基于界面跟踪方法的汽蚀模型和算法的有效性验证

针对两相附着汽蚀流动机理,基于界面跟踪方法发展了新的汽蚀模型和算法。所发展的汽蚀模型和算法不仅考虑了液相／气相界面处的压力差,而且考虑了耦合Reynolds-Averaged Navier-Stokes方程求解技术得到的流场压力梯度信息来迭代计算附着汽蚀形状。采用具有试验数据的半球形头部圆柱体汽蚀绕流作为算例来验证所提出的汽蚀模型和算法的有效性。采用不同的网格数和松弛因子数值验证了发展的汽蚀模型和算法的有效性。三种汽蚀数下的数值计算结果得到的压力系数分布与试验数据完全吻合。结果表明所提出的汽蚀模型和算法能够准确模拟出汽蚀发生点和汽蚀长度。     

绕水翼片状空化流动结构的数值与实验研究

采用数值与实验相结合的方法研究了水翼片状空化流动结构.实验采用高速录像技术观察了片状卒泡形态,应用LDV测量了翼型周围的湍动能和速度分布;采用N-S方程和基于空泡动力学方程的空化模型计算了绕水翼片状空化流场.结果表明:在片状空化阶段,翼型吸力面上附着很薄的一层透明空泡,空泡彤态呈现于指状;随着空化数的减少,空泡尾部水汽交界面相互作用增强,并且空泡尾部出现大的旋涡,影响了空泡尾部区域压力和速度分布,片状空泡尾部的水汽混合区出现不稳定现象,同时存在小的空泡团脱落.数值模拟得到的水翼片状空化流动现象和实验观察到的结果基本一致,验证了计算模型和数值方法的可靠性.     

两种气液两相流模型的应用和比较

本文应用MUSIG模型[1]和均一直径模型对某溶液堆台架模型堆芯内气液流动传热进行了数值模拟.在MUSIG模型中堆芯内离散相气泡被分为5组不同直径的气泡,用于分析堆芯内气泡的流动变化和大小分布,采用Luo and Svendsen[2]和Prince and Blanch[3]模型描述不同直径气泡组间的破裂和聚合.在均一直径模型中,堆芯内的气体被考虑为同一直径的组分,并且不考虑其破裂与聚合现象.计算得到采用两个模型的模拟结果并且对其进行了对比研究.结果显示应用MUSIG模型的计算结果与台架实验结果吻合更好.     

大气泡散射的几何物理模型数值计算

米氏模型和德拜级数展开模型在气泡尺寸达到μm量级时不能很好地解释海水中大尺寸空泡的后向散射现象，计算速度较慢且容易溢出。提出使用几何物理光学模型解释大尺寸空泡的后向散射现象，给出了一种基于几何物理光学模型的球形大气泡散射特性的数值计算方法，并与德拜级数展开式进行了比较，计算结果基本吻合。该方法在计算大尺寸空泡的后向散射时不受微粒尺寸参数和折射率的限制，且避免了复杂的级数形式，与基于米散射理论或德拜级数展开的算法相比，具有更快的运算速度。最后给出了一些计算实例。     

基于汽相/液相界面跟踪方法的弯管内空化流动的数值预测

采用发展的基于Reynolds-Averaged Navier-Stokes方程和液相/气相界面跟踪方法的单相空化模型和数值迭代算法,数值预测了二维弯管内空化卒泡的形状和相应的压力系数分布.将发展的二维空化模型和算法推广到i维空化流动的数值模拟,预测了三维非对称弯管内的三维空泡形状和流动特性.数值预测三维非对称弯管内空化流动时的卒泡位置和相应压力系数分布符合理论分析结论.研究结果证明了所发展的空化模型和算法能够应用于三维空化流动时的数值预测.     

离心泵叶轮内空化流动的数值预测

采用发展的基于液相/气相界面跟踪方法和Reynolds-Averaged Navier-Stokes方程求解技术的空化模型和算法,数值模拟了无空化和空化系数为0.07时离心泵水力性能与流量系数之间的关系,预测了离心泵叶轮内发生的多区域空化流动现象.研究结果表明,在空化系数0.07时,存在临界流量系数.此时离心泵水力性能突然下降.在空化系数0.07下,流量系数接近临界流量系数时,离心泵叶轮的压力面和吸力面均发生附着空化空泡,导致离心泵水力性能迅速下降.研究结果证明了所发展的空化模型和算法能够应用于离心泵空化流动时的性能预测.     

三维小空化数空泡流数值方法

采用均质平衡流空泡模型,引入基于求解液体质量份数输运方程的空泡流模型,提出新的压力密度耦合的压缩性方法和控制气液两相转换的源项,求解汽水混合介质的RANS方程和带低雷诺数修正的k-ε模型,实现了小空化数(σ=0.2~0.01)下水下航行体的空泡流数值模拟.得到清晰的空泡形态特征与内部结构,以及空泡长度和最大直径随空化数的变化规律,给出航行体运动阻力系数与空化数之间的变化关系.将计算结果与解析结果及实验数据进行比较表明,该方法保证了极小空化数下自然空泡流计算的稳定性与收敛性,提高了空泡形态特性的预报精度.     

改进粒子水平集法及其应用

针对传统粒子水平集方法的缺陷提出一种虚拟粒子对距离函数的修正方法.改进算法针对虚拟粒子与待修正网格点的各种相对位置进行误差修正,可以避免虚拟粒子在非界面法线方向移动而产生的误差.数值求解涡旋流场问题显示:改进算法的界面捕捉性能得到显著提高.在此基础上,结合投影法求解Navier-Stokes方程组,数值模拟存在高密度比(10³量级)及粘性比(10²量级)的不可压缩两相流问题(气泡上升及自由面不稳定性问题).计算结果与已有结果吻合良好,详细分析主要控制参数对瑞利-泰勒不稳定性现象的影响.     

自由上浮气泡运动特性的光滑粒子流体动力学模拟

基于虚功原理, 在Hu X Y等和Grenier N等的研究结果基础上推导了多相流光滑粒子流体动力学(smoothed particle hydrodynamics, SPH)控制方程, 采用精度较高的黏性力和表面张力模型, 发展了一套适用于具有大密度比和大黏性比界面的多相流SPH方法. 首先, 通过施加人工位移修正, 适当背景压力和异相界面力, 使得计算全程粒子分布相对均匀, 改善了界面处的失稳现象, 防止了异相界面处粒子的非物理性穿透; 在此基础上, 利用方形流体团振荡模型对表面张力模型进行了验证, 数值结果与解析解甚为吻合; 然后采用上浮气泡经典数值算例对比研究了不同黏性力计算方法、不同核函数的适用性以及人工位移修正的效果; 最后, 对单个气泡的上浮、变形、撕裂以及垂向两个气泡的追赶、融合等现象进行了模拟, 初步揭示了气泡上浮过程中各种有趣物理现象的细节过程和动力学机理.     

界面不稳定性数值模拟中的虚拟流动方法

研究了流体界面不稳定性的一类数值模拟方法——虚拟流动方法(Ghost Fluid Method).在算法中直接针对多维问题设定虚拟区域的流动参数,在流体力学方程的计算中采用了非分裂型的高分辨SCB格式,最后利用该方法完成了R-M和R-T不稳定性问题的数值计算,得到了满意的计算结果.     

Cavitation bubble collapse in a vicinity of a liquid-liquid interface – Basic research into emulsification process

The initial motivation for the study was to gain deeper understanding into the background of emulsion preparation by ultrasound (cavitation). In our previous work (Perdih et al., 2019) we observed rich phenomena occurring near the liquid-liquid interface which was exposed to ultrasonic cavitation. Although numerous studies of bubble dynamics in different environments (presence of free surface, solid body, shear flow and even variable gravity field) exist, one can find almost no reports on the interaction of a bubble with a liquid-liquid interface. In the present work we conducted a number of experiments where single cavitation bubble dynamics was observed on each side of the oil-water interface. These were accompanied by corresponding simulations. We investigated the details of bubble interface interaction (deformation, penetration). As predicted, by the anisotropy parameter the bubble always jets toward the interface if it grows in the lighter liquid and correspondingly away from the interface if it is initiated inside the denser liquid. We extended the analysis to the relationships of various bubble characteristics and the anisotropy parameter.Finally, based on the present and our previous study (Perdih et al., 2019), we offer new insights into the physics of ultrasonic emulsification process.     

Interactions between a cavitation bubble and solidification front under the effects of ultrasound: Experiments and lattice Boltzmann modeling

The phenomena of melting and dendritic fragmentation are captured by using an in-situ device during the ultrasound-assisted solidification of a succinonitrile-acetone (SCN-ACE) alloy. The experimental results show that the dendrite arms detach from primary trunk due to the melting of the solid phase, which is caused by a moving ultrasound cavitation bubble. To quantify the interactions between the ultrasound cavitation bubble and the solidification front, a coupled lattice Boltzmann (LB) model is developed for describing the fields of temperature, flow, and solid fraction, and their interactions. The multi-relaxation-time (MRT) scheme is applied in the LB model to calculate the liquid-gas flow field, while the Bhatnagar–Gross–Krook (BGK) equation is executed to simulate the evolution of temperature. The kinetics of solidification and melting are calculated according to the lever rule based on the SCN-ACE phase diagram. After the validation of the LB model by an analytical model, the morphologies of the cavitation bubble and solidification front are simulated. It is revealed that the solidification interface melts due to the increase of the temperature nearby the cavitation bubble in ultrasonic field. The simulated morphologies of the cavitation bubble and solidification front are compared well with the experimental micrograph. Quantitative investigations are carried out for analyzing the melting rate of the solidification front under different conditions. The simulated data obtained from LB modeling and theoretical predictions reasonably accord with the experimental results, demonstrating that the larger the ultrasonic intensity, the faster the melting rate. The present study not only reveals the evolution of the solidification front shape caused by the cavitation bubbles, which is invisible in the ultrasound-assisted solidification process of practical alloys, but also reproduces the complex interactions among the temperature field, acoustic streaming, and multi-phase flows.     

Numerical modelling of acoustic cavitation threshold in water with non-condensable bubble nuclei

Numerical modelling of acoustic cavitation threshold in water is presented taking into account non-condensable bubble nuclei, which are composed of water vapor and non-condensable air. The cavitation bubble growth and collapse dynamics are modeled by solving the Rayleigh-Plesset or Keller-Miksis equation, which is combined with the energy equations for both the bubble and liquid domains, and directly evaluating the phase-change rate from the liquid and bubble side temperature gradients. The present work focuses on elucidating acoustic cavitation in water with a wide range of cavitation thresholds (0.02–30 MPa) reported in the literature. Computations for different nucleus sizes and acoustic frequencies are performed to investigate their effects on bubble growth and cavitation threshold. The numerical predictions are observed to be comparable to the experimental data in the previous works and show that the cavitation threshold in water has a wide range depending on the bubble nucleus size.     

Cavitation in a lubrication flow between a moving sphere and a boundary

A heavy sphere is free to move inside a rotating horizontal cylinder filled with viscous liquid. The steady motion is essentially Stokesian, and the sphere rotates at a fixed location with a lubrication layer between the ball and the wall. The symmetry of the flow field suggests there will be no force to balance the normal component of the ball's weight. However, we show that a normal force can arise when a cavitation bubble is present. The bubble size was measured as a function of the cylinder rotation rate and agrees well with a model which uses the force and torque balances on the sphere.     

曲面固壁附近空化泡溃灭动力学的 流体体积数值研究*

流体体积法(VOF)可以便捷、高效地实现对多相流界面的捕捉和追踪。本文基于VOF方法,对单个空化泡在曲面固壁附近的运动进行了数值模拟,从实验对比、压力场、速度场、温度场演化、溃灭时间、射流速度、固壁温度等方面分析了空化泡溃灭过程的热动力学影响。结果表明,数值模拟得到的空化泡形态演化与实验观测到的现象一致,随着位置参数、泡内外压差及曲面固壁尺寸的改变,空化泡热动力学行为也将发生变化,受到流体运动及射流冲击的影响,溃灭瞬间产生的高温高压使得曲面固壁温度升高。本文研究的曲面固壁附近空化泡溃灭效应,揭示了空化泡与曲面固壁间的相互作用规律,对学术研究及工程应用都具有重要意义。     

气液段塞流气弹区相界面结构理论研究

根据气液段塞流气弹区相界面结构特征将气弹分为气弹头、气弹体、水跃面和气弹尾四部分,并根据各自的流动和界面结构特征分别进行模化,建立了描述不同倾角的圆管内气液段塞流气弹区相界面结构的一维理论模型.该模型的计算结果表明气液混合Froude数、管道倾角和气弹长度显著影响气液段塞流气弹区相界面结构,计算与实验结果吻合良好.     

浓度边界层中成长汽泡的界面特性

对于双组分池内核态沸腾，热边界层中生成的汽泡，同时也处于浓度边界层中．本文建立了边界展中汽泡表面张力模型，对温度和浓度引起的表面张力变化进行分析．结果表明，表面张力从汽泡顶部到底部是递增的，从而形成液体沿界面自上而下的Marangoni流．通过汽泡底部微层相界面方程的求解，探讨了微层变形特征，由此分析汽泡脱离机理．微层变形与文献[1]中指出的接触线的变化从本质上是一致的．     

Ultrasonic cavitation at liquid/solid interface in a thin Ga–In liquid layer with free surface

Cavitation in thin layer of liquid metal has potential applications in chemical reaction, soldering, extraction, and therapeutic equipment. In this work, the cavitation characteristics and acoustic pressure of a thin liquid Ga–In alloy were studied by high speed photography, numerical simulation, and bubble dynamics calculation. A self-made ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and a max output power of 1000 W during the cavitation recording experiment. The pressure field characteristic inside the thin liquid layer and its influence on the intensity, types, dimensions, and life cycles of cavitation bubbles and on the cavitation evolution process against experimental parameters were systematically studied. The results showed that acoustic pressure inside the thin liquid layer presented alternating positive and negative characteristics within 1 acoustic period (T). Cavitation bubbles nucleated and grew during the negative-pressure stage and shrank and collapsed during the positive-pressure stage. A high bubble growth speed of 16.8 m/s was obtained and evidenced by bubble dynamics calculation. The maximum absolute pressure was obtained at the bottom of the thin liquid layer and resulted in the strongest cavitation. Cavitation was divided into violent and weak stages. The violent cavitation stage lasted several hundreds of acoustic periods and had higher bubble intensity than the weak cavitation stage. Cavitation cloud preferentially appeared during the violent cavitation stage and had a life of several acoustic periods. Tiny cavitation bubbles with life cycles shorter than 1 T dominated the cavitation field. High cavitation intensities were observed at high ultrasonication power and when Q235B alloy was used because such conditions lead to high amplitudes on the substrate and further high acoustic pressure inside the liquid.