近自由面非定常空化流动特征与稳定性机理研究

近自由面非定常空化流动是水面航行器提高航速的重要障碍,主要表现为自由面/空化区相互作用,影响空泡的非稳态特征,空化流动的兴波效应也值得关注。以往研究大多针对定常问题,非定常的算法及实验验证手段的报道很少。本研究建立了水/气/汽三相并包含空化相变效应的大涡模拟数值方法,基于小尺寸水下发射水箱完成了典型的验证实验。计算得到的非定常空泡形态演化与实验结果吻合良好,验证了计算方法的精度和可靠性。在此基础上,分析了空泡形态非稳态演化的特征与自由面/空化相互作用规律,讨论了自由面对空泡流动的约束效果和非稳态空泡的兴波效应。进一步改变不同的浸没深度,发现在航行体距离自由表面非常接近的情况下,会发生通气云状空泡和通气超空泡现象。在通气效应和回射流变薄两种因素的影响下,空泡在距离自由表面更近的条件下能够变得更稳定,自由面可能对空泡发展和稳定性产生整体的影响,值得未来进一步深入研究并探索工程应用的前景

横向来流内单孔气泡形成的理论与实验研究

水下气泡的运动是水下航行器减阻和化工领域中的基础问题。本研究针对横向来流条件下气泡形成的现象进行了研究,主要包括循环水槽实验和理论模型的建立与求解。首先用开有通气孔的平板模型在循环水槽中进行实验,并配合高速摄像设备记录气泡各时刻的形态。然后依据力平衡、运动学方程和量纲分析建立了气泡膨胀阶段和分离阶段的理论模型,利用四阶Runge-Kutta算法并结合初始条件进行数值求解,得到气泡在各个时刻的质心位置、倾斜角度和气泡尺寸大小。最后将数值计算结果与实验结果进行对比,结果表明两者具有良好的一致性,均方根误差为5.38%,进而证实了理论模型的可靠性

挡流条对轴对称航行体空化演化过程的影响研究

空泡的准周期性非稳态演化过程中,空泡的不断脱落和溃灭等会对航行体产生不利影响。本文讨论了一种在航行体前端附近设置挡流条来增加非稳态演化过程中空泡的稳定性的方式,并基于分离式霍普金森压杆发射实验研究了挡流条对空泡初生、发展、脱落及溃灭的全过程的影响。试验表明挡流条会改变空泡的演化过程,如使肩部空泡缩短,提前形成回射流,并在一定程度上增加非稳态演化过程中空泡的稳定性

Effects of open space on neighbourly social interaction : the case of Hong Kong

As a Global city, overall neighborly social interactions in Hong Kong are quite low. People living in community merely interact with each other and not sustain neighborliness. Social interactions in neighborhood level contribute the feelings of belonging and sense of community which are important factors to build up social capital. According to environment psychology, physical environment is a crucial mechanism to influence social interaction. Then open space as focal point of neighborhood environment may have important effects on neighborly social interaction. The goal of this dissertation is to examine the inter-connection of open space and social interaction and provide better open space planning strategies and design principles to boost neighborly social interaction. Firstly open space’s effect in social interaction is identified in theoretical framework. Then current open space planning and social interaction pattern of Hong Kong are reviewed in empirical background. Based on theoretical framework and empirical background, analysis of this study is launched with case studies of two public housing estate. The analysis is divided into two parts. First part is to confirm that open space with good quality could be act as catalyst to boost neighborly social interaction. Second part is to discover appropriate design of open space to stimulate social interaction. Then recommendations are provided, including strategies for establishing comprehensive open space planning and open space design guideline in terms of enhancing social interaction.published_or_final_versionUrban Planning and DesignMasterMaster of Science in Urban Plannin

离散时空直接建模思想及其在模拟多尺度输运中的应用

统一气体动理学格式是基于离散空间直接建模的思想构建的多尺度数值格式。本文对统一气体动理学格式近十年的发展进行总结,并对未来的发展方向进行展望。统一气体动理学格式的建模思路突破了传统偏微分方程数值离散求解的制约,回归物理建模的出发点,基于守恒定律在离散时空有限尺度的控制体上进行建模,利用网格界面处的动理学方程时间演化解构建数值通量,从而构造出有限控制体上取决于网格尺度和时间步长的气体动力学控制方程。统一气体动理学格式建模有两个关键点:一是宏观守恒量与微观分布函数耦合演化,二是通过界面处的多尺度时间演化解构建数值通量。统一气体动理学格式是一种多尺度数值格式,根据网格努森数能够准确捕捉从稀薄到连续不同流域的流体物理。从某种意义上说气体动理学格式提供了有效的随不同网格努森数变化的连续性方程,即连续流的纳维-斯托克斯(N-S)方程和稀薄流的波尔兹曼(Boltzmann)是统一气体动理学格式在网格努森数很小和很大情况小逼近的两个极限方程。对于连续流的黏性边界层问题的捕捉,统一动理学格式不要求网格尺度小于粒子平均自由程。统一气体动理学格式成功应用于多尺度气体输运,等离子体输运,中子、光子输运,以及气固离散两相流等领域的数值模拟,在计算精度和计算效率上都体现出明显优势。尤其对于等离子体的输运计算,统一气体动理学格式提供了一个在连续变化尺度上的模拟方法,包括从求解电子、离子的自由输运的Vlasov动理学方程到连续流域内的双流体方程以及磁流体方程。本文总结了统一气体动理学格式的建模思想,数值性质,以及格式在不同领域的应用。 The direct modeling methodology provides a framework for multiscale modeling of transport processes, based on which the unified gas kinetic scheme (UGKS), the discrete unified kinetic scheme (DUGKS), and unified gas kinetic wave-particle (UGKWP) method have been developed over the last decade. The methodology of direct modeling is to construct the numerical governing equations on a discrete control volume by taking into account the contribution of both particle transport and collision process. The two important ingredients of the direct modeling methodology are the coupling evolution of the macroscopic quantities and microscopic distribution function, and the utilization of the local evolution solution in the construction of numerical flux. Based on the direct modeling methodology, we construct a continuous spectrum of governing equations in the whole flow regimes, which automatically recovers the collision-less Boltzmann and Navier-Stokes equations in their corresponding limiting regimes. In this paper, we are going to review the direct modeling methodology, the construction of schemes, and the multiscale and unified preserving properties. We will also review the applications of the schemes in the transport process of gas, plasma, photon, and disperse multiphase flow, and give an outlook of the future developments. © 2020, The Editorial Board of Acta Aerodynamica Sinica. All right reserved