爆炸与冲击中的实际物质状态方程

本文综述了爆炸力学计算中涉及的实际介质的状态方程,这些物质结构复杂,其描述方法与金属的状态方程有些不同.用物理力学观点及半经验、半理论方法进行描述.①用普遍的热力学方法导出介质的状态方程表达式,并用实测数据计算了有关的参量;②Grneisen系数值的计算以及它与体积和温度的关系;③多孔介质的优态方程;④从理论上系统地导出了波后卸载方程;⑤探讨了原予统计模型的边界势.      

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.      

物理增强的流场深度学习建模与模拟方法

流体运动理论上可用Navier?Stokes方程描述, 但由于对流项带来的非线性, 仅在少数情况可求得方程解析解. 对于复杂工程流动问题, 数值模拟难以高效精准计算高雷诺数流场, 实验或现场测量难以获得流场丰富细节. 近年来, 人工智能技术快速发展, 深度学习等数据驱动技术可利用灵活网络结构, 借助高效优化算法, 获得对高维、非线性问题的强大逼近能力, 为研究流体力学计算方法带来新机遇. 有别于传统图像识别、自然语言处理等典型人工智能任务, 深度学习模型预测的流场需满足流体物理规律, 如Navier?Stokes方程、典型能谱等. 近期, 物理增强的流场深度学习建模与模拟方法快速发展, 正逐渐成为流体力学全新研究范式: 根据流体物理规律选取网络输入特征或设计网络架构的方法称为物理启发的深度学习方法, 直接将流体物理规律显式融入网络损失函数或网络架构的方法称为物理融合的深度学习方法. 研究内容涵盖流体力学降阶模型、流动控制方程求解领域.      

一种修正的低温流体空化流动计算模型

为了更准确地预测低温流体的空化流动特性, 基于Kubota空化模型, 对蒸发和凝结源项进行修正, 建立了一种考虑热力学效应的空化模型. 分别采用原始和修正的Kubota空化模型, 计算了绕对称回转体液氮的空化流动, 通过与实验结果的比较对修正的空化模型进行了评价. 结果表明, 与原Kubota空化模型比较, 修正的空化模型由于考虑了热力学效应, 计算获得的蒸发量减小, 凝结量增大, 空穴长度减小, 空穴界面形态呈模糊状态.计算结果与实验结果更加一致, 说明修正的空化模型能准确的描述低温流体空化过程的质量传输过程, 能够更准确模拟低温流体中的空化流动特性.      

基于数据驱动的大气压射频放电等离子体数值模拟研究

随着人工智能技术的进步,结合低温等离子体的物理特点,数据驱动技术由于其独特的优势在低温等离子体的研究中正逐渐兴起.本研究以深度神经网络(DNN)模型在大气压射频放电中的计算研究为例,讨论了数据驱动方法在低温等离子体模拟研究中的应用.对于低温等离子体的研究而言,数据驱动研究所需要的数据可以来自于实验诊断和数值计算,根据等离子体物理特性的不同也可以选择不同的数据驱动模型.粒子模型与流体模型是低温等离子体研究中常用的两类计算模型,基于这两者的模拟数据组成的训练集, DNN可以实现对大气压射频放电的动理学特性等各种特性的实时预测.首先通过将流体模型与粒子模型计算结果与DNN模型的预测结果相比较,验证了DNN模型在给定精度下的有效性.然后基于流体模拟数据,利用DNN探究了α和γ模式下输入电流密度和放电间隙对大气压射频放电特性的影响,最后借助于粒子模拟数据构建的训练集,讨论了大气压射频微放电的频率效应,特别是电子能量分布函数(EEDF)的演化.预测结果表明,经过大约1 h的训练后, DNN只需要耗时0.01 s左右就能以极高的计算精度(与数值模拟之间的相对误差小于0.5%)获得电子密度、电场强度和...     

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.     

基于同步交替法求解流体-弹性板耦合作用

基于预测-多步校正方法提出了流固耦合同步交替求解方法.采用ALE描述流固耦合问题.基于SUPG有限元对流体动量方程进行空间离散,以消除对流项引起的数值振荡.时间域上的积分采用预测-多步校正(predictor-multicorrector)方法.对流体和结构同时进行预测,在多步校正的过程中对流体和固体域动力学方程进行交替求解,从而在时间上达到同步推进.基于上述同步交替法,编程计算了流体-弹性板耦合作用问题,取得了良好的数值计算结果.     

超临界CO_2-水两相流与CO_2毛细捕获:微观孔隙模型实验与数值模拟研究

CO_2毛细捕获机制是CO_2地质封存中的关键科学问题,然而有关孔隙尺度下(微米极)超临界CO_2毛细捕获的研究较少.采用高压流体-显微镜-微观模型实验装置,开展超临界CO_2条件(8.5 MPa,45?C)下CO_2驱替水(排水)和水驱替CO_2(吸湿)实验,采用高分辨率照相机采集CO_2水两相流运动图像,并借助光学显微镜直接观测孔隙尺度下CO_2毛细捕获特征.同时,采用计算流体动力学方法对实验过程进行三维数值模拟.数值模拟不仅反映了实验过程中两相流驱替锋面的推进过程,还刻画了孔隙尺度下被捕获的CO_2液滴/团簇三维空间形态特征.最后,基于数值模拟给出了CO_2初始饱和度与残余饱和度曲线,即毛细捕获曲线,并对比分析了3种毛细捕获曲线预测模型(即Jurauld模型、Land模型和Spiteri模型)的优劣.分析表明,Jurauld模型的描述能力稍优于Land模型,Spiteri模型的描述能力较弱.由于Land模型只需单个参数,且参数具有明确的物理意义,因此在实际工程中,建议优先采用Land模型.     

Mie-Grneisen状态方程可压缩多流体流动的PPM方法

采用流体体积分数的混合型多流体数值模型，将piecewise parabolic method (PPM)方法应用于可压缩多流体流动的数值模拟，拓展了以前提出的模型和数值方法，使它能够处理一般的Mie-Grneisen状态方程。采用双波近似和两层迭代算法求解一般状态方程的Riemann问题；并根据多流体接触界面无振荡原则设计高精度计算格式，对典型的纯界面平移问题可以从理论上证明本算法在接触间断附近压力和速度没有振荡，而且数值模拟结果表明界面数值耗散也被控制在2~3个网格之内。模拟了多种复杂的可压缩多流体流动，算例结果表明本文方法可以有效地处理接触间断、激波等物理问题，且具有耗散小精度高的特点。     

斜波压缩下锡的相变和层裂行为

获取不同热力学路径下锡的动态响应实验数据,是深入研究其相变和损伤物理过程的基础.利用小型磁驱装置CQ-4完成了金属锡的斜波加载实验,获取了锡含有相变和层裂损伤物理信息的实验数据.实验结果显示,在加载段锡依次经历了弹塑性转变和β-γ相变两种物理过程,屈服强度约0.194 GPa,相变压力随着锡厚度的增加从7.54 GPa减小到7.14 GPa.在卸载段出现了明显的层裂损伤,层裂强度约1.1 GPa,与相同加载压力下冲击实验结果有巨大差异,层裂片厚度约0.38 mm.结合由锡的多相Helmholtz自由能计算的多相状态方程、Hayes相变动力学方程和损伤度理论,对斜波压缩实验过程进行一维流体动力学数值模拟,计算结果可以很好描述锡的弹塑性转变、相变和层裂三个物理过程.     

Flux vector splitting solutions for coupling hydraulic transient of gas-liquid-solid three-phase flow in pipelines

The gas-liquid-solid three-phase mixed flow is the most general in multiphase mixed transportation. It is significant to exactly solve the coupling hydraulic transient problems of this type of multiphase mixed flow in pipelines. Presently, the method of characteristics is widely used to solve classical hydraulic transient problems. However, when it is used to solve coupling hydraulic transient problems, excessive interpolation errors may be introduced into the results due to unavoidable multiwave interpolated calculations. To deal with the problem, a finite difference scheme based on the Steger-Warming flux vector splitting is proposed. A flux vector splitting scheme is established for the coupling hydraulic transient model of gas-liquid-solid three-phase mixed flow in the pipelines. The flux subvectors are then discretized by the Lax-Wendroff central difference scheme and the Warming-Beam upwind difference scheme with second-order precision in both time and space. Under the Rankine-Hugoniot conditions and the corresponding boundary conditions, an effective solution to those points located at the boundaries is developed, which can avoid the problem beyond the calculation region directly induced by the second-order discrete technique. Numerical and experimental verifications indicate that the proposed scheme has several desirable advantages including high calculation precision, excellent shock wave capture capability without false numerical oscillation, low sensitivity to the Courant number, and good stability.     

Model for Gas–Condensate Phase Equilibria for Estimating C7+ Critical Properties Using Genetic Algorithm

A reliable prediction of the phase behavior is necessary in determining the compositions of the gas and liquid phases at various pressures. These calculations require stepwise computational procedure using a cubic equation of state (EOS). Since the heavy components in the petroleum mixtures have the strongest effect on the characteristics of the fluids, critical properties must be estimated for the petroleum fractions making up heptanes-plus. A phase equilibria calculation of a gas–condensate system with Peng–Robinson equation of state was done by satisfying the condition of chemical equilibrium. A genetic algorithm was used to determine the optimum critical properties of heptanes-plus (C₇₊) fraction. The predictions of the model are compared with the experimental results of the constant volume depletion (CVD) test.     

Numerical solution of shallow water magnetohydrodynamic equations with non-flat bottom topography

The governing equations of shallow water magnetohydrodynamics describe the dynamics of a thin layer of nearly incompressible and electrically conducting fluids for which the evolution is nearly two-dimensional with magnetic equilibrium in the third direction. A high-resolution central-upwind scheme is applied to solve the model equations considering non-flat bottom topography. The suggested method is an upwind biased non-oscillatory finite volume scheme which doées not require a Riemann solver at each time step. To satisfy the divergence-free constraint, the projection method is used. Several case studies are carried out. For validation, a gas kinetic flux vector splitting scheme is also applied to the same model.     

Compositional Streamline Simulation: A Parallel Implementation

Compositional reservoir simulators are needed to model oil recovery from petroleum reservoirs by miscible gas injection. This article describes the development and application of a parallel version of a compositional streamline simulator. A compositional streamline module is developed and integrated with an existing finite-difference simulator. Finite-difference calculations including pressure solution are performed on a single processor. The movement of fluids (which includes streamline tracing, mappings, flux calculations, and one-dimensional solver) is done along streamlines in the streamline module. The streamline module is parallelized by distributing streamlines among different processors because computations along any streamline are independent of other streamlines and no communication is required. Flux calculation along streamlines is computationally expensive primarily due to flash calculations that are performed to distribute components among the hydrocarbon phases. Simultaneous solution of this time-consuming step results in reduction of total CPU time. Communication (gathering) across the streamlines or processors is achieved by using message passing interface (MPI). Test runs are conducted for different examples to investigate the performance of the parallel streamline simulator. Results indicate that significant reduction in CPU time can be obtained by distributing streamlines on different processors.     

Multiphase flow model developed for simulating gas hydrate transport in horizontal pipe

The hydrate formation or dissociation in deep subsea flow lines is a challenging problem in oil and gas transport systems. The study of multiphase flows is complex while necessary due to the phase changes (i.e., liquid, solid, and gas) that occur with increasing the temperature and decreasing the pressure. A one-dimensional multiphase flow model coupled with a transient hydrate kinetic model is developed to study the characteristics of the multiphase flows for the hydrates formed by the phase changes in the pipes. The multiphase flow model is derived from a multi-fluid model, while has been widely used in modelling multiphase flows. The heat convection between the fluid and the ambient through the pipe wall is considered in the energy balance equation. The developed multiphase flow model is used to simulate the procedure of the hydrate transport. The results show that the formation of the hydrates can cause hold-up oscillations of water and gas.     

A high‐resolution method for compressible two‐fluid flows and simulation of three‐dimensional shock–bubble interactions

A high‐resolution method is developed to capture the material interfaces of compressible two‐fluid flows in multiple dimensions. A fluid mixture model system with single velocity and pressure is used, and viscous effect can also be taken into account. A consistent thermodynamic law based on the assumption of pressure equilibrium is employed to describe the thermodynamic behaviors of the pure fluids and mixture of two components. The splitting and unsplit Eulerian formulations of piecewise parabolic method are extended to numerically integrate the hyperbolic part of the model system, whereas the system of diffusion equations is solved using an explicit, central difference scheme. The block‐structured adaptive mesh refinement (AMR) capability is built in the hydrodynamic code to locally improve grid resolution. The resulting method is verified to be at least second‐order accurate in space. Numerical results show that the discontinuities, particularly contact discontinuities, can be resolved sharply. The use of AMR allows flow features at disparate scales to be resolved sufficiently. In addition, three‐dimensional shock–bubble interactions are simulated to investigate effects of Mach number on bubble evolution. The flow structures including those peculiar to three‐dimensional bubble are resolved correctly, and some physical phenomena with increasing Mach number are reported. Copyright © 2012 John Wiley & Sons, Ltd.     

Direct numerical simulations of gas–solid–liquid interactions in dilute fluids

The dynamic interactions between gas bubbles, rigid particles and liquid can lead to profound nonlinearities in the aggregate behavior of a multiphase fluid. Predicting the nonlinear dynamics of the multiphase mixture hence requires understanding how the phases interact at the scale of individual interfaces, but these interactions are notoriously difficult to resolve in models. The goal of this paper is to develop and validate a computational method capable of capturing the complex flow interactions between gas bubbles and rigid particles immersed in a Newtonian liquid. We focus on multiphase systems that are dilute enough for the solid and gas components to move through and be moved by the ambient liquid. We use level sets with a topology-preserving advection scheme to track the gas interfaces. To include the motion of the rigid particles, we couple distributed Lagrange multipliers to an immersed-boundary method. The high viscosity contrast between the liquid and the gas requires both time splitting and approximate factorization to efficiently solve the governing equations consisting of the conservation of mass, momentum and energy. To resolve interactions between interfaces that vary drastically in size, we refine our mesh adaptively in the vicinity of the boundary.     

Simulating gas–liquid multiphase flow using meshless Lagrangian method

A multiphase flow model has been established based on a moving particle semi‐implicit method. A surface tension model is introduced to the particle method to improve the numerical accuracy and stability. Several computational techniques are employed to simplify the numerical procedure and further improve the accuracy. A particle fraction multiphase flow model is developed and verified by a two‐phase Poiseuille flow. The multiphase surface tension model is discussed in detail, and an ethanol drop case is introduced to verify the surface tension model. A simple dam break is simulated to demonstrate the improvements with various modifications in particle method along with a new boundary condition. Finally, we simulate several bubble rising cases to show the capacity of this new model in simulating gas–liquid multiphase flow with large density ratio difference between phases. The comparisons among numerical results of mesh‐based model, experimental data, and the present model, indicate that the new multiphase particle method is acceptable in gas–liquid multiphase fluids simulation. Copyright © 2014 John Wiley & Sons, Ltd.     

基于机器学习的页岩气采收率预测方法

页岩气是指以吸附和游离时而还有流体相的状态赋存于泥页岩中的非常规天然气,我国探明储量丰富,地域分布广泛,埋藏深度普遍在3000米以下.页岩气开采的关键技术是水平井和水力压裂,而高效开采面临的更大困难和挑战是预测采收率.如果能够预测采收率,一是可以评估当前储层改造程度,二是可以获悉当前的施工参数对产气量的直接影响,便于动...