随机粗糙微通道内流动特性研究

采用计算流体动力学的方法, 研究了微通道内气体在速度滑移和随机表面粗糙度耦合作用下的流动特性. 其中, 利用二阶速度滑移边界条件描述气体的边界滑移, 利用分形几何学建立随机粗糙表面. 研究发现, 综合考虑二阶速度滑移边界条件和随机表面粗糙度在较大的平均Knudsen数范围内 (0.025-0.59) 得到的计算结果与实验数据符合得很好, 而一阶速度滑移边界条件只在平均Knudsen数较小时(<0.1)符合实验结果. 随机表面粗糙度对气体在边界处的滑移有显著影响, 相对粗糙度越大, 速度滑移系数越小. 并针对计算结果, 给出了滑移系数与相对粗糙度近似满足的关系. 随机粗糙表面对气体流动过程中的压强、速度、Poiseuille数也有显著影响. 关键词： 随机表面粗糙度 二阶速度滑移边界条件 分形 微通道     

纳米硅通道内滑移现象的分子动力学模拟

采用分子动力学方法研究了水在硅通道中的流动现象。通过模拟Couette剪切流动,得到了通道中流体的速度分布,同时考察了壁面吸附气体以及壁面具有纳米结构情况下的流动。模拟表明气体层以及微结构的存在可以改变硅表面的滑移性质,进而可以通过在壁面构造纳米结构以及在通道中充入气体来改变和控制纳米通道中的流体输运。     

压缩性和稀薄性对微通道气体流动换热的影响

本文对于微通道内稀薄气体二维可压缩滑移流动建立了数学模型,采用连续介质流动控制方程与壁面速度滑移和温度跳跃边界条件相组合描述该问题,并利用SIMPLE算法求解,所得结果与文献进行了对比,在相同条件下得到了较高的一致性.文中利用该计算模型的计算结果分析了气体的压缩性和稀薄性对微通道气体流动的影响,结果表明在所计算工况下稀薄性的影响更大。     

直接模拟蒙特卡罗方法在微通道流动模拟中的应用

直接模拟蒙特卡罗方法是一种求解稀薄气体流动换热新的数值方法。本文采用该方法对Kn数跨越速度滑移区和过渡区的三个微通道内的流动进行了数值模拟,给出了通道内速度、压力及局部阻力系数的变化曲线.为了表明通道横纵比对流动的影响,还对每个算例在不同的横纵比下进行了比较。结果表明,微通道内的流动特性不仅与Kn数有关,而且与通道的横纵比也有很大的关系。     

粗糙微通道内气体流动的分子动力学研究

为研究粗糙度对微通道内气体流动及其边界滑移性质的影响,采用分子动力学模拟方法研究了氩气在0.1 μm铂通道内的流动,通道表面粗糙度由三角粗糙元阵列构成。气体流动的边界条件决定于2个准则数: A/λ和Kn=λ/L(其中A为壁面粗糙度、λ为气体分子的平均自由程、L为流动系统的尺度)。Maxwell基于Kn的滑移模型当A/λ<0.25时适用;A/λ<1时,气体流动存在边界速度滑移,体现出稀薄效应,A/λ≈1时为无滑移,A/λ>1时为等效负滑移.     

矩形微槽道气体流动的速度分布

矩形微槽道的各个流向截面可以局部近似为平面Poiseuille流动,应用信息保存(IP)方法和直接模拟Monte Carlo(DSMC)方法计算了从连续介质区到自由分子流区的平面Poiseuille流动,利用其结果对Beskok-Karniadadis公式和质量流率动理论因子进行修正和重新拟合,给出在整个稀薄气体流动领域都适用的微槽道气体流动速度分布.     

粗糙纳通道内流体流动与传热的分子动力学模拟研究

为研究粗糙表面对纳尺度流体流动和传热及其流固界面速度滑移与温度阶跃的影响,本文建立了粗糙纳通道内流体流动和传热耦合过程的分子动力学模型,模拟研究了粗糙通道内流体的微观结构、速度和温度分布、速度滑移和温度阶跃并与光滑通道进行了比较,并分析了固液相互作用强度和壁面刚度对界面处速度滑移和温度阶跃的影响规律. 研究结果表明,在外力作用下,纳通道主流区域的速度分布呈抛物线分布,由于流体流动导致的黏性耗散使得纳通道内的温度分布呈四次方分布. 并且,在固体壁面处存在速度滑移与温度阶跃. 表面粗糙度的存在使得流体剪切流动产生了额外的黏性耗散,使得粗糙纳通道内的流体速度水平小于光滑通道,温度水平高于光滑通道,并且粗糙表面的速度滑移与温度阶跃均小于光滑通道. 另外,固液相互作用强度的增大和壁面刚度的减小均可导致界面处速度滑移和温度阶跃程度降低. 关键词： 速度滑移 温度阶跃 流固界面 粗糙度     

DSMC方法的压力边界条件实现

提出了一种实现DSMC压力边界条件的新方法,新方法不仅可以避免传统"通量法"造成的计算发散问题,又较之"入口平均法"有更快的收敛速度.使用这种方法,对不同压差驱动下等壁温微通道内的气体流动进行了模拟,并与传统的基于连续介质假设的滑移理论所得结果进行了比较.     

微通道内气体流动的格子Boltzmann方法研究

采用格子Boltzmann方法模拟了微通道在滑移区内不同Knudsen数下的微气体Poiseuille流,分析了微气体流动的速度分布以及流量与压降的关系,并给出了相对滑移长度和Poiseuille数随Knudsen数的变化特性。研究结果表明,微气体Poiseuille流的速度轮廓呈抛物线分布,但是边界速度大于0,出现速...     

波纹通道内过渡领域流动特性的数值研究

本文基于离散速度方向模型,数值研究了过渡领域内气体在正弦波纹通道中的流动特性。首先,对模型的控制方程进行了坐标转换,提出了分子在曲边边界上的反射处理方法,将原模型计算范围拓展到了不规则区域。基于此,采用结构化网格和二阶迎风格式对正弦波纹通道内处于过渡领域的气体流动进行了数值研究。结果表明,与连续介质领域和滑移领域不同,过渡领域内通道截面最大速度在Kn=1附近出现极小值;随着Kn数的增加,壁面滑移速度随之增加,而摩擦常数随之降低;此外,通道的渐扩过程滑移速度以及摩擦常数均降低,渐缩过程与此相反。     

通道中串列旋转圆柱的格子Boltzmann-虚拟区域方法的模拟

通过结合格子Boltzmann方法(LBM)和虚拟区域(Fictitiou sDomain)思想,建立格子Boltzmann-虚拟区域(LB-DF/FD)方法.采用两套网格系统,欧拉网格用于流体,拉格朗日网格用于固体.原有的LBM在计算运动固体的受力方面存在数据振荡,LB-DF/FD方法改进了此缺陷.为验证该方法,模拟圆柱绕流、圆形颗粒在无限长通道中平动及在无限大流场中转动三种情况,结果与其他数值解及理论解符合得很好.利用该方法模拟低雷诺数下通道中串列旋转圆柱周围的流场,分析圆柱间距(g)及雷诺数(Re)对流场结构的影响.给出Re=0.001,0.1和10下,0.2≤g≤8.0的流线结构、圆柱升力、阻力以及力矩等数值结果.结果表明,g对流场的结构及圆柱的受力有显著影响,Re对圆柱阻力及Stokes单元数目的影响较大.     

A momentum exchange-based immersed boundary-lattice Boltzmann method for simulating incompressible viscous flows

A momentum exchange-based immersed boundary-lattice Boltzmann method is presented in this Letter for simulating incompressible viscous flows. This method combines the good features of the lattice Boltzmann method (LBM) and the immersed boundary method (IBM) by using two unrelated computational meshes, an Eulerian mesh for the flow domain and a Lagrangian mesh for the solid boundaries in the flow. In this method, the non-slip boundary condition is enforced by introducing a forcing term into the lattice Boltzmann equation (LBE). Unlike the conventional IBM using the penalty method with a user-defined parameter or the direct forcing scheme based on the Navier–Stokes (NS) equations, the forcing term is simply calculated by the momentum exchange of the boundary particle density distribution functions, which are interpolated by the Lagrangian polynomials from the underlying Eulerian mesh. Numerical examples show that the present method can provide very accurate numerical results.     

基于区域分裂的非均匀Lattice Boltzmann方法

基于区域分裂技术,设计了一种使用非均匀网格的LatticeBoltzmann（LB）模型,其基本思想是：将流场分解为若干个规划的子区域,在每个子区域使用均匀网格上的LB模型求解,子区域的边界条件利用插值方法决定,用该模型对二维圆柱绕流问题进行了模拟,并与其它方法的结果作了比较。     

滴形颗粒在通道中沉降的格子Boltzmann-虚拟区域方法模拟

采用格子Boltzmann-虚拟区域方法对滴形颗粒在垂直通道中的沉降过程进行直接数值模拟.通过格子Boltzmann方法求解N-S方程,流体与固体之间的相互作用通过虚拟区域方法描述.研究雷诺数在10^-2到100范围内颗粒形状因子对其摩擦系数和阻力系数的影响.为便于比较,给出了圆形颗粒沉降的结果.结果发现,当雷诺数小于1的时候,颗粒的摩擦系数始终保持常数,而当雷诺数大于1时摩擦系数随雷诺数的增大而增大.此外,当雷诺数小于约30时圆形颗粒的摩擦系数和阻力系数均小于滴形颗粒,而当雷诺数大于30时情况正好相反.颗粒周围的压力分布证明了这一结论.     

An interaction potential based lattice Boltzmann method with adaptive mesh refinement (AMR) for two-phase flow simulation

The lattice Boltzmann method (LBM) for two-phase flow simulation is often hindered by insufficient resolution at the interface. As a result, the LBM simulation of bubbles in bubbling flows is commonly limited to spherical or slightly deformed bubble shapes. In this study, the adaptive mesh refinement method for the LBM is developed to overcome such a problem. The approach for this new method is based on the improved interaction potential model, which is able to maintain grid-independent fluid properties in the two-fluid phases and at the interface. The LBM–AMR algorithm is described, especially concerning the LBM operation on a non-uniform mesh and the improved interaction potential model. Numerical simulations have been performed to validate the method in both single phase and multiphase flows. The 2D and 3D simulations of the buoyant rise of bubbles are conducted under various conditions. The agreement between the simulated bubble shape and velocity with experiments illustrates the capability of the LBM–AMR approach in predicting bubble dynamics even under the large bubble deformation conditions. Further, the LBM–AMR technique is capable of simulating a complex topology change of the interface. Integration of LBM with AMR can significantly improve the accuracy and reduce computation cost. The method developed in this study may appreciably enhance the capability of LBM in the simulation of complex multiphase flows under realistic conditions.     

基于树网格的格子Boltzmann方法以及曲线边界的处理

将格子玻尔兹曼方法(LBM)和BFECC技术相结合,在四叉树网格上实现,并用于二维不可压缩流动的求解.二维四叉树(三维八叉树)网格是一种非均匀网格,利用树结构进行数据存储,具有非常高的检索效率.BFECC是一种数值技巧,可以把求解输运方程的奇数阶格式通过误差修正提高一阶精度.基于四叉树网格的LBM,引入BFECC后,能有效减小输运过程中非均匀网格插值产生的误差,在总体上实现二阶精度,和经典的LBM方法相当.最后给出二维顶盖驱动方腔流动算例和二维圆柱绕流算例,从中可以看出格式的有效性.     

A lattice Boltzmann method for a two-phase flow containing solid bodies with viscoelastic membranes

A lattice Boltzmann method (LBM) for two-phase flows containing solid bodies with viscoelastic membranes is proposed. The method is based on the two-phase LBM, in which one phase is regarded as the solid phase. In the present model, the membrane is assumed to be composed of identical particles that are connected to their neighboring particles by elastic springs to take account of stretching and compression effects. The method is applied to two representative problems, namely the behavior of a viscoelastic body under shear flow and the motion of a viscoelastic body in a Poiseuille flow. Tank-tread motion and axial migration, which are both characteristic of the motion of viscoelastic bodies, are simulated by using the method. These results indicate that the method is capable of simulating the complex behavior of viscoelastic bodies in capillaries, such as the motion of red blood cells in blood flows.     

A solution-adaptive lattice Boltzmann method for two-dimensional incompressible viscous flows

A stencil adaptive lattice Boltzmann method (LBM) is developed in this paper. It incorporates the stencil adaptive algorithm developed by Ding and Shu [26] for the solution of Navier–Stokes (N–S) equations into the LBM calculation. Based on the uniform mesh, the stencil adaptive algorithm refines the mesh by two types of 5-points symmetric stencils, which are used in an alternating sequence for increased refinement levels. The two types of symmetric stencils can be easily combined to form a 9-points symmetric structure. Using the one-dimensional second-order interpolation recently developed by Wu and Shu [27] along the straight line and the D2Q9 model, the adaptive LBM calculation can be effectively carried out. Note that the interpolation coefficients are only related to the lattice velocity and stencil size. Hence, the simplicity of LBM is not broken down and the accuracy is maintained. Due to the use of adaptive technique, much less mesh points are required in the simulation as compared to the standard LBM. As a consequence, the computational efficiency is greatly enhanced. The numerical simulation of two dimensional lid-driven cavity flows is carried out. Accurate results and improved efficiency are reached. In addition, the steady and unsteady flows over a circular cylinder are simulated to demonstrate the capability of proposed method for handling problems with curved boundaries. The obtained results compare well with data in the literature.     

凝固前沿和气泡相互作用的大密度比格子玻尔兹曼方法模拟

建立了二维双组分两相流的大密度比格子玻尔兹曼方法 (lattice Boltzmann method, LBM)模型. 该模型基于改进的Shan-Chen伪势多相流LBM模型, 结合采用不同时间步长的方法, 实现密度比达800以上的气液两相流模拟. 为了对模型进行验证, 模拟了在不同气液相互作用系数和密度比条件下气泡内外压力差与其半径之间的关系, 其结果满足Laplace定律. 将所建立的大密度比LBM与介观尺度的元胞自动机(cellular automaton, CA)和有限差分法(FDM)相耦合, 用LBM模拟气液两相流, 用CA方法模拟固相生长, 用有限差分法模拟温度场, 采用LBM-CA-FDM耦合模型对定向凝固过程中凝固前沿的气泡与液-固界面之间的相互作用进行模拟研究. 结果表明, 绝热气泡的存在影响了温度场分布, 使得凝固前沿接近气泡时, 液-固界面凸起, 在不同的固相生长速度条件下, 出现凝固前沿淹没气泡或气泡脱离凝固前沿的不同情况, 模拟结果与实验结果符合良好. 关键词： 格子玻尔兹曼方法 元胞自动机 凝固 气泡     

Heuristic optimality criterion algorithm for shape design of fluid flow

This paper presents a heuristic optimality criterion algorithm for shape design of fluid flow. In this algorithm, the lattice Boltzmann method (LBM) is utilized to calculate the flow field of a fluid domain which is divided into elemental cells. A heuristic optimality criterion is applied for cells at the solid–fluid interface, i.e. the dynamic pressure for fluid cells and the viscous stress on their neighboring solid cells. An automatic program is processed step by step to exchange the positions of solid and fluid cells identified by the optimality criterion, with the objective of decreasing the flow resistance at the constraint of constant fluid volume. To illustrate the procedure of this algorithm for shape design of fluid flow, two simple examples are presented: one with fluid flowing through a right angle elbow and the other through a converging T-junction. Numerical results show that this algorithm can successfully reduce the total pressure drop of the system, demonstrating its potential applications in engineering optimal design.