Thesis (Ph. D.)--University of Washington, 2000. / Vita. Includes bibliographical references (p. 93-98).

Thesis (Ph. D.)--Colorado State University, Dept. of Atmospheric Science, 1997. / Includes bibliographical references (p. [179]-199).

De nombreux travaux indiquent qu’il est important d’étudier les impacts physico-chimiques de la convection profonde tropicale. Nous avons utilisé le modèle méso-échelle 3D non-hydrostatique CATT-BRAMS pour étudier le transport de traceurs dans la troposphère tropicale au-dessus de l’Amérique du Sud. J’ai effectué une validation de l’outil en complément d’une étude en saison sèche et dans les basses couches menée au CPTEC (Brésil). Les résultats obtenus dans ces travaux indiquent un comportement météorologique globalement correct. Le transport en résultant montre une sur-estimation du transport d’espèces chimiques dans la moyenne troposphère et une sous-estimation dans la haute troposphère. Cela vient d’un déclenchement trop fréquent de la convection restreinte, de la paramétrisation de la convection profonde et de la représentation de leurs interactions. Une adaptation du modèle pour la saison humide est nécessaire. A l’échelle locale des difficultés venant d’une sensibilité importante de la paramétrisation au relief sont rencontrées. Le CATT-BRAMS évolue vers un modèle avec chimie, le C-CATT-BRAMS. Les premiers résultats obtenus indiquent un fort impact de l’initialisation et des conditions aux limites pour les espèces NO et O3. Quelques soient l’initialisation ou les conditions aux limites utilisées, on observe une augmentation du rapport de mélange de ces espèces au cours du temps. Cela peut provenir d’une sur-estimation des émissions `a la surface dans le modèle, en particulier pour les méga-cités. Il est important de poursuivre la validation de cet outil afin de pouvoir étudier l’impact physico-chimique de la convection profonde tropicale avec ce modèle. / Many works show it is important to study the phyical and chemical impacts of tropical deep convection. We used the 3D mesoscale non-hydrostatic model CATT-BRAMS to study the tracers transport in the tropical troposphere above South America. I validated the tool parallel to a study done in CPTEC (Brazil) for the dry season and in the lower troposphere. The results obtained in this work indicate a globaly correct meteorological behaviour. The associated transport show an over estimation of the chemical species transport in mid-troposphere and an under estimation in the upper troposphere.This comes from a to frequent triggering of shallow convection, from the deep convection scheme and from the representation of their interactions. An adaptation of the model for the wet season is necessary. At local scale, difficulties because of a high deep convection scheme sensitivity to the orography are encountered. The CATT-BRAMS model evolve to a model with chemistry, the C-CATT-BRAMS. The first results obtained indicate a strong impact of initialisation and boudary conditions on species NO and O3. Whatever be the initialisation or the boundary conditions, we observe an increase of the mixing ratio along time for these species. This can come from an over estimation of surface emissions in the model, especialy for megacities. It is important to continue the validation of this tool in order to be able to study the physical and chemical impacts of tropical deep convection with this model.

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Cloud Resolving Models use microphysics parameterisation schemes for the simulation of clouds. The thesis reports on the introduction of two single-moment Bulk Microphysics Parameterisation (BMP) schemes in the Nonhydrostatic - coordinate Model (NSM). The rst BMP is known as the PURDUE-LIN scheme, and can be used with ve (excluding graupel) or six (including graupel) classes of the water substance. The second scheme was developed using the PURDUELIN scheme as a starting point, and is known as SBU-YLIN. Graupel and snow share a category and processes in the latter scheme. Simulations of two hours in length are made, with convection initiated through inserting a warm thermal into a cooler environment, using a six-class and ve-class PURDUE-LIN and the SBU-YLIN BMPs. The simulations are performed at various horizontal resolutions of 500 m, 1 km and 2 km. The six-class PURDUE-LIN scheme simulates more rainfall than the ve-class PURDUE-LIN and the SBU-YLIN schemes. The SBU-YLIN scheme generally rains the least, looses the least water vapour to hydrometeors and warms up the least. The PURDUE-LIN schemes simulate two convective cells in a no shear environment. The maximum updrafts associated with the rst cell (triggered by the warm perturbation) are similar in all the simulations. The second cell is triggered by a cold pool. While the cold pool is stronger in the six-class PURDUE-LIN scheme simulations, the updrafts in the second cell are stronger in the ve-class PURDUE-LIN simulation. The SBU-YLIN scheme generally simulates just one cell because of a weak cold pool. Simulations were also made for three di erent periods dominated by suppressed convection with deep convection at the beginning and end of the three periods, forced with large scale tendencies observed during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The NSM is able to capture di erences in the suppressed and deep convection periods. Qualitatively, the simulations provide new insight into the interplay between cloud microphysics and cloud dynamics, and points out the potential for better describing the uncertainty range associated with projections of future climate change, through the improvement and stochastic application of cloud microphysics schemes. / Thesis (PhD)--University of Pretoria, 2013. / gm2013 / Geography, Geoinformatics and Meteorology / unrestricted

This thesis concentrates on diffusion-ordered spectroscopy (DOSY): a technique in liquid state nuclear magnetic resonance (NMR) spectroscopy that is able to distinguish different chemical species by their size in solution, and measure species’ diffusion coefficients. DOSY is an invaluable tool to analyse and resolve the signals of mixtures of chemicals in solution. DOSY measures an ensemble of nuclei in a mixture, using pulsed field gradients (PFGs) to encode, and later decode, the positions of spins. This provides information on the root mean square distance that spins diffuse during a delay between a pair of PFGs, which allows the determination of diffusion coefficients. However, DOSY is susceptible to a number of complications. Where a sample convects, the distance molecules move during the delay is greater than for simple diffusion, and the estimate of the diffusion coefficient becomes greater than the true value. Rayleigh-Bénard convection has been known in NMR for a long time, and is known to result from heating the base of an NMR sample, giving rise to negative vertical temperature gradients. Here, cooling the base (and inadvertently the sides) of the sample is also shown to cause convection from transverse temperature gradients, known as Hadley convection and commonly observed in atmospheric circulation. The theory for convection from transverse temperature gradients is given and recommendations to reduce convection in NMR are made. DOSY analysis can also be challenging where signals from different species overlap. In such instances, it can help to reduce the overlap, which can be achieved by collapsing multiplets into singlet signals: pure shift NMR is a technique that uses band-selection in order to refocus scalar coupling during the free induction decay, leaving pure chemical shift information. However, PFGs can vary in strength across the sample, which can have an effect on band selective experiments, such as Zangger-Sterk pure shift experiments, in the measured peak amplitudes, and where DOSY is combined with the Zangger-Sterk pure shift sequence, the apparent diffusion coefficients are also altered, seemingly as a function of chemical shift. A correction for this is given, simply using a DOSY and a DOSY pure shift experiment, as an alternative to the current literature method (gradient mapping).An investigation into an emulsion, agrochemically important for the delivery of active ingredients (fertiliser,pesticides, etc.) to plants, was then undertaken, in the hopes of finding restricted diffusion, but instead discovering substantial chemical exchange on the diffusional timescale. Finally a program was written for the automated acquisition of DOSY data, without the need for a human controller to calibrate DOSY parameters for the optimum acquisition.

In gas quenching, metal parts are rapidly cooled from hightemperatures, and the convection heat transfer coefficientdistributions are of importance for the hardness and thedistortion (the shape nonuniformities) of the quenched parts.Thermochromic liquid crystals (TLC) and a thin foil techniques,were investi- gated and used for studies of a circular cylinderin axial flows, affected and not affected by upstream owmodifying inserts. Quadratic prisms in cross ows were alsostudied, a single prism, two prisms arranged in-line, and forfour prisms arranged in a square pattern. In this study,particle image velocime- try (PIV) was used for visualizationof the flow, giving physical insight to the convection heattransfer data. Further, relations of the typeNu=CReewere established. The TLC and thin foil techniques werealso used to indicate the dimensions of separated flowregions. Descriptors:Fluid mechanics, wind-tunnel, turbulence,gas quenching, con- vection heat transfer, thermochromic liquidcrystals, calibration, temperature measurement errors, thinfoils, particle image velocimetry, cylinder in axial flow, flowmodifying inserts, quadratic prisms in cross flow

No description available.

In this work we studied the convective heat transfer in a magnetic fluid in both zero and applied magnetic fields. The natural convection is observed in a quasi-one dimensional magnetic fluid in a horizontal temperature gradient. The horizontal non-homogeneous magnetic fields were applied across the sample cell either parallel or anti-parallel to the temperature gradient. The temperature profile was measured by eight thermocouples and temperature sensitive paint. The flow velocity field and streamlines were obtained by optical flow method. Calculated Nusselt numbers, Rayleigh number, and Grashof number show that the convective flow is the main heat transfer mechanism in applied fields in our geometry. It was found that when the field gradient is parallel with temperature gradient, the fields enhance the convective heat transfer while the fields inhibit it in anti-parallel configuration by analyzing the temperature difference across the sample, flow patterns, and perturbation Q field in applied fields. Magnetic Rayleigh number and magnetic Grashof number show that the thermomagnetic convections dominate in high magnetic fields. It is shown that the physical nature of the field effect is corresponding to the magnetic body force which is perpendicular to the gravity in our experiments. When the direction of the magnetic body force is same with temperature gradient in parallel configuration, the body force increases the convective heat transfer; while it has opposite effect in anti-parallel configuration. Our study will not only shed light on the fundamental mechanisms for thermomagnetic convection but also help to develop the potential field-controlled heat transfer devices.