明渠紊流不同尺度相干结构实验研究

发布时间：2018-04-14 06:41

本文选题：明渠紊流 + 相干结构　；参考：《清华大学》2014年博士论文

【摘要】：自然界中普遍存在的明渠紊流包含各种尺度的相干结构，结构及其间的相互作用控制了紊动的生成、传输、耗散等重要过程。传统的明渠紊流相干结构研究继承湍流统计理论的基本假设，认为不同尺度相干结构具有独立的生成和维持机制，但近年来其他壁面湍流的研究成果开始揭示出各尺度间密切而复杂的相互关系。本文研究的主要目的在于促进对明渠紊流相干结构的深入认识，以尺度划分、相互作用等现代观点建立相干结构研究的统一框架。使用自主研发的高频粒子图像测速系统量测了不同雷诺数和弗劳德数的明渠紊流，得到了纵垂面和水平面的独立及连续流场，主要取得了以下创新成果： 1、基于预乘谱分析结果，建立了明渠紊流相干结构尺度划分体系，从统计意义上将相干结构划分为小、大、超大三种尺度，小尺度以u*/ν度量，超大尺度结构大于3h，并常能达到10h的量级，大尺度结构则介于二者之间； 2、提出基于流场几何特征的结构化涡量方法，从明渠紊流中识别出小尺度横向涡，与其他类型壁面湍流相比，自由水面会增加横向涡的数量，，但将抑制其强度和尺度；在瞬时流场中观察到了大量符合发夹涡特征的顺向涡，其条件平均流场表明，发夹涡模型在明渠紊流中仍然有效； 3、使用本征正交分解方法分析得出，明渠紊流中的大尺度结构是发夹涡群，超大尺度现象主要表现为全水深范围的大规模Q2\Q4事件和外区水平面的高低速流带，并比其他壁面湍流更强；分析结果支持超大尺度流向涡模型； 4、基于泰勒冻结假定的连续流场以及横向涡数量与POD模态分量的联合分布，得到了超大尺度流向涡与发夹涡群间的互反馈维持机制；运用希尔伯特变换和滤波技术分析了各种尺度间的相互关系，发现了明渠紊流中的超大尺度流向涡对内区小尺度运动的调制现象，并提出了超大尺度流向涡通过床面剪切对发夹涡的诱发机制； 5、基于明渠紊流中相干现象的尺度划分、各尺度相干结构模型及其相互关系，提出明渠紊流相干结构的统一物理模型，模型中的各尺度结构均具有合理的生成与维持机制，能够解释明渠紊流中的主要相干现象。
[Abstract]:Open channel turbulence in nature contains coherent structures of various scales. The structure and its interaction control the generation, transmission, dissipation and other important processes of turbulence.The traditional study of turbulent coherent structures in open channels inherits the basic hypothesis of turbulent statistical theory and holds that coherent structures with different scales have independent generation and maintenance mechanisms.However, in recent years, the research results of other wall turbulence begin to reveal the close and complex interrelation between different scales.The main purpose of this paper is to promote the deep understanding of turbulent coherent structures in open channels and to establish a unified framework for the study of coherent structures from the perspective of scale division and interaction.The open channel turbulence with different Reynolds number and Froude number is measured by using the self-developed high frequency particle image velocimetry system. The independent and continuous flow fields of vertical plane and horizontal plane are obtained. The main achievements are as follows:1. Based on the results of premultiplicative spectrum analysis, a scale partition system of turbulent coherent structures in open channel is established. In statistical sense, coherent structures are divided into three scales: small, large, large and large.The large scale structure is larger than 3 h and can reach the order of 10 h, while the large scale structure is somewhere between the two.2. A structured vorticity method based on the geometric characteristics of the flow field is proposed to identify small scale transverse vortices from open channel turbulence. Compared with other types of wall turbulence, the free water surface will increase the number of transverse vortices, but its intensity and scale will be restrained.A large number of forward vortices with the characteristics of hairpin vortex are observed in the instantaneous flow field. The conditional mean flow field shows that the hairpin vortex model is still valid in open channel turbulence.3. By using the method of eigenorthogonal decomposition, it is concluded that the large scale structure in the open channel turbulence is hairpin vortex group, and the large scale phenomenon is mainly characterized by the large scale Q 2\ Q 4 events in the whole depth range and the high and low velocity flow zone in the horizontal plane of the outer region.The results support the large scale flow vortex model.4. Based on the continuous flow field assumed by Taylor freezing and the joint distribution of the number of transverse vortices and the POD modal components, the mutual feedback maintenance mechanism between the super-large scale flow vortex and hairpin vortex group is obtained.By using Hilbert transform and filtering technique, the interrelation between various scales is analyzed, and the modulation phenomenon of the large scale flow vortex in open channel turbulence to the small scale motion in the inner region is found.The mechanism of hairpin vortex induced by large scale flow vortex through bed shear is proposed.5. Based on the scale division of coherent phenomena in open channel turbulence, the coherent structure model of each scale and its relationship, a unified physical model of coherent structure in open channel turbulence is proposed.The main coherent phenomena in open channel turbulence can be explained.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TV133

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