We present the first detailed 3D hydrodynamic implicit large eddy simulations of turbulent convection of carbon burning in massive stars. Simulations begin with radial profiles mapped from a carbon-burning shell within a 15M circle dot 1D stellar evolution model. We consider models with 128(3), 256(3), 512(3), and 1024(3) zones. The turbulent flow properties of these carbon-burning simulations are very similar to the oxygen-burning case. We performed a mean field analysis of the kinetic energy budgets within the Reynolds-averaged Navier-Stokes framework. For the upper convective boundary region, we find that the numerical dissipation is insensitive to resolution for linear mesh resolutions above 512 grid points. For the stiffer, more stratified lower boundary, our highest resolution model still shows signs of decreasing sub-grid dissipation suggesting it is not yet numerically converged. We find that the widths of the upper and lower boundaries are roughly 30 per cent and 10 per cent of the local pressure scaleheights, respectively. The shape of the boundaries is significantly different from those used in stellar evolution models. As in past oxygen-shell-burning simulations, we observe entrainment at both boundaries in our carbon-shell-burning simulations. In the large Peclet number regime found in the advanced phases, the entrainment rate is roughly inversely proportional to the bulk Richardson number, Ri(B) (alpha Ri(B)(-alpha) a, 0.5 less than or similar to alpha less than or similar to 1.0). We thus suggest the use of Ri(B) as a means to take into account the results of 3D hydrodynamics simulations in new 1D prescriptions of convective boundary mixing.

In this work, we use the recently developed equilibrium generating functional and systematic derivative expansion approach to hydrodynamics to explore three new regimes of relativistic hydrodynamics. First, we derive the equations of motion and write the constitutive relations to first order in derivatives for relativistic fluids coupled to an external vector field. Next, for relativistic fluids in strong magnetic fields B ~ O(1), we derive the equations of motion and present the constitutive relations to first order in derivatives. From the resulting system of equations, we find the hydrodynamic modes for these systems. We also find the constraints on the transport coefficients due to the entropy production argument and derive the corresponding Kubo formulas. Finally, we repeat the same analysis for relativistic fluids coupled to dynamical electromagnetic fields with <B> ~ O(1). / Graduate

Cette thèse traite de la dynamique des jets et projections liquides faisant suite à l'effondrement de cavités à la surface libre. Une large gamme d'échelle sera considérée : des cavités submillimétriques à décimétriques. La dépendance de la vitesse d'éjection du jet avec la taille de cavité et les propriétés du liquide nous permet de déterminer les lois d'échelle pertinentes et ainsi d'extraire l'essence des mécanismes physiques en jeu. Dans une première partie, les jets sont créés par l'éclatement de petites bulles : la force motrice qui permet de combler la cavité est alors la capillarité. Nous avons mis en évidence les rôles contre-intuitifs de la gravité et de la viscosité. Les aérosols éjectés par ce mécanisme ont ensuite été caractérisés, de leurs tailles à leurs évaporations. Dans une seconde partie, le pendant gravitaire à été considéré via une expérience modèle permettant l'étude de jets issus de cavités centimétriques sculptées à l'air comprimé. Une dynamique originale traduisant une focalisation de liquide et une dissipation localisée à la base du jet a été mise en évidence. Ces jets modèles ont ensuite été comparés à ceux issus de l'impact d'une bille dans un liquide visqueux. Dans la troisième partie, le champ de vitesse dans les jets étirés évoluant dans un régime inertiel a été étudié expérimentalement et théoriquement. La vitesse inertielle en z/t (z la position dans le jet et t le temps) communément utilisé a été caractérisée en fonction de l'alimentation en liquide à la base du jet. / This thesis deals with the dynamics of jets and liquid projections occuring when a cavity collapses at a free surface. A full range of scales will be investigated: from submilimetric cavitis to decimetric ones. The jet velocity dependence with cavity size and liquid properties allow us to establish the right scaling laws and thus to point out the relevant physical mecanisms at play. In a first chapter, jets are created by the bursting of small bubbles: the cavity collapses because of capillary forces. We highligthed the counterintuitive role of both gravity and viscosity. This mecanism gives birth to aerosols, which have been characterized from their size to their evaporation. In a second chapter, we considered the gravity-fed equivalent: a model experiment was set up to study jets from relaxing centimetric cavity shaped by blowing air at a free surface. We evidenced an unusual dynamics revealing the liquid focalisation in the jet and a localized dissipation at the base of the jet. A comparison of those model jets with the ones ejected after a solid objet impacts a viscous liquid was then performed. In the third chapter, we studied experimentally and theoretically the velocity field of inertial stretched jets. The inertial velocity z/t (with z the position in the jet et t the time) commonly used has been investigated depending on the jet feeding at its base.

La rotation a un impact majeur sur la structure et l'évolution des étoiles. En particulier, elle est connue pour être responsable de processus de mélanges macroscopiques des éléments chimiques et de transport de moment cinétique au sein des zones radiatives des étoiles. Dans la première partie de cette thèse, nous montrons comment l'état actuel de la modélisation stellaire justifie une nouvelle approche bi-dimensionnelle qui ne repose pas sur les hypothèses usuelles de rotation faible ou de rotation différentielle sphérique. Nous développons, dans la deuxième partie, un modèle simplifié (approximation de Boussinesq) d'étoiles en rotation rapide, en deux dimensions, où la rotation différentielle qui s'instaure et la circulation méridienne associée sont calculées de manière cohérente. Nous y identifions les paramètres pertinents à la description de l'écoulement induit par une contraction gravitationnelle dans un environnement stratifié de manière stable. Dans la troisième partie, nous démontrons que cet écoulement de spin-up l'emporte sur l'écoulement barocline à l'issue d'un temps de Kelvin-Helmholtz. La rotation différentielle adopte un profil universel cylindrique et la circulation méridienne est celle d'un écoulement de spin-up. Une couche de Stewartson s'établit aussi sur le cylindre tangent au noyau du modèle et pourrait être la source d'un couplage efficace de la rotation du noyau et de celle de l'enveloppe d'une étoile en fin de Séquence Principale. Dans la dernière partie de cette thèse, nous étudions Achernar, étoile en rotation rapide de type Be à l'aide du code compressible ESTER. Les modèles obtenus tendent à montrer que l'étoile est en contraction gravitationnelle post-Séquence Principale. Pour en rendre compte, le code ESTER a été modifié afin de suivre l'évolution chimique de l'étoile sur une échelle de temps nucléaire. / Rotation greatly impacts stellar structure and evolution. Particularly, it is known to be responsible of macroscopic mixings of chemical elements and transport of angular momentum within the radia­tive zones of stars. In the first part of this thesis, we show how the actual state of stellar modeling calls for a bi-dimensional approach going beyond usual assumptions of slow rotation and spherical differential rotation. We develop, in the second part of this thesis, a simplified model (Boussinesq approximation) of rapidly rotating stars in two dimensions, where the establishing differential rotation and associated meridional circulation are solved self-consistently. We describe the relevant parameters of the flow induced by a gravitational contraction in a stably stratified environment. In the third part, we demonstrate that this spin-up flow outweighs the baroclinic flow on a Kelvin-Helmholtz timescale. The differential rotation adopts an universal cylindrical profile and the meridional circulation is typical of the spin-up flow. A Stewartson layer appears too upon the tangent cylinder to the core and could explain an efficient coupling between the rotation of the core and the one of the envelope for stars at the end of the Main Sequence. In the last part of this thesis, we study Achernar, a rapidly rotating Be star, with the full-compressible ESTER code. Models we obtain tend to show that the star is undergoing a post Main Sequence gravitational contraction. To account this, the ESTER code has been modified to follow the chemical evolution of stars on a nuclear timescale.

Fluorine gas is produced industrially by electrolysing hydrogen fluoride in a potassium acid fluoride electrolyte. Fluorine is produced at the carbon anode, while hydrogen is produced at the mild-steel cathode. The fluorine produced has a wide range of uses, most notably in the nuclear industry where it is used to separate 235U and 238U. The South African Nuclear Energy Corporation (Necsa) is a producer of fluorine and requested an investigation into the hydrodynamics of their electrolysis cells as part of a larger national initiative to beneficiate more of South Africa’s large fluorspar deposits. Due to the extremely corrosive and toxic environment inside a typical fluorine electrolysis reactor, the fluid dynamics in the reactor are not understood well enough. The harsh conditions make detailed experimental investigation of the reactors extremely dangerous. The objective of this project is to construct a model that can accurately predict the physical processes involved in the production of fluorine gas. The results of the simulation will be compared to experimental results from tests done on a lab-scale reactor. A good correlation between reality and the simulacrum would mean engineers and designers can interrogate the inner operation of said reactors safely, effortlessly and economically. This contribution reports a time-dependent simulation of a fluorine-producing electrolysis reactor. COMSOL Multiphysics was used as a tool to construct a two dimensional model where the charge-, heat-, mass- and momentum transfer were fully coupled in one transient simulation. COMSOL is a finite element analysis software package. It enables the user to specify the dimensions of his/her investigation and specify a set of partial differential equations, boundary conditions and starting values. These equations can be coupled to ensure that the complex interaction between the various physical phenomena can be taken into account - an absolute necessity in a model as complex as this one. Results produced include a set of time dependent graphics where the charge-, heat-, mass- and momentum transfer inside the reactor and their development can be visualized clearly. The average liquid velocity in the reactor was also simulated and it was found that this value stabilises after around 90 s. The results of each transfer module are also shown at 100 s, where it is assumed that the simulation has achieved a quasi-steady state. The reactor, on which the model is based, is currently under construction and will be operated under the same conditions as specified in the model. The reactor, constructed of stainless steel, has a transparent side window through which both electrodes can clearly be seen. Thus the bubble formation and flow in the reactor can be studied effectively. Temperature will be measured with a set of thermocouples imbedded in PTFE throughout the reactor. The electric field will similarly be measured using electric induction probes. / Dissertation (MEng)--University of Pretoria, 2012. / Chemical Engineering / unrestricted

In this thesis we address the problem of understanding the star formation process in giant molecular clouds in a galactic context. Most simulations of molecular clouds to date use an oversimplified set of initial conditions (turbulent spheres/boxes or colliding flows). Full galactic scale models are able to generate molecular clouds with complex morphologies and velocity fields but they fail to reproduce in detail the effects that occur at sub-pc scales (e.g. stellar feedback). Our goal is to build the bridge between these two scenarios, and to model the star formation process in molecular clouds produced in a galactic context. We extract our molecular clouds from full-scale galactic simulations, hence we need to increase the resolution by two orders of magnitude. We introduce the details of the program used to simulate molecular clouds in Chapter 2, and describe in detail the method we follow to increase the resolution of the galactic clouds. In Chapter 3 we compare our simulated galactic clouds with the more conventional approach of using turbulent spheres. We create turbulent spheres to match the virial state of three galactic clouds. We perform isothermal simulations and find that the velocity field inherited from the full-scale galactic simulations plays an important role in the star formation process. Clouds affected by strong galactic shear produce less stars compared with clouds that are compressed. We define (and test) a set of parameters to characterise the dynamical state of our clouds. To include stellar feedback in our simulations we need to introduce a cooling/heating algorithm. In Chapter 4 we analyse how the different velocity fields of our clouds change the temperature distribution even in the absence of feedback. To study the formation of molecules we need to model the chemistry of H2 in our clouds. We also add CO chemistry, and produce synthetic observations of our clouds. The effect of feedback from winds and supernovae in galactic clouds is studied in Chapter 5. We analyse the effect of winds in clouds with very different velocity fields. We find that the effect of winds is stronger in highly virialised, high star forming clouds, with clouds with weak galactic shear, compared to unbound shear-dominated clouds. The steady and continuous action of the winds appears to have a greater effect than the supernovae. In summary, the inherited properties from the galaxy have an impact on many relevant processes in star formation, influencing gravitational collapse, the formation of filamentary structures, the temperature field of the cloud, and have a considerable effect on the impact of feedback in the clouds.

The knowledge of flow hydrodynamics within the next decades is of particular importance in many practical applications. In this study, a methodological improvement has been made to the evaluation of the flow hydrodynamics under climate change. This research, indeed, proposes an approach which includes the methods that can consider the climate change impact on the flow in estuaries, gulfs, etc. It includes downscaling methods to project the required climate variables through the next decades. Here, two statistical downscaling methods, namely, Nearest Neighbouring and Quantile-Quantile techniques, are developed and implemented in order to predict the wind speed over the study area. Wind speed has an essential role in flow field and wave climatology in estuaries and gulfs. In order to make the proposed methodology computationally efficient, the flow in the estuary is simulated by a large-scale model. The finite volume triangular C-grid is analysed and shown to have advantages over the rectangular (finite difference) one. The dispersion relation analysis is performed for both gravity and Rossby waves that have crucial effects in oceanic models. In order to study the unstructured characteristic of the triangular grids, various isosceles triangles with different vertex angles are considered. Moreover, diverse well-known second-order time stepping techniques such as Leap-Frog, Adams-Bashforth and improved Euler are studied in combination with the C-grid semi discrete method. The fully discrete method is examined through several numerical experiments for both linear and non-linear cases. The results of the large-scale model provide the boundary conditions to the local coastal model. In order to model the flow over a local coastal area, a well-balanced positivity preserving central-upwind method is developed for the unstructured quadrilateral grids. The quadrilateral grid can effectively simulate complex domains and is shown to have advantages over the triangular grids. The proposed central-upwind scheme is well-balanced and preserve the positivity. Therefore, it is capable of modelling the wetting and drying processes that may be the case in many local coastal areas. It is also confirmed that the proposed method can well resolve complex flow features. The local model incorporates the outputs of the downscaling and large-scale flow models and evaluates the flow hydrodynamics under changing climate.

Shippagan Gully is a highly dynamic tidal inlet located on the Gulf of St-Lawrence near Le Goulet, New Brunswick. This tidal inlet is highly unusual due to the fact that the inlet has two open boundaries with phase lagged tidal cycles that drives flow through the inlet. Over the past few decades, the shipping activities through the inlet have been threatened due to the narrowing of the navigation channel caused by deposited sediment on the east side of the channel. Many engineering projects have been undertaken at Shippagan Gully in order to try and mitigate the deposition problem. However, these attempts have either been unsuccessful or the engineered structures have deteriorated over the years. This study uses the CMS-Flow and CMS-Wave numerical models to provide guidance concerning the response of the inlet to various potential interventions aimed at improving navigation safety.

This PhD thesis reports an investigation into medical coronary stent cutting using three different types of lasers and associated physical phenomena. This study is motivated by a gap in the current knowledge in stent cutting identified in an extensive literature review. Although lasers are widely used for stent cutting, in general the laser technology employed is still traditionally based on millisecond pulsed Nd:YAG lasers. Although recent studies have demonstrated the use of fibre lasers, picosecond and femtosecond lasers for stent cutting, it has been preliminary studies.To further understand the role of new types of lasers such as pulsed fibre lasers, picosecond and femtosecond pulsed lasers in stent cutting, these three lasers based stent cutting were investigated in this project. The first investigation was on a new cutting method using water assisted pulsed (millisecond) fibre laser cutting of stainless steel 316L tubes to explore the advantages of the presence of water compared to the dry cutting condition. Significant improvements were observed with the presence of water; narrower kerf width, lower surface roughness, less dross attachment, absence of backwall damage and smaller heat affected zone (HAZ). This technique is now fully commercialised by Swisstec, an industrial project partner that manufactures stent cutting machines.The second investigation used the picosecond laser (with 6 ps pulse duration in the UV wavelength range) for cutting nickel titanium alloy (nitinol) and platinum iridium alloy. The main achievement in this study was obtaining dross-free cut as well as clean backwall, which may eliminate the need for extensive post-processing. Picosecond laser cutting of stents is investigated and reported for the first time. The third area of investigation was on the use of a femtosecond laser at 100 fs pulse duration for cutting nickel titanium alloy tubes. It was found that dry cutting degraded the cut quality due to debris and recast formation. For improvement, a water assisted cutting technique was undertaken, for the first time, by submerging the workpiece in a thin layer of water for comparison with the dry cutting condition. The final part of the thesis presents a three dimensional numerical model of the laser micromachining process using smoothed particle hydrodynamics (SPH). The model was used to provide better understanding of the laser beam and material interaction (with static beam) including the penetration depth achieved, phase changes, melt ejection velocity, also recast and spatter formation. Importantly, the model also simulated the wet machining condition by understanding the role of water removing the melt ejected during the process which avoided backwall damages. Results with the fibre laser in millisecond pulse duration were used for the validation purposes. The conclusions reached in this project and recommendations for future work are enclosed.The work has resulted in the publication of 3 journal papers and 2 additional journal paper submissions.

Cette thèse a pour objectif de détecter les structures turbulentes aux grandes échelles présentes dans une couche de mélange de type Rayleigh-Taylor incompressible à faible nombre d'Atwood. Diverses grandeurs statistiques conditionnées par la présence de ces structures ont été obtenues, et il est désormais possible de les comparer avec les résultats des modèles statistiques turbulents dits bi-structure, tel le modèle 2SFK développé au CEA. Afin de réaliser les simulations numériques directes du mélange turbulent, un code numérique tridimensionnel incompressible à densité variable a été développé. Ce code a été parallélisé dans les trois directions. Plusieurs méthodes de détection de structure ont été conçues et testées. Bien que toutes ces méthodes présentent différents intérêts, seule la plus efficace vis-à-vis de nos critères de détection a été gardée pour faire des simulations à forte résolution (plus d'un milliard de mailles, 1024^3). Un filtrage temporel de la vitesse verticale est utilisé dans cette méthode de détection afin de : 1) corriger les distorsions dues aux points d'arrêt et zones de recirculation dans l'écoulement, 2) minimiser l'effet de la turbulence aux petites échelles et mieux mettre en évidence les grandes échelles, 3) introduire un effet mémoire permettant de prolonger la bimodalité du champ de détection depuis les zones laminaires extérieures jusqu'au centre de la zone de mélange turbulent. Plusieurs simulations numériques directes 1024^3 ont été effectuées. Les résultats viennent conforter ceux obtenus avec le modèle bi-structure 2SFK et justifient une étude plus poussée des grandeurs statistiques en vue de sa validation. / This thesis aims at detecting large-scale turbulent structures in incompressible Rayleigh-Taylor mixing layers at low Atwood number. Various statistical quantities conditioned by structure presence have been obtained and it is now possible to compare them with results from two-structure statistical turbulent models such as the 2SFK model developed at CEA. In order to produce direct numerical simulations of the turbulent mixing, a three-dimensional, incompressible, variable-density numerical code was developed. This code is parallelized in the three directions. Several structure detection methods have been designed and tested. Although all these methods are of interest, only the most efficient with respect to our detection criteria has been retained for simulations at high resolution (over a billion cells, 1024^3). A time filtering of vertical velocity is used in this method to: 1) correct distortions due to stagnation points and recirculation zones in the flow, 2) minimize small-scale turbulence effects and better highlight large-scales, 3) introduce a memory effect in order to extend bimodality of the detection field from the external laminar zones up to the centre of the turbulent mixing zone. Several direct numerical simulations at 1024^3 have been achieved. Results support those obtained with two-structure 2SFK model and justify further studies for its validation.