published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

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The evolution of the large scale structures and the mean field were investigated in axisymmetric and plane mixing layers. Some aspects of the linear instability of an axisymmetric jet have been demonstrated. The axisymmetric geometry admits two additional length scales with relation to the two-dimensional shear layer: the radius of the jet column and the azimuthal wavelength. The importance of these two length scales in governing the instability of an axisymmetric jet was explored. The special case of a thin axisymmetric shear layer was analyzed and the results stressing the evolution of different azimuthal modes were compared with some phase-locked data which was produced by subjecting the jet to axisymmetric and helical excitation. The importance of the initial spectral distribution in a natural jet was demonstrated when it is used as an input to the amplification curve obtained from linear stability theory to predict a measured spectral distribution at a further downstream location. The inclusion of the nonlinear terms in the stability analysis reveals two main interactions: mean flow-wave interaction and wave-wave interaction. The modification of the mean flow of an axisymmetric jet was examined by exciting two azimuthal modes simultaneously. The interaction resulted in an azimuthal modulation of the mean velocity profile having a cosine shape. Effectively, the geometry of the jet was modified without changing the geometry of the nozzle. The coupling between an excited periodic disturbance and the mean flow was analyzed and the spatial evolution of both were compared with experimental results obtained in a plane mixing layer. The behavior of the concommittant Reynolds stresses is discussed in detail. The conditions under which one disturbance will transfer energy to another were derived and demonstrated in an axisymmetric jet. The interaction between a large amplitude plane wave with a weak subharmonic component was shown to enhance the amplification rate of the subharmonic. It was further shown that the nonlinear interaction between two azimuthal modes can produce a third azimuthal mode which was not initially present in the flow. The coupling between a fundamental wave and its subharmonic in a parallel plane mixing layer was demonstrated numerically.

本論文將詳述對湍流熱對流系統改變邊界條件的三組實驗。 / 第一組實驗中，在寬高比等於1 的圓柱形對流槽中，通過平滑板和由金字塔狀規則排列的粗糙表面的板的組合，我們對五個對流槽進行測量。对實驗測量得到Nu 和Ra 的关系式，进行两个指数函数关系式的叠加--分别是由GL 模型提出的主体主导的（1/2）指数和边界层主导的(1/4)指数关系式。两个关系式的系数之比表明，粗糙板可以提高主体的贡献，而且将系统从边界层主导转变到主体主导。而對各個板上的測量顯示，改變平板的條件，比如恒定熱量或者恒定溫度條件，以及同一個對流槽中另外一塊板是否粗糙板，該平板的傳熱沒有發生變化。而對於粗糙版，讓它處於恒定熱量或者恒定溫度條件，得到不同的結果；同一個槽中另外一塊板是平板或者粗糙板，該粗糙板的傳熱也會受影響。 / 第二組實驗中，在平滑板组成的对流槽，和两块粗糙板组成的对流槽中，我们添加高分子聚合物，进一步研究添加的效果。对于平滑板的对流槽，研究观察到热传递效率Nusselt 数在添加聚合物之后降低。而且，添加的聚合物越多，Nu 降低越多，在聚合物浓度大约120ppm 的时候，Nu 降低了接近12%，而且保持不变。而对于粗糙板组成的对流槽，当聚合物浓度大于120ppm 的时候，Nu 提高了大约4%。 / 第三組實驗，在两块平滑板组成的圆柱形对流槽中，我们也对底板边界层内的速度场进行了实验测量。结果表明，在倾斜角度小于1°的时候，速度边界层的厚度δ[subscript v]与雷诺数的指数关系，与理论给的平流边界层的结果接近。当倾斜角度比较大的时候，边界层厚度与雷诺数的指数随着角度减小。 在不同瑞利数下，同一个倾斜角度时，测量得到的水平速度剖面通过标准化后，得到的形状是一致不变的。而对于不同的角度，标准化的形状之间，是有区别的。 / In this thesis three sets of experiments of turbulent Rayleigh-Bénard convection with modified boundary conditions are presented. / The first set of measurements were made in cylindrical cells with aspect ratio one and with various combinations of smooth and rough plates in the form of regularly-arrayed pyramids. The experimental results suggest that the Nu - Ra relationship can be represented by the combination of two power laws, corresponding to the bulk dominant regime (exponent=1/2) and boundary layer dominant one (exponent=1/4) of the Grossmann-Lohse model. The behaviors of the coefficients of the two power laws suggest that the roughness of the plate can enhance the contribution of bulk and push the system to change from the boundary dominant state to bulk dominant state. A further examination of the individual plates reveal that the heat transport properties of smooth plates are insensitive to the surface and boundary conditions of the other plate of the same cell, whether smooth or rough, or whether under constant flux or constant temperature. The heat transport properties of the rough plates, on the other hand, appear to depend on surface and boundary conditions of the other plate of the same cell. / In the second set of experiments we study the effect of polymer additives in two Rayleigh-Bńard convection cells, one with smooth top and bottom plates and the other with rough top and bottom plates. For the cell with smooth plates, a reduction of the measured Nusselt number (Nu) was observed. Furthermore, the amount of Nu reduction increases with increasing polymer concentration (c), reaching ~12% for c = 120 ppm and an apparent level-off thereafter. For the cell with rough plates, however, an enhancement (~4%) of Nu was observed when the polymer concentration is greater than 120 ppm. / The third set of experiments investigates the properties of the velocity boundary layer in turbulent Rayleigh-Bénard convection in a cylindrical cell when it is tilted with respect to gravity. It is found that at small tilt angles (θ ≤ 1°), the measured viscous boundary layer thickness δ[subscript v] scales with the Reynolds number Re with an exponent close to that for a Prandtl-Blasius laminar boundary layer. For larger tilt angles, the scaling exponent of δ[subscript v] with Re decreases with θ. The normalized mean horizontal velocity profiles measured at the same tilt angle but with different Ra are found to have an invariant shape. But for different tilt angles, the shape of the normalized profiles is different. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Wei, Ping = 改變邊界條件下的熱湍流對流系統 : 傳熱性質及邊界層的實驗研究 / 韋萍. / "November 2012." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 148-158). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese. / Wei, Ping = Gai bian bian jie tiao jian xia de re tuan liu dui liu xi tong : chuan re xing zhi ji bian jie ceng de shi yan yan jiu / Wei Ping. / Table of Contents --- p.vii / List of Figures --- p.xxii / List of Tables --- p.xxiv / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background of Turbulence --- p.1 / Chapter 1.2 --- Rayleigh-Bénard convection system --- p.4 / Chapter 1.2.1 --- Physical picture --- p.4 / Chapter 1.2.2 --- Structure of Rayleigh-Bénard convection --- p.8 / Chapter 1.2.3 --- Previous investigation about Rayleigh-Bérnard Convection --- p.11 / Chapter 1.3 --- Motivation of the thesis --- p.15 / Chapter 1.4 --- Organization of this thesis --- p.17 / Chapter 2 --- Experimental apparatus and method --- p.18 / Chapter 2.1 --- Experimental apparatus --- p.18 / Chapter 2.1.1 --- The convection cell with smooth conducting plate --- p.18 / Chapter 2.1.2 --- The rough conducting plate --- p.20 / Chapter 2.2 --- Experimental techniques --- p.21 / Chapter 2.2.1 --- Temperature measurements --- p.21 / Chapter 2.2.2 --- Particle Image Velocimetry (PIV) --- p.23 / Chapter 2.3 --- Correction of heat flux measurment --- p.25 / Chapter 3 --- Convective Heat Transfer in Bulk- and Boundary-Dominated Regimes in Turbulent Thermal Convection --- p.28 / Chapter 3.1 --- Introduction --- p.29 / Chapter 3.2 --- Apparatus and experimental method --- p.31 / Chapter 3.3 --- Results and discussion --- p.34 / Chapter 3.3.1 --- Homogenity of temperature measurement over plate --- p.34 / Chapter 3.3.2 --- The heat transport measurement Nu --- p.40 / Chapter 3.3.3 --- Different heat transport behaviors over smooth and rough plates --- p.41 / Chapter 3.3.4 --- Fluctuations of local temperature --- p.47 / Chapter 3.3.5 --- Velocity and Reynolds number measurement --- p.48 / Chapter 3.3.6 --- Comparison with theoretical model and understanding . --- p.50 / Chapter 3.4 --- Summary --- p.52 / Chapter 4 --- Enhanced and Reduced Heat Transport in Turbulent Thermal Convection with Polymer Additives --- p.54 / Chapter 4.1 --- Introduction --- p.55 / Chapter 4.2 --- Experimental setup and methods --- p.58 / Chapter 4.2.1 --- The convection cell and experimental parameters --- p.58 / Chapter 4.2.2 --- Characterization of Polymer --- p.59 / Chapter 4.2.3 --- Measurement of the polymer solution viscosity --- p.62 / Chapter 4.3 --- Results --- p.65 / Chapter 4.3.1 --- The Nusselt number measurement --- p.65 / Chapter 4.3.2 --- Fluctuations of Nu and the local temperature --- p.72 / Chapter 4.3.3 --- Velocity and Reynolds number behavior --- p.83 / Chapter 4.4 --- Discussions --- p.90 / Chapter 4.5 --- Summary --- p.94 / Chapter 5 --- Viscous boundary layer properties in turbulent thermal convection in a cylindrical cell: the effect of cell tilting --- p.97 / Chapter 5.1 --- Introduction --- p.98 / Chapter 5.1.1 --- Boundary layer measurements in turbulent thermal convection --- p.98 / Chapter 5.1.2 --- Organization of the chapter --- p.102 / Chapter 5.2 --- Experimental apparatus --- p.103 / Chapter 5.2.1 --- Convection cell --- p.103 / Chapter 5.2.2 --- PIV measurement --- p.104 / Chapter 5.3 --- Results and discussion --- p.104 / Chapter 5.3.1 --- Temperature profile and fluid properties --- p.106 / Chapter 5.3.2 --- Velocity profiles and the Reynolds number scaling --- p.106 / Chapter 5.3.3 --- The viscous boundary layer and its scaling with Ra and Re --- p.114 / Chapter 5.3.4 --- Fluctuations and statistical properties of the velocity field in the boundary layer --- p.121 / Chapter 5.3.5 --- Properties of shear stresses and near-wall quantities --- p.125 / Chapter 5.3.6 --- Dynamical scaling and the shape of velocity profiles in the boundary layer --- p.130 / Chapter 5.4 --- Summary --- p.138 / Chapter 6 --- Conclusion --- p.141 / Chapter 6.1 --- Conclusion of our work --- p.141 / Chapter 6.1.1 --- Comparison of the thermal convection with modified plates --- p.142 / Chapter 6.1.2 --- The effect of polymer additives in smooth and rough cells --- p.143 / Chapter 6.1.3 --- Velocity field with tilting angles --- p.144 / Chapter 6.2 --- Perspectives for further investigation --- p.146 / Bibliography --- p.148

by Xin-liang Qiu. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (p. 98-103). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese. / by Xin-liang Qiu.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Noel T. Clemens Vita. Includes bibliographical references. Also available from UMI.

Thesis (M.S. in E.S.M.)--School of Civil and Environmental Engineering, Georgia Institute of Technology, 2004. Directed by Donald R. Webster. / Includes bibliographical references (leaves 118-121).

Thesis (Ph. D.)--University of Hong Kong, 1988. / Also availalbe in microfilm.

Thesis--M. Phil., University of Hong Kong, 1984.

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