连续分层海洋中内波传播的一种数值模式

基于Euler方程,使用有限体积法建立了一种密度为连续分层情况下、适应水深变化的水域中内波传播的数值模式.为了使计算格式能够达到二阶精度,对流项的处理使用了TVD (total variation diminishing)格式.将SIMPLE算法引入连续分层海洋中内波的数值计算,为了简化计算并方便地适应多种TVD格式,在计算预估速度场时采用了显式格式,而没有采用传统的隐式格式;鉴于在原始的SIMPLE算法中没有涉及到由于密度扰动而引起的静水压力场的改变问题,给出了该问题的计算方法.因此改进了SIMPLE算法.出流边界的处理采用阻尼消波和Sommerfeld辐射条件相结合的方式,以使内波得到有效的衰减和释放.将等水深水域的数值解和理论解进行了比较,两者吻合较好;并对存在潜堤时数值计算的不同时刻密度变化的空间分布进行了详细的定性分析.计算结果表明,所建立的数值模式能有效地模拟内波的传播和变形.      

三维湍流流动的涡量-向量势法数值模拟

本文采用涡量-向量势法,结合常规的 k-ε双方程湍流模型,并运用重新推导的固体壁面涡量边值计算法,对扁平射流煤粉燃烧器内的冷态三维流场进行了数值模拟。计算结果与实验数据相吻合。本文工作证明,用涡量-向量势法求解三维湍流问题是可行的,该法具有不应忽视的优越性,值得进一步开发。     

湍流加速火焰的三维数值模拟

火焰在设有障碍物的管内传播时会自身加速，并可能导致爆炸。本文基于湍流κ-ε模型和改进的EBU—Arrhenius反应模型，对该现象进行了三维空间的数值模拟。计算结果反映了障碍物、湍流和火焰之间相互作用的正反馈机理，描绘了火焰在管内加速传播的三维图像。     

非稳态沙波纹流场的数值模拟

风成沙波纹是沙质地表在风力作用下形成的最基本的地貌类型.本文研究了沙波纹在形成过程中,风场对波纹的形态变化的响应.采用目前比较流行的有限体积法,通过求解非稳态情况下的Navier-Stokes方程来计算风场;地表形态采用阶梯网格进行逼近,为了对地表形态变化进行有效的控制,根据相关参数采用正弦函数作为沙波纹的基本形状;最后根据计算结果详细讨论了沙波纹从形成到稳定的发展过程中地表流场的行为变化特征.     

k-ωSST湍流模式三维激波分离流动修正

“激波?边界层分离”是航空气动领域的典型湍流非平衡流动问题, 准确模拟激波分离对于跨声速飞行器气动性能评估和优化设计具有重要意义. 然而传统涡黏性湍流模式中涡黏性系数的定义方式并不适用于非平衡流动, k-ω SST湍流模式为此引入的Bradshaw假设在应用于三维强逆压梯度和较大分离流动时反而限制了雷诺应力的生成, 导致包括k-ω SST在内的常用涡黏性湍流模式均无法对此类流动进行准确模拟. 同时, 现有的非线性雷诺应力本构关系也并不能有效提高模拟精度. 为此, 针对k-ω SST模式分别提出了基于Bradshaw假设和基于长度尺度的两种激波分离流动修正方法. 前者通过提高Bradshaw常数的方式放宽了对雷诺应力生成的限制, 后者则从湍流长度尺度概念出发, 利用混合长度理论、湍动能生成/耗散之比和一种新定义的长度尺度之比构造了ω方程耗散项修正函数, 提高了模式在三维激波分离流动中的建模长度尺度. 两种方法对ONERA M6机翼跨声速大攻角流动均能得到较雷诺应力模式更好的模拟结果. 进一步的雷诺应力分析表明, 三维激波分离流动中“主雷诺应力分量”的概念不再成立, 各雷诺应力分量大小接近. 网格收敛性分析、对其他攻角状态的验证以及湍流平板边界层壁面律验证进一步确认了所提出的两种修正方法的合理性、有效性和通用性.      

圆柱周围湍流和含沙多相流的SPH数值模拟

湍流和多相流是流体力学中最具挑战性的两个主题,湍流多相流的实验和数值模拟更是一项艰巨的挑战。此外,对颗粒干沉积方面的多相流、多尺度、多物理耦合特征的风沙流的综合实地观测仍然很少。因此,本文综合考虑湍流、多相流与多物理耦合等方面,采用以圆柱为干扰物产生对流涡流的强制干扰技术,以塔克拉玛干沙漠地带中和田至若羌铁路的过沙桥桥墩为研究背景。为摆脱有限元软件中由网格大变形或失真引起的各种问题,采用SPH方法的宏观界面追踪和微观单点追踪相结合的方式,初步揭示了以单相对流涡流为风场背景的含沙多相流环境下的圆柱周围复杂的流场变化以及对颗粒干沉积运动的影响。采用数值模拟与现场实验相结合的方式,着重对计算域边界壁面和圆柱壁面对空气单相流中对流涡流的成形运动及其特征分析、两相流中对流涡流在圆柱周围的夹沙运动模拟及其特性分析、两相流中对流涡流的夹沙率以及边界壁湍流对沙粒干沉积效率展开分析研究。     

三维非结构聚合多重网格法数值模拟研究

在三维非结构网格上应用聚合式多重网格技术来加速Euler方程的收敛过程．自行设计了一种高效率的网格聚合方法．采用四重三维非结构网格，在每一层网格上采用有限体积法进行计算．通过对M6翼型的数值求解验证了多重网格加速收敛的高效性．     

激波和湍流相互作用的数值模拟

本文综述了关于激波和湍流相互作用数值模拟的近期研究进展, 主要包括激波和均匀各向同性湍流、激波和湍流边界层、激波和射流以及激波和尾迹的相互作用. 激波和湍流相互作用特性受到诸多因素的影响,如激波的强度、位置、形状和流动边界以及来流的湍流状态和可压缩性等. 激波和湍流的相互作用会引起流场结构、激波特性和湍流统计特性的显著变化. 最后简要讨论了激波和湍流相互作用数值研究需要关注的一些问题.      

湍流燃烧数值模拟研究的综述

对湍流燃烧数值模拟的研究进行了综合评述,其中涉及直接数值模拟（ＤＮＳ）、大涡模拟（ＬＥＳ）、随机涡模拟、概率密度函数输运方程模拟、条件矩模型、简化概率密度函数模型、关联矩模型以及基于简单物理概念的一些唯象模型等几个重要方面．对全面了解湍流燃烧数值模拟的研究现状及前景提出了看法．     

亚临界雷诺数下圆柱受迫振动的数值研究

在Navier-Stokes方程和k-ω湍流模型的基础上,利用流线迎风有限元方法结合ALE动网格技术对亚临界雷诺数下的圆柱受迫振动问题开展了数值模拟研究。本文的数值模型成功模拟了Re=5000条件下,圆柱发生受迫振动时尾迹区内的2S,2P和P+S尾流模式;对Re=10000情况下,无量纲振幅分别为0.3,0.4,0.5的圆柱受迫振动问题开展了数值模拟,分析了给定振幅条件下圆柱受力随振动频率的变化关系以及受迫振动的锁定区间。以上数值计算结果与Gopalkrishnan (1993)的实验结果基本符合。研究结果表明,二维数值模型能够基本正确地反映出圆柱发生受迫振动时的涡激振动特性以及有关的受力变化趋势,为今后进一步开展三维数值分析工作奠定了基础。     

A numerical study of the turbulent flow around the stern of ship models

The present work is concerned with the numerical calculation of the turbulent flow field around the stern of ship models. The finite volume approximation is employed to solve the Reynolds equations in the physical domain using a body-fitted, locally orthogonal curvilinear co-ordinate system. The Reynolds stresses are modelled according to the standard k-ε turbulence model. Various numerical schemes (i.e. hybrid, skew upwind and central differencing) are examined and grid dependence tests have been performed to compare calculated with experimental results. Moreover, a direct solution of the momentum equations within the near-wall region is tried to avoid the disadvantages of the wall function approach. Comparisons between calculations and measurements are made for two ship models, i.e. the SSPA and HSVA model.     

The turbulent flow field around a circular cylinder

The flow field around a circular cylinder mounted vertically on a flat bottom has been investigated experimentally. This type of flow occurs in several technical applications, e.g. local scouring around bridge piers. Hydrogen bubble flow visualization was carried out for Reynolds numbers ranging from 6,600 to 65,000. The main flow characteristic upstream of the cylinder is a system of horse-shoe vortices which are shed quasi-periodically. The number of vortices depends on Reynolds number. The vortex system was found to be independent of the vortices that are shed in the wake of the cylinder. The topology of the separated flow contains several separation and attachment lines which are Reynolds number dependent. In the wake region different flow patterns exist for each constant Reynolds number.     

AN EXTENDED k-ε MODEL FOR NUMERICAL SIMULATION OF WIND FLOW AROUND BUILDINGS

It is assumed in this paper that for a high Reynolds number nearly homogeneouswind flow, the Reynolds stresses are uniquely related to the mean velocity gradientsand the two independent turbulent scaling parameters k and E. By applying dimensionalanalysis and owing to the Cayley-Hamilton theorem for tensors, a new turbulenceenclosure model so-called the axtended k-ε model has been developed. The coefficientsof the model expression were detemined by the wind tunnel experimental data ofhomogeneous shear turbulent flow. The model was compared with the standard k-εmodel in in composition and the prediction of the Reynold’s normal Stresses. Using thenew model the numerical simulation of wind flow around a square cross-section tallbuilding was performed. The results show that the extended k-ε model improves theprediction of wind velocities around the building the building and wind pressures on the buildingenvelope.     

Three-dimensional coupled numerical model of creeping flow of viscous fluid

A three-dimensional coupled numerical model is developed to describe creeping flow in a computational domain that consists of a thick viscous layer overlaid with a thin multilayered viscous sheet. The density of the sheet is assumed to be lower than that of the layer. The model couples the Stokes equations describing the flow in the layer and the Reynolds equations describing the flow in the sheet. We investigate the long-time behavior of the flow in the sheet by using an asymptotic method and derive an ordinary differential equation for the sheet boundary displacements and the velocities at the interface between the sheet and the layer. The Stokes and Reynolds equations are coupled by applying the resulting equation as an internal boundary condition. Numerical implementation is based on a modified finite element method combined with the projection gradient method. The computational domain is discretized into rectangular hexahedra. Piecewise square basis functions are used. The model proposed enables different-type hydrodynamic equations to be coupled without any iterative improvements. As a result, the computational costs are reduced significantly in comparison with available coupled models. Numerical experiments confirm that the three-dimensional coupled model developed is of good accuracy.     

Thermodynamic analysis and numerical resolution of a turbulent - fully ionized plasma flow model

This paper is devoted to the modeling and numerical resolution of a non-dissipative compressible turbulent plasma flow model involving three temperatures (turbulence, ions and electrons). The first step is to derive such a model. To do this, an analysis of the Reynolds averaged Euler equations (the k-model) is carried out. It is shown that thermodynamic requirements enable the derivation of an equation of state for turbulent variables. This equation of state is of the same type as those of an ideal gas. In this context, the various thermodynamic variables of turbulence can be obtained (energy, pressure, temperature etc.). This hyperbolic conservative model has exactly the same structure as the two temperatures plasma model of Zeldovich. Thanks to the clear structure of these two models, the turbulent plasma model is derived and involves three temperatures. The second step is to derive an accurate numerical scheme for its solution. A linearized Riemann solver and a positive HLLC type solver are derived and embedded into a conventional Godunov scheme. It is shown that this method requires important corrections to preserve contact discontinuities and temperatures monotonicity. The corrections are based upon a non-conservative formulation of the turbulence and electrons energy equations, while total energy conservation is preserved. The modified method behaves correctly with contacts and shocks.Received: 11 February 2002, Revised: 19 June 2003, Accepted: 7 August 2003, Published online: 14 October 2003 Correspondence to: R. Saurel     

Direct numerical simulation of turbulent pipe flow

Fully developed turbulent pipe flow at low Re-number is studied by means of direct numerical simulation (DNS). In contrast to many previous DNS's of turbulent flows in rectangular geometries, the present DNS code, developed for a cylindrical geometry, is based on the finite volume technique rather than being based on a spectral method. The statistical results are compared with experimental data obtained with two different experimental techniques. The agreement between numerical and experimental results is found to be good which indicates that the present DNS code is suited for this kind of numerical simulations.     

Variational model of a turbulent rotating flow

A model of effectively viscous turbulent flows satisfying the Navier-Stokes equations and certain slip conditions at the walls is analyzed. The turbulent viscosity is determined on the basis of the principle of minimum energy dissipation rate, whose significance and conditions of applicability are discussed in detail. A new separated turbulent flow model is outlined. The problem of turbulent flow in a porous rotating tube is solved. The existence of two metastable flow regimes is predicted: one with an axial circulation zone, the other straight-through. In the case of a strongly swirled flow the first of these has a greater probability of realization; however, as the rotation weakens, in a certain critical situation the circulation zone collapses, after which the flow can only be straight-through. Despite the absence of empirical content, every aspect of the proposed theory is in good agreement with the experimental research on vortex chamber flows.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 22–32, May–June, 1985.     

Direct numerical simulation of a freely decaying turbulent interfacial flow

Whereas Large Eddy Simulation (LES) of single-phase flows is already widely used in the CFD world, even for industrial applications, LES of two-phase interfacial flows, i.e. two-phase flows where an interface separates liquid and gas phases, still remains a challenging task. The main issue is the development of subgrid scale models well suited for two-phase interfacial flows. The aim of this work is to generate a detailed data base from direct numerical simulation (DNS) of two-phase interfacial flows in order to clearly understand interactions between small turbulent scales and the interface separating the two phases. This work is a first contribution in the study of the interface/turbulence interaction in the configuration where the interface is widely deformed and where both phases are resolved by DNS. To do this, the interaction between an initially plane interface and a freely decaying homogeneous isotropic turbulence (HIT) is studied. The densities and viscosities are the same for both phases in order to focus on the effect of the surface tension coefficient. Comparisons with existing theories built on wall-bounded or free-surface turbulence are carried out. To understand energy transfers between the interfacial energy and the turbulent one, PDFs of the droplet sizes distribution are calculated. An energy budget is carried out and turbulent statistics are performed including the distance to the interface as a parameter. A spectral analysis is achieved to highlight the energy transfer between turbulent scales of different sizes. The originality of this work is the study of the interface/turbulence interactions in the case of a widely deformed interface evolving in a turbulent flow.