Inner flow behavior in an oscillating liquid droplet

A numerical simulation was conducted to investigate the inner flow in an oscillating liquid droplet. A 2D model was proposed to describe the interfacial and inner flow of oscillating droplets with shear stress induced by outside air flow. The 2D inner flow field was reconstructed to explore the liquid flow velocity and pressure profile and better understand the oscillating mechanisms. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20274     

单桩柱和波浪双向流固耦合的数值模拟

以单桩基支撑结构为对象,建立模拟波浪和单桩柱结构的三维模型,采用k-ε湍流模型和VOF运动界面追踪方法,实现了桩柱和波浪之间的双向流固耦合模拟,得到了波面三维显示图、流固交界面压力分布图和桩柱结构的受力图。还对桩柱上的最大等效应力做了研究,其变化范围是15 704~16 320 Pa,根据Von Mises屈服准则可知,桩柱在这种波浪条件下是稳定的。     

Unstructured finite volume method for water impact on a rigid body

A new method is presented for the water impact simulation, in which the air-water two phase flow is solved using the pressure-based computational fluid dynamics method. Theoretically, the air effects can be taken into account in the water structure interaction. The key point of this method is the air-water interface capture, which is treated as a physical discontinuity and can be captured by a well-designed high order scheme. According to a normalized variable diagram, a high order discrete scheme on unstructured grids is realised, so a numerical method for the free surface flow on a fixed grid can be established. This method is implemented using an in-house code, the General Transport Equation Analyzer, which is an unstructured grid finite volume solver. The method is verified with the wedge water and structure interaction problem.     

A coupled volume-of-fluid and level set (VOSET) method for computing incompressible two-phase flows

A coupled volume-of-fluid and level set (VOSET) method, which combines the advantages and overcomes the disadvantages of VOF and LS methods, is presented for computing incompressible two-phase flows. In this method VOF method is used to capture interfaces, which can conserve the mass and overcome the disadvantage of nonconservation of mass in LS method. An iterative geometric operation proposed by author is used to calculate the level set function ? near interfaces, which can be applied to compute the accurate curvature κ and smooth the discontinuous physical quantities near interfaces. By using the level set function ? the disadvantages of VOF method, inaccuracy of curvature and bad smoothness of discontinuous physical quantities near interfaces, can be overcome. Finally the computing results made with VOSET method are compared with those made with VOF and LS methods.     

NUMERICAL INVESTIGATION OF FLOW PATTERNS IN DIFFERENT PUMP INTAKE SYSTEMS

A 3-D numerical model for pump intake is established based on the Navier-Stokes equations with the RNG turbulence model and the VOF method to simulate the free surface.The applicability of the proposed model is validated by a test case of non-symmetric pump-intake bay.The predicted locations,structures and shapes of all vortices are in good agreement with those observed in experiments,though with some differences in vorticity strengths.The flow pattern and the efficiency of five types of pump intake systems are studied.The discharge and the velocity uniformity of the intake system are used as indices to evaluate its performance.     

NUMERICAL SIMULATION OF SLOSHING IN RECTANGULAR TANK WITH VOF BASED ON UNSTRUCTURED GRIDS

A new method for sloshing simulation in a sway tank is present, in which the two phase interface is treated as a physical discontinuity, which can be captured by a well-designed high order scheme. Based on Normalized Variable Diagram (NVD), a high order discretization scheme with unstructured grids is realized, together with a numerical method for free surface flow with a fixed grid. This method is implemented in an in-house code General Transport Equation Analyzer (GTEA) which is an unstructured grids finite volume solver. The present method is first validated by available analytical solutions. A simulation for a 2-D rectangular tank at different excitation frequencies of the sway is carried out. A comparison with experimental data in literature and results obtained by commercial software CFX shows that the sloshing load on the monitor points agrees well with the experimental data, with the same grids, and the present method gives better results on the secondary peak. It is shown that the present method can simulate the free surface overturning and breakup phenomena.     

Experiments and numerical simulations on transport of dissolved pollutants around spur dike

The flow field around a spur dike has three-dimensional characteristics.In order to analyze the influence of the flow field on pollutant transport,based on a compressive volume of fluid(VOF)scheme,the three-dimensional transient compressive pollutant transport model(CPTM)and the cubic equation(CE)bounded differencing scheme were developed.For the calibration and validation of CPTM,laboratory experiments were carried out in a flume with a non-submerged spur dike.The spur dike was angled at 60°,90°,and 120° from the upstream direction.The simulation results agreed with the experimental results.The simulations and experiments showed that the distribution of pollutant concentration was determined by circumfluence and the main flow.Concentration decay in the circumfluence zone was slower than that in the main flow.Downstream of the spur dike,the concentration fluctuation became intensive with the increase of spur dike angle.     

数字微流控芯片半月形驱动电极的设计

为了降低数字微流控芯片的驱动电压,将传统的方形驱动电极结构设计为半月形,并研究了不同参数的半月形驱动电极降低驱动电压的效果。首先,根据介电湿润的基本原理分析了不同驱动电极形状对降低驱动电压的影响。然后,通过流体体积法(VOF)对液滴的运动过程进行建模和数值仿真;根据数值仿真结果对比分析了不同结构参数的半月形驱动电极随驱动时间的运动过程。最后,设计了4种不同结构参数的半月形驱动电极芯片,并对其驱动液滴的效果进行了试验验证。结果表明:研制的4种半月形驱动电极微流控芯片中,电极圆弧直径等于电极长度结构的芯片其驱动电压比其他3种电极结构的芯片的驱动电压至少降低了15.6%,而且可以在16V的驱动电压下使1μL去离子水液滴的运动速度达到1.6cm/s,是设计为半月形驱动电极中的最优设计。实验数据证明了电极圆弧直径等于电极长度的半月形驱动电极结构可有效降低微流控芯片的驱动电压。     

泄洪洞工作闸室及陡坡明渠段体型优化数值模拟

对九甸峡水电站右岸泄洪洞工作闸室,包括圆直段、圆变方段、方洞段、弧形闸门段、渐变段和明渠段流场进行了数值模拟。获得了水面线、压力、空化数、流速等水力要素。成功捕捉到了压力洞出口处的水翅、突扩跌坎处侧空腔和底空腔。计算结果与实测数据符合较好,根据计算结果,提出了能解决原方案负压区严重和明渠陡坡空蚀破坏问题的泄洪洞体型修改方案。     

绕二维对称水翼的通气空泡流数值研究

本文通过有限体积离散方法求解Reynolds平均Navler—Stokes方程，采用VOF法追踪气一液两相交界面。研究了在高压低速来流，即本质上不发生自然空化的条件下，绕二维对称水翼的非定常通气空泡流动。分析了物理参数——通气方向角和攻角，对通气空泡外形、稳定性及水动力参数的影响。计算结果表明：在通气空泡的形成过程中，通气方向角存在着一个临界值，使得空泡形态由片状空泡过渡到超空泡；特定的参数组合将会诱发空泡表面出现波动现象。而且，当攻角做周期性变化时，水动力参数的变化也随之满足周期性规律，且出现阶段性振荡。