical and numerical study of the unsteady airflow inside a solar chimney was performed. The conservation and transport equations that describe the flow were modeled and solved numerically using the finite volumes technique in generalized coordinates. The numerical results were physically validated through comparison with the experimental data. The developed model was used for airflow simulation in solar chimneys with operational and geometric configurations different from those found in the experimental prototype. Analysis showed that the height and diameter of the tower are the most important physical variables for solar chimney design.     

Incompressible flow computations over moving boundary using a novel upwind method

In this paper a novel method for simulating unsteady incompressible viscous flow over a moving boundary is described. The numerical model is based on a 2D Navier–Stokes incompressible flow in artificial compressibility formulation with Arbitrary Lagrangian Eulerian approach for moving grid and dual time stepping approach for time accurate discretization. A higher order unstructured finite volume scheme, based on a Harten Lax and van Leer with Contact (HLLC) type Riemann solver for convective fluxes, developed for steady incompressible flow in artificial compressibility formulation by Mandal and Iyer (AIAA paper 2009-3541), is extended to solve unsteady flows over moving boundary. Viscous fluxes are discretized in a central differencing manner based on Coirier’s diamond path. An algorithm based on interpolation with radial basis functions is used for grid movements. The present numerical scheme is validated for an unsteady channel flow with a moving indentation. The present numerical results are found to agree well with experimental results reported in literature.     

Nonlinear Advection–Diffusion–Reaction Phenomena Involved in the Evolution and Pumping of Oil in Open Sea: Modeling,Numerical Simulation and Validation Considering the Prestige and Oleg Naydenov Oil Spill Cases

The main goal of this article is to improve upon a previous model used to simulate the evolution of oil spots in the open sea and the effect of a skimmer ship pumping oil out from the spots. The concentration of the pollutant is subject to the effects of wind and sea currents, diffusion, and the pumping action of a skimmer (i.e., cleaning) ship that follows a pre-assigned trajectory. This implies that the mathematical model is of the advection–diffusion–reaction type. A drawback of our previous model was that diffusion was propagating with infinite velocity; in this article, we use an improved model relying on a nonlinear diffusion term, implying that diffusion propagates with finite velocity. To reduce numerical diffusion when approximating the advection part of the model, we consider second order discretization schemes with nonlinear flux limiters. We consider also absorbing boundary conditions to insure accurate results near the boundary. To reduce CPU time we use an operator-splitting scheme for the time discretization. Finally, we also introduce the modeling of coastlines and dynamic sources of pollutant. The novel approach we advocate in this article is validated by comparing our numerical results with real life measurements from the Oleg Naydenov and the Prestige oil spills, which took place in Spain in 2015 and 2002, respectively.     

A fourth order finite volume scheme for turbulent flow simulations in cylindrical domains

A finite volume scheme which is based on fourth order accurate central differences in the spatial directions and on a hybrid explicit/semi-implicit time stepping scheme was developed to solve the incompressible Navier-Stokes equations on cylindrical staggered grids. This includes a new fourth order accurate discretization of the velocity at the singularity of the cylindrical coordinate system and a new stability condition. The new method was applied in the direct numerical simulations (DNS) of the fully developed non-swirling turbulent flow through straight pipes with circular cross-section for the Reynolds number Re_τ = 360 based on the friction velocity u_τ and the pipe diameter. The obtained results are expressed in terms of statistical moments of the velocity components and are presented in comparison with those obtained with a second order accurate scheme and by measurements. It is shown that the fourth order spatial discretization leads to improved higher order statistical moments, while the first and the second order moments are more or less insensitive to the spatial discretization order.     

Computational study of flow in a rocket-based combined cycle (RBCC) engine inlet

A two-dimensional axisymmetric viscous flow analysis of a rocket-based combined cycle (RBCC) engine inlet is performed using a time-marching numerical scheme to solve the Reynolds-averaged Navier-Stokes equations. The flow configuration is a subscale model of the engine inlet which was previously tested in the 1X1 supersonic wind tunnel at NASA Glenn Research Center. The computed results are compared with the experimental data which include static pressure profiles along the centerbody and the cowl surface of the inlet. The computational results show a reasonably good agreement with the experimental data.     

Simulations of the wind field in Athens with the nonhydrostatic mesoscale model MEMO

MEMO is a fully vectorized nonhydrostatic mesoscale model using terrain-following coordinates. The numerical solution is based on second-order discretization applied on a staggered grid which is allowed to be non-equidistant in all directions. Special care is taken that conservative propeprties are preserved within the discrete model equations. The discrete pressure equation is solved with a direct elliptic solver in conjuction with a generalized conjugate gradient method. Advective terms are treated with an explicit, monotonicity-preserving discretization scheme with only small implicit diffusion. Turbulent diffusion is described using an one-equation turbulence model, while at roughness height similarity theory is applied. An efficient scheme is applied to calculate radiative transfer. The algebraic surface heat budget equation and an one dimensional heat conduction equation are solved to obtain the surface temperature over land and the soil temperature. In the frame of the APSIS exercise A the model MEMO was used to simulate the mesoscale flow in the Athens basin on May 25, 1990. Being in satisfactory agreement with observations, the results indicate that weak pressure gradients accompanied by warm advection aloft may lead to stagnant conditions and thus to severe air pollution episodes in Athens.     

Quasi-wavelet method for time-dependent fractional partial differential equation

In this decade, many new applications in engineering and science are governed by a series of fractional partial differential equations. In this paper, we propose a novel numerical method for a class of time-dependent fractional partial differential equations. The time variable is discretized by using the second order backward differentiation formula scheme, and the quasi-wavelet method is used for spatial discretization. The stability and convergence properties related to the time discretization are discussed and theoretically proven. Numerical examples are obtained to investigate the accuracy and efficiency of the proposed method. The comparisons of the present numerical results with the exact analytical solutions show that the quasi-wavelet method has distinctive local property and can achieve accurate results.     

汽车空气流量计校准系统的设计

本文介绍了一种基于风洞原理的汽车空气流量计校准系统的研制,该系统可消除汽车空气流量计生产制造过程的细微不一致导致的误差;在连续可调,高精度,稳定的气流条件下,计算机直接将每只汽车空气流量计的插值计算函数模型的参数写入内部的非易失存储空间来完成校准工作,并且论述了系统工作原理、硬件设计和软件设计,设计出了一种高校准精度,高效率,适合进行汽车空气流量计规模化生产的校准系统。