液封液桥内热毛细对流的数值模拟

建立了液封液桥(不相溶混的双层同轴液柱)内热毛细对流的物理数学模型，采用涡量-流 函数法、利用有限差分格式对微重力条件下液封液桥内热毛细对流进行了数值模拟，得到了 双层液柱主流区的温度场和流场，证实了液封技术能削弱液桥主流区的热毛细对流，从而提 高浮区晶体生长质量，找出了液封厚度以及内、外层流体物性参数比对液桥内热毛细对流的 影响规律.     

液桥中不同Prandtl数流体的热毛细对流

本文研究从上面加热的液桥中不同 pr 数流体的热表面张力驱动对流。在 Ma 数相同的条件下,不同 pr 数流体液桥中的温度分布和流场结构定性相似,但定量结果不同。小pr数(pr<<1)流体液桥中的粘性边界层远小于热边界层,最大流函数所在位置向冷端偏移,有较大的流动速度。结果表明,Ma 数是描述这种流动的敏感参数。     

具有强SORET效应的充分发展的混合流体行进波对流

混合流体Rayleigh-Benard对流是研究对流稳定性,时空结构和非线性特性的典型模型之一。本文利用流体力学扰动方程组的数值模拟,讨论了偏离传导状态具有强SORET效应的混合流体行进波对流的温度场和浓度场的成长过程,分析了充分发展对流情况下的对流振幅,Nusselt数及混合参数与相对瑞利数的关系。并给出了行进波相速度对相对瑞利数的依赖关系。结果说明混合参数的曲线与行进波相速度的分布曲线是类似的。文末,给出了垂直速度,温度和浓度场的分布并讨论了相对瑞利数对场的分布及不同场之间的相位差的影响。     

Bingham流体双自由面热毛细液层的稳定性分析

热毛细对流是流体界面温度分布不均导致的表面张力梯度驱动的流动. 它主要存在于空间等微重力环境或小尺度流动等表面张力占主导的情况中. 在很多工业领域, 如晶体生长、聚合物加工、喷墨打印、微流控, 产品质量都与热毛细对流密切相关. 空间3D打印是太空制造的重要技术, 可以支持空间站的在轨长期有人照料的运行和维护, 实现按需制造. 本文以聚合物流体的空间3D打印为应用背景, 采用线性稳定性理论研究了Bingham流体双自由面热毛细液层的稳定性, 得到了在不同Bingham数(B)下的临界Marangoni数(Ma_c)与Prandtl数(Pr)的函数关系,分析了临界模态的流场和能量机制. 研究发现: 该流动的临界模态包括流向波和斜波模态, 与B, Bi和两界面垂直方向上的温差(Q)相关. B和Bi的增加会增强热毛细对流的稳定性. 当Q = 0时, 扰动温度分布分成对称和反对称两种情况. 当Q > 0时, Pr的增加会减弱流动稳定性. 在小Pr情况下, 扰动温度分布在整个流场, 在大Pr情况下, 扰动温度在栓塞区为零. 能量分析表明: 扰动动能的主要能量来源是表面张力做功, 但小Pr数下基本流也有一定贡献.      

波纹竖板层流降膜蒸发传热的分析

对高粘度液体在等温正弦形波纹壁面上的自由降落与蒸发建立了摄动分析模型。得到了流动的分析解和蒸发传热的数值解。考察了壁面波纹的波幅和波数、液膜表面张力及贝克利数对流动与传热的影响,结果表明,加大波纹的波幅、适当选择波数、减小贝克利数可增强传热,而表面张力对蒸发传热的影响较小。     

管内搅拌流传热模型及实验研究

基于搅拌流压力波动特性及搅拌流与环状流的相似性,假设搅拌流也分为近壁液膜区和气体核心区两部分,但是两区界面呈三角函数波动;在此基础上,根据传热学基本理论首次建立了搅拌流传热模型,并给出了模型中所需参数的计算方法,从理论上解决了搅拌流温度场的计算问题;为进一步研究搅拌流传热特点,进行了大量室内搅拌流传热实验。对比实验结果表明:本文建立的传热模型得到的对流换热系数计算值与实验值的总体误差在±10%以内,大部分在±5%以内,模型准确度较高,且计算值的变化规律与实验值相符合;本模型与均相和分相搅拌流传热模型相比更接近实际流动。另外,通过实验研究了液体流量和气体流量对对流换热的影响规律,结果表明由于搅拌流独特的流动结构,液体流量和气体流量均对搅拌流对流换热的影响不大,在实际工程运算中可以忽略这两种因素的影响。     

机械密封动环外周表面织构换热机理及结构优化

本文中以接触式机械密封为研究对象，考虑端面摩擦热，建立了包含密封环及密封腔的三维轴对称传热模型，并采用Fluent软件进行了传热分析，得到了密封环及密封腔的流场、温度场、速度场和Nusselt Number(Nu数)分布情况. 通过分析织构及其周边的流场、流态和温度场，揭示了织构的对流换热机理，主要研究了等边三角形织构的深径比、排数、排间距、排列方式和方向角等几何因素对端面温度的影响，并对等边三角形的结构参数进行了优选. 结果表明:动环外周表面织构具有增强对流换热效果和降低密封环整体温度的能力；织构的型式、深径比、个数以及排列方式均会对其换热效果产生影响，其中织构个数和旋转角影响较大，且三角形织构具有较强的换热能力，为今后高性能机械密封的优化设计及应用提供了理论依据.      

双自由面溶质?热毛细液层的不稳定性

溶质?热毛细对流是流体界面的浓度和温度分布不均导致的表面张力梯度驱动的流动, 它主要存在于空间微重力环境、小尺度流动等表面张力占主导的情况中, 例如晶体生长、微流控、合金浇筑凝固、有机薄液膜生长等. 对其流动进行稳定性分析具有重要意义. 本文采用线性稳定性理论研究了双自由面溶质?热毛细液层对流的不稳定性, 得到了两种负毛细力比(η)下的临界Marangoni数与Prandtl数(Pr)的函数关系, 并分析了临界模态的流场和能量机制. 研究发现: 溶质?热毛细对流和纯热毛细对流的临界模态有较大的差别, 前者是同向流向波、逆向流向波、展向稳态模态和逆向斜波, 后者是逆向斜波和逆向流向波. 在Pr较大时, Pr增加会降低流动稳定性; 在其他参数下, Pr增加会增强流动稳定性. 在中低Pr, 溶质毛细力使流动更加不稳定; 在大Pr时, 溶质毛细力的出现可能使流动更加稳定; 在其他参数下, 溶质毛细力会减弱流动稳定性. 流动稳定性不随η单调变化. 在多数情况下, 扰动浓度场与扰动温度场都是相似的. 能量分析表明: 扰动动能的主要能量来源是表面张力做功, 但其中溶质毛细力和热毛细力做功的正负性与参数有关.      

二元工质Marangoni对流的实验研究与数值模拟

用实验和数值模拟方法研究了正值表面张力温度系数的二元醇类特殊水溶液Marangoni对流流动. 首先通过实验测量确定正戊醇水溶液表面张力在特定的浓度分布和温度区间内具有明显随温度升高而增加的变化区域, 然后利用特定浓度配比的正戊醇水溶液,采用PIV方法实验观测了矩形液池中二元工质液层在水平温差驱动下的Marangoni对流, 发现了不同于常规的反向热毛细对流流动, 测量的表面速度分布与相同工况的数值模拟结果进行了比较, 发现二者变化趋势一致. 实验观测和理论结果的比较进一步验证了表面张力温度系数为正值时二元工质液层的热毛细流动输运特性.      

半浮区液桥热毛细振荡流

采用非定常、三维直接数值模拟方法研究大Pr数半浮区液桥热毛细对流从定常流向振荡流的过渡过程．文中详细描述了热毛细振荡流的起振和振荡特征,给出了液桥横截面上振荡流的流场和温度分布．在地面引力场条件下计算的结果与地面实验的结果进行比较,得出液桥水平截面上的流场和温度分布图样以一定的速度旋转,自由表面固定点处流体的环向流速正、负交替变化的一致结论．     

Newton's method for solving heat transfer,fluid flow and interface shapes in a floating molten zone

Newton's method is applied to the finite volume approximation for the steady state heat transfer, fluid flow and unknown interfaces in a floating molten zone. The streamfunction/vorticity and temperature formulation of the Navier–Stokes and energy equations and their associated boundary conditions are written in generalized curvilinear co-ordinates and conservative law form with the Boussinesq approximation. During Newton iteration the ILU(0) preconditioned GMRES matrix solver is applied for solving the linear system, where the sparse Jacobian matrix is estimated by finite differences. Nearly quadratic convergence of the method is observed. Sample calculations are reported for sodium nitrate, a high-Prandtl-number material (Pr = 9.12). Both natural convection and thermocapillary flow as well as an overall mass balance constraint in the molten zone are considered. The effects of convection and heat input on the flow patterns, zone position and interface shapes are illustrated. After the lens effect due to the molten zone is considered, the calculated flow patterns and interface shapes are compared with the observed ones and are found to be in good agreement.     

Numerical modeling of time-dependent three-dimensional flows and heat and mass transfer in a cylindrical liquid bridge in the absence of gravity

Numerical modeling of the temperature distribution, the velocity fields, and the dopant (Ge:Si) concentration during germanium crystal growth by the floating zone method under microgravity is carried out. The time-dependent three-dimensional problem is solved using the difference method. The deformations of the free surface of a liquid bridge and of crystallization and melting fronts are neglected. The relaxation of axisymmetric flow regimes to a steady state is observed, whereas the unsteadiness of three-dimensional flows develops with time.     

INSTABILITY OF COUPLED THERMO-SOLUTE CAPILLARY CONVECTION IN LIQUID BRIDGE AND CONTROL BY ROTATING MAGNETIC FIELD

浮区法因具有无坩埚接触污染的生长优点而成为生长高完整性和高均匀性单晶材料的重要技术.但熔体中存在的毛细对流会给浮区法晶体生长带来极大挑战,这是由于对流的不稳定会导致晶体微观瑕疵的产生和宏观条纹等缺陷的形成.为了提高浮区法生长单晶材料的品质,研究浮区法晶体生长中毛细对流特性及如何控制其不稳定性显得尤为重要.本文采用数值模拟的方法对半浮区液桥内Si_xGe_1-x体系中存在的热质毛细对流展开研究并施加旋转磁场对其进行控制.结果表明：纯溶质毛细对流表现为二维轴对称模式,温度场主要由热扩散作用决定,而浓度场则由对流和溶质扩散共同支配;纯热毛细对流呈现三维稳态非轴对称流动,浓度分布与熔体内热毛细对流的流向密切相关,等温线在对流较大的区域发生弯曲;耦合溶质与热毛细对流则为三维周期性旋转振荡流.施加旋转磁场后,熔体周向速度沿径向向外增大,熔体内浓度场和流场均呈现二维轴对称分布.     

超疏水小球低速入水空泡研究

物体入水问题是一类复杂的流固耦合问题,具有广泛的工程应用背景.物体在跨越自由液面入水的过程中,在一定的条件下,会向水中卷入空气形成空泡,空泡的运动还可能形成指向物体的射流,从而对物体的受力及其运动过程产生影响.超疏水表面能够在物体入水过程中形成多尺度流固耦合作用,进而影响物体的运动和宏观流动现象.而对于小尺度的小球低速入水问题,表面和界面力往往起主导作用.为了在更广的参数空间获得超疏水小球入水空泡类型和小球的运动特性,采用高速摄影实验方法,研究了半径0.175$\sim$10mm的超疏水小球低速入水及空泡动力学行为,获得了小球漂浮振荡、准静态空泡、浅闭合空泡、深闭合空泡和表面闭合空泡5种类型的动力学行为,探讨了这些运动行为与韦伯数We}和邦德数Bo之间的关系,并推导了小球漂浮振荡与下沉现象的无量纲关系.研究结果表明:超疏水小球的入水及空泡动力学行为主要与韦伯数We和邦德数Bo有关.在邦德数Bo $<$ $O$ (10$^{-1})$范围内,表面张力对流动的影响显著,随着韦伯数We}的增大,小球入水及空泡动力学行为依次经历漂浮振荡、准静态闭合、浅闭合、深闭合和表面闭合;在邦德数$O$ (10$^{-1})$<$ Bo} $<$O(1)$范围内,漂浮振荡现象不再发生;当邦德数$Bo>O(1)$后,浅闭合现象也不再发生;小球漂浮振荡与下沉现象的临界关系可以用相似律关系描述.      

Parallel finite element simulation of mooring forces on floating objects

The coupling between the equations governing the free‐surface flows, the six degrees of freedom non‐linear rigid body dynamics, the linear elasticity equations for mesh‐moving and the cables has resulted in a fluid‐structure interaction technology capable of simulating mooring forces on floating objects. The finite element solution strategy is based on a combination approach derived from fixed‐mesh and moving‐mesh techniques. Here, the free‐surface flow simulations are based on the Navier–Stokes equations written for two incompressible fluids where the impact of one fluid on the other one is extremely small. An interface function with two distinct values is used to locate the position of the free‐surface. The stabilized finite element formulations are written and integrated in an arbitrary Lagrangian–Eulerian domain. This allows us to handle the motion of the time dependent geometries. Forces and momentums exerted on the floating object by both water and hawsers are calculated and used to update the position of the floating object in time. In the mesh moving scheme, we assume that the computational domain is made of elastic materials. The linear elasticity equations are solved to obtain the displacements for each computational node. The non‐linear rigid body dynamics equations are coupled with the governing equations of fluid flow and are solved simultaneously to update the position of the floating object. The numerical examples includes a 3D simulation of water waves impacting on a moored floating box and a model boat and simulation of floating object under water constrained with a cable. Copyright © 2003 John Wiley & Sons, Ltd.     

Viscous effects in liquid encapsulated liquid bridges

An analytical derivation of the surface deflections and the streamfunctions for the flow inside a liquid encapsulated liquid bridge has been derived using an asymptotic expansion about a small capillary number. The model assumes an initially flat and cylindrical interface under the assumption that the densities of both fluids are equal. To simplify the analysis, the top and bottom walls are assumed to be stress-free and the Reynolds number is assumed to be negligible. Flow is generated either by a moving outer wall (shear-driven flow) or by applying a temperature difference across the top and bottom walls (Marangoni-driven flow). The resulting equations show that for the shear-driven flow, as the viscosity ratio increases, the surface deflections increase monotonically. For the Marangoni-driven flow there exist values of the viscosity ratio where the surface deflections reach a minimum and then switch signs. This investigation shows that it may be possible in more realistic systems to use an outer encapsulating liquid of the proper viscosity ratio to stabilize the liquid–liquid interface during float zone crystal growth.     

Methods of Solving Nonlinear Problems of Hydrodynamic Impact in Bounded Regions

The plane problem of separation impact on a plate floating on the surface of an ideal incompressible fluid in a bounded vessel is considered. In this problem the zone of contact between the body and the fluid is not known in advance and must be determined together with the fluid flow. As a result, the problem formulated is nonlinear and belongs to the class of free-boundary problems. The effect of vessel walls of different shapes on the fluid particle separation zone formed on the plate surface is studied. As examples, the problems for a layer or a truncated circular meniscus are considered.     

Interface damage and its effect on vibrations of slab track under temperature and vehicle dynamic loads

This paper presents a three-dimensional finite element model to investigate the interface damage occurred between prefabricated slab and CA (cement asphalt) mortar layer in the China Railway Track System (CRTS-II) slab track system. In the finite element model, a cohesive zone model with a non-linear constitutive law is introduced and utilized to model the damage, cracking and delamination at the interface. Combining with the temperature field database obtained from the three-dimensional transient heat transfer analysis, the interface damage evolution as a result of temperature change is analyzed. A three-dimensional coupled dynamic model of a vehicle and the slab track is then established to calculate the varying rail-supporting forces which are utilized as the inputs to the finite element model. The non-linearities of the wheel–rail contact geometry, the wheel–rail normal contact force and the wheel–rail tangential creep force are taken into account in the model. Setting the maximum interface damaged state calculated under temperature change as the initial condition, the interface damage evolution and its influence on the dynamic response of the slab track are investigated under the joint action of the temperature change and vehicle dynamic load. The analysis indicates that the proposed model is capable of predicting the initiation and propagation of cracks at the interface. The prefabricated slab presents lateral warping, resulting in severe interface damage on both the sides of the slab track along the longitudinal direction during temperature drop process, while the interface damage level does not change significantly under vehicle dynamic loads. The interface damage has great effects on the dynamic responses of the slab track.     

On one model of cyclic ejections of magma in the process of the explosive volcanic eruption

The dynamics of the structure of a cavitating magma flow behind the decompression wave front is experimentally studied by the method of hydrodynamic shock tubes. It is demonstrated that a discrete system of intensely cavitating zones with alternation of low and high densities of the gas phase can be formed at a certain regime of shock-wave loading of the examined fluid sample. Based on the results of a numerical analysis of the formation of an anomalous zone in the cavitating magma flow with anomalously high flow characteristics exceeding the values of these characteristics outside this zone at least by an order of magnitude, a model of an instantaneous transformation of the cavitating magma in the anomalous zone to a gas-droplet system, its ejections, and formation of a free surface on the interface is proposed. A numerical analysis shows that the characteristic wave structure and the anomalous saturation zone are fairly rapidly reconstructed in the vicinity of this free surface of the flow part remaining in the conduit after the ejections, and the above-mentioned jumps of flow characteristics are again formed in the anomalous zone.