二维水翼空化初生瞬态特性研究

发布时间：2018-12-07 08:49

【摘要】：空化是发生在液体中复杂的非定常流动物理现象,涉及到多相流、可压缩以及相间交换等多个方面,对水力机械、水利工程、船舶工程、水下兵器和核工业等众多领域均有显著的影响。为了更加准确的对空化性能进行预测和评估,很有必要针对空化初生时的瞬态机理特性进行研究,分析空化初生时的瞬态特性。本文分别以液氩、水作为微观研究对象,研究液氩、水内部单个空化核生长过程中的热力学参数变化。以NACA4412翼形为宏观研究对象,将空化机理微观研究结论应用到宏观二维水翼的空化瞬态特性变化中,对其进行数值模拟计算,并运用试验研究方法验证瞬态二维水翼空化的模拟计算结果的准确性。主要内容如下:1.查阅相关资料,介绍了空化、气泡成核等相关的理论基础知识及国内外最新研究进展。并介绍了分子动力学(MD)研究方法,以及相变和气体状态方程的理论基础知识,叙述了分子动力学(MD)分子力场、边界条件以及分子动力学(MD)模拟的求解过程,阐述了在计算过程中的各种系综的定义以及适用类型,并详细介绍了分子动力学软件的选择以及应用,同时采用LAMMPS软件完成本文第三、四章的模拟计算工作,在模拟计算中采用Lennard-Jones(12-6)力场,边界条件采用周期性边界条件。2.在NVT系综下,对存在不同初始尺寸空化核的Lennard-Jones流体进行了研究,得到NVT系综下空化核发展的演变过程,并对其分子势能、系统密度、分子径向分布函数、系统压力以及系统总能量等相关的热力学参数进行分析,结果表明:在空化初生阶段,由于流体的局部压力降低而造成液相分子不断的进入空化核,促使了空化核的生长。通过分子势能研究发现,空化核位置处的分子势能较高,而液相分子势能在空化核生长初期变化较大。在空化核成长后期,液相分子势能相对较为平稳。液相密度和界面密度变化比较明显,而空泡中心区域的密度变化甚微。液相区域径向分布函数的峰强度就越大,峰宽度越窄,该区域分子间距rbin越小。随着空化核初始尺寸的减小,系统压力和总能量达到平衡的时间越长,平衡后的压力值和能量值也就越小。而且在空化核生长初期,系统的总能量和压力的变化波动较大。3.在NPT系综下,对存在空化核的水分子计算域进行了研究,得到空化核初始尺寸与系统初始压力对空化初生的影响关系,并对空化核发展的演变过程、系统压力以及系统能量、分子径向分布函数等相关的热力学参数进行分析,结果表明:在系统初始压力一定时,空化核尺寸存在临界值,当空化核初始尺寸小于该临界值时,即使液体受到负压的影响,由于分子之间的作用力和氢键力束缚空化核不能生长,因此空化不会发生。空化在微观层面上不仅是压力和空化核参数的变化,更重要的原因是系统初始压力造成系统能量增大,结构失稳,从而造成空化核的变化。在内部空化核尺寸一定时,外部压力存在一临界值,当外部压力值大于该临界值时,计算域内部不会产生相应的负压,或者产生负压值较小,不足以使系统失稳,在分子之间的作用力和氢键力束缚下空化核不能生长,因此空化不会发生。伴随着空化核的生长,系统能量和分子势能总有增大的趋势,而该增大趋势影响了计算域内部结构的稳定性,促使空化核迅速增大。4.通过第四章中纳米尺度空化核生长变化过程以及目前研究的微米尺度空化核生长过程对Z-G-B空化模型进行了优化,并在此基础上选用RNG k-ε湍流模型以及优化后的空化模型对二维水翼空化初生过程进行模拟计算。在空化初生时,由于不同攻角产生的压力场略有不同,因此空化核周围液体失稳的可能性不同,造成水翼上表面空化核生长发育的几率不同。攻角越大,产生的低压极值越小,空化核周围液体所受到的负压逐渐增大,从而势能增大,空化核快速生长。通过微观空化核研究发现,在不同压力场作用下,空化核生长几率不同,因此在不同攻角影响下,空泡发展规模和强度不同。在空化发展过程中,因尾部高压产生的回射流对空泡的形状以及脱落有着重要影响。空化形成后期,低压和高压的相互影响直接造成脱落空泡破裂与发展,因而影响到水流的流态分布以及旋涡的产生。空化初生时只是受到低压作用的影响,而空化再生则受到低压作用和紊乱流场的双重作用。
[Abstract]:Cavitation is a complex non-constant-flow physical phenomenon in the liquid, which involves many aspects such as multi-phase flow, compressible and phase-to-phase exchange, and has a remarkable influence on the hydraulic machinery, hydraulic engineering, ship engineering, underwater weapon and nuclear industry. In order to predict and evaluate the cavitation performance more accurately, it is necessary to study the characteristics of the transient mechanism at the time of cavitation, and to analyze the transient characteristics at the time of cavitation. In this paper, the thermodynamic parameters in the growth of a single cavitation nucleus in liquid argon and water are studied by using liquid argon and water as the micro-research object. In this paper, the micro-study of the cavitation mechanism is applied to the change of the cavitation transient characteristics of the macro-two-dimensional hydrofoil, and the accuracy of the simulation results of the transient two-dimensional hydrofoil cavitation is verified by means of the experimental research method. The main content is as follows: 1. The relevant data, such as cavitation, bubble nucleation, etc., and the latest research progress at home and abroad are introduced. In this paper, the molecular dynamics (MD) method, the theoretical basic knowledge of the phase change and the gas state equation are introduced, the molecular dynamics (MD) molecular fields, boundary conditions and the solution process of the molecular dynamics (MD) simulation are described. The definition and application type of various ensemble in the calculation process are described, and the selection and application of the molecular dynamics software are introduced in detail. At the same time, the simulation calculation of the third and fourth chapters of this paper is accomplished by using the LAMPS software, and the Lennard-Jones (12-6) force field is used in the simulation calculation. The boundary conditions are periodic boundary conditions. In the NVT ensemble, the NVT-Jones fluid with different initial size cavitation cores was studied to obtain the evolution of the development of the cavitation nuclei under the NVT ensemble, and the molecular potential energy, the system density and the molecular radial distribution function were obtained. The thermodynamic parameters, such as the system pressure and the total energy of the system, are analyzed. The results show that, in the initial stage of cavitation, the flow of the liquid phase molecules into the cavitation nucleus due to the decrease of the local pressure of the fluid promotes the growth of the cavitation nucleus. The results show that the molecular potential energy at the position of the cavitation nucleus is high, and the potential energy of the liquid phase changes greatly in the initial stage of the cavitation nucleus. The potential energy of the liquid phase is relatively stable in the later stage of the growth of the cavitation nucleus. The change of the density of the liquid phase and the density of the interface is obvious, and the density of the cavity of the cavity has little change. The larger the peak intensity of the radial distribution function in the liquid phase region, the narrower the peak width, and the smaller the region molecular space rbin. As the initial size of the cavitation nucleus decreases, the longer the system pressure and total energy reach equilibrium, the smaller the pressure and energy values are balanced. and the variation of the total energy and pressure of the system is large in the early stage of the growth of the cavitation nuclei. The influence of the initial size of the cavitation nucleus and the initial pressure of the system on the cavitation is obtained under the NPT ensemble, and the evolution of the development of the cavitation nucleus, the system pressure and the system energy are obtained. The thermodynamic parameters such as the molecular radial distribution function and so on are analyzed. The results show that when the initial pressure of the system is constant, the critical value of the size of the cavitation core, when the initial size of the cavitation nucleus is less than the critical value, even if the liquid is affected by the negative pressure, Cavitation does not occur due to the forces between the molecules and the hydrogen bond forces that bind the cavitation core to be unable to grow. Cavitation is not only the change of pressure and cavitation core parameters at the micro level, but more importantly, the initial pressure of the system causes the system energy to increase and the structure is unstable, thus causing the change of the cavitation core. when the internal cavitation core is of a certain size, the external pressure is a critical value, and when the external pressure value is larger than the critical value, a corresponding negative pressure is not generated in the calculation domain, or the negative pressure value is small, so that the system is unstable, the cavitation nuclei can not grow under the force of the molecules and the hydrogen bond forces, so that the cavitation does not occur. With the growth of the cavitation nucleus, the total energy and molecular potential energy of the system have a tendency to increase, and the increasing trend affects the stability of the internal structure of the computational domain, which causes the cavitation nuclei to increase rapidly. The Z-G-B cavitation model was optimized by the process of nano-scale cavitation nucleus growth in the fourth chapter and the current study of the micro-scale cavitation nuclear growth process. On the basis of this, the RNG k-turbulent flow model and the optimized cavitation model are used to calculate the cavitation primary process of the two-dimensional hydrofoil. When the cavitation is primary, the pressure field generated by the different attack angles is slightly different, so the possibility of liquid instability around the cavitation nucleus is different, and the probability of the development of the cavitation nuclei on the surface of the hydrofoil is different. The higher the attack angle, the smaller the low-pressure extreme value, and the negative pressure of the surrounding liquid of the cavitation nucleus is gradually increased, so that the potential energy is increased and the cavitation nucleus grows rapidly. The results show that under different pressure fields, the probability of cavitation core growth is different, and the development scale and intensity of the cavitation are different under the influence of different attack angle. In the process of cavitation development, the jet-back jet produced by the tail high pressure has an important influence on the shape and the shedding of the cavity. In the late stage of cavitation formation, the interaction of low pressure and high pressure directly causes the drop-off cavitation and development, thus affecting the flow state distribution of the water flow and the generation of the vortex. The cavitation regeneration is only affected by the low pressure, while the cavitation regeneration is the dual function of the low pressure and the disturbance flow field.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O35

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