空泡流现象的非平衡分子动力学模拟和机理研究

【作者】 龚博致；

【导师】 张秉坚；

【作者基本信息】 浙江大学 ， 物理化学， 2008， 博士

【摘要】 流体中物体超高速运动的微观机理是现代科技最前沿的研究领域之一,无论在军事还是在民用技术中都具有极高的应用价值,特别是超空泡现象的研究,对水下航行体的减阻效应和超高速鱼雷的设计至关重要,有着重要的科学价值。因此本工作得到了国家自然科学基金项目《超高速流体的汽液相变与界面现象研究》(20473073)的资助。本论文运用非平衡分子动力学模拟方法,研究了微观体系条件下流体分子与固体壁面高速相对运动过程中流体流粒子的分布规律、状态性质、界面现象和摩擦情况等,特别是对开放体系超空泡现象的计算机模拟的实现,对超空泡流动中分子运动规律的观察和微观机理的探讨,为真实环境中物体超高速运动的研究提供了重要的基础数据。为了逐步深入研究不同体系不同条件下的流动,我们模拟了从简单窄孔到开放体系,从简单分子到甲烷和水等真实流体,从单系统到复合系统等模型,应用NEMD方法计算了流体压力、密度、速度、界面张力、摩擦系数等物理化学性质,通过对流场细节的观察,提出了一系列有关高速流动现象的形成机理,并与宏观流体力学理论进行了关联。为了验证分子动力学模拟程序和所建分子模型,本工作首先利用所建立的NEMD模型,代入平衡流动条件,模拟了宏量条件下甲烷的PVT关系,模拟结果与实验测量值或文献模拟值都能较好地吻合,说明本工作的模拟方法和设置基本正确。随后,对微孔无阻碍流进行了非平衡模拟,获得了符合宏观水动力学的模拟结果,验证了模拟程序与模型在非平衡流动模拟中的可行性。在上述研究的基础上,通过设定阻碍物模型,模拟计算了受限于窄缝孔中甲烷流体在160K温度、不同力场作用以及不同阻碍物条件下的阻碍流流场细节。模拟结果发现在高速条件下阻碍流会在流经阻碍物时形成高密区和低密区,同时引发相应的压力变化。流速对低密区形成有重要影响;阻碍物前端形态会影响流体两相形成和前端压力梯度,因此钝头阻碍物与其他流线形阻碍物相比将受到更大的压差阻力,但对两相界面表面张力影响不大;增加阻碍物长度或外力场强度会增大两相间表面张力。同时流动中观察到的摆动波现象符合Rayleigh-Taylor理论,表明该模型可以作为数值模拟的补充,研究宏观现象的微观机理。为了将流体模型应用到更有实际意义的超空泡流动模拟中,本工作引入了新的开放体系模型。通过将EMD模拟体系与NEMD模拟体系结合,模拟系统由硅酸盐狭缝孔扩展到开放体系,壁面限制作用被消除,外加力场改为恒流速流动,使得流体更为贴近真实流动状念。通过引入局部空化数σ_l概念,模拟体系被划分为多个独立方格,以观察超空泡的形成过程以及不同因素对超空泡现象的影响。模拟结果表明,微观条件下,空泡流的传统判据空化数σ＜0.1仍然成立。产生低局部空化数的σ_l区域与实际上的空泡发生区域在空间上并不重叠;低σ_l区域可以引起微小气泡的形成,虽然并不足以成为气化核,但也能给流体带来周期性的密度降;当低σ_l区域在运动速度方向上的尺度达到2流体分子直径时,可以在其后方形成稳定的超空泡;水下物体运动速度是引起超空泡的决定因素。在以上模拟工作成功的基础上,进行了水分子的超空泡模拟,试图进一步挖掘该模型的实用价值。应用SPC/E水分子模型,模拟并研究了不同空化器设置和不同运动速度下,超空泡流动的密度分布、局部空化数分布、空泡含气百分比和表面摩擦系数,并于数值模拟结果进行了对比。统计结果表明,平头空化器在相同流速条件下较为容易产生优良超空泡,空泡内含气百分比较高,能进一步降低壁面摩擦力。空化器外形对减阻效应有很大影响;超空泡现象可以降低航行体50%-90%的表面摩擦力。同时通过与数值模拟结果的对比,验证了模型的可行性。本论文完成了既定的研究目标,为日后进一步研究更加复杂和更加实际的空泡流体系,以及为研究流体复杂流动状态的微观机理提供了有力的研究工具。更多还原

【Abstract】 This work is financially supported by National Natural Science Foundation of China Project 20473073. This project was aimed to study high-velocity flow mechanism in molecular scale, which was very important in both industrial and military application. One of the most important phenomena was supercavitation, which directly affect the research progress on high-velocity torpedo. The traditional experiment and numerical simulation were unable to provide insight for molecular scale flow, so we involve Non-equilibrium Molecular Dynamics simulation to study these phenomena.In this work, we studied multiple flows in micro scale system and their molecular distribution, physical and chemical properties and interfacial behaviors. Particularly the simulations for open system supercavitation studied molecular scale mechanism and behavior of this phenomena, provided valuable data for supercavitation study in real system.In order to study flow in different systems and conditions, our simulation configuration varied from simple slitpore to open system, from simple LJ particle to real molecule like methane and water, from simple system to dual systems coupling. NEMD methods were applied to calculate fluid properties like pressure, density, velocity, interfacial tensor and friction coefficient. By studying detail data in flow field, several mechanism about high-velocity flow were found and associated with macro scale fluid dynamics.In order to verify the simulation model and NEMD program, methane fluid was studied in macro volume system using Equilibrium Molecular Dynamics conditions for its PVT relation. The result agreed with previous literature. Also force field driven flows in slitpore were studied using NEMD, the results were agreeable with hydrodynamics theory, showing the program was able to reproduce and predict flow phenomena in molecular scale.With verified model, we simulated different obstructed methane flows in slitpore, under 160K. Different driving force fields and shape of obstacles were applied to investigate their effect on flow field detail. Simulation results showed that under high-velocity, obstructed flow would form high-density and low-density area around obstacle, causing corresponding reaction on pressure. Flow velocity was the critical factor on the formation of low-density area, which can be treated as a diluted phase. Front shape of obstacle would affect the formation of diluted phase and frontal pressure gradient but has less effect on interfacial tensor. Thus flat-head obstacle was receiving more frontal drag force from press gradient. Increasing the length of obstacle or driving force field intensity would greatly increasing the system press, preventing gas phase to form. Oscillation wave was predicted in the simulation and agreed with Rayleigh-Taylor criterion, provided a molecule scale insight for macro scale flow phenomena.Since supercavitation drag reduction was very important for both industrial and military application, we introduced open system model to expand our simulation from slitpore. By coupling EMD simulation with NEMD simulation, confine effect of wall boundary was removed, and constant flow velocity could be used instead of driving force field, making the whole simulation system better reflect flow in real circumstance. By introducing local cavity numberσ_l, simulation cell was divided into multiple bins in order to study the formation of cavitation. Simulation results showed that the conversional cavity criterionσ<0.1 was applicable in molecule scale. Lowσ_l area was spatially separated with actually cavity, and generating micro bubble that was quickly filled with surrounding fluids. These bubbles were not able to nucleate for gas phase but still causing periodical density drop. When lowσ_l area developed beyond 2 fluid molecular diameter, stable supercavitation would like to form. Velocity of underwater object was the critical factor for supercavitation.With valid model for supercavitation simulation, we carried out cavity flow for water. By applying SPC/E water molecular model, different cavitators and object velocities were simulated. Local density profile, local cavity number profile, gas volume percentage of cavity and skin friction coefficient was recorded and compared with numerical simulation result. Results showed that flat-head cavitator was easer to generate cavity with high gas percentage, hence further lowing friction under similar σthan other streamline cavitators. Drag reduction of supercavitation can range from 50% to 90% depending on different shapes of cavitator.In this work, we have reached our aim to create a theory tool for studying flow mechanism in molecular scale. This work will benefit consequent works, which can focus on more complicated cavitation flow setup.更多还原