本文選題：明渠紊流 + 相干結構��； 參考：《清華大學》2014年博士論文

【摘要】：自然界中普遍存在的明渠紊流包含各種尺度的相干結構，結構及其間的相互作用控制了紊動的生成、傳輸、耗散等重要過程。傳統(tǒng)的明渠紊流相干結構研究繼承湍流統(tǒng)計理論的基本假設，認為不同尺度相干結構具有獨立的生成和維持機制，但近年來其他壁面湍流的研究成果開始揭示出各尺度間密切而復雜的相互關系。本文研究的主要目的在于促進對明渠紊流相干結構的深入認識，以尺度劃分、相互作用等現(xiàn)代觀點建立相干結構研究的統(tǒng)一框架。 使用自主研發(fā)的高頻粒子圖像測速系統(tǒng)量測了不同雷諾數(shù)和弗勞德數(shù)的明渠紊流，得到了縱垂面和水平面的獨立及連續(xù)流場，主要取得了以下創(chuàng)新成果： 1、基于預乘譜分析結果，建立了明渠紊流相干結構尺度劃分體系，從統(tǒng)計意義上將相干結構劃分為小、大、超大三種尺度，小尺度以u*/ν度量，超大尺度結構大于3h，并常能達到10h的量級，大尺度結構則介于二者之間； 2、提出基于流場幾何特征的結構化渦量方法，從明渠紊流中識別出小尺度橫向渦，與其他類型壁面湍流相比，自由水面會增加橫向渦的數(shù)量，，但將抑制其強度和尺度；在瞬時流場中觀察到了大量符合發(fā)夾渦特征的順向渦，其條件平均流場表明，發(fā)夾渦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結構是發(fā)夾渦群，超大尺度現(xiàn)象主要表現(xiàn)為全水深范圍的大規(guī)模Q2\Q4事件和外區(qū)水平面的高低速流帶，并比其他壁面湍流更強；分析結果支持超大尺度流向渦模型； 4、基于泰勒凍結假定的連續(xù)流場以及橫向渦數(shù)量與POD模態(tài)分量的聯(lián)合分布，得到了超大尺度流向渦與發(fā)夾渦群間的互反饋維持機制；運用希爾伯特變換和濾波技術分析了各種尺度間的相互關系，發(fā)現(xiàn)了明渠紊流中的超大尺度流向渦對內區(qū)小尺度運動的調制現(xiàn)象，并提出了超大尺度流向渦通過床面剪切對發(fā)夾渦的誘發(fā)機制； 5、基于明渠紊流中相干現(xiàn)象的尺度劃分、各尺度相干結構模型及其相互關系，提出明渠紊流相干結構的統(tǒng)一物理模型，模型中的各尺度結構均具有合理的生成與維持機制，能夠解釋明渠紊流中的主要相干現(xiàn)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.
【學位授予單位】：清華大學
【學位級別】：博士
【學位授予年份】：2014
【分類號】：TV133

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