SPH方法和MPS方法模拟溃坝问题的比较分析

SPH (Smoothed Particle Hydrodynamics)方法和MPS (Moving Particle Semi-Implicit)方法是两种常用的无网格粒子法,在处理自由面大变形问题方面有较大的灵活性,然而这两种方法在数值格式、计算效率和收敛性等方面有许多不同之处.本文以二维溃坝问题为例对SPH方法和MPS方法进行了比较分析,结果表明:SPH方法易于给出更为清晰、光滑的自由面形状,而MPS方法给出的粒子分布较为凌乱;在收敛性上,随着初始粒子间距的减小,SPH和MPS均趋于收敛,但MPS方法收敛得更快些;对于时间步长,在满足CFL条件且计算稳定的情况下,对结果影响不是很大;在计算效率上,SPH方法具有较高的效率,适合求解大型复杂流动问题,而MPS方法计算量较大.通过对SPH方法和MPS方法的比较分析,为具体问题计算选用不同的无网格粒子方法提供了选择依据.     

用MPS方法数值模拟低充水液舱的晃荡

液体晃荡是一种非线性的自由面流动现象.在低充水率下,由于液体具有很大的运动空间,晃荡现象具有强烈的非线性特征,自由面的变形较大,剧烈时甚至会翻卷和破碎,这给数值模拟带来很大挑战.本文基于移动粒子半隐式法(Moving Particle Semi-Implicit,MPS)对低充水率的液舱晃荡问题进行了数值模拟.结果表明:当激励频率等于共振频率时,液体撞击到了液舱的顶板,并出现了飞溅现象,此时液体对侧舱壁产生了较大的抨击压力;在低于共振频率时,自由面出现了破碎波及波前的翻卷和融合等现象,抨击压力相对较小.计算结果表明,MPS方法能够很好地预测晃荡引起的拍击现象,数值计算得到的压力能够与实验结果很好地吻合.在自由面的追踪上,MPS方法具有很大的灵活性,能够很好地处理自由面的破碎、融合及液体的飞溅等流动现象.     

SPH方法在自由面问题中的应用

作为无网格粒子法,SPH法在处理大变形、自由面流动问题时具有显著的优势.介绍了SPH法的基本数值方法,并基于SPH法数值模拟了2个二维溃坝问题,将计算结果与试验数据进行了比较,结果表明:SPH法在处理自由面时具有很强的适应性.尽管水面发生了翻卷、破碎等剧烈的变化,但SPH法仍然能够较好地捕捉到这些流动现象,同时数值模拟得到的水头位置和自由面形状均能与试验结果相吻合,表明SPH法在处理自由面问题时具有较高的准确性及可靠性.     

基于虚单元法求解渗流自由面的曲线拟合法

渗流在坝坡稳定及坝坡变形中起关键作用,渗流自由面的确定是渗流计算的主要内容之一。目前,求解渗流自由面经常采用虚单元法,但是该法需不断调整通过自由面的单元,迭代效率慢且易造成单元畸形。对有限元法求解的自由面节点水头做曲线拟合,无需调整单元,即可得到高精度的渗流自由面曲线。应用所提方法求解工程实例,计算结果表明该方法精度高、稳定性好、计算效率高,计算结果与实际自由面形状更吻合。     

基于能量损失率最小原理求解稳定渗流场的自由面曲线

提出了一种基于渗流域能量损失率最小原理求解自由面的有限单元计算方法,将求解渗流自由面问题转化为求解稳定渗流场能量损失率最小值问题。对渗流域进行有限单元划分,首先基于能量损失率最小原理计算渗流逸出点,然后由下游边界到上游边界逐层单元求解自由面节点的位置,连接各层自由面点以及逸出点得到完整的渗流自由面。采用该方法计算了有电模拟试验解的矩形坝、有甘油模型试验解的矩形坝、有解析解的梯形坝的自由面曲线,并与试验解或解析解进行了对比。结果表明提出的自由面求解方法具有很高的计算精度。     

基于GPU加速的MPS方法模拟三维急弯河道溃坝流动

溃坝流动是一种灾害性的复杂流动现象,该文将GPU加速技术与MPS方法相结合,采用该课题组自主开发的MPSGPU-SJTU求解器对三维急弯细长河道中的溃坝流动问题进行了数值模拟.监测河道中轴线处自由液面水位高度与速度场分布情况表明,数值模拟结果与实验结果吻合较好.此外,该文对比了CPU与GPU的计算时间,验证了采用GPU...     

有自由面渗流分析的虚单元法

有自由面渗流分析的有限元法一般采用移动网格法，近年来出现了几种固定网格法在扩大了的域上求解，本文在上述方法的基础上，提出了有自由面渗流分析的虚单元法，该法优于上述方法。     

MPS方法数值模拟土石坝溃决流动

土石坝溃决流动一般为逐渐溃坝流动,对逐渐溃坝流动进行数值模拟分析有重要现实意义。该文使用自主开发的无网格粒子法求解器MLParticle-SJTU,对土石坝溃决的逐渐溃坝流动进行了数值模拟。首先建立了溃坝过程中坝体高度随时间变化的模型,在此基础上引入MPS方法,计算了溃坝过程中的流动形态以及流量,计算结果与其他研究者的模拟结果相吻合。此外,还采用二维模型计算考察了溃坝时间对溃坝水流流动形态及壁面砰击压力的影响,并对其中几个工况进行了三维模拟。     

经典水跃水气两相流特性的数值模拟

水跃是急流快速过渡到缓流时水面突然跃起的局部水力现象,研究它的掺气和流动特性具有重要的工程和理论意义。使用二维混合模型通过雷诺平均Navier-Stokes方程求解平均流动,通过标准k-ε模型实现湍流封闭,通过改进后的VOF法计算掺气浓度分布并追踪波动自由面,系统研究了3个弗劳德数的经典水跃。对比已有的实验数据,发现模型能够准确预测水跃的自由面特征、时均速度和掺气浓度分布及其自相似性。在水工建筑物设计中,需要考虑水跃的水气两相流特性而实验测量不现实时,该模型可提供解决方法。     

基于CIP方法的液滴冲击薄膜数值模拟

该文基于自主研发的气-液二相流数学模型开展了液滴冲击液膜的数值模拟,采用紧致插值曲线(CIP;constrained interpolation profile)方法离散Naiver-Stokes方程,完成速度-压力场的求解,并分析了速度场、压力场随时间变化的过程。采用VOF/WLIC(volume of fluid/weighed line interface calculation)方法重构了自由面,并与SOLA-VOF方法及CIP方法得到的自由面进行比较,验证VOF/WLIC方法对于液滴冲击液膜问题的适用性。通过调整气、液二相流的黏度比值,分析比较了不同黏度比值对模拟结果的影响。结果表明,CIP方法可精准重现液滴冲击过程中的复杂自由面流动现象。     

Comparative study of MPS method and level-set method for sloshing flows

This paper presents a comparative study of a meshless level-set method in the simulation of sloshing flows. The numerical moving particle semi-implicit （MPS） method and a grid based schemes of the MPS and level-set methods are outlined and two violent sloshing cases are considered. The computed results are compared with the corresponding experimental data for validation. The impact pressure and the deformations of free surface induced by sloshing are comparatively analyzed, and are in good agreement with experimental ones. Results show that both the MPS and level-set methods are good tools for simulation of violent sloshing flows. However, the second pressure peaks as well as breaking and splashing of free surface by the MPS method are captured better than by the level-set method.     

A numerical study of violent sloshing problems with modified MPS method

A numerical study on violent liquid sloshing phenomenon in a partially filled rectangular container is carried out by using moving particle semi-implicit(MPS) method. The present study deals with the implementation of five modifications all together over the original MPS method. The modifications include improved source terms for pressure Poisson equation, special approximation technique for the representation of gradient differential operator, collective action of mixed free surface particle identification boundary conditions, effecting Neumann boundary condition on solving the PPE and fixing judiciously the parting distance among particles to prevent collision. The suitability of the kernel function used in the original MPS method along with these five modifications is investigated for violent sloshing problems. The present model ensures a good agreement between numerical results with the existing experimental observations. The model is successfully applied to a partially filled tank undergoing horizontal sinusoidal excitation to compute the sloshing wave amplitudes and pressure on tank walls. The assessment of dynamic behaviour manifested in terms of base shear, overturning moment and impact pressure load exerted on tank ceiling induced by violent sloshing motion using MPS method is not reported in the open literature and has been efficiently carried out in the present study.     

SURFACE WAVE PATTERNS AND INSTABILITY IN A VERTICALLY OSCILLATING CIRCULAR CYLINDRICAL VESSEL

The natural frequency of surface wave, which has been derived from a vertically oscillating circular cylindrical vessel in inviseid fluid, was modified by considering the influence of surface tension and weak viscosity. Many flow patterns were found at different forced frequencies by numerical computation. In addition, the nonlinear amplitude equation derived in inviseid fluid was modified by adding viscous damping and the unstable regions were determined by stability analysis.     

无压渗流有限元分析的改进初流量法

目前常用的裂隙岩体渗流等效连续介质模型中在初流量域的积分误差是由于积分中选择高斯点作为初流量判定依据和积分值所致。因此本文引入连续调整系数法对利用初流量法求解自由面的计算精度问题进行了改进,并利用矩形均质坝渗流的实例对改进模型计算精度和收敛性进行了验证。实例表明,经改进的初流量法稳定性和收敛性良好。     

Turbulence characteristics in free-surface flow over two-dimensional dunes

The turbulent structure of open-channel flows over two-dimensional dunes is investigated numerically using large-eddy simulation (LES), in order to improve our understanding of the interaction between the dune-generated turbulence and the free surface dynamics. The filtered Navier–Stokes equations in the LES model have been discretised using the finite volume method, with a dynamic sub-grid model being employed for the unresolved scales of turbulence. The partial cell treatment has been implemented in a Cartesian grid form to deal with the dune topography. Both the volume of fluid method and rigid lid approach have been employed in the numerical framework to investigate the effects of the free surface treatment on the flow characteristics. The numerical model predicted mean flow velocities, turbulence intensities and Reynolds stresses have been compared with experimental measurements published in the literature, with a detailed analysis being undertaken to assess the accuracy of the model results and the effects of the free surface treatment on the velocity and turbulence predictions. The instantaneous flow structure has been investigated, with emphasis being focused on the free surface dynamics and coherent structures.     

边坡稳定性分析的Sarma模式及其可靠度计算方法

推导了边坡Sarma力学模式稳定性分析的有关公式，揭示了其稳定性系数是隐式函数的特点，研究了该隐式稳定性系数的迭代计算方法。针对Sarma模式和经典响应面法的特点，将响应面中的有限元数值模拟以Sarma模式中隐含稳定性系数的迭代方法代替，而隐含稳定性系数迭代计算过程中随机变量的抽样则遵从响应面中的规则，将传统的响应面思路与Sarma模式结合起来，形成了一种新的边坡稳定可靠性响应面分析方法。最后采用此方法分析了某一边坡工程的稳定可靠性，并把分析过程与结果与蒙特卡洛模拟法计算结果进行对照，表明该方法计算效率较高，原理简单，精度满足要求。     

泵站内部流动分析方法研究进展

泵站是调水、供水、灌溉和排水系统重要基础设施,其水力性能直接影响系统的安全性、稳定性和工程效益。泵站内部流动分析涉及泵站、流体动力学和数值理论等多个学科,分析方法已由传统的一维半经验半理论阶段发展到了基于计算流体动力学的现代三维黏性阶段。本文综述了泵站内部流动分析过程中涉及的湍流模型、几何模型、网格模型、数值离散模型和解算模型,阐述了研究泵站进水池表面旋涡、泥沙与空化特性、水力激振特性、水力瞬变特性及水力设计的基本方法和过程,总结了泵站流动分析方法方面的最新研究成果,展望了未来发展趋势及主要研究课题。     

Study of fluid resonance between two side-by-side floating barges

The hydrodynamic interaction between two vessels in a side-by-side configuration attracted research attentions in recent years. However, because the conventional potential flow theory does not consider the fluid viscosity, in the hydrodynamic results, the wave elevations were overestimated in the narrow gap under resonance conditions. To overcome this limitation and investigate the complex fluid flow around multiple bodies in detail, this study examines the fluid resonance between two identical floating barges using a viscous flow analysis program FLOW-3D. The volume of fluid method is implemented for tracking the free surface, and a porous media model is used near the outflow boundary to enhance the wave absorption. A three-dimensional numerical wave basin is established and validated by comparison with the waves generated using theoretical values. On this basis, a computational fluid dynamics(CFD) simulation of the two barges in a resonance wave period is performed, and the wave elevations, the fluid flow around the barges, and the motions of the barges are discussed. The numerical simulation is verified by comparison with results of corresponding experimental data.     

分岔管道三维湍流水力特性数值模拟

建立了分岔管道水动力学的计算模型,求解三维贴体坐标系下水流控制方程。计算中采用修正的k-ε湍流模型,应用有限体积法离散方程,物理变量采用同位网格布置,控制体表面上各变量的值用界面动量插值法计算,速度-压力耦合计算采用SIMPLEC算法。本文应用该数值模型模拟了三维弯道水流的水力特性,并与模型试验实测值进行了比较,结果吻合较好。最后应用该数值模型预测了35°分岔管道连接区域三种流动情况的水力特性。     

GPU加速的SPH方法在溢洪道水流模拟中的应用

基于拉格朗日描述的光滑粒子动力学方法（SPH）擅长于处理自由面剧烈变化的水流现象,十分适合水利工程中泄洪等问题的数值模拟。然而,SPH方法通常采用均匀分布的粒子对流体计算域进行空间离散,对于工程问题而言需要的粒子数量较多、计算量大。为了突破SPH方法在实际大规模计算中的适用范围,采用C++和CUDA混合编程的技术,借助GPU实现了对SPH方法的并行加速。通过WES三圆弧段组成的光滑溢洪道过流问题,验证了GPU加速的SPH方法的计算精度和可靠性,计算效率相对原始的SPH仿真过程提高了61.8倍。最后,将GPU加速的SPH方法应用于水利工程的溢洪道泄流问题,分别模拟了光滑溢洪道和台阶式溢洪道流动特性,通过自由面的演化过程及泄流沿程截面上的速度分布状态,对比分析了台阶对泄流现象的影响。 〖HT5”H〗关〓键〓词：〖HT5”K〗