Dissertations / Theses: 'Convection'

1

Stich, Dennis. "Convection initiation." Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-157194.

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Clark, Adam James. "Predictability associated with convection-allowing and convection-parameterizing forecasts." [Ames, Iowa : Iowa State University], 2009. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3380372.

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Colin, Maxime. "Convective memory, and the role of cold pools." Electronic Thesis or Diss., Sorbonne université, 2018. http://www.theses.fr/2018SORUS312.

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Les paramétrisations représentent encore mal la variabilité spatio-temporelle de la convection. Cela peut provenir du fait qu’elles supposent que la convection peut être diagnostiquée à partir des conditions de grande échelle. La mémoire de la convection, traduisant la dépendance de la convection en sa propre histoire, pourrait permettre de résoudre ce problème. Cette thèse propose de distinguer la mémoire micro-état (non résolue) et la mémoire macro-état (de grande échelle). On a recours à un modèle résolvant les nuages, un Modèle de Circulation Générale (GCM) en 1D et en 3D, et un modèle simple proie-prédateur. Ils sont placés soit en Equilibre Radiatif-Convectif soit en condition de macro-état fixé, et on analyse leur réponse à des perturbations d'homogénéisation. La mémoire de la convection est de l’ordre d’une heure à un jour. Elle apparaît fortement liée à l'organisation convective dans l'espace. La mémoire du micro-état est essentiellement stockée dans les structures micro-état de couche-limite de la vapeur d'eau et de la température, la mémoire de la vapeur d'eau étant dominante. De plus, le micro-état convectif est intrinsèquement instable, ce qui confirme que la connaissance de l'état de grande échelle n'est pas suffisante pour prédire la convection. On montre ensuite que le GCM de référence a déjà des formes encourageantes de mémoire. Une modification simple du GCM rend les poches froides moins froides et ainsi moins aptes à déclencher la convection. La pluie du GCM modifié devient ainsi plus intermittente, résolvant un biais typique des modèles. Cette étude promeut l'introduction de variables pronostiques dans les GCMs pour représenter la mémoire du micro-état
Convective parameterizations struggle to represent the spatiotemporal variability of convection. This may be because they assume that convection can be diagnosed from the large-scale state, without knowing the convective history. The concept of convective memory, which states that convection depends on its own history, could help overcome this issue. A new framework suggests a distinction between microstate (unresolved) memory and macrostate (large-scale) memory. Using a hierarchy of models either in Radiative-Convective Equilibrium or under fixed-macrostate conditions, the thesis analyses the recovery to homogenisation perturbations. It exploits a Cloud-Resolving Model, a General Circulation Model (GCM) in 1D and in 3D, and a simple predator-prey model. The results show that convective memory plays a role on time scales of up to a day. Convective memory in time is dramatically enhanced by convective organisation in space. Microstate memory is found to be mostly stored in boundary layer microstate structures of water vapour and temperature, with a dominant water vapour memory. Furthermore, the convective microstate is shown to be inherently unstable, which confirms that knowledge of the macrostate conditions is not sufficient to predict convection. The standard version of the GCM already shows a reasonable level of convective persistence. A simple modification of the GCM convection scheme, meant to improve cold pools over oceans, makes cold pools less cold and thus weaker to trigger convection. This leads to more intermittent precipitation, partly correcting a typical GCM bias. Overall the thesis fosters introducing prognostic variables into GCMs and suggests ways to do it

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Berg, Niclas, Gustav Johansson, and Maja Sandberg. "Rayleigh-Bénard convection." Thesis, KTH, Mekanik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-105486.

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This report considers Rayleigh-Bénard convection, i.e. the 　ow between two large parallel plates where the lower one is heated. The change in density due to temperature variations gives rise to a 　ow generated by buoyancy. This motion is opposed by the viscous forces in the 　uid. The balance between these forces determines whether the 　ow is stable or not and the goal of this report is to nd a condition giving this limit as well as analyzing other aspects of the 　ow. The starting point of the analysis is the incompressible Navier- Stokes equations and the thermal energy equation upon which the Boussinesq approximation is applied. Using linear stability analysis a condition for the stability is obtained depending solely on a nondimensional parameter, called the Rayleigh number, for a given wavenumber k . This result is conrmed to be accurate after comparison with numerical simulations using a spectral technique. Further non-linear two- and three-dimensional simulations are also performed to analyze dierent aspects of the 　ow for various values of the Rayleigh number.

Examensarbete inom teknisk fysik, grundnivå

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Leppinen, David Michael. "Aspects of convection." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627366.

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Ramesh, Chandra D. S. "Turbulent Mixed Convection." Thesis, Indian Institute of Science, 2000. https://etd.iisc.ac.in/handle/2005/236.

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Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale. In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.

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Ramesh, Chandra D. S. "Turbulent Mixed Convection." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/236.

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Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale. In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.

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8

Lizée, Arnaud. "Contribution à la convection vibrationnelle : contrôle actif de la convection naturelle." Aix-Marseille 2, 1995. http://www.theses.fr/1995AIX22013.

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On considere un fluide en cavite carree differentiellement chauffee, soumis a l'action de la force de gravite et d'une force de volume due a un champ vibratoire. Pour de tres grandes frequences de vibrations, on realise une etude parametrique de l'ecoulement convectif, en utilisant les equations moyennees de la convection vibrationnelle. On met en evidence l'effet du nombre de rayleigh vibrationnel et de la direction de l'axe des vibrations sur le controle de la convection naturelle. On donne alors le domaine d'application physique de ces resultats. Une etude comparative de ces resultats, par simulation directe des equations de boussinesq permet de degager un critere sur la frequence adimensionnelle a partir de laquelle l'emploi des equations moyennees est justifie. Cette etude est realisee pour un nombre de prandtl de 0,71 et un nombre de rayleigh de 10#4. Elle montre les divers regimes possibles pour la convection en cavite carree en fonction de la frequence des vibrations. Par ailleurs, on etudie la stabilite lineaire de la convection vibrationnelle, pour une geometrie cylindrique ; le fluide etant soumis a l'action d'un flux de chaleur interne uniformement reparti. Pour cette etude, on emploie la methode matricielle couplee a la methode de galerkin. Une comparaison de l'emploi de fonctions de bessel et de polynomes est realisee pour le cas axisymetrique

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9

Eriksson, Stefan. "Global Magnetospheric Plasma Convection." Doctoral thesis, Stockholm : Tekniska högsk, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3230.

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10

Adams, Thomas M. "Turbulent convection in microchannels." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/19421.

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11

Courtney, R. C. "Hot spotsand mantle convection." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303911.

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12

Fox, Nicola Justine. "Ionospheric convection during substorms." Thesis, Imperial College London, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.281720.

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13

Jones, Alastair Stephen. "Convection heat transfer problems." Thesis, Keele University, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267356.

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Tian, Wenshou. "Orographic effects on convection." Thesis, University of Leeds, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.250911.

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15

Impey, M. D. "Bifurcation in Lapwood convection." Thesis, University of Bristol, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.234799.

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16

Brummell, Nicholas H. "Modal methods of convection." Thesis, Imperial College London, 1988. http://hdl.handle.net/10044/1/46977.

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17

Brown, A. R. "Modelling turbulent sheared convection." Thesis, University of Surrey, 1995. http://epubs.surrey.ac.uk/843794/.

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Large-eddy simulations of the atmospheric boundary layer have been performed over a range of stabilities between neutral and free convective conditions. The variation of various non-dimensionalized turbulence statistics over this stability range is presented and the results are compared with observations where possible. The robustness of the model results is also assessed by comparing those from high and low resolution simulations, and by reference to a number of additional sensitivity tests. The simulation results for the variation with stability of the mean wind and temperature profiles and various similarity coefficients are presented. The large-eddy model datasets are then used to evaluate critically the performance of a number of simple closure schemes suitable for use in boundary layer parametrizations in large-scale weather forecasting and climate prediction models. The potential significance of the shortcomings of the simplest mixing length schemes is discussed, and an assessment is made of the types of closure most likely to give a significant improvement in performance without an excessive computational overhead. Results are also presented from large-eddy simulations of the baroclinic boundary layer. The effects of the shear in the geostrophic wind on scaled turbulence statistics and the mean wind profiles are discussed. It is shown that this shear does not lead to significant degradation of the performance of two simple closure models, in either neutral or convective conditions. Finally simulation results for the entrainment flux at the top of the boundary layer are presented. A parametrization of this flux is developed, based on the boundary layer root mean square vertical velocity.

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Maczyńska, Borońska Katarzyna. "Motifs tridimensionnels dans la convection de Rayleigh-Benard cylindrique." Paris 7, 2005. http://www.theses.fr/2005PA077073.

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Dalbert, Anne-Marie. "Convection naturelle et convection mixte dans les thermosiphons verticaux chauffés à flux constant." Poitiers, 1991. http://www.theses.fr/1991POIT2270.

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Le present memoire propose une etude de l'ecoulement laminaire en convection naturelle et mixte dans un canal vertical bidimensionnel forme de deux plaques planes, verticales, paralleles et chauffees a flux constant. Les parametres de l'ecoulement sont mis en evidence a partir de la resolution, par une methode semi-analytique, des equations de convection mixte, en regime etabli. Les transferts parietaux ainsi que les profils de vitesse et de temperature dans le thermosiphon sont calcules en convection naturelle par une methode numerique aux differences finies. La structure de l'ecoulement et le role moteur de la pression motrice sont discutes en fonction des valeurs du nombre de grashof et du facteur de dissymetrie de flux de chaleur parietal. Les resultats numeriques sont de meme valides par comparaison avec des mesures experimentales effectuees sur deux dispositifs. Le nombre de reynolds de l'ecoulement et les transferts convectifs parietaux sont calcules dans la zone de developpement des couches limites, en convection mixte, avec le meme type de methode numerique. Trois cas sont envisages: convection forcee favorisant la convection naturelle, convection forcee contrariant la convection naturelle avec ecoulement resultant ascendant ou descendant. Le probleme du blocage ou du renversement de l'ecoulement est discute dans le cas ou la convection forcee et la convection naturelle agissent en sens contraire l'une de l'autre. Les resultats theoriques de l'etude sont appliques a la modelisation d'un systeme passif de climatisation par evaporation d'eau dans une circulation naturelle d'air. Ce type de refroidissement trouve une application interessante dans les pays au climat chaud et relativement sec

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Hama, Katsuhiko. "Film boiling heat transfer in various liquids under natural convection and forced convection conditions." Kyoto University, 2006. http://hdl.handle.net/2433/143793.

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21

Shiomi, Junichiro. "Control of oscillatory thermocapillary convection." Doctoral thesis, KTH, Mechanics, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3655.

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The possibility to stabilize the oscillatory thermocapillaryconvection is demonstrated using a proportional feedbackcontrol. This topic has a strong industrial motivation inconnection with a container-less crystal growth method calledthe floating-zone technique. The thermocapillary oscillation isknown to cause detrimental striations, microscopicinhomogeneity of the dopant distribution, in the final productof the crystal growth process. The feedback control is realizedby locally modifying the surface temperature by using the localtemperature measured at dierent locations fed back through asimple control law. Placing sensor/actuator pairs (controllers)in a strategical manner using the knowledge of the modalstructures, a simple cancellation scheme can be constructedwith only a few controllers. In this method, the state can bestabilized without altering the base flow appreciably whichcould be advantageous compared with other available controlmethods targeting the base convection.

As an initial study of such kind of control method, thisthesis work explores the possibility of applying the control insimplified geometries such as the annular configuration and thehalf-zone for high Prandtl number liquids by means ofexperiments, numerical simulations, and formulation of a simplemodel equation system. Successful suppression of theoscillation was obtained especially in the weakly nonlinearregime where the control completely suppresses theoscillations. With a right choice of actuators, even with thelocal control, it was shown that it is possible to modify thelinear and weakly-nonlinear properties of the three-dimensionalflow system with linear and weakly nonlinear control. On theother hand, the method exhibits certain limitations. Dependingon the geometry of the system and actuators, the limitation canbe caused by either the enhancement of nonlinear dynamics dueto the finite size of the actuators or the amplification of newlinear modes. The former case can be attenuated by increasingthe azimuthal length of the actuators to reduce the generationof broad wavenumber waves. In the latter case, having an ideaof the structures of the newly appearing modes, thedestabilization of those modes can be delayed by optimizing theconfiguration of controllers. On the whole, the oscillation canbe attenuated significantly in a range of supercritical Maup to almost twice the critical value.

Keywords:Fluid mechanics, Marangoni convection,thermocapillary convection, annular configuration, half-zone,feedback control, flow visualization, low dimensional model,bifurcation.

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Tisserand, Jean-Christophe. "Convection thermique : transport et mélange." Phd thesis, Ecole normale supérieure de lyon - ENS LYON, 2010. http://tel.archives-ouvertes.fr/tel-00612953.

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Dans ce manuscrit, nous nous sommes intéressés au phénomène de convection thermique turbulente. La première partie du manuscrit analyse l'influence de la rugosité sur l'efficacité du transport thermique dans des cellules de convection de Rayleigh-Bénard cylindrique et rectangulaire. Au vu des mesures thermiques, une transition claire apparaît lorsque l'épaisseur de la couche limite thermique est égale à la hauteur caractéristique de la rugosité. Au delà de la transition, un nouveau régime apparaît et il ne peut être expliqué par une simple augmentation de la surface d'échange de la plaque rugueuse avec le fluide. Nous proposons ici un modèle qui rend compte de ces observations. Dans la seconde partie, la convection thermique turbulente a été étudiée dans un canal vertical long. Dans cette configuration, des mesures de vélocimétrie par imagerie de particules (PIV) ont montré que l'écoulement était décomposable en deux colonnes de fluide : le fluide chaud montant sur la partie droite du canal et le froid descendant sur la partie gauche ou réciproquement. Nous avons étudié de façon systématique l'influence de la puissance injectée sur l'écoulement aux temps longs (retournement global des colonnes de fluide) et courts (obtention des profils de vitesse et de leur fluctuation et mesure de la vitesse des panaches thermiques). Dans une troisième et dernière partie, en plus de la puissance, nous ajoutons un second degré de liberté : l'angle d'inclinaison du canal. Des mesures PIV et des mesures thermiques nous ont permis d'étudier la compétition entre la turbulence et la stratification dans ce canal incliné. Un modèle rendant compte des mesures turbulentes est également proposé.

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Wissmeier, Ulrike. "The Physics of Tropical Convection." Diss., lmu, 2009. http://nbn-resolving.de/urn:nbn:de:bvb:19-106407.

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Rusaouën, Éleonore. "Échanges turbulents en convection thermique." Thesis, Lyon, École normale supérieure, 2014. http://www.theses.fr/2014ENSL0937/document.

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La problématique des échanges turbulents en convection thermique est abordée au travers de deux écoulements complémentaires en eau, Prandtl compris entre 2.5 et 6.5.Le premier est une cellule de Rayleigh-Bénard asymétrique. La plaque chaude située au bas de la cellule présente des rugosités périodiques de taille contrôlée. La plaque froide en haut de la cellule est, au contraire, lisse. Le flux de chaleur est imposé à la plaque chaude. Dans la gamme de forçage explorée dans ce travail, les deux plaques sont indépendantes ce qui permet une comparaison in-situ entre une plaque lisse et une plaque rugueuse. Par comparaison avec des résultats de référence, nous mettons en évidence une augmentation importante du nombre de Nusselt (flux de chaleur adimensionné) associé à la plaque rugueuse alors que celui de la plaque lisse demeure inchangé. Cette augmentation semble pilotée par la hauteur des rugosités choisies.Le second écoulement étudié ici est un écoulement en canal thermique. La géométrie du dispositif permet de s'affranchir des conditions aux limites à l'injection du flux de chaleur. L'expérience étant inclinable, il est possible d'étudier les effets de la stratification sur l'écoulement, puisqu'elle est de plus en plus importante avec l'augmentation de l'inclinaison. En fonction du couple (flux de chaleur, inclinaison), quatre régimes d'écoulements distincts peuvent apparaître. Nous proposons une modélisation pour trois d'entre eux, dont un modèle d'écoulement turbulent. Ce modèle est ensuite validé à plus haut Reynolds dans une nouvelle expérience à grande échelle, qui permet d'explorer le régime de turbulence inertielle induit par l'écoulement
The question of turbulent exchanges in thermal convection is studied with two kind of complementary flows in water, Prandtl number is kept between 2.5 and 6.5.The first one is an asymmetrical Rayleigh-Bénard cell. The hot bottom plate of the cell is covered with periodical obstacles of controlled size, whereas the cold top plate is kept smooth. The thermal flux is imposed at the hot plate. Considering the range of thermal forcing imposed to the cell, the two plates can be considered independent, which allows in-situ comparison between the smooth plate and the rough one. Comparison with reference results underlines a huge increase of the Nusslet number (non dimensional thermal transfer) of the rough plate whereas the transfer of the smooth plate remains unchanged. This increase seems to be controlled by the height of the obstacles.The second flow studied here is a thermal channel. The geometry of the apparatus allows the channel to be free of the influence of boundary conditions. By tilting the experiment, it is possible to study the influence of stratification on the flow. The larger the inclination is, the larger is the stratification. Considering the (heat flux, inclination) couple, four regimes of flow can be identified. We detailed a model for three of them, including a turbulent flow model. This model is then tested at higher Reynolds number in a new experiment at large scale. In this case the inertial range of turbulence is investigated

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Tera, Sridhar R. "Turbulence modeling of solar convection." abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1446423.

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Hoefsloot, Hubertus Cornelis Josef. "Marangoni convection under microgravity conditions." [S.l. : [Groningen : s.n.] ; University of Groningen] [Host], 1992. http://irs.ub.rug.nl/ppn/.

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Thayer-Calder, Katherine. "Downdraft impacts on tropical convection." Thesis, Colorado State University, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3565466.

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Downdrafts are an integral part of the convective cycle, and have been observed and documented for more than a hundred years. But many questions still surround convective downdrafts and their most difficult to observe properties. These questions have made the parameterization of convective downdrafts in global climate models (GCMs) very difficult. Designers of parameterizations have resorted to a wide range of assumptions and unverified hypotheses in their models of convective downdrafts.

In the last ten years, computing resources have advanced to a point where large domain, high resolution cloud resolving models (CRMs) can easily be run for long simulations. This study uses several simulations with 1 km horizontal resolution from the System for Atmospheric Modeling (SAM) v6.8.2 to examine convective downdrafts. We look at Radiative-Convective Equilibrium (RCE), a 21 day case from TOGA-COARE, Weak Temperature Gradient (WTG) simulations with varied shear profiles, and Lagrangian Parcel data to consider many difficult to observe properties of downdrafts.

We consider a variety of assumptions and questions that arise in the development of convective parameterizations. Our results show that downdrafts are an important mass flux in all simulations, and that cold pools organize convective systems and enhance updraft Convective Available Potential Energy (CAPE). We examine the ability for downdrafts to help couple deep convection to high relative-humidity regions in the tropics, and find that entrainment is likely a more important process in this relationship. We discuss the impact of downdrafts in maintaining boundary layer quasi-equilibrium, and find that, in our simulations, environmental entrainment has a larger impact on low-level most static energy. Finally, we show results from Lagrangian parcel data that illuminate our downdrafts as existing in an unsaturated state, with increasing buoyancy as they descend. We show that many of our downdrafts have positive buoyancy perturbations, suggesting the presence of warm downdrafts and under-shooting bottoms in heavily precipitating tropical systems.

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Kilroy, Gerard. "Numerical studies of tropical convection." Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-164143.

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Idealized numerical model experiments are presented to investigate the convective generation of vertical vorticity in a tropical depression. The calculations are motivated by observations made during the recent PREDICT field experiment to study tropical cyclogenesis, and by a desire to understand the aggregation of vorticity debris produced by deep convection in models of tropical cyclogenesis to form a monopole vortex. One aim is to isolate and quantify the effects of low to mid level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. Another aim is to isolate the effects of a unidirectional boundary layer wind profile on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. A third aim is to isolate the effects of a vortex boundary-layer wind profile on tropical deep convection, focussing especially on the morphology of vertical vorticity that develops. The growing convective updraughts, that are initiated by a near surface thermal perturbation, amplify locally the ambient rotation at low levels by more than an order of magnitude and this vorticity persists long after the updraught has decayed, supporting the results of an earlier study. The results of calculations with dry air aloft do not support a common perception that the dry air produces stronger downdraughts. In calculations where the vertical wind shear changes sign at some level near the top of the boundary layer, as occurs in warm-cored disturbances such as tropical depressions or tropical cyclones, it was found that the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This feature is quite unlike the structure in a typical middle-latitude `supercell' storm. These results provide an essential first step to understanding the interaction between deep convective elements in a tropical depression or tropical cyclone. An increase in the magnitude of boundary-layer shear was found to have the dual effect of weakening the development of the initial thermal, which is detrimental to vertical vorticity production by stretching and tilting, while at the same time increasing the magnitude of horizontal vorticity that can be tilted. The results provide a basis for appraising a recent conjecture concerning the role of storm splitting in explaining the contraction of the eyewall in tropical cyclones.

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Jenkins, D. R. "Nonlinear convection and solidification models." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372654.

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30

Bernoff, A. J. "Transitions from order in convection." Thesis, University of Cambridge, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383057.

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31

Julien, Keith Anthony. "Strong spatial resonance in convection." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.386110.

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Rust, Arlene Elizabeth. "Thermohaline convection in polar seawater." Thesis, Imperial College London, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.321658.

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Juel, Anne. "Magnetohydrodynamic convection in molten gallium." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389009.

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Jennings, Richard Leonard. "Stellar convection and dynamo theory." Thesis, University of Newcastle Upon Tyne, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.346446.

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Jones, Alison Tracey. "Combined convection in vertical ducts." Thesis, University of Leeds, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305521.

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Keen, D. J. "Combined convection in heat exchangers." Thesis, University of Leeds, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235252.

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Gray, M. E. B. "Geostrophic adjustment following deep convection." Thesis, University of Reading, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318585.

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Weeks, Mark Alexander. "Plane-layer convection and magnetoconvection." Thesis, University of Exeter, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.248180.

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Cole, Steven James. "Nonlinear rapidly rotating spherical convection." Thesis, University of Exeter, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.410805.

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Richardson, Chris N. "Mantle convection and melt generation." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263527.

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Halliday, Oliver John. "Atmospheric convection and gravity waves." Thesis, University of Leeds, 2018. http://etheses.whiterose.ac.uk/22414/.

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Novev, Yavor Kirilov. "Natural convection in electrochemical systems." Thesis, University of Oxford, 2018. http://ora.ox.ac.uk/objects/uuid:b8badcfd-e376-4ff6-b2da-b8f821871777.

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This thesis is concerned with modelling natural convective flows and specifically with their role in electrochemistry. The studies described here demonstrate that many electroanalytical techniques are prone to non-negligible natural convective effects, thus making the standard assumption for purely diffusional mass transport inapplicable. The chosen approach focusses on investigating idealized systems and establishing orders of magnitude for the quantities of interest. The complexity of the observed natural convective flows and their strong dependence on factors such as container geometry serve as compelling arguments for rigorously excluding natural convection in experimental measurements. The text is structured as follows. Chapter 1 introduces the theoretical framework used in the rest of the text and gives an outline of the electrochemical techniques to which the results in later chapters apply. Chapter 2 surveys the literature on natural convection in electrochemistry and emphasizes recent developments. Chapter 3 studies the natural convection induced by the intrinsic heat of an electrochemical reaction, specifically its effect on mass transport in chronoamperometry and cyclic voltammetry. Chapters 4-6 deal exclusively with coupled heat and momentum transport. Chapter 4 considers the thermal convective flows that arise in an idealized cell for scanning electrochemical microscopy (SECM) and the surrounding air under conditions of imperfect thermostating. Chapter 5 is dedicated to thermal convection in an SECM cell that is being thermostated from below through a solid substrate. This chapter demonstrates the influence of the spatial distribution of substrate thermal conductivity on the observed flows and highlights this effect by using a simpler model of the SECM cell than Chapter 4. Chapter 6 investigates the thermal convection in a novel thermostated cell for electrochemical measurements. Chapter 7 contains the main conclusions from the studies described in the thesis. Appendices A, B and C provide additional data for Chapters 3, 5 and 6, respectively.

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Carney, Thomas J. Ph D. (Thomas Joseph) Massachusetts Institute of Technology. "Convection enhanced electrochemical energy storage." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/120204.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 119-136).
Electrochemical energy storage will play a pivotal role in our society's energy future, providing vital services to the transportation, grid, and residential markets. Depending on the power and duration requirements of a specific application, numerous electrochemical technologies exist. For the majority of the markets, lithium-ion (Li-ion) batteries are the state-of-the-art technology owing to their good cycle life and high energy density and efficiency. Their widespread penetration, however, is limited by high production cost and inherent safety concerns. Understanding the solid-electrolyte interphase (SEI) which governs the performance and lifetime of these batteries is critical to developing the next generation Li-ion batteries. As an alternative to Li-ion, redox flow batteries store energy in solutions of electroactive species, which are housed in external tanks and pumped to a power-converting electroreactor. This configuration decouples power and energy, improving the safety and flexibility of the system, however, flow battery energy density is inherently lower than Li-ion and expensive ion-selective membranes are required for efficient operation. As a contrast to Li-ion and redox flow batteries, convection batteries harnesses the key benefits of Li-ion batteries and redox flow batteries while overcoming their individual limitations. By incorporating thick electrodes into the cell, the energy density is increased and the cost of the system is reduced. To overcome the diffusive losses in the thick electrodes, electrolyte is pumped through the electrodes, enabling uniform ion transport throughout the porous structure. However, thick electrodes can lead to large ohmic losses in the cell resulting in lower energy efficiency. In this thesis, I discuss my work on understanding the SEI in Li-ion batteries, highlighting the thermodynamics of its origin, characterization of its structure, and strategies for future development. I then detail my work understanding redox active molecules from molecule characterization and mechanistic generation to redox flow cell level engineering. Finally, I highlight my work in the development of the convection battery technology explaining the synthesis of active materials, thick electrode design, and fabrication of the prototype convection cell architecture. Taken together, these projects highlight the theme of achieving low-cost electrochemical energy storage through various technical pathways.
by Thomas J. Carney.
Ph. D.

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Mott, Richard William. "Problems in turbulent buoyant convection." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609998.

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Chiu-Webster, Sunny. "Horizontal convection and glass furnaces." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611923.

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Bascoul, Guillaume Pierre. "Double-diffusive convection in stars." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613006.

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Ihle, Bascuñán Christian. "Spatiotemporal Features of Natural Convection." Tesis, Universidad de Chile, 2011. http://repositorio.uchile.cl/handle/2250/102700.

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Esta tesis, consistente en una recopilación de artículos de investigación originales autocontenidos, se ocupa del estudio de los mecanismos físicos que explican algunas características de la dinámica de convección térmica con aplicación a convección penetrativa, frecuentemente observada en lagos y reservorios chilenos. Este fenómeno consiste en la aparición de un campo de flujo derivado del enfriamiento superficial de una masa de de fluido donde potencialmente puede existir una estratificación de densidad previa. Si bien este problema ha sido extensivamente estudiado empleando experimentos de pequeña escala (desde 1 mm hasta unos pocos centímetros), no es el caso para sistemas naturales de mayor tamaño, donde los flujos son comúnmente turbulentos y la dinámica asociada está además acoplada con perturbaciones espaciotemporales, incluyendo temperatura ambiente y vientos locales. El presente trabajo se ocupa de algunas de estas interrogantes, incluyendo las condiciones requeridas para la aparición de convección penetrativa bajo condiciones de borde térmicas que dependen del tiempo y suponiendo ausencia de viento. Primero, se consideró el caso más simple de un enfriamiento superficial repentino, modelado como una capa horizontal infinita, inicialmente en reposo, de fluido de Boussinesq. La siguiente fase de este estudio consistió en la elaboración de un modelo teórico simplificado, propuesto como una base para dar cuenta de la estabilidad de sistemas de pequeña escala frente a patrones de forzamiento térmico sinusoidales, buscando así un símil al efecto de enfriamiento vespertino o nocturno en lagos en los casos donde además hay turbulencia media nula antes del comienzo del flujo convectivo. Un segundo aspecto de este trabajo de tesis fue el estudio del efecto de la presencia de fuentes y sumideros térmicos cercanos. Para condiciones débiles de calentamiento y enfriamiento, se ha encontrado que el estudio de esta configuración es equivalente al estudio de la interacción entre plumas térmicas y corrientes de densidad en régimen laminar. Se ha perseguido los objetivos mencionados empleando una combinación de métodos, incluyendo simulaciones numéricas, técnicas analíticas de perturbación para el estudio de la estabilidad de los sistemas referidos modelados a través de las ecuaciones de Navier-Stokes y energía, además de la realización de experimentos. En este último caso, se propone una técnica de medición simultánea de los campos vectoriales de velocidad (usando PIV) y gradiente de densidad (usando schlieren sintético). La naturaleza inherentemente delicada de los experimentos llevados a cabo hizo necesario el desarrollo de sistemas de control ad-hoc. Como resultado de estas actividades, ha sido posible vincular las propiedades del fluido con parámetros adimensionales (incluyendo los números de Prandtl y Rayleigh), para dar cuenta de los tiempos de inicio de convección y frecuencia de forzamiento térmico en la superficie (entre otros). Del estudio de inhomogeneidades espaciotemporales, se encontró que las plumas térmicas bidimensionales laminares pueden sobrevivir el impacto con una corriente de gravedad modificando, sin embargo, su posición original.

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Juillet, Fabien. "Control of convection-dominated flows." Palaiseau, Ecole polytechnique, 2013. http://pastel.archives-ouvertes.fr/docs/01/00/94/63/PDF/PhD_Juillet_Fabien.pdf.

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Cette thèse s'intéresse à la mise en œuvre d'une technique de contrôle d'écoulements, d'un point de vue à la fois numérique et expérimental. L'objectif de cette technique est la réduction de perturbations au sein d'écoulements dominés par la convection. Dans ce but, trois aspects sont développés au sein d'un première partie. On observe tout d'abord que dans de tels systèmes l'information voyage essentiellement vers l'aval. Pour cette raison les perturbations doivent être mesurées le plus tôt possible, en plaçant les capteurs en amont. Cette idée intuitive est étudiée quantitativement en introduisant le concept de " longueur de visibilité ". Ensuite, une description de l'écoulement est obtenue à l'aide de technique d'identification de systèmes. Ces outils présentent l'avantage de construire des modèles en se fondant uniquement sur des données accessibles au sein d'une expérience. Enfin, une approche de contrôle du type feed-forward étant particulièrement appropriée pour ces écoulements, une comparaison théorique et numérique avec la théorie classique LQG (Linear Quadratic Gaussian) est menée. Dans une seconde partie, ces trois aspects sont pris en compte dans une procédure d'identification et de contrôle qui est simplifiée de manière à faciliter une mise en place expérimentale. En particulier, les réponses impulsionnelles du système sont identifiées puis utilisées directement dans le calcul de la loi de contrôle. Cette technique repose alors uniquement sur de simples minimisations par moindres carrés et présente l'avantage d'être fondée sur des quantités aux interprétations physiques claires, telles que des vitesses de convection ou des fréquences caractéristiques. Enfin, dans une dernière partie, la procédure est appliquée expérimentalement au contrôle de perturbations dans une écoulement de Poiseuille à Re =870. Dans cette expérience, l'amplitude du signal mesuré par un capteur objectif a pu être réduite de 45%
In this thesis, a flow control procedure is developed numerically and is then implemented experimentally. The purpose of this procedure is to reduce the amplitude of perturbations in convection dominated flows. To design such a technique three aspects are analyzed in a first part. Since information in convection-dominated flows essentially travel downstream, incoming perturbations are better described by placing sensors upstream. This intuitive idea is studied quantitatively by introducing the concept of visibility length. In addition, a description of the flow dynamics is obtained using system identification techniques. These tools have the advantage of providing models based solely on experimentally accessible data and are therefore directly applicable to real flows. Finally, a feed-forward control approach is found to be most appropriate and a comparison with the classical linear quadratic gaussian technique is presented from numerical and theoretical point of views. In a second part, these three aspects are then taken into account in the design of a feed-forward identification and control procedure, which is then simplified to be more amenable to practical implementations in experiments. In particular, the system impulse responses are first identified, and are then directly used for the computation of the control law. Hence, the technique only relies on simple least-squares minimizations and has the advantage of manipulating quantities that have clear physical meanings, such as perturbation convective speeds and characteristic frequencies. Thus, in a last part, the control procedure is applied experimentally to the quenching of natural disturbances in a plane channel flow at Re = 870. Results show that the magnitude of the signal recorded by the objective sensor can be reduced by up to 45%

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Kirshbaum, Daniel. "Shallow convection in orographic precipitation /." Thesis, Connect to this title online; UW restricted, 2004. http://hdl.handle.net/1773/10091.

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Al-Kiffai, Ameera Nema. "The role of convection on spreading speeds and linear determinacy for reaction-diffusion-convection systems." Thesis, Swansea University, 2015. https://cronfa.swan.ac.uk/Record/cronfa42766.

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This thesis is concerned with spreading speeds and linear determinacy for both discretetime recursion models Un+1 = Q[un] and reaction-diffusion-convection systems (PDE) under a co-operative assumption. In this thesis we are interested in the role of convection terms in propagation and linear determinacy. Such reaction-diffusion-convection systems have monotone travelling-wave solutions of the form w(x - ct) that describe the propagation of species as a wave with a fixed speed c, connecting two equilibria, a stable equilibrium beta and an unstable equilibrium 0 of the reaction term. The concept of spreading speeds was introduced by Aronson and Weinberger in [2] as a description of asymptotic speeds of spread, and in fact they showed that this spreading speed can be characterized as a minimal travelling wave speed. We discuss a characterization theory of spreading speeds of the PDE system in terms of critical travelling wave speeds. We present sufficient conditions involving both the reaction and convection terms of the PDE system for spreading speeds to equal values obtained from the linearization of the travelling-wave problem of the PDE system about the unstable equilibrium 0. These conditions guarantee the linear determinacy for the discrete-time recursion models and the PDE systems. As a result of the asymmetry in propagation that is caused by the convection terms in the PDE system, and a corresponding lack of reflection invariance in the abstract system un+1 = Q[un], we present separate conditions for non-increasing and non-decreasing initial data, called right and left conditions respectively, and we consider right and left spreading speeds. Weinberger, Lewis and Li in [42] allowed there to be more equilibria other than 0 and beta, in which case different components may spread at different speeds. This implies the need for both slowest and fastest spreading speeds, called right and left slowest (fastest) spreading speeds corresponding, to non-increasing and non-decreasing initial data respectively. We also give sufficient conditions on the reaction and convection terms such that right (left) slowest spreading speed equals right (left) fastest spreading speed for the PDE system, which implies that the system has a right (left) single spreading speed. Examples are included that illustrate the key propositions and theorems, for instance, the existence of reaction and (non-trivial) convection terms for which the right and left linear determinacy conditions are simultaneously satisfied, as well as a system that is right (left) linearly determinate in absence of convection terms, but it is not left linearly determinate in the presence of a convection term.

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Dissertations / Theses on the topic 'Convection'

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