The subject of this thesis is the development of an accurate, general and robust numerical method capable of predicting the flow behavior of two-phase immiscible fluids, separated by a well defined interface. In the quest of an accurate and robust numerical method for the modeling of two-phase flows, one has to keep in mind the intrinsic properties and difficulties associated with the problem: (i) those flows are mostly three-dimensional, (ii) some flows are steady, others unsteady, (iii) the interface might encounter a lot of topology changes (like merger or break-up), (iv) large jumps of density and viscosity might exist across the interface (e.g. ratio of density of 1/1000 for water and air), (v) surface tension forces may play a very important role in the interface dynamics. Hence, the influence of this force should be accurately evaluated and incorporated into the model, (vi) mass conservation is of primary importance. All these issues are addressed in this thesis, and special techniques are proposed for their treatment, which enables to construct the desired computational method. The chosen computational method combines a pressure stabilized finite element method for the Navier Stokes equations with a discontinuous Galerkin (DG) method for the level set equation. Such a combination of those two numerical methods results in a simple and effective algorithm that allows to simulate diverse flow regimes presenting large density and viscosity ratios (ratio up to 1/1000).

Flavonoids are plant secondary metabolites that play diverse roles in plants and human health. These compounds in most part exist in the glucosylated form. Grapefruit accumulates high levels of glucosylated flavonoids. Plant secondary product glucosyltransferases (GTs) catalyze the glucosylation reaction, but due to low homology at both the nucleotide and amino acid sequence level of different GTs, it is not possible to ascribe function based on sequence only. The hypotheses that PGT clones 9 and 11 are plant secondary product GTs and are biochemically regulated were tested. PGT 9 has been cloned into Pichia pastoris using the pPICZA and pPICZAα vectors, expressed, enriched, and screened for GT activity with a variety of phenolic substrates. Initial screens show catechol, gentisic acid, vanillin, and p-hydroxylphenylacetic acid as potential substrates for the PGT 9 protein. PGT 11 has been successfully cloned into pPICZA for transformation into yeast, expression, and subsequent characterization.

Flavonoids are a class of plant secondary metabolites that fulfill many functions in planta and also benefit human health. Glucosylation of flavonoids, catalyzed by glucosyltransferases (GTs), help flavonoids perform their important roles. Citrus paradisi (grapefruit) is known to be active in flavonoid metabolism and possesses a variety of secondary product GTs. This research is designed to test the hypothesis that PGT10 is a flavonoid glucosyltransferase. PGT10 was cloned, expressed, purified, and tested for GT activity with a variety of phenolic substrates. Efforts to identify the reaction catalyzed by PGT10 continue. A second project was designed to test the hypothesis that multiple bands obtained by RT-PCR of PGT 5/6 found in root cDNA represented a set of homologous genes. Hypothesis has been confirmed by the homologous fragments found. Additional structural information is presented.

The evolution of numerical methods and computational facilities allow re- searchers to explore complex physical phenomenons such as multiphase flows. The specific regime of incompressible, turbulent, bubbly two-phase flow (where a car- rier fluid is infused with bubbles or particles) is also receiving increased attention due to it’s appearance in major industrial processes. The main challenges arise from coupling individual aspects of the physics into a unified model and to provide a robust numerical framework. The presented work aimed at to achieve the second part by employing the most frequently used dispersed two-phase flow model and another incompressible, turbulent single phase solver as a base flow provider for coupled Lagrangian or surface tracking tools. Among the numerical techniques, the finite element method is a powerful can- didate when the need arises for multiphysics simulations (for example coupling with an electrochemical module) where the counterpart has a node based ap- proach. Stabilization schemes such as PSPG/SUPG/BULK provide remedies for the pressure decoupling and the inherent instability of the central discretization when applied for convective flow problems. As an alternative to unsteady solvers based upon an explicit or a fully im- plicit nonlinear treatment of the convective terms, a semi-implicit scheme results in a method of second order accurate in both space and time, has absolute linear stability and requires only a single or two linear system solution per time step. The application of the skew symmetric approach to the convective term further stabilizes the solution procedure and in some cases it even prevents divergence. The Eulerian-Eulerian two-phase flow model poses various issues to be over- come. The major difficulty is the density ratio between the phases; for an ordinary engineering problem it is in the order of thousands or more. The seemingly minus- cule differences in the formulation of the stabilizations can cause very different end results and require careful analysis. Volume fraction boundedness is of concern as well, but it is treatable by solving for its logarithm. Since the equations allow jumps (even separation of the phases) in the volume fraction field, discontinuity capturing techniques are also needed. Besides the standard ’spatial’ stabilization temporal smoothing is also necessary, otherwise the limitation in time step size becomes too stringent. Designing a flow solver is one side of the adventure, but verification is equally important. Comparison against analytical solution (such as the single and two- phase Taylor-Green testcase) provides insight and confirmation about the mathe- matical and physical properties. Meanwhile comparing with real life experiments prove the industrialization and usability of a code, dealing with low quality meshes and effective utilization of computer clusters. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished