液滴倾斜撞击液膜液冠演化规律

采用可调节表面张力的大密度比格子玻尔兹曼伪势模型模拟了液滴倾斜撞击液膜的过程,分析了不同雷诺数、韦伯数、液膜厚度和碰撞角对液冠演化的影响。结果表明: 碰撞时能量损失随着雷诺数增大而减小,但随着液膜厚度增大而增大;上下游液冠高度均随着雷诺数、韦伯数和碰撞角的增大而增大;同时上下游液冠高度随着液膜厚度的变化呈现先增大后减小的趋势,液膜厚度为0.25倍液滴半径时达到最大值。液冠延伸长度同样随着雷诺数和碰撞角增大而增大,但并不随着韦伯数的增大而变化,同时液冠延伸长度随着液膜厚度增加而减小。     

基于LBM伪势模型下的三维大密度液滴撞击壁面数值研究

伪势多相MRT-LBM(多重松弛时间格子玻尔兹曼方法)模型由于理论简单,自动形成两相界面等特点,被大量应用于多相流中的液滴撞击以及气液相变等数值模拟问题。该文基于二维和三维大密度伪势MRT-LBM模型,研究了三维条件下的单液滴和连续双液滴撞击壁面过程。结果表明:三维MRT-LBM能很好地实现对单液低撞击过程的模拟,且能精确地捕捉不同润湿特性下的撞击过程。该模型在液滴碰撞的模拟中与实验结果吻合程度较高,较高的液滴雷诺数与亲水程度有利于液滴在壁面上的铺展。     

液滴冲击流动液膜的格子Boltzmann模拟

采用单相格子Boltzmann方法模拟了二维液滴冲击流动液膜的含自由面的流动过程。由定义的质量与密度的比例系数来追踪气液自由面的变化。表面张力通过计算界面格点处作用在流体上的汽液相间作用力加入到模型中。数值模拟了速度比(液膜流动速度与液滴撞击速度的比值)和液膜相对厚度(液膜实际厚度与液滴直径的比值)不同时,液滴冲击流动液膜的过程。与液滴冲击静止液膜时的计算结果和理论分析结果进行对比,分析了液滴冲击流动液膜后产生的流动现象及其内在机理,产生的水花的铺展半径和溅起高度随时间的变化规律,并讨论了飞溅产生条件。     

格子Boltzmann方法模拟双液滴同时冲击固体表面液膜

采用单相自由面格子Boltzmann方法数值模拟了两个液滴撞击同种薄膜的流动过程。计算格子的液体比例系数来追踪气液界面的形状,表面张力效应通过分布函数的重构来引入。数值模拟中分别考虑了液滴以不同速度、不同中心间距撞击液膜的情况。数值计算了两个液滴同时撞击液膜产生水花、发生飞溅,水花相遇并发生碰撞后的流动现象。分析了液滴冲击速度及其水平距离对上述流动现象的影响。     

基于伪势模型的表面张力对空化泡溃灭的影响

采用可调节表面张力的大密度比、大黏滞系数比格子玻尔兹曼伪势空化模型模拟了近壁区空化泡溃灭过程,并进一步分析了表面张力和初始空化泡内外压差对空化泡溃灭过程中流场分布的影响,探究了表面张力变化对空化泡溃灭时产生的微射流和溃灭压力演化的影响。结果表明:在近壁区空化泡溃灭过程中,随着表面张力减小,加剧了气液交界面的变形,导致微射流更为集中。同时空化泡在溃灭过程中蓄积的表面能减小,在溃灭时刻迅速释放后,减弱了空化泡溃灭强度,空化泡溃灭最大微射流流速和最大压力均随着表面张力的减小而减小,导致空化泡溃灭时间缩短,增加了壁面引起的Bjerknes力,加剧空化泡朝向壁面溃灭的趋势。     

汽液黏滞系数对空化泡溃灭过程影响的数值模拟

利用格子玻尔兹曼方法(LBM)伪势模型模拟了近壁区空化泡溃灭过程,分析了不同汽相黏滞系数和液相黏滞系数对空化泡溃灭过程中的最大微射流流速、最大溃灭压力、最大溃灭时间的影响。结果表明,相同液相黏滞系数条件下,改变汽相黏滞系数,空化泡溃灭时产生的最大微射流流速、最大溃灭压力和最大溃灭时间不变;保持汽相黏滞系数不变,空化泡溃灭时最大溃灭压力、最大微射流流速均随着液相黏滞系数的增大而减小,但最大溃灭时间随着液相黏滞系数的增大而增大。     

泄槽底部掺气坎后水流掺气浓度分布模型试验

针对泄槽底部掺气坎后的掺气浓度分布规律较为复杂、研究成果较少的问题,为了更清楚地探究泄槽底部掺气坎后上游直段、反弧段及下游直段水流掺气浓度分布规律,采用含有反弧段的泄槽进行模型试验研究。试验结果表明:上游直段、反弧段及下游直段水流中不同水深处掺气浓度的沿程变化规律是不相同的;其他条件不变时,水流掺气浓度随掺气坎高度的增加而增大,随反弧段反弧半径的减小而减小;掺气坎高度和反弧半径对掺气设施的有效保护范围有一定的影响,适当提高掺气坎的高度和反弧段的半径对增大掺气设施的有效保护长度有利。     

脉冲爆震发动机燃料空气辅助雾化机理的研究

燃料的雾化、与空气的混合是提高脉冲爆震发动机燃料燃烧爆轰效率的前提。文中利用两相流VOF方法研究了燃料喷雾初始时刻液膜的运动及破碎过程,讨论了空气速度、气液比对液膜破碎的影响,揭示了爆震发动机中燃料空气辅助雾化的机理。结果显示,对于来流速度30 m/s、燃料喷射速度23 m/s的条件,喷雾初期燃料射流在锥形壁面上呈现连续的液膜。该液膜在喷嘴内部涡结构及燃料表面张力的影响下,在距离中心锥顶点7 cm位置处液膜开始破碎,破碎后燃料液滴的粒径与破碎时液膜的厚度满足d_D∝t~(1/3)关系。同时,针对可爆混气形成的二次雾化过程,基于大涡模拟(LES)方法,讨论流场涡和燃料颗粒运动的耦合作用。分析受到涡扩散及惯性力的作用,不同St数颗粒在流场中的分布情况。在气液相互作用中,液滴不断破碎。当初始气液比从0.5增大到3时,爆震室流场距离喷嘴出口26.88 mm位置处燃料颗粒平均粒径减小了约45%。     

含不凝气体蒸汽波节管内流动与换热特性研究

通过数值模拟,研究空气含量、Re及波节高度对含空气的水蒸汽管内凝结特性的影响。模拟结果表明:随空气含量的增加壁面平均传热系数减小,光滑管的减小幅度大于波节管;波节管内流体的流动与换热过程均呈现振荡趋势;在单个波节内,波节下游的壁面平均传热系数约为波节上游的3倍,波节下游在波节管强化传热中起着决定性作用;随着波节高度增加,波节管内壁面平均传热系数先增大后减小,拐点处波节高度值为0.007m;随着波节高度增加,波节管内摩擦阻力系数先增加后趋于稳定甚至减小,拐点处波节高度值为0.009 m。     

上游圆柱固定条件下串列三圆柱涡激振动响应和尾流特性

应用基于嵌入式迭代浸入边界法对雷诺数Re=100下串列三圆柱涡激振动进行了数值模拟,其中上游圆柱固定不动,中游和下游圆柱仅作横向振动。三圆柱间距相同,间距比L~*= 1.2、2.0和5.0,折合流速U_r=3.0-25.0,质量比m~*=2.0。研究发现,各间距比下,串列三圆柱存在强烈的相互作用,中游和下游圆柱的振幅明显大于单圆柱涡激振动的情况。随着L~*的增大,圆柱振幅整体上减小,除个别工况外,下游圆柱的振幅均大于中游圆柱的情况。当间距比较小(L~*=1.2、2.0)时,St数随折合流速的增加而缓慢减小;当间距比较大(L~*=5.0)时,St数几乎不再随折合流速变化,固定在St=0.15上。对尾流的研究发现,当振幅较小时,上游圆柱的剪切层将三圆柱包裹在一起,尾流与绕流时相似,表现为经典的卡门涡街;当振幅较大时,上游圆柱的旋涡/剪切层撞击/重附着于下游圆柱上,圆柱之间存在强烈的相互作用,尾流表现为并排的两列旋涡。     

SIMULATION OF THE TWO PHASE FLOW OF DROPLET IMPINGEMENT ON LIQUID FILM BY THE LATTICE BOLTZMANN METHOD

A Lattice Boltzmann Method (LBM) with two-distribution functions is employed for simulating the two-phase flow induced by a liquid droplet impinging onto the film of the same liquid on solid surface.The model is suitable for solution of twophase flow problem at high density and viscosity ratios of liquid to vapor and phase transition between liquid and its vapor.The roles of the vapor flow,the density ratio of liquid to vapor and the surface tension of the droplet in the splashing formation are discussed.It is concluded that the vapour flow induced by the droplet fall and splash in the whole impinging process may affect remarkably the splash behaviour.For the case of large density ratio of liquid to vapor a crown may engender after the droplet collides with the film.However,for the case of small density ratio of liquid to vapor a "bell" like splash may be observed.     

A LATTICE BOLTZMANN METHOD FOR SIMULATION OF A THREE- DIMENSIONAL DROP IMPACT ON A LIQUID FILM

A single-phase free surface tracking model based on the Lattice Boltzmann Method （LBM）, which has capability of simulating liquid-gas system with the assumption that the gas phase has only negligible influence on the liquid phase, is utilized to simulate the flow of a drop impacting on a liquid film. Three typical outcomes in the flows, i.e., deposition, crown and splash, which have been observed in the previous experiments, are obtained in the present three dimensional numerical simulations. The numerical results are consistent with the experimental and analytical results available.     

Tracking methods for free surface and simulation of a liquid droplet impacting on a solid surface based on SPH

With some popular tracking methods for free surface, simulations of several typical examples are carried out under various flow field conditions.The results show that the Smoothed Particle Hydrodynamics (SPH) method is very suitable in simulating the flow problems with a free surface.A viscous liquid droplet with an initial velocity impacting on a solid surface is simulated based on the SPH method, and the surface tension is considered by searching the free surface particles, the initial impact effect is co...     

周期性加热Poiseuille-Rayleigh-Benard流动中局部行波的研究

该文通过二维流体力学基本方程组的数值模拟,研究了水平来流和周期性加热对腔体内流体的运动和动力学特性的影响。当相对瑞利数r(28)9时,随着水平来流雷诺数Re逐渐增加,腔体内的流体发生三种不同的运动,即局部对流运动、局部行波运动和水平流运动。对局部行波运动的周期pT进行研究,局部行波运动的稳定周期随水平来流雷诺数的增大而减小,随相对瑞利数的减小而减小;相对瑞利数越小,稳定周期随水平来流雷诺数变化的增长率越大。局部行波运动的区间(35)Re依赖水平来流雷诺数的强度,当水平来流雷诺数Re大于(35)Re的上限时流体为水平流运动,而当水平来流雷诺数Re小于(35)Re的下限时流体为局部对流运动;对于不同的相对瑞利数r,局部行波运动的区间(35)Re是不同的。随着水平来流雷诺数增大,发现腔体内上游行波区域对流圈的减少速度比下游行波区域更快。     

用SST-DES和SST-URANS方法数值模拟亚临界雷诺数下三维圆柱绕流问题

基于剪切应力运输(shear stress transport,SST)的分离涡模拟(detached-eddy simulation,DES)在近壁面的流动区域采用SST模型,而在其他区域采用亚格子模型求解流场。本研究中分别应用SST-DES和SST-URANS方法对亚临界雷诺数Re=3900下的三维圆柱绕流进行了数值模拟,比较分析了两种数值方法下圆柱表面的时均压力分布、圆柱后方的漩涡脱落特征、圆柱下游流场的时均速度剖面分布等,并将数值模拟结果同前人物理试验比较,结果表明SST-DES方法在亚临界雷诺数下的三维圆柱绕流数值模拟问题上比SST-RANS方法具有更好的效果。     

Velocity and concentration measurements in initial region of submerged round jets in stagnant environment and in coflow

Velocity and concentration fields are measured in submerged round jets in a stagnant environment and in coflow using laser Doppler anemometry and laser-induced fluorescence. Measurements are made in the initial region within distances of 40 jet exit diameter at jet Reynolds number between 1000 and 5000 and coflow-to-jet velocity ratio from 0 to 0.43. Different behaviors of jet spreading and dilution are found in jets at three different ranges of Reynolds number in which the jets are classified as initially laminar, transitional or turbulent. In the zone of established flow, the jet centerline velocity and concentration decay with downstream distance at different rates in the three groups of jets. For jets in coflow, axial development of normalized forms of centerline mean excess velocity and mean concentration at different velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale. Turbulence properties inside a jet are increased by the presence of a strong coflow. Inside the zone of flow establishment, some strange features are observed on jet turbulence properties. The length of zone of flow establishment increases from the turbulent jets, to the transition jets and to the laminar jets. The zone lengths for concentration are shorter than those for velocity by one to two jet exit diameters. Both lengths are shortened further in the presence of a coflow. For jets a stagnant environment and in the strong jet flow region of jets in coflow, jet widths increase linearly with downstream distance in transitional and turbulent jets. Self-similarity of radial profiles of mean velocity or excess velocity, mean concentration, turbulence intensities and concentration fluctuation level is explored in the zone of established flow.     

Streambed instabilities around a bridge pier in a dredged channel

We conducted flume experiments to study the morphological behaviour around a circular pier located downstream of a mining pit. Severe transverse erosion at the pier base as well as lowering of channel bed upstream of the pier was observed because of channel dredging. Streambed instability was analysed based on two non‐dimensional parameters, namely, pier exposure factor and normalized upstream incision depth. The maximum values of both these parameters were observed when the mining pit was dredged closest to the pier. We also observed growth in streambed instability with an increase in flow Reynolds number.     

入湖浅水三角洲形成过程实验模拟分析

本文基于自然模型法,对入湖浅水三角洲的形成发展过程进行实验模拟,从河型和流场变化的角度,分析其河道演变过程规律。通过改变上游来沙和下游水位,研究不同边界条件对三角洲形成过程中河道摆动及河型发育的影响。实验结果表明:(1)河槽在顺直、分汊及无河槽的形态间交替演变,下游淤积的河坝会导致地形抬高、反水压力以及溯源淤积现象,促进河道发生周期性横向摆动。(2)三角洲表面流场形态及流速大小随河型变化,且与演变阶段有关。(3)上游多沙和下游高水位,都能使河型周期历时变短,主流摆动角度变大,摆动点位置向上游移动,较多来沙可以使河道形态由鸟足状向辐射状发育。在这一演化过程中,来沙影响占主导而水位影响居次要,水位变化条件不会从根本上改变河道形态的类型。