基于边界层转捩直接数值模拟的湍流生成与维持机理研究

发布时间：2018-01-05 09:36

本文关键词：基于边界层转捩直接数值模拟的湍流生成与维持机理研究　出处：《南京航空航天大学》2016年博士论文　论文类型：学位论文

【摘要】：边界层转捩不仅在自然和工程流动中起着关键性的作用,而且由于其具有相对有序的涡系结构,正日益在湍流生成、发展与维持机理的研究中扮演着重要的角色。本文针对可压缩平板边界层转捩,采用直接数值模拟技术建立整个转捩过程的流场数据库,并对流场数据进行详细的验证与分析。在此基础上,对转捩过程中的涡系结构、转捩机理以及一些重要的物理现象进行研究和探索,针对湍流提出了一些新的理解和认识。论文的具体研究内容及主要成果包括:(1)对转捩过程中涡系结构的发展,尤其是Λ涡、发卡涡、流向涡和涡包的生成和演化机理进行详细的研究。发现在转捩过程中,流场中的一次涡结构,通过其诱导的上喷和下扫等运动,将无粘区的能量带到边界层底部,再通过剪切层不稳定性,产生更小尺度的涡结构。根据这一重要现象,首次提出边界层转捩是有序的多层涡结构“建造”过程,而不是通过大涡“破裂”产生小尺度涡结构的过程。这一观念与传统湍流转捩理论存在很大区别,为湍流转捩理论研究和模型建立提供了新的思路和方向。(2)对湍流理论中的一些重要概念进行梳理和探讨,包括湍流猝发与间歇性,转捩过程中总涡量的变化,涡是否是涡量的集中等。首次详细研究了湍流转捩过程中的间歇式猝发与涡系结构运动和发展的关系,证明猝发并非来源于某种流动不稳定性或者涡“破碎”,而是有序的涡“建造”过程在欧拉视角的表现。其次,尽管转捩过程中边界层总涡量基本没有变化,但流场内会同时产生正负相抵的涡量,这些涡量分别驱使涡旋运动,因此,并不能将边界层转捩简单看作是涡量重新分配的过程。同时,本文引进了“体积涡量”的概念,来说明转捩过程涡量变化的原因。另外,人们普遍认为涡是当地涡量的集中,然而根据直接数值模拟数据库,Λ涡恰恰位于当地涡量最小的位置。(3)基于高精度的直接数值模拟数据库,对涡显示方法进行研究,探讨了涡量和涡之间的关系,并提出将涡量分为旋转部分和非旋转部分的思想。根据这一概念,并结合流体刚性旋转时耗散最小的事实,提出了新的涡显示方法:Ω方法。与目前比较流行的Q和λ_2方法相比,Ω方法具有以下优点:捕捉精度高并且容易应用;物理含义更清晰;不需要针对每个具体问题给定阈值,使用Ω=0.52.左右总能得到较好的涡捕捉效果;能够同时捕捉强涡和弱涡(具有高动态范围)。这一新的涡显示方法,将能够很好地推动对湍流涡系结构的研究。
[Abstract]:Boundary layer transition not only plays a key role in natural and engineering flows, but also because of its relatively orderly vortex structure, it is increasingly generated in turbulence. The development and maintenance mechanism plays an important role. In this paper, the flow field database of the whole transition process is established by direct numerical simulation for the compressible plate boundary layer transition. Based on the detailed verification and analysis of the flow field data, the vortex structure, transition mechanism and some important physical phenomena in the transition process are studied and explored. Some new understanding and understanding of turbulence are put forward. The specific research contents and main results of this paper include the development of vortex structure in transition process, especially the A vortex and hairpin vortex. The generation and evolution mechanism of flow vortex and vortex-pack are studied in detail. It is found that the primary vortex structure in the flow field is induced by the upward jet and downsweep motions during the transition process. The energy of the viscidity zone is brought to the bottom of the boundary layer and then through the instability of the shear layer to produce a smaller scale vortex structure. According to this important phenomenon. It is proposed for the first time that boundary layer transition is a process of "building" ordered multi-layer vortex structures, rather than a process of producing small-scale vortex structures through large vortex "rupture". This concept is very different from the traditional theory of turbulent transition. It provides a new idea and direction for the theoretical study and modeling of turbulent transition. (2) some important concepts in turbulence theory are combed and discussed, including turbulence burst and intermittency. For the first time, the relationship between intermittent burst and vortex structure motion and development during turbulent transition is studied in detail. It is proved that the burst does not originate from some flow instability or vortex "fragmentation", but rather from the Euler angle of view of the ordered vortex "construction" process. Secondly, although the total vorticity of the boundary layer does not change in the transition process, there is no change in the total vorticity in the boundary layer. However, both positive and negative vortices are produced in the flow field, which drive the vortex motion respectively. Therefore, the boundary layer transition can not be regarded as a process of vorticity redistribution. At the same time, the boundary layer transition can not be considered simply as a process of vorticity redistribution. In this paper, the concept of "volumetric vorticity" is introduced to explain the reason of vorticity variation during transition. In addition, it is generally considered that vorticity is the concentration of local vorticity, but based on the direct numerical simulation database. Based on the high precision direct numerical simulation database, the vorticity display method is studied, and the relationship between vorticity and vorticity is discussed. The idea of classifying vorticity into rotating part and non-rotating part is put forward. According to this concept and the fact that the fluid is rotated rigidly, the dissipation is minimized. In this paper, a new vortex display method, 惟 method, is proposed. Compared with the popular Q and 位 s _ 2 method, 惟 method has the following advantages: high capture accuracy and easy application; The physical meaning is clearer; It is not necessary to set a threshold value for each specific problem and use 惟 0. 52. The effect of vortex capture can always be better. It can capture both strong and weak vortices at the same time (with high dynamic range). This new vortex display method will be able to promote the study of turbulent vortex system structure.
【学位授予单位】：南京航空航天大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O357.5

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