液固两相圆柱绕流尾迹内颗粒扩散分布的离散涡数值研究

基于离散涡方法求得的非定常水流场和颗粒的Lagrange运动方程,数值模拟了稀疏液固两相圆柱绕流尾迹内颗粒的扩散分布．获得了流动的涡谱与3种不同St数颗粒（St=0.25,1.0,40）在流场中的分布．通过引入扩散函数来定量表示颗粒在流场中的纵向扩散强度,并计算得到了不同St数颗粒的扩散函数随时间的变化．数值结果揭示出了液固两相圆柱绕流尾迹中的颗粒扩散分布与颗粒的St数和尾涡结构密切相关:1) 中小St数(St=0.25～4.0)颗粒在运动过程中不能进入涡核区,而在旋涡结构的外沿聚集,且颗粒的St数愈大,其越远离涡核区域;2） 在圆柱绕流尾迹区域内,中小St数(St=0.25～4.0)颗粒的纵向扩散强度随其St数的增大而减小．     

气泡间距对受污染球形气泡界面性质和尾流的影响

为了理解气泡间相互作用对受污染气泡水动力学特性的影响,基于改进的停滞帽模型,以表面活性剂作为污染介质,详细研究了不同气泡间距下气泡的界面参数、周围流场和尾涡特性.通过求解气泡界面与流域间的吸附和解析方程,考虑局部流动以及Marangoni效应的影响,形成稳定的污染界面.借助Langmuir方程将界面切应力与界面表面活性剂浓度相关联,实现气泡界面切应力的求解.研究发现,改变两气泡的间距,不会显著影响气泡1的界面参数,而对气泡2的界面参数影响巨大.气泡1尾涡向气泡2上游界面的逼近是气泡2界面参数改变的主要原因,该尾涡对气泡2界面上表面活性剂分布的影响与对流作用相反,其可以把流向气泡2尾部的表面活性剂拖回气泡上游界面,从而影响气泡2的界面参数分布,并出现了低影响与高影响阶段.而且气泡1的尾涡长度和涡中心垂直位置的值受气泡2上游界面浓度和气泡间距的共同影响,气泡2各尾涡参数值随上游界面浓度的增加而减小直至为零.     

Marangoni效应对等曲率弯曲界面凝固过程的影响

用渐近方法和相场数值模拟分别研究了球状晶体和柱状晶体凝固过程中,固液界面表面张力随温度的变化对界面运动的影响．结果表明Marangoni效应将增大临界晶核半径,减缓界面运动速度．在静止熔体中,枝晶尖端生长速度随Marangoni数线性下降．渐近展开和相场模拟得到的结果定性是一致的．     

不可压缩流体三维Rayleigh-Taylor不稳定性的大涡模拟

对不可压缩流体三维Rayleigh-Taylor不稳定性问题建立被动标量输运模型,用大涡模拟方法计算了正弦初始扰动和随机初始扰动下不稳定性发展各个阶段的瞬时速度场和标量场,以及混合过程中计算尺度和亚格子尺度上的平均湍流脉动能、平均剪切应力和被动标量通量;分析了界面形状、被动标量浓度分布的演化规律及气泡、尖钉速度和混合层宽度随时间的变化规律,计算结果与其他数值模拟和实验结果相吻合,验证了大涡模拟方法应用于该问题的可行性.     

狭窄流域内气泡破裂现象数值模拟

基于边界积分法建立狭窄流域内气泡破裂数值模型,开发相应的计算程序,分别模拟对称破裂与非对称破裂两类典型工况并与相应实验结果进行对比,计算值与实验值吻合很好,表明三维数值模型的有效性．从狭窄流域内气泡运动的基本现象入手,基于开发的程序系统地研究气泡的对称破裂与非对称破裂,在已有数值研究成果和实验数据基础上,提出气泡破裂的可行性准则,研究分裂后子气泡的动力学特性,并分析距离参数对气泡破裂特性及子气泡动力学行为的影响,总结相应规律,旨在为相关气泡破裂特性研究提供参考．     

壁面效应对剪切稀化流体内气泡上浮特性的影响

数值研究了壁面效应对剪切稀化流体内气泡上浮运动特性的影响,气液两相的界面捕捉采用流体体积(VOF)法,剪切稀化流体流变特性和气液相间表面张力的计算分别采用Carreau模型和连续表面张力模型.详细研究了不同流变指数下,壁面效应对气泡形状、液相流场和气泡终端速度的影响.结果表明,强的壁面效应或弱的剪切稀化程度会限制气泡的变形和尾涡的形成,使气泡的终端速度减小;气泡终端速度最易受壁面效应的影响;强的壁面效应和强的剪切稀化程度会导致高剪切速率区域出现在壁面附近,引起壁面附近液相表观黏度大幅度的下降.     

水下爆炸气泡融合动态特性研究

在自然界中,气泡融合是一种重要的物理现象．基于势流理论,采用边界积分法,对水下爆炸气泡融合进行数值模拟研究．依据已有的数值研究成果和实验数据,建立气泡融合三维动力学数值模型,数值模拟结果与实验值吻合良好．应用该文开发的三维计算程序,分析距离、水深等特征参数对气泡融合动态特性的影响,得到了一些规律性的曲线和结论,旨在为水下爆炸气泡融合机理和动态特性研究提供理论参考．     

大尺度大气运动的阶段性

大气运动中重力波与长波共存,由于这两种波频散速度在时间上相差一个量级,所以大气过程显出阶段性.当长波频散后大气中再无扰动,运动便趋向常态,因此大气运动便出现了三个时间尺度,形成三个运动阶段.本文采用多时间尺度方法,得到了三个阶段的运动性质,提出大气运动的双适应现象:第一阶段重力波频散结果,产生了地转适应,第二阶段长波频散结果产生了位涡适应。位涡适应过程是非线性过程,利用数学分析方法和数值计算方法证明了位涡适应过程存在,并指出它包括了地转平衡,地转适应是位涡适应的初级阶段。     

黏性分层流中椭球体俯仰振荡研究

以黏性密度分层流下椭球体自由俯仰振荡衰减过程为研究内容,建立了密度连续分层流数值计算模型.通过对经典小球黏性绕流场的数值模拟和增阻系数的计算验证了数值模型的正确性.以初始45°攻角下的椭球体俯仰振荡过程为研究对象,采用基于Aitken亚松弛适应算法的双向流固耦合方法,数值模拟了不同内Froude(弗汝德)数Fri下椭球体俯仰衰减振荡的动态绕流场.数值研究结果表明,俯仰振荡将上下搅动周围流体,在椭球体上下两侧对称形成四个密度涡环,密度的垂向分层限制了涡环的垂向传播,也加速了涡环的消失,这种限制助长了水平运动的发展,远场尾涡流场将以水平波动的形式传播.在较高的内Froude数Fri和Reynolds(雷诺)数Re下,双向耦合抑制了数值震荡.研究还发现,随着来流速度的增加,阻力系数不增反降,这说明,对于自由俯仰振荡的椭球体,负阻尼现象仍然会出现.     

槽道湍流中拟涡能输运对反向控制的瞬时响应

利用直接数值模拟研究了槽道湍流中脉动拟涡能输运对反向控制的瞬时响应． 发现流向和展向拟涡能的衰减首先由拉伸产生项的抑制引起,而法向拟涡能的减小是因为控制阻碍了平均剪切的倾斜．在控制的初始阶段,流向拟涡能的演化远远落后于其它两个分量的变化．法向涡量快速单调减小,并对其它两个分量的减弱起到了重要作用．     

Numerical simulation of the interaction between vortex ring and bubble plume

In present paper a three-dimensional Vortex-In-Cell method with two-way coupling effect was developed to study the bubble plume entrainment by a vortex ring. In this method the continuous flow was calculated by the three-dimensional Vortex-In-Cell method and the bubbles are tracked through bubble motion equation. Two-way coupling effect between continuous flow and dispersed bubbles is considered by introducing a vorticity source term, which is induced by the change of void fraction gradient in each computational cell. After validated by the comparison between experimental measurements and simulation results for the motion of vortex rings and the rising velocity of bubble plume, present method is implemented to simulate the interaction between an evolving vortex ring and a rising bubble plume. It was found that there is little effect of the bubble entrainment to the total circulation of vortex ring while the effect of bubble entrainment to the vortex ring structure is quite obvious. The bubble entrainment by the vortex ring not only changed the vorticity distribution in the vortex structure, but also displaced the positions of the vortex cores. The vorticity in the lower vortex core of the vortex ring decreases more than that in the upper vortex core of the vortex ring while the vortex core in the upper part of the vortex ring is displaced to the center of vortex ring by the entrained bubbles. Smaller bubbles are easier to be entrained by the large scale vortex structure and the transportation distance is in inverse proportion to bubble diameter.     

Numerical simulation of laminar flow past a circular cylinder

The present paper focuses on the analysis of two- and three-dimensional flow past a circular cylinder in different laminar flow regimes. In this simulation, an implicit pressure-based finite volume method is used for time-accurate computation of incompressible flow using second order accurate convective flux discretisation schemes. The computation results are validated against measurement data for mean surface pressure, skin friction coefficients, the size and strength of the recirculating wake for the steady flow regime and also for the Strouhal frequency of vortex shedding and the mean and RMS amplitude of the fluctuating aerodynamic coefficients for the unsteady periodic flow regime. The complex three dimensional flow structure of the cylinder wake is also reasonably captured by the present prediction procedure.     

Coherent structure in flow over a slitted bluff body

The focus of the present investigation is resolution of the coherent structure in the near wake behind a slitted bluff body. The bluff body is two-dimensional with gap ratio from 0.12 to 0.48. The evolution of the structure was numerically investigated using the renormalization group (RNG) k–ε model at Reynolds number of 470,000. Two types of coherent structure are identified: At low gap ratio 0.12, the structure is characterized by a flip–flopping gap flow; at high ratio 0.22–0.48, the gap flow deflects to one side with an asymmetrical wake. The coherent structure is divided by the gap flow into two zones called the primary recirculation zone and the secondary recirculation zone. The coherent structure is intimately related to the gap ratio, and the structure of small gap ratio is different from that of large gap ratio because the interaction between two zones relates to the gap ratio. To explain the vortex shedding, a mechanism that single vortex of large size suddenly immerses between two shear layers was proposed. Experimental results using point-to-point method and particle-image velocimetry (PIV) measurements in a close wind tunnel were also carried out to confirm the observation from the numerical study. The evidence shows that the numerical results are of good agreement with the experiments. The comparison between the RNG k–ε model and the large eddy simulation also indicates that the RNG k–ε model is adequate in computing the bluff body flow.     

Investigation on the effect of trailing edge ejection on a turbine cascade

This paper presents a detailed experimental and numerical investigation for a turbine cascade with different trailing edge ejection. The numerical simulation is based on Three-Dimensional Navier–Stokes equations coupled with an effective ejection model, where a high resolution non-oscillatory scheme, LU-SGS implicit algorithm and Baldwin-Lomax turbulence model are employed. The experiments presented in this paper focused on a transonic turbine cascade performance with different ejection to validate the numerical simulation results. The results show that the blowing ratio has a small effect on the Mach number distribution and exit flow angle with two slot types. However the energy loss coefficient increases initially, and subsequently has a decrease tendency with the increasing of blowing ratio. The ejection from the symmetry slot blows away the vortex at the blade trailing edge and strengthens the mixing between the wake and main flow. The ejection from the pressure side cutback only clears up the vortex near the slot surface, and has small effect on the flow field near the trailing edge.     

Vortex shedding behind a rising bubble and two-bubble coalescence: A numerical approach

In this work, we present the computational results on the wake instability in wobbling bubble regime as well as on the coalescence of two bubbles in different shape regimes. This is a continuation of our previous studies on the dynamics of a single gas bubble rising in a viscous liquid (see [A. Smolianski, H. Haario, P. Luukka, Computational Study of Bubble Dynamics, Research Report 86, Lappeenranta University of Technology, Finland]), and we use the same, finite-element/level-set/operator-splitting method that was proposed in [A. Smolianski, Numerical Modeling of Two-Fluid Interfacial Flows, Ph.D. Thesis, University of Jyväskylä, 2001]. The numerical method allows to simulate a wide range of flow regimes, accurately capturing the shape of the deforming interface of the bubble and the surface tension effect, while maintaining a good mass conservation. Due to the highly unstable and small-scale nature of the considered problems there are very few experimental investigations, but the comparison with available experimental data confirms a good accuracy of our numerical predictions. Our studies show that plausible results can be obtained with two-dimensional numerical simulations, when a single buoyant bubble or a coalescence of two bubbles is considered.     

Numerical exploration of new features on three-dimensional transition of a cylinder wake

The three-dimensional transition of the wake flow behind a circular cylinder is studied in detail by direct numerical simulations using 3D incompressible N-S equations for Reynolds number ranging from 200 to 300. New features and vortex dynamics of the 3D transition of the wake are found and investigated. At Re = 200, the flow pattern is characterized by mode A instability. However, the spanwise characteristic length of the cylinder determines the transition features. Particularly for the specific spanwise charac-     

Simulation of particle dynamics in a viscous fluid near a plane wall

A method was developed (and implemented as a software package) for the numerical simulation of the dynamics of a large number of particles in a viscous flow near a plane wall. The fluid motion is considered in the Stokes approximation with allowance for the hydrodynamic interaction of all the particles. The velocities and trajectories of the particles are computed, and fluid streamlines are constructed. In the case of even two particles falling onto the wall, it is found that a complex vortex flow is formed between them. This gives rise to the particles having a velocity component along the wall. The dynamics of a group of several dozen particles moving parallel and perpendicular to the wall are simulated.     

Numerical modelling of EHD effects on heat transfer and bubble shapes of nucleate boiling

In this article, mathematical and numerical models are developed to study pure electrohydrodynamic (EHD) effects on heat transfer and bubble shapes when an initial bubble attached to a superheated horizontal wall in nucleate boiling. In the modelling of EHD effects on heat transfer, an undeformed bubble is considered; the electric body force and Joule heat are added to the momentum and energy equations; governing equations for heat, fluid flow and electric fields are coupled numerically and solved using a non-orthogonal body-fitted mesh system with necessary interfacial treatments at the gas–liquid boundary. While, to study the pure effect of EHD on the deformation of the bubble, the evaluation of a deformable bubble without heat transfer is simulated by volume of fluid (VOF) method based on an axial symmetric Cartesian coordinate system. The simulations indicate that EHD can effectively enhance heat transfer rate of nucleate boiling by influencing the motion of the ring vortex around the bubble and that bubble can be elongated due to the pull in axial direction and push in the negative radial direction by the electric field force.