A direct‐forcing pressure‐based lattice Boltzmann method for solving fluid–particle interaction problems

A direct‐forcing immersed boundary‐lattice Boltzmann method (IB–LBM) is developed to simulate fluid–particle interaction problems. This method uses the pressure‐based LBM to solve the incompressible flow field and the immersed boundary method to handle the fluid–particle interactions. The pressure‐based LBM uses the pressure distribution functions instead of the density distribution functions as the independent dynamic variables. The main idea is to explicitly eliminate the compressible effect due to the density fluctuation. In the IB method, a direct‐forcing method is introduced to capture the particle motion. It directly computes an IB force density at each lattice grid from the differences between the pressure distribution functions obtained by the LBM and the equilibrium pressure distribution functions computed from the particle velocity. By applying this direct‐forcing method, the IB–LBM becomes a purely LBM version. Also, by applying the Gauss theorem, the formulas for computing the force and the torque acting on the particle from the flows are derived from the volume integrals over the particle volume instead of from the surface integrals over the particle surface. The order of accuracy of the IB–LBM is demonstrated on the errors of velocity field, wall stress, and gradients of velocity and pressure. As a demonstration of the efficiency and capabilities of the new method, sedimentation of a large number of spherical particles in an enclosure is simulated. Copyright © 2010 John Wiley & Sons, Ltd.     

Incompact3d: A powerful tool to tackle turbulence problems with up to O(105) computational cores

Understanding the nature of complex turbulent flows remains one of the most challenging problems in classical physics. Significant progress has been made recently using high performance computing, and computational fluid dynamics is now a credible alternative to experiments and theories in order to understand the rich physics of turbulence. In this paper, we present an efficient numerical tool called Incompact3d that can be coupled with massive parallel platforms in order to simulate turbulence problems with as much complexity as possible, using up to O(10⁵) computational cores by means of direct numerical simulation (DNS). DNS is the simplest approach conceptually to investigate turbulence, featuring the highest temporal and spatial accuracy and it requires extraordinary powerful resources. This paper is an extension of Laizet et al.(Comput. Fluids 2010; 39 (3):471–484) where the authors proposed a strategy to run DNS with up to 1024 computational cores. Copyright © 2010 John Wiley & Sons, Ltd.     

基于离散元模型的土石混合体直剪试验分析

土石混合体是由高强度块石和低强度土体组成的一类特殊工程地质材料,其力学性质可通过直剪试验进行确定.论文针对土石混合体的细观材料特性,分别采用球形颗粒单元和非规则组合颗粒单元模拟土体和块石材料,对其在不同含石量和颗粒粘结强度下的直剪试验过程进行离散元分析.计算结果表明,土石混合体的抗剪强度随着含石量和粘结强度的增加而增加;通过不同法向应力下直剪试验的离散元分析,确定了不同含石量下土石混合体材料的内摩擦角和粘聚力.论文研究结果有助于进一步揭示土石混合体的抗剪强度特性.     

Assessment of a discontinuous Galerkin method for the simulation of vortical flows at high Reynolds number

This paper focuses on the assessment of a discontinuous Galerkin method for the simulation of vortical flows at high Reynolds number. The Taylor–Green vortex at Re = 1600 is considered. The results are compared with those obtained using a pseudo‐spectral solver, converged on a 512³ grid and taken as the reference. The temporal evolution of the dissipation rate, visualisations of the vortical structures and the kinetic energy spectrum at the instant of maximal dissipation are compared to assess the results. At an effective resolution of 288³, the fourth‐order accurate discontinuous Galerkin method (DGM) solution (p = 3) is already very close to the pseudo‐spectral reference; the error on the dissipation rate is then essentially less than a percent, and the vorticity contours at times around the dissipation peak overlap everywhere. At a resolution of 384³, the solutions are indistinguishable. Then, an order convergence study is performed on the slightly under‐resolved grid (resolution of 192³). From the fourth order, the decrease of the error is no longer significant when going to a higher order. The fourth‐order DGM is also compared with an energy conserving fourth‐order finite difference method (FD4). The results show that, for the same number of DOF and the same order of accuracy, the errors of the DGM computation are significantly smaller. In particular, it takes 768³ DOF to converge the FD4 solution. Finally, the method is also successfully applied on unstructured high quality meshes. It is found that the dissipation rate captured is not significantly impacted by the element type. However, the element type impacts the energy spectrum in the large wavenumber range and thus the small vortical structures. In particular, at the same resolution, the results obtained using a tetrahedral mesh are much noisier than those obtained using a hexahedral mesh. Those obtained using a prismatic mesh are already much better, yet still slightly noisier. Copyright © 2013 John Wiley & Sons, Ltd.     

High‐order ADER‐WENO ALE schemes on unstructured triangular meshes—application of several node solvers to hydrodynamics and magnetohydrodynamics

In this paper, we present a class of high‐order accurate cell‐centered arbitrary Lagrangian–Eulerian (ALE) one‐step ADER weighted essentially non‐oscillatory (WENO) finite volume schemes for the solution of nonlinear hyperbolic conservation laws on two‐dimensional unstructured triangular meshes. High order of accuracy in space is achieved by a WENO reconstruction algorithm, while a local space–time Galerkin predictor allows the schemes to be high order accurate also in time by using an element‐local weak formulation of the governing PDE on moving meshes. The mesh motion can be computed by choosing among three different node solvers, which are for the first time compared with each other in this article: the node velocity may be obtained either (i) as an arithmetic average among the states surrounding the node, as suggested by Cheng and Shu, or (ii) as a solution of multiple one‐dimensional half‐Riemann problems around a vertex, as suggested by Maire, or (iii) by solving approximately a multidimensional Riemann problem around each vertex of the mesh using the genuinely multidimensional Harten–Lax–van Leer Riemann solver recently proposed by Balsara et al. Once the vertex velocity and thus the new node location have been determined by the node solver, the local mesh motion is then constructed by straight edges connecting the vertex positions at the old time level tⁿ with the new ones at the next time level t^{n + 1}. If necessary, a rezoning step can be introduced here to overcome mesh tangling or highly deformed elements. The final ALE finite volume scheme is based directly on a space–time conservation formulation of the governing PDE system, which therefore makes an additional remapping stage unnecessary, as the ALE fluxes already properly take into account the rezoned geometry. In this sense, our scheme falls into the category of direct ALE methods. Furthermore, the geometric conservation law is satisfied by the scheme by construction. We apply the high‐order algorithm presented in this paper to the Euler equations of compressible gas dynamics as well as to the ideal classical and relativistic magnetohydrodynamic equations. We show numerical convergence results up to fifth order of accuracy in space and time together with some classical numerical test problems for each hyperbolic system under consideration. Copyright © 2014 John Wiley & Sons, Ltd.     

考虑压力作用的煤吸附/解吸CH_4变形试验研究

气体压力是煤吸附/解吸瓦斯的重要影响因素之一。因此,在实验室通过改变压力差,进行了一次升压/降压和等梯度逐次升压/降压方式吸附/解吸试验。通过比较分析了两种吸附/解吸方式煤变形量大小,研究了两种吸附/解吸方式下煤残余变形量的变化规律。研究结果表明,等梯度逐次升压吸附煤膨胀变形量大于同一压力水一次升压吸附煤膨胀变形量,而两种解吸试验中煤收缩变形量大小关系与之相反,残余变形量大小关系与之相同;同一压力水平下,变压力梯度降压解吸好于等梯度逐次降压解吸。     

石油基电流变液中链结构的显微观察

利用显微观察技术连续记录了直流与交流电场下石油基电流变液中的显微结构变化,比较了两种电场下成链、链生长、断链的差异,这种差异包括颗粒的运动、链的形状以及断链时有无崩裂现象,并且分析了造成这种差异的原因,认为交流场下出现的链崩裂现象是由约翰逊-拉贝克效应造成的,通过石油基电流变液在直流、20Hz、50Hz三种电场条件下显微链结构的对比,得出结论横跨两极的链总的横截面积是影响剪应力大小的一个重要因素。     

论如何构建我国的风险投资机制

我国经济的发展需要投资而且基本具备发展条件。我国应充分借鉴国外发达国家的成功政策和经验并与中国的实际情况相结合，构建和完善我国的投资机制，制定法律法规，优化社会服务支持系统，减少投资风险，普遍提高国人的风险投资意识，最大程度地吸引社会资金投入高新技术风险企业。     

直流绝缘子串电压分布测量方法及其分布特性

提出了一种新型测量直流系统绝缘子串电压分布的静电计式仪器和测量方法．仪器采用振动式探头，把绝缘子的直流电压变成交流信号，处理后，语音直接报出绝缘子的电压值．通过在含有５ 片绝缘子的绝缘子串上的各种试验，研究了绝缘子串电压分布的影响因素及电压分布特征．影响直流系统绝缘子串电压分布的因素与交流系统不同，污秽、空气湿度和电压极性对分布电压都有影响．电压分布是一个动态的过程，随各种因素的变化而变化，绝缘子电压分布有很大的分散性．干燥情况下，直流系统绝缘子串电压分布更加不均匀，导线端绝缘子电压更高．     

推广的Bernstein多项式逼近的正逆定理

讨论推广的Bernstein多项式在空间C逼近的正逆定理，得到了Bernstein多项式的相应结果．