Numerical simulation of steady compressible flows using a zonal formulation. Part II: viscous flows

Viscous flow simulations are usually based on the Navier–Stokes equations representing the balance of mass, momentum, and energy. For many high Reynolds number flows, the viscous effects are only limited to small regions in the neighborhood of solid surfaces and in the wakes. We present here a zonal formulation with a reduced system of equations in the outer inviscid flow region. As in part I of this work, the velocity components are calculated from a generalized form of the Cauchy–Riemann equations with non-vanishing vorticity only in the inner region, where the governing equations including the viscous terms are solved. The viscous effects are transferred to the Cauchy–Riemann equations through a forcing function (vorticity), and the process is repeated until convergence. Preliminary results are presented and compared to standard Navier–Stokes calculations for two and three dimensional flow problems.     

Numerical simulation of viscous flows in hydraulic turbomachinery by the finite element method

Finite element simulations of 2-D blade-to-blade channel flows and of swirling flows in a conical diffuser are described. Algorithms use state-of-the-art features of the FEM and are shown to provide helpful capabilities for the design of hydraulic turbomachinery. Results of the computation include a flow pattern with laminar instabilities characteristic of the onset of turbulence. Numerical experiments are conducted up to a Reynolds number of 2000.     

Numerical simulation of viscous flow over non-smooth surfaces

The incompressible viscous flow over several non-smooth surfaces is simulated numerically by using the lattice Boltzmann method. A numerical strategy for dealing with a curved boundary with second-order accuracy for velocity field is presented. The drag evaluation is performed by the momentum-exchange method. The streamline contours are obtained over surfaces with different shapes, including circular concave, circular convex, triangular concave, triangular convex, regular sinusoidal wavy and irregular sinusoidal wavy, are obtained. The flow patterns are discussed in detail. The velocity profiles over different kinds of non-smooth surfaces are investigated. The numerical results show that the lattice Boltzmann method is reliable, accurate, easy to implement, and can provide valuable help for some engineering practices.     

Large-eddy simulation of multi-component compressible turbulent flows using high resolution methods

The ability of a finite volume Godunov and a semi-Lagrangian large-eddy simulation (LES) method to predict shock induced turbulent mixing has been examined through simulations of the half-height experiment [Holder and Barton. In: Proceedings of the international workshop on the physics of compressible turbulent mixing, 2004]. Very good agreement is gained in qualitative comparisons with experimental results for combined Richtmyer-Meshkov and Kelvin-Helmholtz instabilities in compressible turbulent multi-component flows. It is shown that both numerical methods can capture the size, location and temporal growth of the main flow features. In comparing the methods, there is variability in the amount of resolved turbulent kinetic energy. The semi-Lagrangian method has constant dissipation at low Mach number, thus allowing the initially small perturbations to develop into Kelvin-Helmholtz instabilities. These are suppressed at the low Mach stage in the Godunov method. However, there is an excellent agreement in the final amount of fluid mixing when comparing both numerical methods at different grid resolutions.     

Numerical analysis of a 2-D viscous sintering problem with non-smooth boundaries

By viscous sintering it is meant the process of bringing a granular compact to a temperature at which the viscosity of the material becomes low enough for surface tension to cause the particles to deform and coalesce, whereby the material transport can be modelled as a viscous incompressible newtonian volume flow. Here a two-dimensional model is considered. A Boundary Element Method is applied to solve the governing Stokes creeping flow equations for an arbitrarily initial shaped fluid region. In this paper we show that the viscous sintering problem is well-conditioned from an evolutionary point of view. However as boundary value problem at each time step, the problem is ill-conditioned when the contact surfaces of the particles are small, i.e. in the early stages of the coalescence. This is because the curvature of the boundary at those places can be very large. This ill-conditioning is illustrated by an example: the coalescence of two equal circles. This example demonstrates the main evolutionary features of the sintering phenomenon very well. A numerical consequence of this ill-conditioning is that special care has to be taken for distributing and redistributing the nodal points at these boundary parts. Therefore an algorithm for this node redistribution is outlined. Several numerical examples sustain the analysis.     

圆管绕流旋涡脱落诱导振动的数值模拟

利用计算流体力学软件Ansys／Flotran CFD,首先对粘性不可压缩流体的固定圆管绕流进行了数值模拟,然后结合逐步积分法完成了同时考虑纵横两向弹性支撑圆管绕流旋涡脱落诱导振动的数值模拟,并通过快速傅立叶变换,得到了弹性支撑圆管和固定圆管的升力及弹性支承圆管横向位移响应的功率谱．通过计算结果分析,得出了一些有价值的结论,可供从事具有圆管绕流构件设备设计的工程技术人员参考．     

高精度自适应hp-FEM在电法测井数值模拟中的应用*

介绍了电法测井数值模拟的建模方法并提出一种新型的自适应高阶有限元方法(hp-FEM)用于求解各向异性地层中的电场问题。电场数值模型在相同的误差精度下,从计算时间和计算自由度的角度对h-FEM、p-FEM和hp-FEM进行了计算比较。数值结果表明,在各向同性和各向异性模式下,该自适应hp-FEM的性能(指数速率收敛)明显优于h-FEM和p-FEM。探讨了算法的收敛性,提出的hp-FEM可以改进现有的正演模拟方法,提高分析的正确性和工作效率,对于减小研究周期有重要的应用价值。     

Numerical analysis of homogeneous condensation in rarefaction wave in a shock tube by the space-time CESE method

A detailed numerical investigation has been carried out for the homogeneous condensation of water vapor in a non-stationary rarefaction wave generated in a shock tube. The space-time CESE method has been adopted to simulate the mutual interaction of condensation with the rarefaction wave for an extremely long time. It is found that the homogeneous condensation in the rarefaction wave has a significant influence on the flow due to the latent heat release and the continuous change of cooling rate. Three stages can be defined in this process: the initial stage which contains the onset of the condensation and the formation of the condensation shock waves in both downstream and upstream directions, the oscillating stage which is characterized as the repeat of quench and onset of condensation in the expansion fan approximately in a logarithm time manner, and the asymptotic stage which the oscillating waves are damping out with time and no apparent condensation shock wave is formed.     

Numerical simulations of viscous rotating flows using a new pressure-based method

This paper describes the development and application of a parabolic finite-volume procedure for the computation of viscous rotating flows having complex geometries. A parabolic numerical scheme has been developed to include the influence of the Coriolis force on pressure corrections. Two-equation turbulence models were employed to account for the effect of turbulence on the flows. Numerical tests for 2-D flows have been conducted, and the results show that computation with the new pressure-based method gives reasonably good agreement with the experiments.     

高压动态校准的水激波管

提出了预压水激波管动态校准方法。通过施加预压给水激波管产生的脉冲信号,在水激波管所产生的频率丰富的脉冲信号基础上叠加不同的静态预压,利用准δ脉冲校准原理,对高压测试系统进行动态校准。对3种高压传感器的测试数据分别建立了数学模型,进行了频谱分析,得出了3种高压传感器的频率响应特性。     

Numerical simulation of two-dimensional sine-Gordon solitons via a split cosine scheme

This paper proposes a split cosine scheme for simulating solitary solutions of the sine-Gordon equation in two dimensions, as it arises, for instance, in rectangular large-area Josephson junctions. The dispersive nonlinear partial differential equation allows for soliton-type solutions, a ubiquitous phenomenon in a large variety of physical problems. The semidiscretization approach first leads to a system of second-order nonlinear ordinary differential equations. The system is then approximated by a nonlinear recurrence relation which involves a cosine function. The numerical solution of the system is obtained via a further application of a sequential splitting in a linearly implicit manner that avoids solving the nonlinear scheme at each time step and allows an efficient implementation of the simulation in a locally one-dimensional fashion. The new method has potential applications in further multi-dimensional nonlinear wave simulations. Rigorous analysis is given for the numerical stability. Numerical demonstrations for colliding circular solitons are given.     

Numerical simulation of radially converging shock waves in a bubble cavity

This paper presents a numerical technique investigating the final stage of focusing a radially converging nonspherical shock wave in the neighborhood of center of the axially symmetric cavitation bubble subjected to strong compression. Hydrodynamic model used includes liquid compressibility, heat conductivity of vapor and liquid, as well as evaporation and condensation on the interphase surface; the realistic wide-range equations of state are used. The calculation is performed on moving grids with explicit accentuation of the bubble surface. This technique is based on the TVD modification of the Godunov second order accuracy scheme in space and time. Its efficiency is due to an allowance for the special features of the problem in the final stage of focusing of nonspherical shock wave in the central part of the bubble. After the value of deformation of the shock exceeds the threshold (i.e., when the shock wave becomes largely nonspherical) in the central field of the bubble the curvilinear radially diverging grid is changed by the rectilinear oblique-angled grid close to Cartesian. At the same moment, the spherical immovable system of the reference frame is changed to a cylindrical system. The recalculation of the cell parameters from grid to grid is made by the method of conservative interpolation. The efficiency of the proposed approach is shown by the test problem’s calculation results and an illustrative example.     

Numerical simulation of shock diffraction on unstructured meshes

Shock diffraction over geometric obstacles is performed on two-dimensional unstructured triangular meshes using the AUSM+ flux-vector splitting scheme. Numerical simulations of shock diffraction using structured grids are reviewed in the literature, as are experimental results corresponding to the flow conditions studied. Present unstructured grid results for popular and challenging two-dimensional shock diffraction problems are presented and compared to experimental data and photographs. Benchmark and example test cases were chosen to cover a wide variety of Mach numbers for weak and strong shock waves, and for square and circular geometries. Both single and multiple obstacles are considered, as are obstacles located in the free field and confined in a channel.     

一维理想磁控等离子体激波管流动仿真

推导了磁场作用下等离子体流动的控制方程,采用加入人工粘性项的MacCormack二阶格式对方程进行了离散.计算了在不同磁场作用下,一维理想磁控等离子体在激波管内的流动情况.计算结果表明,在施加磁场作用后,激波管内的波系结构发生了明显的改变,磁场强度在激波的影响下发生了重新分布.     

Numerical simulation of high Rayleigh number convection

A direct numerical simulation of thermal convection between horizontal plane boundaries has been performed, at a Rayleigh number Ra=9800Ra _c , where Ra _c is the critical Rayleigh number for the onset of convection. The flow is found to be fully turbulent, and analysis of the probability distributions for temperature fluctuations indicates that this is within the hard turbulence regime, as defined by the Chicago group. Good agreement is shown to exist between their experiments and the present simulation.     

内置转子强化管抗污垢性能数值模拟

采用数值模拟方法研究光管及内置螺旋叶片转子强化管的抗污垢性能,得到换热管颗粒污垢体积分数分布,研究流体流速对强化管抗污垢性能的影响.结果表明:与在相同条件下的光管相比,强化管管内沉积的污垢明显减少;由于颗粒自身重力影响,底部沉积的污垢体积分数高于换热管顶部;随着流体流速的增加,强化管底部的颗粒污垢体积分数有所减小,顶部的颗粒污垢体积分数有所增大,并且强化管的颗粒污垢体积分数不断趋近于入口处设置的颗粒污垢体积分数.     

Numerical simulation of the failure of composite materials by using the grid-characteristic method

This is an overview of the existing criteria of the failure of the composite materials and of the results of the application of some of them to simulate a low-speed hit on the composition material for the three-dimensional statement of the problem. Simulation is made by means of the grid-characteristic method. Reasons are given for the selection of specific criteria and they are compared with each other.     

Parallel-multigrid computation of unsteady incompressible viscous flows using a matrix-free implicit method and high-resolution characteristics-based scheme

A three-dimensional parallel unstructured non-nested multigrid solver for solutions of unsteady incompressible viscous flow is developed and validated. The finite-volume Navier–Stokes solver is based on the artificial compressibility approach with a high-resolution method of characteristics-based scheme for handling convection terms. The unsteady flow is calculated with a matrix-free implicit dual time stepping scheme. The parallelization of the multigrid solver is achieved by multigrid domain decomposition approach (MG-DD), using single program multiple data (SPMD) and multiple instruction multiple data (MIMD) programming paradigm. There are two parallelization strategies proposed in this work, first strategy is a one-level parallelization strategy using geometric domain decomposition technique alone, second strategy is a two-level parallelization strategy that consists of a hybrid of both geometric domain decomposition and data decomposition techniques. Message-passing interface (MPI) and OpenMP standard are used to communicate data between processors and decompose loop iterations arrays, respectively. The parallel-multigrid code is used to simulate both steady and unsteady incompressible viscous flows over a circular cylinder and a lid-driven cavity flow. A maximum speedup of 22.5 could be achieved on 32 processors, for instance, the lid-driven cavity flow of Re = 1000. The results obtained agree well with numerical solutions obtained by other researchers as well as experimental measurements. A detailed study of the time step size and number of pseudo-sub-iterations per time step required for simulating unsteady flow are presented in this paper.     

Inertia based filtering of high resolution images using a GPU cluster

The scheme of inertia based anisotropic diffusion is a very powerful noise reducing and structure preserving image processing operator. This paper presents an implementation of this time consuming filter process on a cluster of Nvidia Tesla high performance computing processors, which can be applied to very large amounts of data in only a few minutes. Applying the inertia based diffusion filter to high resolution image stacks of neuron cells provides fully automatic geometric reconstructions of these images on a scale of <1μm. Such a high throughput and automatic image processing tool has great impact on various research areas, in particular the fast growing field of computational neuroscience, where one encounters increasing amount of microscopy data that needs to be processed.     

利用格式塔的高分辨率遥感影像建筑物提取

目的 格式塔心理学的理论基础为通过对事物的部分感知,实现对事物整体的认识。本文将该思想应用到建筑物提取中,提出一种兼顾目标细节及整体几何特征的高分辨率遥感影像建筑物提取方法。方法 首先,利用SIFT算法提取特征点作为候选边缘点;然后定义格式塔序列连续性原则判别边缘点,从而得到边缘点点集;并由边缘点点集拟合边缘,实现遥感影像建筑物提取。结果 利用提出算法,对WorldView-2遥感影像进行建筑物提取实验。通过与基于多尺度分割和区域合并的建筑物提取算法对比可以看出,提出算法能够更加准确、完整地提取出建筑物。采用分支因子、遗漏因子、检测率和完整性4个定量化指标对实验结果的定量评价,本文算法的检测率和完整性均大于对比算法,且本文算法的检测率均在95%以上,验证了提出基于格式塔理论的高分辨率遥感影像建筑物提取算法的有效性和准确性。结论 基于格式塔的高分辨率遥感影像建筑物提取算法能够准确刻画建筑物细节特征,同时兼顾建筑物整体几何轮廓,准确提取高分辨率遥感影像中的建筑物。本文算法针对高分辨率遥感影像,适用于提取边缘具有直线特征的建筑物。使用本文算法进行遥感影像建筑物提取时,提取精度会随分辨率降低而降低,建议实验影像分辨率在5 m以上。