PTT黏弹性流体的光滑粒子动力学方法模拟

运用光滑粒子流体动力学(smoothed particle hydrodynamics, SPH)方法对基于PHan-Thien-Tanner (PTT)模型的黏弹性流动进行了数值模拟. 首先, 利用SPH方法模拟了基于PTT模型的平板 Poiseuille流, 通过与文献结果的比较, 验证了SPH方法模拟黏弹性流动的准确性和有效性; 随后, 基于PTT模型对黏弹性自由表面流-液滴碰撞问题进行了SPH模拟, 研 究了PTT模型中拉伸参数对碰撞过程的影响. 为了解决张力不稳定问题, 采用简化的 人工应力公式. 数值结果表明, SPH方法可有效而灵活地模拟黏弹自由表面流问题.     

Oldroyd-B黏弹性液滴碰撞过程的数值模拟

复杂的流变特性使凝胶推进剂的雾化过程存在一定困难,这制约了它的发展.聚合物胶凝剂的加入使凝胶推进剂具有黏弹性,从而在雾化时会产生黏弹性液滴,因此为了进一步认识凝胶推进剂的雾化机理、提高凝胶推进剂的雾化性能,对黏弹性液滴的碰撞行为进行数值模拟研究.针对凝胶推进剂雾化过程中出现的液滴撞击现象,考虑流体具有的黏弹性效应,采用...     

Oldroyd-B黏弹性液滴弹跳行为的改进SPH模拟

基于光滑粒子流体动力学SPH(Smoothed Particle Hydrodynamics)方法对Oldroyd-B黏弹性液滴撞击固壁面产生的弹跳行为进行了模拟与分析。首先,为了解决SPH模拟黏弹性自由表面流出现的张力不稳定性问题,联合粒子迁移技术提出了一种改进SPH方法。然后,对Oldroyd-B黏弹性液滴撞击固壁面产生的铺展行为进行了改进SPH模拟,与文献结果的比较验证了方法的有效性。最后,通过降低Reynolds数捕捉到了液滴的弹跳行为;并在此基础上,分析了液滴黏度比、Weissenberg数和Reynolds数对液滴弹跳行为的影响。结果表明,改进SPH方法可有效地模拟黏弹性自由表面流问题;液滴黏度比、Weissenberg数和Reynolds数对液滴最大回弹高度均有显著的影响。     

三维溃坝流的光滑粒子动力学方法模拟

采用光滑粒子动力学SPH(Smoothed Particle Hydrodynamics)方法对三维溃坝流问题进行了数值模拟。为了逼真地模拟出坝内水体与壁面间相互作用而产生的水花飞溅、融合以及近壁面流动等现象,加入了混合长度形式的湍流模型。为了有效地防止粒子穿透固壁,提出了一种新型的适合三维数值模拟的固壁边界处理方法。应用SPH方法对三维溃坝流进行了数值模拟,并分别考虑了未添加障碍物和添加圆柱障碍物两种情形。计算结果表明,改进SPH方法能够精细地捕捉溃坝流在不同时刻的自由液面,并获得稳定而精确的数值结果。     

一种基于黎曼解处理大密度比多相流SPH的改进算法

多相流界面存在密度、黏性等物理场间断,直接采用传统光滑粒子水动力学(smoothedparticle hydrodynamics,SPH)方法进行数值模拟,界面附近的压力和速度存在震荡.一套基于黎曼解能够处理大密度比的多相流SPH计算模型被提出,该模型利用黎曼解在处理接触间断问题方面的优势,将黎曼解引入到SPH多相流计算模型中,为了能够准确求解多相流体物理黏性、减小黎曼耗散,对黎曼形式的SPH动量方程进行了改进,又将Adami固壁边界与黎曼单侧问题相结合来施加多相流SPH固壁边界,同时模型中考虑了表面张力对小尺度异相界面的影响,该模型没有添加任何人工黏性、人工耗散和非物理人工处理技术,能够反应多相流真实物理黏性和物理演变状态.采用该模型首先对三种不同粒子间距离散下方形液滴震荡问题进行了数值模拟,验证了该模型在处理异相界面的正确性和模型本身的收敛性;后又通过对Rayleigh--Taylor不稳定、单气泡上浮、双气泡上浮问题进行了模拟计算,结果与文献对比吻合度高,异相界面捕捉清晰,结果表明,本文改进的多相流SPH模型能够稳定、有效的模拟大密度比和黏性比的多相流问题.      

一种基于黎曼解处理大密度比多相流SPH的改进算法

多相流界面存在密度、黏性等物理场间断,直接采用传统光滑粒子水动力学(smoothed particle hydrodynamics, SPH)方法进行数值模拟,界面附近的压力和速度存在震荡.一套基于黎曼解能够处理大密度比的多相流SPH计算模型被提出,该模型利用黎曼解在处理接触间断问题方面的优势,将黎曼解引入到SPH多相流计算模型中,为了能够准确求解多相流体物理黏性、减小黎曼耗散,对黎曼形式的SPH动量方程进行了改进,又将Adami固壁边界与黎曼单侧问题相结合来施加多相流SPH固壁边界,同时模型中考虑了表面张力对小尺度异相界面的影响,该模型没有添加任何人工黏性、人工耗散和非物理人工处理技术,能够反应多相流真实物理黏性和物理演变状态.采用该模型首先对三种不同粒子间距离散下方形液滴震荡问题进行了数值模拟,验证了该模型在处理异相界面的正确性和模型本身的收敛性;后又通过对Rayleigh–Taylor不稳定、单气泡上浮、双气泡上浮问题进行了模拟计算,结果与文献对比吻合度高,异相界面捕捉清晰,结果表明,本文改进的多相流SPH模型能够稳定、有效的模拟大密度比和黏性比的多相流问题.     

微流道内椭圆粒子黏弹性聚焦数值模拟研究

黏弹性聚焦技术借助微尺度黏弹性流体的惯性和弹性耦合效应,能够实现生物粒子在流道中心的单一位置聚焦排列,被认为是未来生物粒子计数以及检测的理想预处理单元,因而引起了广泛的关注．自然界中的生物粒子往往是非球形的,故而研究不同形状粒子在黏弹性流体中的迁移特性具有十分重要的价值．本文通过格子玻尔兹曼方法耦合浸入边界法,对椭球粒子在直流道内黏弹性流体中的聚焦行为进行了系统的数值模拟研究．结果表明,面积相同但长径比不同的椭圆粒子在黏弹性流体中有不同的旋转周期与迁移速度．长径比更大的粒子旋转周期更长,且长径比大于3.5 的粒子甚至不再有明显的旋转．长径比更大的粒子上下两侧的黏弹性力分布更加平缓,受到指向流道中心的弹性力更小,使得粒子横向迁移速度更慢从而导致了长径比不同的椭圆粒子聚焦至流道中心所需时间的差异．此外,Weissenberg 数Wi 的增加同样能够减弱粒子的旋转,使得长径比稍小的粒子也能和长径比为1.0 的圆形粒子产生明显的分离．上述数值模拟的结论,为不同长径比粒子在黏弹性流体中的聚焦与分选应用提供了重要的理论指导．     

NUMERICAL SIMULATION OF WIND-BLOWN SAND MOVEMENT BASED ON SPH

运用光滑粒子流体动力学(smoothed particles hydrodynamics, SPH)方法对沙粒和气流的相互耦合运动特性进行了分析,研究提出了风沙流的SPH数值方法并进行了数值模拟. 首先提出了风沙流的SPH建模方法和基本理论,建立了风沙流动的SPH数值模拟平台. 其次通过建立风沙流的SPH 模型并施加边界条件,对自然风作用下沙粒的运动情况进行了数值模拟,详细分析了沙粒运动轨迹及特性,最后通过与相关研究成果对比分析,验证了完善后的SPH方法有效性. 通过考虑气流场的可变性,在风沙流SPH计算模型中引入了加载(起风)和卸载(停风)方式,观察并对比分析了沙粒的运动轨迹和特性. 为进一步研究风沙流的实时动态非线性行为提供了SPH理论基础和数值分析方法.     

改进的SPH边界处理方法与土体大变形模拟

沙土滑坡往往会造成重大的人身财产损失,研究这类土体大变形问题对防灾工程具有指导意义。光滑粒子流体动力学SPH（Smoothed Particle Hydrodynamics）方法是一种拉格朗日型无网格粒子法,十分适用于模拟大变形问题。在SPH方法中,合适的边界处理方法一直是个难点,传统的边界虚粒子法或排斥力法较难模拟复杂边界。本文引入了一种能处理任意形状边界的方法——统一半解析壁面边界条件处理方法USAW（unified semi-analytical wall boundary conditions）,通过在控制方程中引入修正因子并保留边界面积分项来弥补边界缺失。为了更准确模拟问题域边界,提出无质量边界粒子的新概念。利用该方法成功模拟了土体滑坡算例,验证了方法的可靠性,并避免了边界零粒子层问题。通过数值模拟,分析了内摩擦角和黏聚力等土体物性参数对滑坡过程的影响。最后,应用该方法研究了滑坡冲击楔形体时的压力响应。     

改进的SPH边界处理方法与土体大变形模拟

沙土滑坡往往会造成重大的人身财产损失,研究这类土体大变形问题对防灾工程具有指导意义。光滑粒子流体动力学SPH(Smoothed Particle Hydrodynamics)方法是一种拉格朗日型无网格粒子法,十分适用于模拟大变形问题。在SPH方法中,合适的边界处理方法一直是个难点,传统的边界虚粒子法或排斥力法较难模拟复杂边界。本文引入了一种能处理任意形状边界的方法——统一半解析壁面边界条件处理方法 USAW(unified semianalytical wall boundary conditions),通过在控制方程中引入修正因子并保留边界面积分项来弥补边界缺失。为了更准确模拟问题域边界,提出无质量边界粒子的新概念。利用该方法成功模拟了土体滑坡算例,验证了方法的可靠性,并避免了边界零粒子层问题。通过数值模拟,分析了内摩擦角和黏聚力等土体物性参数对滑坡过程的影响。最后,应用该方法研究了滑坡冲击楔形体时的压力响应。     

An enhanced treatment of boundary conditions in implicit smoothed particle hydrodynamics简

In this work, an enhanced treatment of the solid boundaries is proposed for smoothed particle hydrodynamics with implicit time integration scheme （Implicit SPH）. Three types of virtual particles, i.e., boundary particles, image particles and mirror particles, are used to impose boundary conditions. Boundary particles are fixed on the solid boundary, and each boundary particle is associated with two fixed image particles inside the fluid domain and two fixed mirror particles outside the fluid domain. The image particles take the flow properties through fluid particles with moving least squares （MLS） interpolation and the properties of mirror particles can be obtained by the corresponding image particles. A repulsive force is also applied for boundary particles to prevent fluid particles from unphysical penetra- tion through solid boundaries. The new boundary treatment method has been validated with five numerical examples. All the numerical results show that Implicit SPH with this new boundary-treatment method can obtain accurate results for non-Newtonian fluids as well as Newtonian fluids, and this method is suitable for complex solid boundaries and can be easily extended to 3D problems.     

Fully resolved simulations of viscoelastic suspensions by an efficient immersed boundary-lattice Boltzmann method

An efficient immersed boundary-lattice Boltzmann method (IB-LBM) is proposed for fully resolved simulations of suspended solid particles in viscoelastic flows. Stress LBM based on Giesekus and Oldroyd-B constitutive equation are used to model the viscoelastic stress tensor. A boundary thickening-based direct forcing IB method is adopted to solve the particle–fluid interactions with high accuracy for non-slip boundary conditions. A universal law is proposed to determine the diffusivity constant in a viscoelastic LBM model to balance the numerical accuracy and stability over a wide range of computational parameters. An asynchronous calculation strategy is adopted to further improve the computing efficiency. The method was firstly applicated to the simulation of sedimentation of a single particle and a pair of particles after good validations in cases of the flow past a fixed cylinder and particle migration in a Couette flow against FEM and FVM methods. The determination of the asynchronous calculation strategy and the effect of viscoelastic stress distribution on the settling behaviors of one and two particles are revealed. Subsequently, 504 particles settling in a closed cavity was simulated and the phenomenon that the viscoelastic stress stabilizing the Rayleigh–Taylor instabilities was observed. At last, simulations of a dense flow involving 11001 particles, the largest number of particles to date, were performed to investigate the instability behavior induced by elastic effect under hydrodynamic interactions in a viscoelastic fluid. The elasticity-induced ordering of the particle structures and fluid bubble structures in this dense flow is revealed for the first time. These simulations demonstrate the capability and prospects of the present method for aid in understanding the complex behaviors of viscoelastic particle suspensions.     

光滑粒子动力学方法的发展与应用

光滑粒子动力学(smoothed particle hydrodynamics,SPH)是一种拉格朗日型无网格粒子方法,已经成功地应用到了工程和科学的众多领域.SPH使用粒子离散及代表所模拟的介质,并且基于粒子体系估算和近似介质运动的控制方程.本文分析和综述了SPH模拟方法的发展历程、数值方法与应用进展.介绍了SPH方法的基本思想;从连续性、边界处理、稳定性和计算效率4个方面阐述了SPH方法的研究现状;介绍了SPH方法近年来在可压缩流动、不可压缩流动以及弹塑性材料高速变形与失效方面的一些典型应用;并对SPH方法的发展与应用进行了预测与展望.      

An improved SPH model for multiphase flows with large density ratios

This paper presents a new smoothed particle hydrodynamics (SPH) model for simulating multiphase fluid flows with large density ratios. The new SPH model consists of an improved discretization scheme, an enhanced multiphase interface treatment algorithm, and a coupled dynamic boundary treatment technique. The presented SPH discretization scheme is developed from Taylor series analysis with kernel normalization and kernel gradient correction and is then used to discretize the Navier‐Stokes equation to obtain improved SPH equations of motion for multiphase fluid flows. The multiphase interface treatment algorithm involves treating neighboring particles from different phases as virtual particles with specially updated density to maintain pressure consistency and a repulsive interface force between neighboring interface particles into the pressure gradient to keep sharp interface. The coupled dynamic boundary treatment technique includes a soft repulsive force between approaching fluid and solid particles while the information of virtual particles are approximated using the improved SPH discretization scheme. The presented SPH model is applied to 3 typical multiphase flow problems including dam breaking, Rayleigh‐Taylor instability, and air bubble rising in water. It is demonstrated that inherent multiphase flow physics can be well captured while the dynamic evolution of the complex multiphase interfaces is sharp with consistent pressure across the interfaces.     

An SPH model for free surface flows with moving rigid objects

This paper presents a computational model for free surface flows interacting with moving rigid bodies. The model is based on the SPH method, which is a popular meshfree, Lagrangian particle method and can naturally treat large flow deformation and moving features without any interface/surface capture or tracking algorithm. Fluid particles are used to model the free surface flows which are governed by Navier–Stokes equations, and solid particles are used to model the dynamic movement (translation and rotation) of moving rigid objects. The interaction of the neighboring fluid and solid particles renders the fluid–solid interaction and the non‐slip solid boundary conditions. The SPH method is improved with corrections on the SPH kernel and kernel gradients, enhancement of solid boundary condition, and implementation of Reynolds‐averaged Navier–Stokes turbulence model. Three numerical examples including the water exit of a cylinder, the sinking of a submerged cylinder and the complicated motion of an elliptical cylinder near free surface are provided. The obtained numerical results show good agreement with results from other sources and clearly demonstrate the effectiveness of the presented meshfree particle model in modeling free surface flows with moving objects. Copyright © 2013 John Wiley & Sons, Ltd.     

An improved KGF‐SPH with a novel discrete scheme of Laplacian operator for viscous incompressible fluid flows

The kernel gradient free (KGF) smoothed particle hydrodynamics (SPH) method is a modified finite particle method (FPM) which has higher order accuracy than the conventional SPH method. In KGF‐SPH, no kernel gradient is required in the whole computation, and this leads to good flexibility in the selection of smoothing functions and it is also associated with a symmetric corrective matrix. When modeling viscous incompressible flows with SPH, FPM or KGF‐SPH, it is usual to approximate the Laplacian term with nested approximation on velocity, and this may introduce numerical errors from the nested approximation, and also cause difficulties in dealing with boundary conditions. In this paper, an improved KGF‐SPH method is presented for modeling viscous, incompressible fluid flows with a novel discrete scheme of Laplacian operator. The improved KGF‐SPH method avoids nested approximation of first order derivatives, and keeps the good feature of ‘kernel gradient free’. The two‐dimensional incompressible fluid flow of shear cavity, both in Euler frame and Lagrangian frame, are simulated by SPH, FPM, the original KGF‐SPH and improved KGF‐SPH. The numerical results show that the improved KGF‐SPH with the novel discrete scheme of Laplacian operator are more accurate than SPH, and more stable than FPM and the original KGF‐SPH. Copyright © 2015 John Wiley & Sons, Ltd.