Sasol's Research and Development Division has identified several proprietary gas-liquid reactions where very low superficial gas velocities « 0.8 cmls) are required to obtain desired conversions in a bubble column reactor. A review of existing literature has shown that research in bubble column reactors is typically conducted in the superficial gas velocity range of 1 - 40 cmls. Traditionally bubble column reactors are designed via the application of empirical correlations which are only valid under specific conditions. There is a danger of under or over design if incorrect nonadjustable parameters such as liquid dispersion coefficients, mass transfer coefficients and gas hold-up values are used. To this extent, a hydrodynamic study was undertaken at superficial gas velocities lower than 0.8 cmls, to determine whether existing correlations are valid in this little investigated superficial gas velocity regime. Three bubble column reactors were designed and set up to perform hydrodynamic studies: • 22 cm inner diameter QVF glass column, 190 cm tall • 30 cm inner diameter 304 stainless-steel column, 200 cm tall • 30 cm inner diameter QVF glass column, 80 cm tall All measurements were undertaken in an air/water system. Gas hold-up measurements revealed that at the investigated gas flow rates, the gas hold-up was less than 1 % and as such was not investigated extensively. Partition plates were installed into the bubble columns and residence time distribution measurements were undertaken. The bubble columns were found to behave identically to the well known tanks in series model (Levenspiel, 1962). Liquid dispersion coefficients were measured via two methods. Batch liquid measurements were undertaken via the method of Ohki and Inoue (1970) and continuous liquid residence time distribution measurements were also undertaken. Data reduction was performed for both methods using the axial dispersion model to regress the liquid dispersion coefficient EL_ Both methods yielded equivalent results. The effect of distributor plate geometry on EL was also investigated and proved not to affect EL. It was found that existing literature correlations developed at higher superficial gas velocities failed to accurately predict the measured dispersion coefficients obtained in this study_ Correlation of the EL values with column diameter and superficial gas velocity showed EL to be a weak function of diameter as compared to existing correlations. This will have a significant effect on scale-up to larger column diameters. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

is shown to follow an exponential distribution. These three models enable operators to explore the impact of various inputs prior to programming the vehicle, thus allowing them to choose the best combination of vehicle parameters that minimize the offset error between the reported and actual locations.

Approved for public release; distribution is unlimited / Naval operations are highly dependent upon environmental conditions that can either adversely affect successful completion or hinder the safety of personnel. Each warfare community has defined environmental thresholds and operating limits that restrict the execution of any intended maneuver. As the warfare environment continues to shift from open ocean to the littoral, environmental prediction and modeling efforts of the shallow water surroundings need to be developed in order to support these operations. A hydrodynamic model, Water Quality Management and Analysis Package (WQMAP), has been developed by Applied Sciences Associates, Inc. that is designed specifically to provide accurate littoral environmental prediction. WQMAP is one of several hydrodynamic models used operationally by the Naval Oceanographic Office (NAVOCEANO) to predict currents and water elevations in littoral regions. Implementations of shallow water hydrodynamic models in foreign waters are usually data-starved for model forcing and validation. In a series of studies, NAVOCEANO intends to model various bays within the continental United States, where sufficient data exists, to study the sensitivity of lack of data on model results. This study will utilize WQMAP to design a hydrodynamic model in San Diego Bay to predict currents in order to investigate the impact of grid resolution on model results, and to provide proper current predictions for Fleet training and operations. / Lieutenant, United States Navy

We study the formation of planetesimals in protoplanetary disks from the gravitational collapse of solid over-densities generated via the streaming instability. To carry out these studies, we implement and test a particle-mesh self-gravity module for the ATHENA code that enables the simulation of aerodynamically coupled systems of gas and collisionless self-gravitating solid particles. Upon employment of our algorithm to planetesimal formation simulations, we find that (when a direct comparison is possible) the ATHENA simulations yield predicted planetesimal properties that agree well with those found in prior work using different numerical techniques. In particular, the gravitational collapse of streaming-initiated clumps leads to an initial planetesimal mass function that is well-represented by a power law, dN / dM(p) proportional to M-p(-p), with p similar or equal to 1.6 +/- 0.1, which equates to a differential size distribution of dN / dR(p) proportional to R-p(-q), with q similar or equal to 2.8 +/- 0.1. We find no significant trends with resolution from a convergence study of up to 512(3) grid zones and N-par approximate to 1.5 x 10(8) particles. Likewise, the power-law slope appears indifferent to changes in the relative strength of self-gravity and tidal shear, and to the time when (for reasons of numerical economy) self-gravity is turned on, though the strength of these claims is limited by small number statistics. For a typically assumed radial distribution of minimum mass solar nebula solids (assumed here to have dimensionless stopping time tau = 0.3), our results support the hypothesis that bodies on the scale of large asteroids or Kuiper Belt Objects could have formed as the high-mass tail of a primordial planetesimal population.

With the ever present desire for ships and boats to run faster while carrying a greater load, a need exists to reduce the drag while simultaneously increasing hydrodynamic lift. Therefore, a need for semi-planing/semi-displacement hullforms exists for vessels to carry relatively high loads (between 500 and 3000 tons) with a general length Froude number range between 0.4 and 1.0. A hybrid method for calculating the lift and drag of semi-planing/semi-displacement hull forms is developed. This is done by separating the kinematic boundary condition into odd and even parts. The odd and even parts of the kinematic boundary condition are solved independently along with the free-surface boundary condition and superimposed for a complete "hybrid" solution. The superimposed solution components relate to Michell's (1898) "thin ship" integral for odd flow and Maruo's (1967) "flat ship" integral for even flow. A generalized form of Michell's (1898) integral is provided for high speed slender bodies by implementing a more realistic near field condition (Ogilvie, 1975) and a wake trench (Vorus, 2009). A generalized form of Maruo's (1967) integral has also been developed. Comparisons of the generalized methods have been made with available model test and/or analytical data. With this, the concept of the Semihull (Vorus, 2005) is revisited. Some results are given concerning the validity of the Semihull as compared to a traditional displacement ship. Hull form optimization is also explored and the deadrise angle distribution proves to be a major factor in calm water hydrodynamic performance.

Sunspots and active regions are two of the many manifestations of the solar magnetic field. This field plays an important role in causing phenomena such as coronal mass ejections, flares, and coronal heating. Therefore, it is important to study the origin of sunspots and active regions and determine the underlying mechanism which creates them. It is believed that flux tubes rising from the bottom of the convection zone can create sunspots. However, there are still unanswered questions about this model. In particular, flux tubes are expected to expand as they rise, hence their strength weakens and some sort of reamplification mechanism must complement this model to match the observational properties of sunspots. To compensate for the absence of such an amplification mechanism, the field strength of the flux tubes, when at the bot- tom of the convection zone, must be far stronger than present dynamo models can explain. In the last few years, there has been significant progress toward a new model of magnetic field concentrations based on the negative effective mag- netic pressure instability (NEMPI) in a highly stratified turbulent plasma. NEMPI is a large-scale instability caused by a negative contribution to the total mean-field pressure due to the suppression of the total turbulent pressure by a large-scale magnetic field. In this thesis, I study for the first time NEMPI in the presence of a dynamo-generated magnetic field in both spherical and Carte- sian geometries. The results of mean-field simulations in spherical geometry show that NEMPI and the dynamo instability can act together at the same time such that we deal with a coupled system involving both NEMPI and dynamo effects simultaneously. I also consider a particular two-layer model which was previously found to lead to the formation of bipolar magnetic structures with super-equipartition strength in the presence of a dynamo-generated field. In this model, the turbulence is forced in the entire domain, but the forcing is made helical in the lower part of the domain, and non-helical in the upper part. The study of such a system in spherical geometry showed that, when the stratification is strong enough, intense bipolar regions form and, as time passes, they expand, merge and create giant structures. To understand the underlying mechanism of the formation of such intense, long-lived bipolar structures with a sharp boundary, we performed a systematic numerical study of this model in plane parallel geometry by varying the magnetic Reynolds number, the scale separation ratio, and Coriolis number. Finally, I investigate the formation of the current sheet between bipolar regions and reconnection of oppositely orientated magnetic field lines and demonstrate that for large Lundquist numbers, S, the reconnection rate is nearly independent of S – in agreement with recent studies in identical settings.

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This thesis is an experimental, analytical and numerical investigation of pipe flows started impulsively from rest and thereafter accelerated exponentially with time. The broad objectives were to reconcile measured base flow and transition inception data with linear stability predictions, [Abbreviated Abstract. Open document to view full version] / A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, June 1995.

A seletividade e eficiência do processo de flotação não estão fundamentadas tão somente nas diferenças das propriedades físico-químicas das superfícies dos minerais, mas também na hidrodinâmica do sistema de concentração. Via de regra, a eficácia da flotação diminui quando o tamanho de partícula ultrapassa 150 micrômetros. Esta pesquisa visou estudar a influência de variáveis hidrodinâmicas sobre mecanismos de flotação de partículas grossas em células mecânicas e suas implicações cinéticas. Os ensaios utilizaram células com agitação mecânica e cada situação experimental esteve associada a um conjunto de variáveis hidrodinâmicas: números de hidrodinâmicos adimensionais, características geométricas do impelidor, velocidade mínima de suspensão da polpa, energia dissipada média e energia de preservação do agregado partícula-bolha. Desta forma, a influência dos parâmetros hidrodinâmicos na flotação de apatita e esferas de vidro (diâmetro médio248 micrômetros) foi determinada na presença de oleato de sódio e acetato de eteramina em pH 10. Em condições hidrodinâmicas pouco turbulenta, a recuperação de partículas grossas decresce em função da falta de suspensão de partículas, por outro lado para condições hidrodinâmicas mais severas a recuperação foi quase nula devido à destruição do agregado partícula/bolha. A cinética de flotação da apatita foi estudada em escala de laboratório sob severas condições de turbulência, os resultados indicaram que partículas grossas de apatita têm flotabilidade inferior a finas, corroborando que a flotação dessas partículas grossas não se ajustou ao modelo de primeira ordem. Estes resultados evidenciaram que grossas demandam um ambiente hidrodinâmico apropriado para flotar eficientemente. / Coarse particles badly float mostly because the bubbleparticle aggregates are not strong enough to prevent the particle detachment from the bubble surface caused by the particle weight and turbulence eddies during the rise of the aggregates in the pulp phase to the froth phase and then to the froth launder. The aim this work was to determine effects and degrees of significance of hydrodynamic variables on coarse particle flotation and their kinetic implications. The tests were carried out in flotation cells with mechanical stirring system and each experimental condition was related to group of hydrodynamic variables: hydrodynamic dimensionless numbers, impeller geometries, minimum impeller rotational speed, energy dissipation rate and detachment energy. The influence of hydrodynamic parameters on flotation of the apatite and glass spheres (diameter average248 micrometers) was determined in the presence of sodium oleate and etheramine acetate at pH 10. For more quiescent hydrodynamic conditions, recovery of coarse particles decreased due to a lack in particle suspension, on the other hand, in the most severe hydrodynamic conditions the recovery was almost nil because the bubbleparticle aggregates detachment. Flotation kinetics of apatite was studied in laboratory scale under strong hydrodynamic conditions, the results indicated that coarse particles of apatite float at a lower rate than finer particles and the flotation kinetics of coarse particles did not fit to first order model. Those results evidenced that coarse particles demand a suitable hydrodynamic environment to float efficiently.

The dissertation presented in this manuscript contributes to river science by providing a detailed overview on the state of the art on the interaction between riparian vegetation and hydrogeomorphological processes, by devising a novel model encompassing most of such processes and by proposing a field methodology aimed at providing means for improving the modelling of such interactions. The state of the art is summarized in an extensive review describing riparian vegetation and hydrogeomorphological processes mutual feedbacks. Such review did not simply seek to describe these feedbacks but, compiling from a large array of results from field, laboratory and modelling studies, provides a set of physical thresholds that trigger system changes. Therefore, processes are not only described terms but also explained with a quantitative approach. Processes description provided the conceptual foundation for the development of the novel simulation model while model parameterization was based on the quantitative information collected in the review. Such novel model, encompasses the main relationships entwining riparian woody vegetation and hydrogeomorphological processes and is able of replicating long term riparian landscape dynamics considering disturbance events, environmental stressor and riparian woody vegetation establishment from seeds and large wood. The manuscript presents the model structure and its conceptual validation by means of hydrological scenarios aimed at testing the coherence of the simulation results with expected system behaviour. Examples of such coherences are vegetation growth rate in response to hydrological regime, entrainment and establishment of large wood in an unconfined river system and vegetation effect on erosion and deposition patterns. Analysis of sedimentation patterns from the modelled results suggested that vegetation flow resistance should be modelled with greater detail. These conclusions pointed the dissertation research towards the testing of a novel class of vegetation flow resistance equations, proposed by different authors, able of describing woody vegetation flow resistance on a physical basis. These equations have the advantage of considering flow stage, plants foliation level and species-specific flexibility. However, the use of such equations is limited by the difficulty of measuring the vegetation properties required as equation-inputs. In order to test if these equations could effectively improve sediment dynamics predictions, a field method was formulated and tested. The field method allows to sample vegetation properties that can be used with these novel class of flow resistance equations. In the manuscript, such method is applied and the resulting vegetation properties used in several modelling scenarios. Such scenario proved that hydraulic variables modelled with these novel flow resistance approaches are more realistic and thus that the model developed during the dissertation could benefit from inclusion of such flow resistance equations in its source code.