Numerical simulation of solitary and random wave propagation through vegetation based on VOF method

A vertical two-dimensional numerical model has been applied to solving the Reynolds Averaged Navier- Stokes （RANS} equations in the simulation of current and wave propagation through vegetated and non- vegetated waters. The k-e model is used for turbulence closure of RANS equations. The effect of vegeta- tion is simulated by adding the drag force of vegetation in the flow momentum equations and turbulence model. To solve the modified N-S equations, the finite difference method is used with the staggered grid system to solver equations. The Youngs＇ fractional volume of fluid （VOF） is applied tracking the free sur- face with second-order accuracy. The model has been tested by simulating dam break wave, pure current with vegetation, solitary wave runup on vegetated and non-vegetated channel, regular and random waves over a vegetated field. The model reasonably well reproduces these experimental observations, the model- ing approach presented herein should be useful in simulating nearshore processes in coastal domains with vegetation effects.

Numerical simulation of a falling droplet surrounding by air under electric field using VOF method:A CFD study

This is a numerical study of a falling droplet surrounding by air under the electric field modeled with finite volume method by means of CFD.The VOF method has been employed to model the two-phase flow of the present study.Various capillary numbers are investigated to analyze the effects of electric field intensity on the falling droplet deformation.Also,the effects of electric potential on the heat transfer coefficient have been examined.The obtained results showed that by applying the electric field at a capillary number of 0.2 the droplet tends to retain its primitive shape as time goes by,with a subtle deformation to an oblate form.Intensifying the electric field to a capillary number of 0.8 droplet deformation is almost insignificantwith time progressing;however,further enhancement in capillary number to 2 causes the droplet to deform as a prolate shape and higher values of this number intensify the prolate form deformation of the droplet and result in pinch-off phenomenon.Ultimately,it is showed that as the electric potential augments the heat transfer coefficient increases in which for electric potential values higher than 2400 V the heat transfer coefficient enhances significantly.

Simulated and Experimental Study of Seabed Sediments Sampling Parameters Based on the VOF Method

Sediments in the seabed hold vital clues to the study of marine geology,microbial communities and history of ocean life,and the remote operated vehicle(ROV)mounted tubular sampling is an important way to obtain sediments.However,sampling in the seabed is a particularly difficult and complicated task due to the difficulty accessing deep water layers.The sampling is affected by the sampler’s structural parameters,operation parameters and the interaction between the sampling tube and sediments,which usually results in low volume and coring rate of sediments obtained.This paper simulated the soft viscous seabed sediments as non-Newtonian Herschel-Bulkley viscoplastic fluids and established a numerical model for the tubular sampling based on the volume of fluid(VOF)method.The influence rules of the sampling tube diameter,drainage area rate,penetration velocity,and sediments dynamic viscosity on coring rate and volume were studied.The results showed that coring volume was negatively correlated with all the parameters except the sampling tube diameter.Furthermore,coring rate decreased with increases in penetration velocity,drainage area rate,and sediments dynamic viscosity.The coring rate first increased and then decreased with increasing of the sampling tube diameter,and the peak value was also influenced by penetration velocity.Then,based on the numerical simulation results,an experimental sampling platform was set up and real-world sampling experiments were conducted.The simulation results tallied with the experimental results,with a maximum absolute error of only 4.6%,which verified that the numerical simulation model accurately reflected real-world sampling.The findings in this paper can provide a theoretical basis for facilitating the optimal design of the geometric structure of the seabed sediments samplers and the parameters in the sampling process.

Vacuum Filling Simulation with Combined Lagranian and VOF Method

Jetting succeeded by accumulation is the characteristic of the vacuum filling,which is different from the conventional pressure-driven flow.In order to simulate this kind of flow,a three-dimensional theoretical model in terms of incompressible and viscous flow is established,and an iterative method combined with finite element method(FEM)is proposed to solve the flow problem.The Lagranian-VOF method is constructed to trace the jetting and accumulated flow fronts.Based on the proposed model and algorithm,a simulation program is developed to predict the velocity,pressure,temperature,and advancement progress.To validate the model and algorithm,a visual experimental equipment for vacuum filling is designed and construted.The vacuum filling experiments with different viscous materials and negative pressures were conducted and compared with the corresponding simulations.The results show the flow front shape closely depends on the fluid viscosity and less relates to the vacuum pressure.

An Irregular Wave Maker of Active Absorption with VOF Method

A numerical irregular wave flume with active absorption of re-reflected waves is simulated by use of volume of fluid （VOF） method. An active ＇absorbing wave-maker based on linear wave theory is set on the left boundary of the wave flume. The progressive waves and the absorbing waves are generated simultaneously at the active wave generating-absorbing boundary. The absorbing waves are generated to eliminate the waves coming back to the generating boundary due to reflection from the outflow boundary and the structures. SIRW method proposed by Frigaard and Brorsen （1995） is used to separate the incident waves and reflected waves. The digital filters are designed based on the surface elevation signals of the two wave gauges. The corrected velocity of the wave-maker paddle is the output from the digital filter in real time. The numerical results of regular and irregular waves by the active absorbing-generating boundary are compared with the numerical results by the ordinary generating boundary to verify the performance of the active absorbing-generator boundary. The differences between the initial incident waves and the estimated incident waves are analyzed.

Application of CLEAR-VOF method to wave and flow simulations

A two-dimensional numerical model based on the Navier-Stokes equations and computational Lagrangian-Eulerian advection remap-volume of fluid （CLEAR-VOF） method was developed to simulate wave and flow problems. The Navier-Stokes equations were discretized with a three-step finite element method that has a third-order accuracy. In the CLEAR-VOF method, the VOF function F was calculated in the Lagrangian manner and allowed the complicated free surface to be accurately captured. The propagation of regular waves and solitary waves over a flat bottom, and shoaling and breaking of solitary waves on two different slopes were simulated with this model, and the numerical results agreed with experimental data and theoretical solutions. A benchmark test of dam-collapse flow was also simulated with an unstructured mesh, and the capability of the present model for wave and flow simulations with unstructured meshes, was verified. The results show that the model is effective for numerical simulation of wave and flow problems with both structured and unstructured meshes.

非定常自由面流动模拟的反扩散VOF算法研究

VOF(Volume of Fluid)方法由于其良好的守恒性和网格适应性,且具有计算资源需求相对较小等优点,成为船舶水动力学领域自由面流动CFD(Computational Fluid Dynamics)模拟的主流方法。但原始的VOF方法存在较为严重的界面扩散问题,导致模拟的界面厚度过大、空间分辨率不够,进而影响流场相关变量的计算精度,这一问题在非定常自由面流动模拟中尤为明显。本文针对上述问题,通过在VOF控制方程中引入人工对流项以达到抑制界面扩散、压缩界面厚度的目的,并采用隐式离散人工对流项的方式提高计算稳定性,形成了反扩散VOF算法。经Zalesak和剪切场等经典算例在不同数量网格下的测试验证,表明反扩散VOF算法能够大幅压缩界面厚度,同时明显减小质量误差。随后的三维无障碍溃坝算例和破舱进水算例,进一步证明了反扩散VOF算法在实际非定常流动模拟中能够更好地捕捉自由面并提高计算精度。

THE VOF METHOD FOR STUDY OF WAVE RUN-UP AND BREAKING ON A SLOPING STRUCTURE

The volume of fluid method (VOF method) for numerical simulations describing wave run-up on a sloping structure including the overturning. breaking and merging phenomena is presented. The flow motions are governed by the classical, two-dimensional Navier-Stokes equation for incompressible fluid. Computational results concerning the time evolution of the free surface and pressure distribution along water bed and slope boundary are given, showing how an initial solitary wave undergoes run-up, overturning, breaking and merging on the slope. It is found that most of the wave energy is lost after the wave breaking and merging.

基于VOF方法的超临界二氧化碳——水两相流动孔隙尺度数值模拟

二氧化碳(CO_(2))捕集与封存技术有利于减少CO_(2)的排放量,近年来针对CO_(2)地质封存形成了从纳米尺度到油气藏尺度的大量研究成果,大多数研究只针对单一维度多孔介质中流动行为开展研究,且物理实验方法受许多不确定性因素影响,十分耗费时间和成本。为了从微观角度深入理解CO_(2)地质封存过程中的渗流行为,提高CO_(2)地质埋存量,基于追踪两相界面动态变化的VOF(Volume of Fluid)方法,分别建立了2D和3D模型,开展了超临界CO_(2)-水两相流动数值模拟研究,对比了不同润湿性、毛细管数、黏度比条件下的CO_(2)团簇分布特征、CO_(2)饱和度变化规律,揭示了孔隙尺度CO_(2)埋存的内在机理。研究结果表明:①随着岩石对CO_(2)润湿性增加,CO_(2)波及范围扩大,同时CO_(2)团簇的卡断频率减少,CO_(2)埋存量增加;②随着毛细管数的增加,驱替模式由毛细指进转变为稳定驱替,CO_(2)埋存量增加;③随着注入超临界CO_(2)黏度逐渐接近水的黏度,两相流体之间的流动阻力降低,促进了“润滑效应”,CO_(2)相的渗流能力提高,CO_(2)埋存量增加;④润湿性、毛细管数、黏度比在不同维度多孔介质模型中对CO_(2)饱和度的影响程度不同。结论认为,基于VOF方法的CO_(2)-水两相渗流模拟研究在孔隙尺度上揭示了CO_(2)地质封存过程中的渗流机理,对CCUS技术的发展有指导意义,也为更大尺度的CO_(2)地质封存研究提供了理论指导和技术支撑。

基于VOF方法的可调参数对气泡聚并过程计算精度与成本的影响

气泡聚并通过改变气液两相间界面面积,影响相间的传热和传质,而聚并时间和液膜厚度是描述2个气泡聚并过程的重要物理参数。采用流体体积(VOF)方法对气泡聚并过程进行数值模拟时,合理设置计算流程中可调参数(例如网格尺寸、最大库朗数以及方程循环次数等),有助于提高求解过程的收敛性、节省计算时间。因此,本文基于OpenFOAM开源软件,采用VOF方法与自适应网格相结合的方法,以同轴两气泡聚并过程为例,探究最大库朗数Co_(max)、相方程循环次数nα和控制方程循环次数n_(pimple)这3个参数对气泡聚并过程数值模拟的计算量与计算精度的影响,同时,得到气泡间液膜厚度在可调参数不同取值时随时间的变化规律。研究结果表明:气泡间液膜的变薄速度与最大库朗数Co_(max)成反比,与相方程循环次数nα和控制方程循环次数n_(pimple)成正比,这主要是由于减小最大库朗数Co_(max)以及增大相方程循环次数nα和控制方程循环次数n_(pimple)提高了计算精度,改善了流体在网格单元上移动滞后的现象,加快了两气泡聚并以及液膜减薄过程;综合考虑计算精度与计算成本,得到了基于VOF方法的气泡聚并数值模拟较优的可调参数组合,即(Co_(max),nα,n_(pimple))=(0.05,3,8)。

A NEW NUMERICAL WAVE FLUME COMBINING THE 0-1 TYPE BEM AND THE VOF METHOD

A new coupling numerical wave model, based on both the Boundary Element Method （BEM） and the Volume Of Fluid （VOF） method, is established by taking advantages of the both methods to solve the wave-structure interaction problems. In this model, the wave transformation in front of structures is calculated by the 0-1 type BEM, and the intense wave motions near the structures are calculated by the VOF method. In this paper, the characteristics of the BEM and the VOF method are discussed first, and then the coupling treatments are described in detail. In the end, the accuracy and the validity of the coupling model are examined by comparing the numerical results with experiment results and other numerical results available for the interactions between regular waves with a monolayer horizontal plate.

基于Projection VOF方法的熔模铸造蜡模充型过程数值模拟

针对熔模铸造蜡模充型过程数值模拟软件自主开发,建立了流动场和温度场数学模型,采用Cross-WLF粘度模型描述蜡料的流变性能,基于Projection VOF方法进行蜡料流动过程速度场、压力场以及自由表面的求解。通过试验测量获取了K512型蜡料的热物理性能和流变性能的相关数据,拟合得到粘度模型参数。分别在蜡料常粘度和变粘度模型两种条件下对扳机铸件蜡模充型过程进行模拟,结果表明,所建立蜡模充型过程数学模型能较好地描述蜡料的动态流变特性和传热行为。

A stencil-like volume of fluid (VOF)method for tracking free interface

A stencil-like volume of fluid （VOF） method is proposed for tracking free interface. A stencil on a grid cell is worked out according to the normal direction of the interface, in which only three interface positions are possible in 2D cases, and the interface can be reconstructed by only requiring the known local volume fraction information. On the other hand, the fluid-occupying-length is defined on each side of the stencil, through which a unified fluid-occupying volume model and a unified algorithm can be obtained to solve the interface advection equation. The method is suitable for the arbitrary geometry of the grid cell, and is extendible to 3D cases. Typical numerical examples show that the current method can give ＂sharp＂ results for tracking free interface.

基于VOF方法的半潜平台润滑油箱摇荡仿真分析

为探究半潜平台运动对燃气轮机机组润滑油运行状态的影响,建立润滑油箱物理模型,利用VOF方法进行润滑油摇摆垂荡复合运动仿真模拟,揭示了半潜平台燃气轮机机组润滑油摇荡运动规律。研究结果表明,对于半潜平台上典型润滑油箱充液系统,摇荡运动周期远离共振周期,润滑油箱在摇荡运动工况下液面上下起伏波动较小。润滑油泵吸油口距离液面最小距离发生在箱体摇摆到最大角度前后,最小距离较共振时有明显增加,保证润滑油泵工作在安全液位高度。

燃料电池部分堵塞通道内液滴的动态行为

为了有效去除通道内液态水,采用流体体积模型(volume of fluid model,VOF)方法研究了燃料电池通道内部分堵塞对液滴运输过程的影响,探讨了堵塞块形状参数、液滴尺寸和气体扩散层(gas diffusion layer,GDL)表面润湿性对通道内两相输运特性的影响.结果表明:堵塞块的存在会影响液滴在通道内运输,堵塞块的引入导致液滴受到y方向的剪切力增大,液滴的运输速率提高,有利于GDL表面液态水的去除;随着堵塞比和纵向比的增大,通道内压降增大,液滴运输速率加快;随着液滴直径和GDL表面接触角的增大,液滴运输速率增大;当液滴直径为0.8 cm和GDL表面接触角为150°时,通道内液滴的运输速率越快.

轴心射流收油环内部油气流动特性的数值模拟研究

为研究环下润滑结构内部油膜迁移及流动特性,针对轴心射流收油环采用VOF (Volume of fluid)方法开展了数值计算,获得了收油环端面油膜动态形成过程,在分析流场特征的基础上,讨论了收油环运转工况及结构参数对内部油膜形态、滑油体积分数、油膜速度和供油孔输油能力的影响规律。结果表明:收油环端面油膜呈圆盘状迁移,边缘破碎形成油滴、油带甩至侧壁面,在供油孔内以“月牙形”分布加速流动,收油环端面油膜厚度随主轴转速增大而减小,随喷嘴流量上升而增加;提高转速降低了供油孔内滑油含量,使孔内油膜加速流动,孔内滑油含量随喷嘴流量的上升而增大,随供油孔径的增加而下降;喷嘴流量与供油孔径的改变对孔内流速影响较小;增加孔径与提高收油环转速可加强供油孔输运能力,8 kr/min下提高喷嘴流量使无量纲输油量Cq平均降低了40.71%,提高孔径使Cq最大提高了57.14%,转速的增加使Cq平均增加25.87%。

液舱运动与流体晃荡耦合运动数值模拟与试验验证

本文针对液舱运动与流体晃荡耦合运动问题,采用基于Hilber-Hughes-Taylor(HHT)格式的隐式直接积分法求解液舱运动,应用VOF法捕捉流体晃荡自由表面运动,建立液舱运动与流体晃荡双向耦合迭代算法。将算法应用到某转动液舱系统的模型试验中,通过数值计算结果与试验结果的比较验证算法的有效性。本文建立的分析方法可为液舱运动与流体晃动耦合运动问题提供一种有效的分析方法。

三维VOF模型及其在溢洪道水流计算中的应用

详细介绍了VOF模型及其应用方法,结合k-ε紊流模型和分段线形方式构造界面的方法,数值求解了带自由水面的三维水流流动问题。以大型水工建筑物溢洪道内水流流动为例,计算了溢洪道内水流流动特性,并将计算结果与实验数据进行对比,结果表明计算值与实测值相吻合,说明VOF模型能够精确地跟踪自由表面,适用于溢洪道等大型水工建筑物内的水流流动的数值模拟。

VOF方法理论与应用综述

分析两相流中采用VOF方法模拟交界面运动变化的基本原理,概述近年来为提高VOF方法计算精度和自由面重构效果所做出的各种改进和在各个领域所取得的研究成果及应用前景.指出VOF方法如在基础理论上进行改进,则在多维流场的数值模拟上将会有更广阔的应用空间.

求解不可压缩两相流的复合Level Set-VOF方法

目前,不可压缩两相流问题的数值求解方法主要有VOF方法和Level Set方法等.根据CLSVOF方法基本思想,通过将VOF和Level Set两种方法结合,实现了复合Level Set-VOF方法追踪不可压缩两相流相界面的模拟计算;对比分析了VOF、Level Set和复合Level Set-VOF方法计算经典两相流问题的结果.研究结果表明,复合Level Set-VOF方法继承了VOF方法精确构造相界面和Level Set方法精确计算表面张力的优点,能够有效削弱两相流求解过程中出现的虚假流动问题,因此可以更加精确地求解不可压缩两相流问题.