高马赫数下激波液滴相互作用的数值模拟研究

本文采用守恒清晰界面多相流数值方法模拟了超声速和高超声速环境下三维液滴的推进、变形和破碎演化过程.数值模拟结果与实验数据的一致性表明了本文所用数值方法和计算程序的准确性, 而网格无关性研究验证了采用的网格分辨率可以捕捉流场和界面的主要特征. 模拟结果验证了高韦伯数下液滴变形破碎过程所遵循的剪切诱导剥离(SIE)破碎机制, 其包含液滴的扁平化和剪切剥离两个主要特征. 而最近发现的SIE破碎机制下的循环破碎机制也在本文得到了验证, 即主液滴从球形液滴破碎为小液滴会经历多个循环重复的破碎阶段, 高韦伯数下液滴的破碎并非一次性剪切剥离的结果, 而是会发生逐层的剪切剥离和破碎. 本文还研究了马赫数对激波冲击液滴加速变形过程的影响. 结果表明, 高韦伯数下不同马赫数的液滴破碎过程具有高度一致性, 并遵循统一的SIE破碎机制.通过对液滴质心位移、速度、加速度和拽力系数的量化统计揭示其运动过程中的统一加速规律. 在激波的驱动下, 液滴并非以一个恒定的加速度做加速运动.在扁平化不明显的前期, 液滴以一个恒定的加速度做加速运动.随着液滴扁平化的发生, 迎风面积的增加导致拽力系数的增大, 进而导致液滴加速度的不断增大.      

液滴在气体介质中剪切破碎的数值模拟研究

液滴变形和破碎是燃料抛撒问题的重要过程.本文将VOF方法和标准k-ε湍流模型组合,建立了计算液滴在气流中变形破碎过程的数值方法.数值模拟了相关的实验,计算得到的液滴破碎过程与实验结果符合较好.在此基础上,分析了几个关键参数(Weber数、Ohnesorge数、液气密度比)对液滴破碎过程的影响.计算结果表明,Weber数...     

相近韦伯数条件下激波波后液滴初期变形的影响机制

以实验结合数值模拟与理论分析的方法,研究韦伯数在2 100~2 700区间内,不同组合流动参数对液滴破碎初期变形的影响与作用机制。实验中通过高速摄影捕捉到一系列具有明显差异的液滴变形模态,表明在相近韦伯数下液滴的初期变形仍受到气流速度、密度等具体流动参数的显著影响。以刚性球体替代液滴进行外流数值模拟,利用球体表面气动力分布推算出的液滴表面变形趋势与实际变形形态吻合,表明液滴的初期变形特征与外流流动分离和涡特征具有一致性。对流场和理论变形数据的分析显示,流动分离发展阶段和稳定阶段对液滴作用力以及它们所诱导的液滴变形特征存在很大差异;分离发展与液滴变形过程的特征时间之比可由气液密度比的平方根表示,它决定了液滴早期变形的基本形态。分离发展阶段所占时间比例越高,即实验中气液密度比越高,则液滴更倾向于发展出单个显著的环形突起,反之则趋于形成多个相对均衡的突起。     

激波诱导高速气流中液滴的初期变形

基于激波管平台和高速摄影方法对平面激波诱导高速气流中液滴的早期变形现象进行实验研究。研究发现在相近的We数或Re数下,实验参数的改变可导致液滴形态发展出现显著差异。这种差异主要体现在背风面的脊状环形突起、褶皱区以及后驻点区的凹凸形态。对刚性圆球外流的数值模拟显示,液滴变形早期形态与外流场结构和表面气动力分布之间存在鲜明的对应关系。最后采用简化理论推导出一组估测液滴早期变形的表达式。将数值模拟所得气动力数据代入计算发现:导致液滴变形的主要驱动力是液滴表面不均匀压力的挤压效应,而不是界面剪切摩擦所引起的切向流动堆积效应,前者高出后者约2个数量级;此外,采用压力作用理论计算所得液滴外形在主要变形特征和变形量级上均可与实验图像很好地吻合。     

激波诱导气流与液幕、液柱相互作用的实验研究

利用激波管对激波诱导气流与液幕、液柱的相互作用进行了实验研究。通过比较发现,这种相互作用下的液体块变形破碎过程与以往对于液滴进行的研究结果很不相同。当激波与液幕相互作用时,阴影照片和直接照相都表明,液幕的变形破碎行为有很强的三维性,较之液滴的变形破坏机理更为复杂,并且在局部区域,初始时刻液幕破碎抛撒的速度相较激波诱导气流速度为快,本文应用一维变截面激波管理论对这一现象进行了理论分析。     

矩形凹槽管道中激波传播的数值研究

基于二维Euler方程,结合五阶加权基本无振荡(weighted essentially nonoscillatory,WENO)格式以及自适应网格加密(adaptive mesh refinement,AMR)技术对入射激波在矩形凹槽管道内传播过程进行了数值模拟。数值结果清晰地显示了入射激波传播过程中与多个矩形凹槽作用以及在凹槽内变化的整个过程,且与已有的实验结果吻合较好。另外,结果还揭示了入射激波与单个凹槽作用时,会发生绕射产生膨胀波,还会发生碰撞从而诱导反射激波。膨胀波会导致入射激波压力降低,而反射激波则导致其升高,但膨胀波的影响占主导作用,因而入射激波波阵面强度出现振荡下降。     

可压缩流向涡与激波轴对称干扰的数值模拟

用ＮＳ方程数值模拟了可压缩流向涡和激波轴对称相互作用现象。数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较,结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征。提出了判断流向涡与运动激波相互作用中旋涡破碎的准则。     

可压缩流向涡与激波轴对称干扰的数值模拟

用ＮＳ方程数值模拟了可压缩流向涡和激波轴对称相互作用现象．数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较．结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征．提出了判断流向涡与运动激波相互作用中旋涡破碎的准则     

可压缩流向涡与激波轴对称干扰的数值模拟1)

用NS方程数值模拟了可压缩流向涡和激波轴对称相互作用现象.数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较.结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征.提出了判断流向涡与运动激波相互作用中旋涡破碎的准则.     

激波诱导火焰变形、混合和燃烧的数值研究

为了深入研究激波诱导的火焰变形以及由此带来的混合和燃烧变化特性,采用带单步化学反应的Navier-Stokes方程和高网格分辨率,对平面入射激波及其反射激波诱导球形火焰变形的现象进行了二维数值研究,计算结果与实验结果较好吻合。研究结果显示,在反射激波作用前,火焰的变形和皱褶主要受入射激波诱导等物理过程影响;而在反射激波与失稳火焰作用后,燃烧放热率、火焰有效面积和界面长度均迅速增加,控制火焰变形的机制逐渐向化学反应(燃烧)过程过渡;在失稳火焰发展的后期,增强的燃烧过程能够削弱火焰界面的皱褶,进而抑制未燃气和可燃气的混合。由此可以得出结论,激波诱导的火焰界面通过变形可促进界面两侧未燃气与可燃气的混合,进而强化燃烧过程,但燃烧的增强却反过来能抑制混合;认识两者之间的关系有助于利用或控制激波 火焰相互作用过程。     

Numerical simulation of the deformation and breakup of a two-core compound droplet in an axisymmetric T-junction channel

The rheological behaviors of a compound droplet in a confined geometry are of importance in many industrial and natural processes. However, a detailed numerical simulation of the finite deformation and its transition to the breakup of the multi-core compound droplet in an axisymmetric T-junction channel is still lacking. The present study is to fill this gap through the numerical simulations of a two-core compound droplet that deforms and breaks up in this channel configuration. The numerical results are obtained by the axisymmetric front-tracking method. Our new finding is that the compound droplet in the channel can experience the finite deformation or the breakup depending on the flow condition or the configuration of the channel. In the finite deformation mode (i.e. non-breakup mode), the droplet rapidly reaches the maximum deformation before approaching the perpendicular rigid wall. The most deformation occurs with the outer droplet, and the inner droplet closer to the wall is less deformed than the other inner core droplet. In the breakup mode, three breakup patterns are recognized: (i) breakup type I occurring when breaking up only the outer droplet; (ii) breakup type 2 occurring when breaking up only the inner droplets; (iii) breakup type 3 occurring when breaking up both inner and outer droplets. The transition from the non-breakup mode to the breakup mode is available when increasing the Reynolds number Re (from 0.16 to 40.0), the capillary number Ca (from 0.04 to 4.0), the size R_o of the outer droplet and the middle-to-outer fluid viscosity ratio μ₂₁, or decreasing the size R_i of the inner droplets, the radial size C₂ of the channel (normalized by the channel axial size C₁) and the interfacial tension ratio of the inner to the outer droplets. The transition diagrams based on some of these parameters are also proposed to provide a more complete picture of the two-core compound droplet behaving in the axisymmetric T-junction channel..     

Numerical study of a compound droplet moving toward a rigid wall in an axisymmetric channel

Dynamical behaviors of compound droplets including those interacting with rigid walls in an axisymmetric channel appear in various industrial and natural processes. However, so far, no detailed investigation has been carried out for such interactions of compound droplets. Motivating from this missing gap, we here numerically study the finite deformation and breakup of an initially concentric compound droplet when it moves toward a rigid wall at the bottom of an axisymmetric vertical channel. The method used is a finite difference-based front-tracking method. The numerical results reveal that when the compound droplet is delivered toward the wall, it is deformed and can break up into smaller droplets. For the cases of finite deformation (i.e. non-breakup), while the outer droplet is radially stretched, the inner droplet first moves downward in the direction of the outer flow but then gets back. Thereby, a thin film is created between the outer and inner interfaces at the droplet top and thus prevents the outer droplet further deforming and breaking up. In contrast, if breakup happens, the outer droplet is further stretched, and most of the middle fluid moves outward toward the outer droplet edge to form a blob. Breakup can be available in one of three patterns: off-axis breakup, on-axis breakup, and inner breakup. The off-axis breakup mode only happens with the outer droplet while the inner breakup mode is only for the inner droplet. Various parameters are investigated to show the transition between a non-breakup mode to a mode of breakup. Such parameters contributing the transition include the Capillary number Ca (varied in the range of 0.01–2.5), the channel aspect ratio (varied in the range of 0.4–2.0), the ratio of the inner to outer droplet radii (varied in the range of 0.3–0.8), the droplet size relative to the channel size (varied in the range of 0.2–0.9), the interfacial tension ratio of the inner to outer interfaces (varied in the range of 0.1–4.0), and the viscosity ratio of the middle to outer fluids (varied in the range of 0.16–6.3). In contrast, some others, e.g. the Reynolds number, the viscosity ratio of the inner to the outer, do not induce any transition. From the numerical results, regime diagrams of breakup and non-breakup based on these parameters are proposed.     

PARAMETRIC STUDY OF DROPLET BREAKUP BEHIND A SHOCK WAVE BY THE SHEET STRIPING MECHANISM

Journal of Applied Mechanics and Technical Physics - The physical grounds of droplet breakup in a flow behind an incident shock wave in accordance with one of possible sheet stripping...     

高韦伯数条件下黏性对液滴变形过程的影响

为探究液滴黏性对变形过程的影响，深入了解液滴在冲击波作用下变形破碎的行为机制。采用高速阴影技术在水平激波管上拍摄了高韦伯数（We=1 100～4 400）条件下，3种黏性硅油液滴的变形过程。结果表明随着黏性的提升:液滴演化出相应特征所需时间增大，同时会出现新的变形特征；液滴空间及位移特征参数的生长速率降低而变形时间、最大变形高度/位移都增大，这是因为提升的黏性力降低了变形速率、耗散了更多的动能并延长了液滴的变形过程；液滴表面最不稳定的Kelvin-Helmholtz波朝着大尺度、低生长率的方向发展，从而实现黏性对变形过程的延缓作用。随着最大变形位移的增大，最大变形高度首先线性增长，之后增幅降低。     

Study of the shock-induced acceleration of hexane droplets

An experimental study of the interaction of a shock wave with a hexane droplet is presented. The main goal of the experiments was to record images of the process and measure basic parameters describing movement, dispersion and evaporation of the droplets engulfed by a shock wave propagating in air. A shock tube with a visualization section was used for this research. Photography of the process allowed one to measure the positions, velocities and sizes of mist clouds created by the interaction processes. Analysis of the pictures shows that there is no qualitative difference between cases for different size droplets, but shock Mach number had a significant effect on the process. Quantitative analysis shows that under certain conditions, a catastrophic breakup mechanism of dispersion occurred. The droplets are shattered into a mist cloud before they achieve mechanical equilibrium with the surrounding gas. The approximate time for the complete dispersion and acceleration of the fuel droplet varies from 300 to 500 μs, and depends both on the droplet diameter and shock velocity. The dispersion time is controlled principally by the droplet diameter, and to a lesser extent, the shock Mach number. This paper is based on work that was presented at the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems, Montreal, Canada, July 31–August 5, 2005.     

The formation of a secondary shock wave behind a shock wave diffracting at a convex corner

This paper deals with the formation of a secondary shock wave behind the shock wave diffracting at a two-dimensional convex corner for incident shock Mach numbers ranging from 1.03 to 1.74 in air. Experiments were carried out using a 60 mm 150 mm shock tube equipped with holographic interferometry. The threshold incident shock wave Mach number () at which a secondary shock wave appeared was found to be = 1.32 at an 81° corner and = 1.33 at a 120° corner. These secondary shock waves are formed due to the existence of a locally supersonic flow behind the diffracting shock wave. Behind the diffracting shock wave, the subsonic flow is accelerated and eventually becomes locally supersonic. A simple unsteady flow analysis revealed that for gases with specific heats ratio the threshold shock wave Mach number was = 1.346. When the value of is less than this, the vortex is formed at the corner without any discontinuous waves accompanying above the slip line. The viscosity was found to be less effective on the threshold of the secondary shock wave, although it attenuated the pressure jump at the secondary shock wave. This is well understood by the consideration of the effect of the wall friction in one-dimensional duct flows. In order to interpret the experimental results a numerical simulation using a shock adaptive unstructured grid Eulerian solver was also carried out. Received 1 May 1996 / Accepted 12 September 1996     

Attenuation of a shock wave passing through a cloud of water droplets

The mitigation of a planar shock wave caused by a cloud of calibrated water droplets was studied both experimentally and numerically. Experiments were carried out, with different shock wave Mach numbers ranging from 1.1 to 1.8, in a vertical shock tube coupled with a droplet generator which produced a well-characterized cloud of droplets of 120, 250 and 500 μm in diameter. By exploiting such an experimental set-up, we successfully measured the attenuation of a normal shock wave when passing through the water droplet cloud. This series of experiments allowed to identify the main parameters of this investigation and a clear dependence between the attenuation of the shock wave and terms governing the regimes of droplet breakup has been found. On the other hand, to support this experimental approach, 1D unsteady calculations were performed in similar configurations. Although the mathematical model based on an Eulerian/Eulerian approach was actually incomplete, the first comparisons between the experiments and the simulations were rather interesting and pointed out the need to improve the physical model, by taking into account the fragmentation and the vaporization of the droplets submitted to the shock wave as well as the size distribution of the water spray.     

Flow visualization of shock/water column interactions

The breakup of a liquid droplet induced by a high speed gas stream is a typical multiphase flow problem. The shock/droplet interaction is the beginning stage of the droplet breakup. Therefore, investigation of the shock/droplet interactions would be a milestone for interpreting the mechanism of the droplet breakup. In this study, a compressible multiphase solver with a five-equation model is successfully developed to study shock/water column interactions. For code validation, interface-only, gas–gas shock tube, and gas–liquid shock tube problems are first computed. Subsequently, a planar shock wave interacting with a water column is simulated. The transmitted wave and the alternative appearances of local high- and low-pressure regions inside the water column are observed clearly. Finally, a planar shock wave interacting with two water columns is investigated. In this work, both horizontal and vertical arrangements of two water columns are studied. It is found that different arrangements can result in the diversity of the interacting process. The complex flow structures generated by shock/water column interactions are presented by flow-visualization techniques.