Dissipative particle dynamics simulation of wettability alternation phenomena in the chemical flooding process

Wettability alternation phenomena is considered one of the most important enhanced oil recovery （EOR） mechanisms in the chemical flooding process and induced by the adsorption of surfactant on the rock surface. These phenomena are studied by a mesoscopic method named as dissipative particle dynamics （DPD）. Both the alteration phenomena of water-wet to oil-wet and that of oil-wet to water-wet are simulated based on reasonable definition of interaction parameters between beads. The wetting hysteresis phenomenon and the process of oil-drops detachment from rock surfaces with different wettability are simulated by adding long-range external forces on the fluid particles. The simulation results show that, the oil drop is liable to spread on the oil-wetting surface and move in the form of liquid film flow, whereas it is likely to move as a whole on the waterwetting surface. There are the same phenomena occuring in wettability-alternated cases. The results also show that DPD method provides a feasible approach to the problems of seepage flow with physicochemical phenomena and can be used to study the mechanism of EOR of chemical flooding.     

高温激波管化学非平衡流动数值模拟研究

为了预测氢氧定容燃烧驱动的高温激波管性能,需要准确分析激波管非定常化学非平衡流动过程.本文在破膜前的驱动段定容燃烧以及破膜后的化学非平衡流动数值模拟中,引入双时间步长方法,发展高温激波管化学非平衡流动数值模拟方法,该方法在时间上具有二阶精度.计算结果与目前存在的激波管流动解析解以及零维化学反应系统的数值解进行了比较,吻合较好.对于典型高温激波管状态,采用有限体积方法离散准一维流动Euler控制方程,并通过将流动过程和化学反应动力学过程耦合求解,获得了激波管内部的化学非平衡流动特征.     

Steady-state two-phase flow through planar and nonplanar model porous media

Transport in Porous Media - A comparative experimental study of ‘steady-state’ two-phase flow in two types of model porous media is made to determine the effects of nonplanarity on the...     

Numerical simulation of silencers

The problem of attenuating the noise from weapons firing is studied experimentally and numerically. As a possible method of attenuating the noise significantly, a silencer with no internal baffles is attached to the M242 cannon. The internal pressures inside the muffler are measured. The near-field overpressures outside the muffler at various polar angles are also measured. A numerical simulation of the flow through the muffler is performed, using Harten's shock-capturing method to solve the Euler equations of ideal compressible flow. The numerical simulation yields a detailed picture of the flow field as displayed by the pressure and Mach contours. Pressure–time curves at selected locations are obtained and compared with experimental data. There is good agreement, except that the numerical simulation generates more vigorous oscillations.     

复杂多相流体的介观模拟: 耗散粒子动力学方法及应用

在悬浮液、乳液和泡沫等复杂多相流体中, 离散分布着大量的纳米至微米尺度的颗粒、液滴和气泡等, 在流动作用下这些离散相物质呈现出复杂的个体或群体运动行为, 进而显著影响这些复杂多相流体的宏观流变和流动行为. 针对这类流体, 开展介观尺度数值模拟成为一种有效且相对经济的研究手段. 其中, 耗散粒子动力学 (dissipative particle dynamics, DPD) 方法是一种具有代表性的介观尺度数值模拟方法, 由于其粒子方法的特质, DPD方法适合用于上述复杂多相流体内部结构的数值建模和数值模拟研究. 本文对近年来DPD方法在颗粒悬浮液、乳液和气泡等复杂多相流体模拟方面研究进展进行了系统的介绍, 深入探讨了DPD方法在复杂多相流体介观模拟方面的针对性改进以及当前存在的不足, 并对DPD方法的研究和应用进行了总结和展望.     

Numerical simulation and homogenization of two-phase flow in heterogeneous porous media

A mathematically rigorous method of homogenization is presented and used to analyze the equivalent behavior of transient flow of two incompressible fluids through heterogeneous media. Asymptotic expansions and H-convergence lead to the definition of a global or effective model of an equivalent homogeneous reservoir. Numerical computations to obtain the homogenized coefficients of the entire reservoir have been carried out via a finite element method. Numerical experiments involving the simulation of incompressible two-phase flow have been performed for each heterogeneous medium and for the homogenized medium as well as for other averaging methods. The results of the simulations are compared in terms of the transient saturation contours, production curves, and pressure distributions. Results obtained from the simulations with the homogenization method presented show good agreement with the heterogeneous simulations.     

Numerical simulation of axisymmetric unsteady incompressible flow by a vorticity-velocity method

A new numerical method for solving the axisymmetric unsteady incompressible Navier-Stokes equations using vorticity-velocity variables and a staggered grid is presented. The solution is advanced in time with an explicit two-stage Runge-Kutta method. At each stage a vector Poisson equation for velocity is solved. Some important aspects of staggering of the variable location, divergence-free correction to the velocity field by means of a suitably chosen scalar potential and numerical treatment of the vorticity boundary condition are examined. The axisymmetric spherical Couette flow between two concentric differentially rotating spheres is computed as an initial value problem. Comparison of the computational results using a staggered grid with those using a non-staggered grid shows that the staggered grid is superior to the non-staggered grid. The computed scenario of the transition from zero-vortex to two-vortex flow at moderate Reynolds number agrees with that simulated using a pseudospectral method, thus validating the temporal accuracy of our method.     

高超声速非定常流动的数值模拟与气动热计算

高超声速飞行器研究中的一个重点问题是飞行器表面的气动加热,它对飞行器的气动、热特性及安全性有重要的影响.受到当前实验技术的限制,地面实验无法准确模拟真实飞行条件,所以采用数值模拟研究气动加热问题成为目前重要的研究手段.本文采用数值方法求解三维N-S方程,得到钝头体再入模型绕流的瞬态流场,驻点温度及表面热流沿轨道变化规律.计算中采用变边界条件模拟沿轨道飞行的非定常性.     

Numerical simulation of a flow around an impulsively started radially deforming circular cylinder

The development of a two‐dimensional viscous incompressible flow generated by a deformable circular cylinder impulsively started into rectilinear motion is studied numerically for the Reynolds numbers equal to 550 and 3000. The vorticity transport equation is solved by a second‐order finite difference method in both directions of the domains. The Poisson equation for the streamfunction is solved by a Fourier–Galerkin method in the direction of the flow that is assumed to remain symmetrical and a second‐order finite difference for the radial direction. The advance in time is achieved by a second‐order Adams–Bashforth scheme. The computed results are compared qualitatively with experimental and numerical results done before in the particular non‐deformable case. The comparison is found to be satisfactory. The influence of the deformation of the cylinder on the flow structure and the drag coefficient is then analyzed. Copyright © 1999 John Wiley & Sons, Ltd.     

Numerical simulation of the process of autoregulation of the arterial blood flow

For describing the autoregulation of the blood flow in an artery under constant transmural pressure a mathematical model that takes into account the multilayer structure of the arterial wall, the diffusion and kinetic processes in the wall, and the nonlinear viscoelastic properties of the wall material is proposed. The limiting case of a thin-walled artery is studied analytically. The arterial-wall viscosity range on which the equilibrium state of the system is stable is estimated. Accurate stationary distributions of the nitric oxide, calcium and myosin concentrations in the arterial wall are found. Numerical simulation of the autoregulation process demonstrated the possibility of arterial adaptation to radius perturbations, the existence of slow oscillations, and system transition to a new equilibrium state with change in blood flow level. The results are in good agreement with the experimental data.     

A sequential numerical chemical compositional simulator

Chemical flooding in the petroleum industry has a larger scale of oil recovery efficiency than water flooding. On the other hand, it is far more technical, costly, and risky. Numerical reservoir simulation can be employed to conduct mechanism study, feasibility evaluation, pilot plan optimization, and performance prediction for chemical flooding to improve recovery efficiency and reduce operational costs. In this article, we study numerical simulation of chemical flooding such as alkaline, surfactant, polymer, and foam (ASP+foam) flooding. The main displacement mechanisms in this type of flooding are interfacial tension lowering, capillary desaturation, chemical synergetic effects, and mobility control. The model of chemical flooding involves such physicochemical phenomena as dispersion, diffusion, adsorption, chemical reactions, and in situ generation of surfactant from acidic crude oil. The numerical simulator is based on a sequential solution approach that solves both pressure and compositions implicitly, and is applied to three experiments, a chemical flow without mass transfer between phases, a laboratory sandstone core, and an ASP+foam displacement problem with mass transfer, and to a real oilfield. A comparison with UTCHEM is also performed. These applications and comparison indicate that this numerical simulator is practical, efficient, and accurate for simulating complex chemical flooding processes.      

翼型空泡周期性流动的数值模拟及机理分析

采用基于正压关系的均质平衡流空化模型和低雷诺数修正的k-ε湍流模式,自行开发了空泡流数值模拟方法和计算软件,对绕翼型空泡的周期性流动现象进行了数值模拟.计算结果与实验数据的对比表明,空泡的宏观特征、流动特性、周期性脱落的斯坦顿数St等与试验结果接近,验证了计算结果的可靠性.空泡在大约一个周期的2/3时间段内成长,并在大约1/3周期时刻发生断裂脱落.这两个特征时间与高速摄像实验结果一致.所取工况对应的组合参数σ/2α=2.865,以翼弦长计算可得St=0.217,与文献的最新试验结果吻合.空泡周期性运动过程中升阻系数也周期性振荡,时均值C<,1>=0.41,C<,d>=0.097,振荡频率与空泡脱落频率一致.对空泡运动过程中流场结构的变化进行了分析,结果表明在大攻角条件下,空泡闭合区后的逆压梯度导致涡的形成及回射流的发展,沿壁面逆向流动的混合介质射流是引起空泡断裂的原因,回射流发展、涡结构变化与空泡非稳态演化过程存在密切的联系,探讨了翼型空泡发生周期性脱落的一些机理.     

Direct simulation of fluid flow with cellular automata and the lattice-Boltzmann equation

With the invention of the Hexagonal Lattice Gas it was hoped that this new technique would facilitate direct simulation of turbulent flow. In the past years, however, we have learned about its barriers on numerical accuracy and computational efficiency, which cannot easily be taken. The work on lattice gases has evolved in the introduction of the lattice-Boltzmann scheme. With the appropriate refinements this scheme provides the essential balance between robustness and numerical accuracy and enables us to simulate three-dimensional time-dependent flows at Reynolds numbers up to 50000.     

Numerical simulation of the unsteady flow over an elliptic cylinder at different orientations

A numerical method is developed for investigating the two‐dimensional unsteady viscous flow over an inclined elliptic cylinder placed in a uniform stream of infinite extent. The direction of the free stream is normal to the cylinder axis and the flow field unsteadiness arises from two effects, the first is due to the flow field development following the start of the motion and the second is due to vortex shedding in the wake region. The time‐dependent flow is governed by the full conservation equations of mass and momentum with no boundary layer approximations. The parameters involved are the cylinder axis ratio, Reynolds number and the angle of attack. The investigation covers a Reynolds number range up to 5000. The minor–major axis ratio of the elliptic cylinder ranges between 0.5 and 0.6, and the angle of attack ranges between 0° and 90°. A series truncation method based on Fourier series is used to reduce the governing Navier–Stokes equations to two coupled infinite sets of second‐order differential equations. These equations are approximated by retaining only a finite number of terms and are then solved by approximating the derivatives using central differences. The results reveal an unusual phenomenon of negative lift occurring shortly after the start of motion. Various comparisons are made with previous theoretical and experimental results, including flow visualizations, to validate the solution methodology. Copyright © 2001 John Wiley & Sons, Ltd.     

Dynamic simulation of arbitrarily shaped particles in shear flow

A new simulation framework was created for modeling the dynamics of arbitrarily shaped particles dispersed in Newtonian fluid. Theoretical complexity usually restricts suspension simulations to those for spheroids. This new simulation is loosely based on the Stokesian Dynamics method including long range hydrodynamic interaction and uses spheres as building components for greater particulates of arbitrary shape. This approach is capable of accurately reproducing the dynamics of an isolated arbitrarily shaped particle. As verification, the simulated results are compared against known results for a rod-like particle. An elongated rod-shaped structure made from linked spheres is shown to reproduce the well-known elongated ellipsoidal particle dynamics described by [Jeffrey Proc R Soc Lond A 102:161–179, 1923]. The predicted orbital period and spin rates for a fiber in shear are reproduced and compare well with theoretical prediction over a wide aspect ratio range. Predicted particle dynamics for other shaped particles are then demonstrated.     

Numerical simulation of supersonic flow past reentry capsules

The flow fields over ARD (ESA's atmospheric reentry demonstrator), OREX (orbital reentry experiments) and spherically blunted cone-flare reentry configurations are numerically obtained by solving time-dependent, axisymmetric, compressible Navier–Stokes equations for freestream Mach numbers range of 1.2–6.0. The fluid dynamics are discretized in spatial coordinates employing a finite volume approach which reduces the governing equations to semi discretized ordinary differential equations. Temporal integration is performed using the multistage Runge–Kutta time-stepping scheme. A local time step is used to achieve steady-state solution. The numerical simulation is carried out on a structured grid. The flow-field features around the reentry capsule, such as bow shock wave, sonic line, expansion fan and recirculating flow in the base region are obtained. A good agreement is found between the calculated value of aerodynamic drag coefficient of the spherically blunted cone/fare reentry configuration with the experimental data. The effects of geometrical parameters, such as radius of the spherical cap, half cone angle, with sharp shoulder edge and with smooth shoulder edge on the flow-field have been numerically investigated for various reentry configuration which will be useful for optimization of the reentry capsule. PACS 47.11.Df, 47.40.Ki     

Molecular dynamics simulation of liquid argon flow at platinum surfaces

The micro Poiseuille flow for liquid argon flowing in a nanoscale channel formed by two solid walls was studied in the present paper. The solid wall material was selected as platinum, which has well established interaction potential. We consider the intermolecular force not only among the liquid argon molecules, but also between the liquid argon atoms and the solid wall particles, therefore three regions, i.e. the liquid argon computation domain, the top and bottom solid wall regions are included for the force interaction. The present MD (Molecular Dynamics) simulation was performed without any assumptions at the wall surface. The objective of the study is to find how the flow and the slip boundaries at the wall surface are affected by the applied gravity force, or the shear rate. The MD simulations are performed in a nondimensional unit system, with the periodic boundary conditions applied except in the channel height direction. Once the steady state is reached, the macroscopic parameters are evaluated using the statistical mechanics approach. For all the cases tested numerically in the present paper, slip boundaries occur, and such slip velocity at the stationary wall surface increases with increasing the applied gravity force, or the shear rate. The slip length, which is defined as the distance that the liquid particles shall travel beyond the wall surfaces to reach the same velocity as the wall surface, sharply decreases at small shear rate, then slightly decreases with increasing the applied shear rate. We observe that the liquid viscosity remains nearly constant at small shear rates, and the Newtonian flow occurs. However, with increasing the shear rate, the viscosity increases and the non-Newtonian flow appears.     

缝洞型多孔介质中多相流的有限体积法数值模拟

本文研究的碳酸盐岩油藏储集体属于缝洞型多孔介质.这类缝洞型多孔介质由裂缝、溶蚀孔洞和低孔隙度低渗透率的基岩组成.裂缝是空隙流体流动的主要通道;溶蚀孔洞大小从几厘米到数米不等,渗透率和孔隙度都很高,是流体主要的储集空间.由于缝洞型多孔介质空隙空间的复杂性和强非均质性,数值计算中基本控制方程的空间离散应采用非结构化网格的计算模型.本文采用有限体积法模拟缝洞型多孔介质中多相流体的流动,并给出了相应的单元中心格式有限体积法的计算公式.裂缝介质和溶洞介质中单元间多相流体的流动考虑为高速非达西流,其质量通量采用Forchheimer定律计算.非线性方程的离散选取全隐式格式,并采用Newton-Raphson迭代进行求解.通过两个二维模型注水驱油的数值模拟,验证了本文方法的有效性.     

Numerical simulation of turbulent free-surface flow in curved channel

This paper presents first results of numerical simulation of turbulent free-surface flow. Simple implementation of surface capturing method is based on the variable density approach. The flow is treated as if there is only one fluid, but with variable material properties (density, viscosity). The switch in these values is done by a function resulting from the mass conservation principle. This approach simplifies the implementation of turbulence model. In this case the SST k−ω model was chosen in modification given by Hellsten.Numerical solution was carried out by finite-volume method with explicit Runge-Kutta time-integration. The artificial compressibility method was used for time-marching search for steady state solution. The whole model was tested on horizontally placed square-sectioned 90^∘ bend, which was partially filled by the water. The main goal of this study was to demonstrate the applicability of this model and solution method for capturing the water-air interface as well as for predicting the turbulent effects in both fluids.