两种湍流模型在风洞拐角流场计算中的应用及比较

为降低风洞拐角压力损失,提高其气动性能,使用雷诺应力模型(Reynolds Stress Model,RSM)和k-ε湍流模型对其流场进行数值模拟,分析和比较两种模型在压力损失因数、壁面摩擦损失和二次流损失上的异同,得出:使用两种模型均能发现拐角压力损失因数并不随雷诺数变化而变化;使用RSM得到的压力损失因数与实验值更为接近;相同雷诺数下,使用RSM计算得到的壁面摩擦损失因数比k-ε湍流模型的大,但两种模型得到的壁面摩擦损失因数变化趋势一致;两种模型计算得到的二次流损失随雷诺数变化规律一致,但是在相同雷诺数下,使用k-ε湍流模型计算得到的二次流损失比RSM大.     

基于二维模型的电梯井道空气流动分析

为研究电梯井道尺寸和电梯运行速度等对井道空气流动的影响,建立二维电梯井道模型,利用FLUENT对井道空气流动进行数值模拟.通过量纲分析和试验设计,针对不同阻塞比和雷诺数,选取60个样本点进行数值模拟,并建立流场参数的响应面.基于响应面的分析可知：轿厢阻力因数和井道空气最大流速比（最大流速与电梯运行速度之比）主要与阻塞比有关,并随阻塞比增大而变大;阻塞比和雷诺数都对轿厢表面的湍流强度和无量纲流场平均涡量都有影响.     

低雷诺数湍流模型的网格特征

低雷诺数模型目前主要应用于二维简单流动的数值仿真中,为研究该湍流模型在三维复杂流动计算中的网格特征,选取不同系列的车身面网格尺寸、车身壁面第一层边界层与壁面法向高度以及边界层层数等3组网格参数,利用ANSYS对阶背式MIRA模型外流场进行数值仿真.数值仿真结果与风洞试验的结果对比表明：数值计算得到的车身表面平均y+值随面网格尺寸增加而呈现减小趋势;网格方案对气动力因数和车身表面压力因数分布影响显著,气动阻力因数仿真值与试验值误差的变化区间为0.83%~7.93%,气动升力因数误差变化区间为10%~104%;气动阻力因数和气动升力因数均随着边界层层数的增加而增大,边界层层数为5时可以得到兼顾气动力因数精度和车身表面压力因数精度的较优仿真结果.     

内燃机中吸气和压缩过程的二维模拟

进行了发动机中空气吸气和压缩过程的二维计算机模拟．使用二维正交曲线坐标系以考虑燃烧室的复杂形状．采用SIMPLEC算法求解流体力学模型方程．论文中分别采用4个湍流模型：标准的k-ε模型,Realizable k-ε模型,雷诺应力模型和亚网格尺度模型．论文中对模拟计算的预测值进行了分析．计算出的流场与实验测定值进行了对比．通过模拟计算得到了内燃机中重要的流动图象．     

二维三温热传导方程组的九点差分格式

１．引言在惯性约束聚变（ＩＣＦ）的二维数值模拟中,常常要耦合求解包括电子热传导、离子热传导和光子热传导的辐射流体力学方程组,其中带有不同能量之间交换的三温热传导方程组的求解占有重要的位置,在高温高压等离子体的二维流场运动中,拉格朗日网格必然会发生扭曲,尤其在网格变形较大的情况下,如何给出解三温热传导方程组的差分格式和解法是值得探讨的一个重要问题．本文给出了能适应各种二维拉格朗日网格的三温热传导方程组的九点差分格式,并与五点差分格式作了对比计算．ｇＺ．三温热传导方程组及其九点差分格式拉格朗日坐标系…     

改进的灰色GM（1，1）预测模型

为进一步提高灰色GM（1,1）模型的模拟精度和预测精度,分析传统GM（1,1）模型存在的缺陷,提出一种改进的GM（1,1）预测模型。对已有GM（1,1）模型的背景值构造公式进一步优化,基于最小二乘法原理改进模型初始值参数的选取策略。对比实验结果表明,改进的模型适用于低增长序列和高增长序列,拓宽了传统GM（1,1）预测模型的应用范围,提高了模拟精度和预测精度。     

高精度曲面建模优化方案研究

目的：为了进一步提高高精度曲面建模（HASM）方法的模拟精度和计算速度,进而拓宽该模型的应用领域。方法：本研究采用新的差分格式计算HASM高斯方程中的一阶偏导数,以HASM预处理共轭梯度算法为例分析了改进的差分格式对HASM的优化效果。结果：数值试验表明：在计算耗时及内存需求不变的情况下,采用新的差分格式的HASM算法可以显著提高单次迭代的模拟精度,同时能够降低关键采样点缺失对模拟结果精度的影响。进一步研究发现,当HASM采用新差分格式与原始差分格式（中心差分）交替迭代时,能够快速降低模拟结果的误差。结论：本文基于HASM模型控制方程的离散差分改进格式,提出了新的HASM模型算法,新算法一方面当达到指定的精度条时能够显著减小计算耗时,另一方面还能降低关键采样点缺失对模拟结果的影响。     

GPU通用计算在LBM方法中的应用

提出了一种结合GPU通用计算与计算流体力学中的LBM算法来模拟二维流场的方法.根据GPU通用计算和LBM方法的基本原理,利用OpenGL的离屏渲染技术FBO和Cg语言,基于LBM方法中的D2Q9模型对二维方腔流进行数值模拟,并设计出基于OpenGL的GPU通用计算的二维流场数值计算框架.实验结果表明,利用GPU模拟与CPU模拟流场的数值结果相当吻合,特别地,利用GPU进行数值模拟实验的速度是利用CPU的4倍左右.     

基于涡量场的二维物体气动力计算

提出了在有限流场区域中根据涡量场计算二维物体气动力的两个公式。在FLUENT数值模拟得到的流场中,运用提出的两个公式,计算了雷诺数为1000的二维无厚度平板的气动力。按照不同方法计算得到的结果接近。提出的两个公式比Noca的FLUX和MOMENTUM公式在形式上简单,便于使用。     

高精度曲面建模优化方案

目的 为了进一步提高高精度曲面建模（HASM）方法的模拟精度和计算速度,进而拓宽该模型的应用领域,提出了新的HASM模型算法。方法 采用新的差分格式计算HASM高斯方程中的一阶偏导数,以HASM预处理共轭梯度算法为例分析改进的差分格式对HASM的优化效果。结果 数值实验表明：在计算耗时及内存需求不变的情况下,采用新的差分格式的HASM算法可以显著提高单次迭代的模拟精度,同时能够降低关键采样点缺失对模拟结果精度的影响。进一步研究发现,当HASM采用新差分格式与原始差分格式（中心差分）交替迭代时,能够快速降低模拟结果的误差。结论 本文算法当达到指定的精度条时能够显著减小计算耗时,同时还能降低关键采样点缺失对模拟结果的影响。     

A numerical study of flow past a cylinder with cross flow and inline oscillation

Two-dimensional fluid flow around an oscillating circular cylinder is studied numerically at different values of oscillation frequency and amplitude. A novel finite element method which uses discretization along the characteristic line is used for simulation. The solver is coupled to a mesh movement scheme using the Arbitrary Lagrangian-Eulerian (ALE) formulation to account for body motion in the flow field. Two cases of cylinder motion have been studied, cross flow and inline oscillation. In both cases, occurrence of lock on is investigated and the bounds of the lock on region are determined. A comparison of the numerical results with the experimental data indicates that 2D simulation is valid up to Re = 300. Beyond that, 3D effects appear. By using flow visualization, effect of a cylinder oscillation on the flow field and wake pattern has been studied. Also, variation of the mean drag coefficient against the oscillation parameters is discussed. The numerical results are in good agreement with the experimental data available in the literature.     

New model and scheme for compressible fluids of the finite difference lattice Boltzmann method and direct simulations of aerodynamic sound

A new scheme for the finite difference lattice Boltzmann method is proposed, in which negative viscosity term is introduced to reduce the viscosity and the calculation time can be remarkably reduced for high Reynolds number flows. A model with additional internal degree of freedom is also presented for diatomic gases such as air, in which an additional distribution function is introduced. Direct simulations of aero-acoustics by using the proposed model and scheme are presented. Speed of sound is correctly recovered. As typical examples, the Aeolian tone emitted by a circular cylinder is successfully simulated even very low Mach number flow. Full three-dimensional sound emission is also given.     

Control of flow using genetic algorithm for a circular cylinder executing rotary oscillation

We propose here a new approach to optimally control incompressible viscous flow past a circular cylinder for drag minimization by rotary oscillation. The flow at Re = 15000 is simulated by solving 2D Navier-Stokes equations in stream function-vorticity formulation. High accuracy compact scheme for space discretization and four stage Runge-Kutta scheme for time integration makes such simulation possible. While numerical solution for this flow field has been reported using a fast viscous-vortex method, to our knowledge, this has not been done at such a high Reynolds number by computing the Navier-Stokes equation before. The importance of scale resolution, aliasing problem and preservation of physical dispersion relation for such vortical flows of the used high accuracy schemes [Sengupta TK. Fundamentals of computational fluid dynamics. Hyderabad, India: University Press; 2004] is highlighted.For the dynamic problem, a novel genetic algorithm (GA) based optimization technique has been adopted, where solutions of Navier-Stokes equations are obtained using small time-horizons at every step of the optimization process, called a GA generation. Then the objective functions is evaluated that is followed by GA determined improvement of the decision variables. This procedure of time advancement can also be adopted to control such flows experimentally, as one obtains time-accurate solution of the Navier-Stokes equation subject to discrete changes of decision variables. The objective function - the time-averaged drag - is optimized using a real-coded genetic algorithm [Deb K. Multi-objective optimization using evolutionary algorithms. Chichester, UK: Wiley; 2001] for the two decision variables, the maximum rotation rate and the forcing frequency of the rotary oscillation. Various approaches to optimal decision variables have been explored for the purpose of drag reduction and the collection of results are self-consistent and furthermore match well with the experimental values reported in [Tokumaru PT, Dimotakis PE. Rotary oscillation control of a cylinder wake. J Fluid Mech 1991;224:77].     

求解Navier-Stokes方程的一类有限元有限体积迎风方法及其实现

Navier-Stokes方程是一类非线性的鞍点问题,在高Reynolds数流的情形下,标准Galerkin有限元方法会导致数值伪震荡.迎风有限元方法在算法结构上表征了流体＂上游＂决定＂下游＂的流动性态,它能够有效地消除高Reynolds数流的对流占优扩散所产生的非物理震荡.基于此,将Navier-Stokes方程的对流项采用有限体积框架下的迎风离散,对其它项仍使用Galerkin有限元离散,研究了二维定常Navier-Stokes方程的数值求解,编程藉助于有限元程序自动生成软件FEPG.通过对方腔流动和圆柱绕流问题与基准测试已有数值结果的比较,验证了所构造方法的可行性和有效性.     

A heuristic approach to effective sensor placement for modeling of a cylinder wake

The effectiveness of a sensor configuration for feedback flow control on the wake of a circular cylinder is investigated in both direct numerical simulation as well as in a water tunnel experiment. The research program is aimed at suppressing the von Kármán vortex street in the wake of a cylinder at a Reynolds number of 100. The design of sensor number and placement was based on data from a laminar two-dimensional simulation of the Navier-Stokes equations for the unforced condition. A low-dimensional proper orthogonal decomposition (POD) was applied to the vorticity calculated from the flow field and sensor placement was based on the intensity of the resulting spatial eigenfunctions. The numerically generated data was comprised of 70 snapshots taken over three cycles from the steady state regime. A linear stochastic estimator (LSE) was employed to map the velocity data to the temporal coefficients of the reduced order model. The capability of the sensor configuration to provide accurate estimates of the four low-dimensional states was validated experimentally in a water tunnel at a Reynolds number of 108. For the experimental wake, a sample of 200 particle image velocimetry (PIV) measurements was used. Results show that for experimental data, the root mean square estimation error of the estimates of the first two modes was within 6% of the desired values and for the next two modes was within 20% of the desired values. This level of error is acceptable for a moderately robust controller.     

基于NS2的802.11n D信道模型实现

分析马尔科夫随机过程信道建模机制,研究802.11n信道建模方案,基于查找信道容量、误包率、信噪比三者之间对应关系表格的方法,在NS2软件中实现802.11n D信道模型仿真模块。通过比较NS2仿真结果和TGn参考模型验证该模块的准确性,为后续相关802.11n协议的研究与改进提供有效的物理层信道仿真基础。     

Space-time discretizing optimal DRP schemes for flow and wave propagation problems

The present paper reports constrained optimization of explicit Runge–Kutta (RK) schemes, coupled with optimal upwind compact scheme to achieve dispersion relation preservation (DRP) property for high performance computing. Essential ideas of optimization employed in arriving at the proposed time integration scheme are extension of the earlier work reported in Rajpoot et al. (J Comput Phys 2010;229:3623–51). This is in turn an application of the correct error evolution equation in Sengupta et al. (J Comput Phys 2007;226:1211–8). Resultant DRP scheme demonstrated the idea for explicit spatial central difference schemes. Present work is similar, extending it for near-spectral accuracy compact schemes. Practical utility of the developed method is demonstrated by solution of model problems and for flow problems by solving Navier–Stokes equation, some of which cannot be solved by conventional schemes, as the problem of rotary oscillation of cylinder.Developed method is calibrated with: (i) flow past a circular cylinder performing rotary oscillation at Re = 150 and (ii) flow inside a 2D lid-driven cavity (LDC) at Reynolds numbers of Re = 1000 and Re = 10,000. Quantitative and qualitative comparisons show excellent match for rotary oscillation cylinder cases with the experimental results of Thiria et al. (J Fluid Mech 2006;560:123–47). Results for LDC for Re = 1000 are compared with that in Botella & Peyret (Comp Fluids 1998;27:421–33) and results for Re = 10,000 are compared with recent published ones showing triangular vortex in the core.     

A Transformation-free HOC Scheme for Incompressible Viscous Flow Past a Rotating and Translating Circular Cylinder

In the present work, a numerical study is made using a recently developed Higher Order Compact (HOC) finite difference scheme to test its capacity in capturing the very complex flow phenomenon of unsteady flow past a rotating and translating circular cylinder. The streamfunction-vorticity formulation of the Navier-stokes equations in cylindrical polar coordinate are considered as the governing equations. In the present investigation, flow is computed for a fixed Reynolds number (Re) 200 and rotational parameter values 0.5, 1.0, 2.07 and 3.25 are considered. Firstly, the flow patterns for different α values and for long time range are computed and qualitative comparisons are made with existing experimental and numerical results. Then, as a further check on the consistency of the experimental and present numerical results, quantitative comparisons are made for the velocity profiles at several locations. All these qualitative and quantitative comparisons show excellent agreements with existing experimental and numerical results.     

High Order Accurate Solution of Flow Past a Circular Cylinder

A high order method is applied to time-dependent incompressible flow around a circular cylinder geometry. The space discretization employs compact fourth-order difference operators. In time we discretize with a second-order semi-implicit scheme. A large linear system of equations is solved in each time step by a combination of outer and inner iterations. An approximate block factorization of the system matrix is used for preconditioning. Well posed boundary conditions are obtained by an integral formulation of boundary data including a condition on the pressure. Two-dimensional flow around a circular cylinder is studied for Reynolds numbers in the range 7 ≤ R ≤ 180. The results agree very well with the data known from numerical and experimental studies in the literature.