This thesis investigates three problems relating to thermoconvective stability in porous media. These are (i) the stability of an inclined boundary layer flow to vortex type instability, (ii) front propagation in the Darcy-B´enard problem and (iii) the onset of Prantdl-Darcy convection in a horizontal porous layer subject to a horizontal pressure gradient. The nonlinear, elliptic governing equations for the inclined boundary layer flow are discretised using finite differences and solved using an implicit, MultiGrid Full Approximation Scheme. In addition to the basic steady state three configurations are examined: (i) unforced disturbances, (ii) global forced disturbances, and (iii) leading edge forced disturbances. The unforced inclined boundary layer is shown to be convectively unstable to vortex-type instabilities. The forced vortex system is found to produce critical distances in good agreement with parabolic simulations. The speed of propagation and the pattern formed behind a propagating front in the Darcy-B´enard problem are examined using weakly nonlinear analysis and through numerical solution of the fully nonlinear governing equations for both two and three dimensional flows. The unifying theory of Ebert and van Saarloos (Ebert and van Saarloos (1998)) for pulled fronts is found to describe the behaviour well in two dimensions, but the situation in three dimensions is more complex with combinations of transverse and longitudinal rolls occurring. A linear perturbation analysis of the onset of Prandtl-Darcy convection in a horizontal porous layer subject to a horizontal pressure gradient indicates that the flow becomes more stable as the underlying flow increases, and that the wavelength of the most dangerous disturbances also increases with the strength of the underlying flow. Asymptotic analyses for small and large underlying flow and large Prandtl number are carried out and results compared to those of the linear perturbation analysis.

Solidified igneous intrusions from originally liquid magma chambers display a large number of different sedimentary features. These features include the gravitational collapse of sidewalls producing slumps and the layering produced by gravitational settling of crystals. In the chamber fluid-dynamic processes such as convection are expected to occur due to cooling at the roof producing dense gravitationally unstable liquid, and the crystallisation of interstitial liquid changing the composition of the remaining liquid possibly reducing the density causing the liquid to rise up. The crystals which form in basaltic magma chambers have a high propensity to be mobilised due to convection and other fluid-dynamic processes including replenishment by a secondary intrusion. Convective mobilisation of plagioclase grains in vertical, tabular intrusions is seen from flat profiles of apparent aspect ratio as a function of dyke width. These flat profiles were formed due to scouring of gravitationally unstable sidewall mushes, and these crystals then become entrained in the convecting liquid. Convection only ceases once the volume of crystals in suspension reaches a critical volume fraction leading to an increase in viscosity, which dampens the vigour of convection. The majority of this study is performing and analysing a number of different experiments to look at the behaviour of different styles of analogue particle piles. Particle piles that are formed of inert, plastic particles are subjected to convection in the particle layer and in the bulk overlying fluid, and different styles of mobilisation depending on the heat flux driving convection and the density profile of the pile are observed. The mobilisation style goes from rolling of particles on the surface, to puffs of particles from the surface being lofted into the interior, followed by large particle fountains and then the entire particle pile being completely disaggregated and lofted into the interior of the chamber as the force driving convection is increased. The initiation of mobilisation can be explained by the fluidisation of a particle pile, whilst the high degrees of mobilisation seen in some high Rayleigh number regimes can be explaining by resuspending particles. In experiments where particle piles have a positive density profile (dense particles overlying low density particles) the underlying low density particles can break through the overlying layer in particle fountains and can be explained by a modified fluidisation parameter. These experiments lack the reactivity and cohesion that realistic crystal piles would have. To try and quantify this, I have also performed a series of experiments looking at the rheology of an ice-sucrose suspension, where ice crystals can sinter and aggregate together. Under sheared conditions the forces required to disaggregate ice aggregates can be calculated, with the viscosity of an ice-sucrose suspension being described by a power-law relationship of shear rate and crystal radius. The particle pile experiments show that mobilisation of equivalent crystal piles in magma chambers should be readily observed. As it is not observed, except in replenished magmatic systems, this suggests that the additional forces coming from cohesion and aggregation in crystal piles prevent mobilisation of magmatic crystals. The replenishment by secondary intrusions can lead to forces which overcome the strength of the pile.

O presente estudo visa dar luz sobre fenômenos ainda pouco explorados que ocorrem durante a estratificação em leitos de jigagem, bem como sobre novas possibilidades de aplicação da técnica. Por meio de testes com misturas de agregados em jigues hidráulico e pneumático descontínuos, foi pela primeira vez examinada a ocorrência de convecção granular em jigues. Os fatores envolvidos em sua formação e a influência sobre a estratificação foram analisados em detalhes. Do mesmo modo, foi examinada a influência da granulometria do leito sobre a estratificação em jigues pneumáticos e a possibilidade de uso do mesmo como um equipamento multi-separador. Os resultados revelam a existência de um fluxo ordenado e estacionário de partículas durante a pulsação do leito, ascendente na porção central e descendente nas bordas do jigue, de modo similar ao fluxo convectivo descritos na literatura para leitos vibrados mecanicamente. Este fluxo convectivo gerou uma distorção no rearranjo do leito estratificado, dando origem a um perfil horizontal de estratificação no qual as partículas leves se concentraram nas bordas e as partículas densas se concentraram preferencialmente na zona central dos estratos. Com base nos resultados, um mecanismo de formação do fluxo convectivo e um modelo preditivo do rearranjo do leito foram propostos. Observou-se também que a diminuição da granulometria e o aumento da distribuição de tamanho dos constituintes do leito teve um efeito positivo sobre a estratificação. Os resultados sugerem que isto pode estar em parte relacionado a maior compactação do leito quando da presença de partículas de menor tamanho. Também, resultados experimentais demonstraram a possibilidade de uso do jigue pneumático para realizar a operação conjunta de separação de materiais rochosos e não rochosos em um único estágio. O desempenho global na remoção de contaminantes não-rochosos foi similar ao de separadores pneumáticos e sistemas de "sorting" automático descritos na literatura. Ainda, a quantidade inicial de contaminantes no sistema teve um efeito desprezível sobre o desempenho. Os resultados obtidos, em particular aqueles relacionados à convecção granular, fornecem novas perspectivas a respeito dos fenômenos envolvidos na estratificação em jigues. Ainda, benefícios práticos podem ser extraídos a partir dos resultados da análise do efeito da granulometria sobre a estratificação e do uso de jigues pneumáticos como multi-separadores. / This study aims to shed some light on some unclear phenomena that occurs during particle stratification in jigs as well as on new possibilities of application of jigging technique. For the first time, the occurrence of granular convection in jigging beds was analyzed by considering the case of hydraulic and pneumatic jigs operating with mixtures of aggregates. The elements involved in the pattern formation and its influence on bed stratification were analyzed in detail. The influence of particle size distribution on bed stratification and the possibility of using pneumatic jigging as a multi-separator were also examined. The experimental results reveal the existence of a steady, ordered flow of particles over bed pulsation, in which particles move upward in the center of the jig bed and downward along the walls of the jig container, similarly to the circulation patterns reported in the literature. The convection gave rise to a distortion in the particle rearrangement, engendering the formation of a horizontal stratification profile in which light particles tended to be concentrated along the walls while dense particles tended to be concentrated preferentially in the center portion of the strata. On the basis of the experimental results, a mechanism of pattern formation and a prediction model were proposed. The results also revealed that the decrease of the particle size and the increase in the particle size distribution had a positive influence on stratification extent. It was suggested by the results that this behavior can be related to the higher bed compaction due to the presence of smaller particles in the bed. Also, experimental results indicated the technical feasibility of the combined use of pneumatic jigging for the removal undesired non-stony materials together with the bed stratification in only one single stage. The reported separation efficiency in the removal of low-density materials was comparable to those of air classifiers and automatic sorting systems. Also, the initial content of contaminants seemed to have a negligible effect on the separation performance. The results obtained, in particular those related to the granular convection, provides new perspectives about the phenomena involved in stratification by jigging. Moreover, practical benefits can be obtained from the results about the influence of particle size on bed stratification and about the usage of pneumatic jigging for multi-separation.

Thesis. 1979. M.Arch.--Massachusetts Institute of Technology. Dept. of Architecture. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ROTCH. / Bibliography: leaves 148-151. / M.Arch.

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Xi Hengdong = 關於熱羽流和Rayleigh-Bénard對流中大尺度環流形成的實驗研究 / 郗恒東. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (leaves 71-75). / Text in English; abstracts in English and Chinese. / Xi Hengdong = Guan yu re yu liu he Rayleigh-Bénard dui liu zhong da chi du huan liu xing cheng de shi yan yan jiu / Xi Hengdong. / Table of Contents --- p.v / List of Figures --- p.xi / List of Tables --- p.xii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Rayleigh-Benard System --- p.1 / Chapter 1.1.1 --- Physical Picture --- p.1 / Chapter 1.1.2 --- Characteristic Parameters --- p.2 / Chapter 1.2 --- Plume and Large Scale Circulation --- p.4 / Chapter 2 --- Experimental Setup and Techniques --- p.8 / Chapter 2.1 --- Apparatus --- p.8 / Chapter 2.1.1 --- Convection Cell --- p.8 / Chapter 2.1.2 --- Other Apparatus --- p.12 / Chapter 2.2 --- Visualization --- p.14 / Chapter 2.3 --- PIV technique --- p.17 / Chapter 2.3.1 --- Image Capture System --- p.20 / Chapter 2.3.2 --- Image Analysis system --- p.26 / Chapter 3 --- Properties of Laminar Plume --- p.30 / Chapter 3.1 --- Shadowgraph and Temperature measurement --- p.30 / Chapter 3.2 --- Velocity Measurement --- p.35 / Chapter 4 --- Onset of Large-scale circulation in turbulent thermal convec- tion --- p.48 / Chapter 5 --- Convection in Rectangular cell --- p.60 / Chapter 6 --- Conclusion --- p.69 / Chapter 6.1 --- Conclusion --- p.69 / Chapter 6.2 --- Perspective for further investigation --- p.71 / Bibliography --- p.72

Ren Liyuan = 小寬高比熱對流中高精度Nusselt數和局部溫度測量 / 任立元. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references (leaves 64-66). / Text in English; abstracts in English and Chinese. / Ren Liyuan = Xiao kuan gao bi re dui liu zhong gao jing du Nusselt shu he ju bu wen du ce liang / Ren Liyuan. / Abstract (in English) --- p.i / Abstract (in Chinese) --- p.ii / Acknowledgements --- p.iii / Table of Contents --- p.iv / List of Figures --- p.vi / List of Tables --- p.ix / Chapters / Chapter I. --- Turbulent Rayleigh-Benard Convection --- p.1 / Chapter 1.1 --- Introduction of Rayleigh-Benard Convection --- p.1 / Chapter 1.2 --- The Convection Equations --- p.2 / Chapter 1.3 --- The parameters --- p.3 / Chapter 1.4 --- Background --- p.5 / Chapter 1.5 --- Synopsis of this thesis --- p.8 / Chapter II. --- Experimental Setup and Methods --- p.9 / Chapter 2.1 --- The Apparatus --- p.9 / Chapter 2.2 --- Thermistor Calibration --- p.12 / Chapter III. --- Local temperature measurements --- p.19 / Chapter 3.1 --- Introduction and motivation --- p.19 / Chapter 3.2 --- Local temperature measurements --- p.20 / Chapter 3.3 --- Temperature time series and histograms --- p.20 / Chapter 3.4 --- Mean temperature profile --- p.24 / Chapter 3.5 --- Summery --- p.35 / Chapter 3.6 --- Appendix A: Data lists for this chapter --- p.36 / Chapter IV --- Heat transport measurement --- p.39 / Chapter 4.1 --- Introduction and Motivation --- p.39 / Chapter 4.2 --- Heat transfer measurements --- p.40 / Chapter 4.3 --- Experimental uncertainties associated with high-precision measurements of Nu --- p.41 / Chapter 4.3.1 --- Cell height measurement --- p.41 / Chapter 4.3.2 --- Temperature measurement --- p.41 / Chapter 4.3.3 --- Thermal source --- p.41 / Chapter 4.3.4 --- Thermal leakage --- p.42 / Chapter 4.3.5 --- Sidewalleffect --- p.44 / Chapter 4.3.6 --- Long time measurement --- p.44 / Chapter 4.4 --- Results and discussion --- p.45 / Chapter 4.4.1 --- Experimental data --- p.45 / Chapter 4.4.2 --- Finite conductivity effect --- p.47 / Chapter 4.4.3 --- Nu dependence on Г --- p.49 / Chapter 4.4.4 --- Nu dependence on Ra --- p.50 / Chapter 4.5 --- Summary --- p.53 / Chapter 4.6 --- Appendix B: Data lists for this chapter --- p.54 / Chapter V. --- Conclusions --- p.62 / References --- p.64

Cheung Yin Har = 瑞利-柏納德湍流對流中Nusselt與縱橫比的關係以及邊界層性質的研究 / 張燕霞. / Thesis submitted in: October 2004. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 115-119). / Text in English; abstracts in English and Chinese. / Cheung Yin Har = Ruili-Bonade tuan liu dui liu zhong Nusselt yu zong heng bi de guan xi yi ji bian jie ceng xing zhi de yan jiu / Zhang Yanxia. / Abstract --- p.i / 摘要 --- p.iii / Acknowledgments --- p.iv / Contents --- p.v / List of Figures --- p.vii / List of Tables --- p.x / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background of turbulence --- p.1 / Chapter 1.2 --- Rayleigh-Benard convection --- p.3 / Chapter 1.3 --- Theoretical background --- p.4 / Chapter 1.3.1 --- The convection equations --- p.4 / Chapter 1.3.2 --- Characteristic parameters --- p.6 / Chapter 1.3.3 --- Reynolds equations --- p.8 / Chapter 1.4 --- Recent developments --- p.10 / Chapter 1.4.1 --- Heat transport --- p.10 / Chapter 1.4.2 --- Large scale flow and thermal plumes --- p.11 / Chapter 1.4.3 --- Boundary layers --- p.12 / Chapter 1.5 --- Motivation --- p.14 / Chapter 1.5.1 --- Nusselt measurements --- p.14 / Chapter 1.5.2 --- Boundary layer properties measurements --- p.14 / Chapter 1.6 --- Synopsis of this thesis --- p.15 / Chapter Chapter 2 --- Experimental setup and measurement techniques --- p.17 / Chapter 2.1 --- The turbulent convection system --- p.17 / Chapter 2.1.1 --- The convection cells --- p.18 / Chapter 2.1.2 --- The temperature probe --- p.21 / Chapter 2.1.3 --- The thermistors --- p.23 / Chapter 2.2 --- Particle Image Velocimetry (PIV) --- p.25 / Chapter 2.2.1 --- Image capture system --- p.27 / Chapter 2.2.2 --- Image analysis system --- p.36 / Chapter Chapter 3 --- Aspect ratio dependence of heat transport and the flow field --- p.39 / Chapter 3.1 --- Motivation for this experiment --- p.39 / Chapter 3.2 --- Heat transfer efficiency measurements --- p.40 / Chapter 3.3 --- Heat correction --- p.44 / Chapter 3.3.1 --- Temperature correction --- p.44 / Chapter 3.3.2 --- Heat current density J correction --- p.45 / Chapter 3.3.3 --- Finite conductivity of plate --- p.50 / Chapter 3.4 --- Aspect ratio dependence --- p.51 / Chapter 3.4.1 --- Without correction of finite conductivity --- p.51 / Chapter 3.4.2 --- With correction of finite conductivity --- p.59 / Chapter 3.5 --- Time-averaged velocity field --- p.65 / Chapter 3.6 --- Summary --- p.70 / Chapter Chapter 4 --- Local temperature and velocity measurements near the boundary layers --- p.71 / Chapter 4.1 --- Motivation for this experiment --- p.71 / Chapter 4.2 --- Temperature profile measurement --- p.72 / Chapter 4.2.1 --- Temperature and fluctuation profiles --- p.73 / Chapter 4.2.2 --- Thermal boundary thickness --- p.77 / Chapter 4.2.3 --- Temperature time series --- p.79 / Chapter 4.2.4 --- PDF --- p.83 / Chapter 4.3 --- Velocity profile measurement --- p.86 / Chapter 4.3.1 --- 2D velocity and fluctuation profiles --- p.86 / Chapter 4.3.2 --- Scaling properties --- p.93 / Chapter 4.4 --- Shear stress --- p.98 / Chapter 4.4.1 --- Viscous and Reynolds stresses --- p.99 / Chapter 4.4.2 --- Laminar or Turbulent? --- p.101 / Chapter 4.5 --- Summary --- p.104 / Chapter Chapter 5 --- Conclusion --- p.106 / Chapter 5.1 --- Heat flux measurement --- p.106 / Chapter 5.2 --- Boundary layers --- p.107 / Chapter 5.3 --- Perspective for further investigation --- p.108 / Appendix A Heat flux measurement for high Prandtl number --- p.109 / Chapter I. --- Experimental conditions --- p.110 / Chapter II. --- Result and discussion --- p.112 / Chapter III. --- Summary and perspective for further investigation --- p.114 / Bibliography --- p.115

Constituant la couche limite supérieure de la convection mantellique, la lithosphère terrestre est à l'interface entre les enveloppes externes et internes de notre Planète. Les interactions multiples entre celle-ci et le manteau sont à l'origine de déformations latérales (tectonique des plaques) et verticales (topographie dynamique) de la surface terrestre. Comprendre comment la formation et l'évolution d'instabilités convectives mantelliques renouvellent sans-cesse la surface est donc primordial pour améliorer nos interprétations d'un grand nombre d'observations de surface, telles que la formation de bassins sédimentaires, le mouvement des continents, la localisation des points chauds, la formation d'anomalies gravimétriques ou encore les variations du niveau marin.Cette thèse propose de développer des modèles numériques de convection mantellique générant defaçon auto-organisée de la tectonique des plaques en surface an d'étudier la façon dont le développement et la dynamique d'instabilités convectives telles que les panneaux de subduction ou les panaches mantelliques modifient la surface, dans un contexte de tectonique de surface approchant le régime terrestre.Dans une première partie, je m'intéresse à l'influence du couplage des mouvements de convection mantellique et de tectonique des plaques sur le développement de topographie dynamique (i.e. les mouvements verticaux de la lithosphère induits par la convection mantellique) à différentes échelles spatio-temporelles. Mes résultats suggèrent que la surface terrestre peut se déformer à toutes les échelles spatiales, du fait de mouvements convectifs de grande ampleur faisant intervenir le manteau entier (> 104 km) ou encore de convection à petite échelle sub-lithosphérique (< 500 km). Les variations temporelles de topographie dynamique s‘étendent de cinq à plusieurs centaines de millions d'années selon la nature des processus convectifs dont elles dérivent. En particulier, la dynamique d'initiation ou d'arrêt des zones de subduction contrôle l'existence d'échelles intermédiaires de topographie dynamique (longueurs d'onde variant entre 500 et 104 km). Ces résultats montrent donc que les interactions entre la dynamique de la lithosphère et la convection mantellique génèrent des motifs spatio-temporels de topographie dynamique complexes et cohérents par rapport aux observations terrestres.Dans un deuxième temps, cette thèse se focalise sur la dynamique des panaches mantelliques, et leurs interactions avec la surface. Je caractérise d'abord précisement le comportement des panaches générés dans nos modèles de convection à la lumière d'observations de surface. Puis, j'étudie la façon dont leurs interactions avec la tectonique de surface et les différentes échelles convectives modifient leurs mouvements latéraux. Enfin, la compréhension de la signature thermique des interactions entre panaches et rides océaniques me permet de proposer une reconstitution des mouvements relatifs entre le panache des Açores et la ride médio-Atlantique. / Earth's lithosphere, which is the upper boundary layer of mantle convection, represents the interface between the external and internal envelopes of our Planet. The multiple interactions between the mantle and lithosphere generate lateral (plate tectonics) and vertical (dynamic topography) deformations of Earth's surface. Understanding the influence of the dynamics of mantle convective instabilities on the surface is fundamental to improve our interpretations of a large range of surface observations, such as the formation of sedimentary basins, continental motions, the location of hotspots, the presence of gravity anomalies or sea-level variations.This thesis aims at developing numerical models of whole-mantle convection self-generating plate-like tectonics in order to study the impacts of the development and the dynamics of mantle convective instabilities (such as slabs or mantle plumes) on the continuous reshaping of the surface.First, I focus on the influence of the coupling between mantle convective motions and plate tectonics on the development of dynamic topography (i.e. surface vertical deformations induced by mantle convection) at different spatial and temporal scales. The results suggest that Earth's surface can deform over large spatial scales (> 104 km) induced by whole-mantle convection to small-scales (< 500 km) arising from small-scale sub-lithospheric convection. The temporal variations of dynamic topography range between five and several hundreds of millions of years depending on the convective instabilities from which they originate. In particular, subduction initiation and slab break-off events control the existence of intermediate scales of dynamic topography (between 500 and 104 km). This reflects that the interplay between mantle convection and lithosphere dynamics generates complex spatial and temporal patterns of dynamic topography consistent with constraints for Earth.A second aim of this thesis is to understand the dynamics of mantle plumes and their interactionswith surface. I first characterize in detail the behaviour of mantle plumes arising in models ofwhole-mantle convection self-generating plate-like tectonics, in light of surface observations. Then, I study how the interactions between surface plate tectonics and mantle convection affect plume motions. Finally, I use observations of the thermal signature of plume/ridge interactions to propose a reconstruction of the relative motions between the Azores mantle plume and the Mid-Atlantic Ridge.

In a world that is constantly in need of a continuous, reliable and sustainable energy supply, concentrating photovoltaic technologies have the potential to become a cost effective solution for large scale power generation. In this light, important progresses have been made in terms of cell’s design and efficiency, but the concentrating photovoltaic industry sector still struggles to gain market share and to achieve adequate economic returns. The work presented in this thesis is focused on the development of innovative solutions for high concentrating photovoltaics receivers. The design, the fabrication and the characterization of a large cell assembly for high concentrations are described. The assembly is designed to accommodate 144 multijunction cells and is rated to supply energy up to 2.6kWe at 500 suns. The original outline of the conductive copper layer limits the Joule losses to the 0.7% of the global power output, by reducing the number of interconnections. All the challenges and the issues faced in the manufacturing stage are accounted for and the reliability of the fabrication has been proven by quality tests and experimental investigations conducted on the prototype. An indoor characterization shows the receiver’s potential to supply a short-circuit current of 5.77A and an open circuit voltage per cell of 3.08V at 500×, under standard test conditions, only 4.80% and 2.06% respectively lower than those obtained by a commercial single-cell assembly. An electrical efficiency of 29.4% is expected at 500 suns, under standard conditions. A prototype’s cost of $0.91/Wp, in line with the actual price of CPV systems, has been recorded: a cost breakdown is reported and the way to further reduce the cost have been identified and is accounted. In a second approach, the design of a natural convective micro-finned array to be integrated in a single cell receiver has been successfully attempted. Passive cooling systems are usually cheaper, simpler and considered more reliable than active ones. After a detailed review of micro-cooling solutions, an experimental investigation on the thermal behaviour of micro-fins has been conducted and has been combined with a multiphysics software model. A micro-finned heat sink shows the potential to keep the CPV temperature below 100°C under standard conditions and the ability to handle the heat flux when the cell’s efficiency drops to zero. Moreover, a micro-finned heat sink demonstrates the potential to introduce significant benefits in terms of material usage and weight reduction: compared to those commercially available, a micro-finned heat sink has a power-to-weight ratio between 6 and 8 times higher, which results in lower costs and reduced loads for the CPV tracker.