三维倾斜平板液氧降膜润湿数值模拟研究

建立三维倾斜平板降膜模型,利用VOF两相流模型计算了液氧降膜的润湿情况,研究了工质物性、倾斜角、液膜入口高度对润湿面积的影响。结果表明:Weber数(We)相同时,液氧和水的润湿比均随Kapitza数(Ka)增大而减小;相同Ka下,液氧和水的润湿比均随We增大而增大,而液氧润湿比一直小于水润湿比,两者的差值也随We增大而增大。拟合得到液氧在液膜入口高度0.4 mm、接触角70°时的界面润湿比经验关联式,拟合值和模拟值相对误差≤±20%;在We=0.76时,液氧的润湿比随倾斜角增大而减小,但降低速率随Ka增大而减小;在倾斜角为90°时,易出现液膜脱离壁面的现象;当We固定时,液氧的润湿比随液膜入口高度增大而增大。     

数值模拟孤立波通过水下孤立方柱的粘性流动

本文用完整二维Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程和ＶＯＦ方法，研究孤立波通过淹没水下孤立直立方柱水域时的波形变化和粘性流场运动。本文对孤立波通过水下 孤立直方柱的情形进行了实例计算。给出了波形随时间的演化图，可以看到反射波、前传波和跟随的振荡型小波列的生成及涡流场的运动演化，并与势流计算结果进行了比较。     

A numerical investigation of electrohydrodynamic (EHD) effects on bubble deformation under pseudo-nucleate boiling conditions

In this article, the electrohydrodynamic (EHD) effects on nucleate boiling are studied by developing a numerical modelling of EHD effect on bubble deformation in pseudo-nucleate boiling conditions. The volume of fluid (VOF) method is employed to track the interface between the gas–liquid two phases; the user-defined code is written and added to the commercial software FLUENT to solve the electric field and the corresponding electric body force. On this basis, the model is applied to study the EHD effects on heat transfer and fluid flows. An initial air bubble surrounded by liquid CCl₄ and attached to a horizontal superheated wall under the action of electric field is studied. The results of the EHD effect on bubble shape evolution are compared with those of available experiments showing good agreement. The mechanism of EHD enhancement of heat transfer and the EHD induced phenomena including bubble elongation and detachment are analyzed in detail.     

A height function based momentum balance model to simulate contact angle dynamics with hysteresis

We report a combined method to deal with the contact angle dynamics with hysteresis. The momentum balance model is applied to obtain the transient contact angle by balancing the inertia and the capillary force where the curvatures are estimated by the height function at the contact line. This integrated approach provides a great convenience that no need for a prior knowledge of the contact angle, and possesses the good property of the sharp interface approximation. In order to facilitate the wider use of the present method, we incorporate a dynamic contact line model to estimate the contact angle when the contact line starts to move. This combination plus the hysteresis region will take the ability to solve most of problems related to wetting with more physical sense. This proposed model is finally validated by the droplet equilibrium, spreading, and sliding tests.     

单模态Richtmyer-Meshkov不稳定性数值模拟

采用VOF(Volume of Fluid)方法和PPM(Piecewise Parabolic Method)方法,发展了可用于可压缩多介质粘性流体动力学问题的数值模拟方法MVPPM(Multi-Viscous-Fluid Piecewise Parabolic Method)。利用MVPPM对多个具有不同初始扰动振幅的二维和三维单模态RM(Richtmyer-Meshkov)不稳定性模型进行了数值计算,并与理论模型的计算结果进行了比较。结果表明,无论二维还是三维情况,当初始扰动振幅相对于波长较小的时候,计算的扰动振幅和增长率与理论模型的计算结果一致。当初始扰动波长不变而振幅逐渐增大时,界面振幅和增长率也逐渐增大。对于具有相同初始扰动的情况,三维计算结果在线性段与二维计算结果相同,但是在非线性段比二维结果大,说明非线性和三维效应在RM不稳定性发展过程中起着重要作用。     

Numerical study of breaking waves by a two‐phase flow model

A two‐phase flow model, which solves the flow in the air and water simultaneously, is presented for modelling breaking waves in deep and shallow water, including wave pre‐breaking, overturning and post‐breaking processes. The model is based on the Reynolds‐averaged Navier–Stokes equations with the k ?ε turbulence model. The governing equations are solved by the finite volume method in a Cartesian staggered grid and the partial cell treatment is implemented to deal with complex geometries. The SIMPLE algorithm is utilised for the pressure‐velocity coupling and the air‐water interface is modelled by the interface capturing method via a high resolution volume of fluid scheme. The numerical model is validated by simulating overturning waves on a sloping beach and over a reef, and deep‐water breaking waves in a periodic domain, in which good agreement between numerical results and available experimental measurements for the water surface profiles during wave overturning is obtained. The overturning jet, air entrainment and splash‐up during wave breaking have been captured by the two‐phase flow model, which demonstrates the capability of the model to simulate free surface flow and wave breaking problems.Copyright © 2011 John Wiley & Sons, Ltd.     

轮胎滑水的CFD计算方法研究

针对轮胎滑水现象,采用VOF模型(Volume of Fluid Model)进行了CFD数值模拟计算。建立了分析对象的物理模型(轮胎模型、水模型和边界条件及初始条件)和RNGk-ε湍流模型;采用多块网格技术对计算域进行了离散,重点模拟分析三种不同入口速度下的动水压力变化和轮胎附近流场变化。最后和国外学者所做的试验和相关文献结果进行了对比,表明本文介绍的方法是可行的,该方法为轮胎产品开发提供了有效经济的技术支持。     

3D Simulations of Freezing Characteristics of Double-Droplet Impact on Cold Surfaces with Different Wettability

In this work, the freezing characteristics of double-droplet impact on three typical wettability surfaces were investigated by coupling the solidification and melting VOF models. Different temperature conditions were adopted to study the influence of icing speed on droplet behavior. Simulation results show that the motion of the double-droplet impact is consistent with that of a single droplet in the early spreading stage but behaves differently in the retraction stage. The wetting area evolution during the impact-freezing process shows different tendency for hydrophilic and hydrophobic surfaces: Compared with single droplets, double droplets have a smaller wetting area factor on hydrophilic surfaces but a larger one on superhydrophobic surfaces. In addition, three typical impact results are observed for the double-droplet impact on a superhydrophobic cold surface: full rebound, adhesive avulsion, and full adhesion, which reflects the interaction of droplet merging and solidification during the impact freezing of the double droplet. These findings may deepen our understanding of the mechanism of impact freezing on a cold surface, it provides reference for the associated applications and technologies in icing/anti-icing.     

A PLIC volume tracking method for the simulation of two‐fluid flows

Numerical methodologies for computer simulations of two‐fluid flows are presented. These methodologies fall into the category of volume tracking methods with piecewise‐linear interface calculation (PLIC). The scope of this work is limited to laminar flows of immiscible, non‐reacting, incompressible Newtonian fluids, without phase change, in planar two‐dimensional geometries. The following novel or enhanced procedures are proposed: a parallelogram scheme for multidimensional advection of the volume‐fraction field; a circle‐fit technique for the orientation of the interface segments and the calculation of curvature; a novel contact angle treatment; and a staggered formulation for volumetric body forces that can accurately balance pressure forces in the vicinity of the interface. In addition, surface‐tension‐derived and hydrostatic‐derived pressure adjustments are introduced as a means of accurately calculating pressure forces in cells that contain the interface, so as to minimize the non‐physical flows that afflict many available volume tracking methods. The proposed method is validated using four test problems that involve simulations of pure advection, a static drop, an oscillating bubble, and a static meniscus. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical analysis of the effect of the gas temperature on splat formation during thermal spray process

Thermal spray coatings are affected by various parameters. In this study, the finite element method with volume of fluid (VOF) procedure is used to investigate the deposition process which is very important for the quality of sprayed coatings. The specific heat method (SHM) is used for the solidification phenomenon. A comparison of the present model with experimental and numerical model available in the literature is done. A series of numerical calculations is carried out to investigate the effect of the surrounding gas temperature on the splat formation. The variation of the surrounding gas temperature has a significant effect on splat morphology and can affect the adhesion of the splat on the substrate.