The primary study of this thesis is the response of the nematic director to pressure driven flow. Dynamic flow experiments using optical conoscopy and pressure gradient measurements are used to explore the physics behind the flow alignment seen to occur for some nematic liquid crystals. New research into the techniques and methods for aligning the director at a glass interface is also presented, the results of which are used towards the latter end of this thesis in the production of a highly novel flow cell. A bespoke technique for fabricating robust liquid crystal flow cells is also presented. The observation of flow alignment for the nematic liquid crystal 5CB is detailed for pressure driven flow via optical conoscopy when the director is initially aligned planar homogeneously at 45◦ to the direction of flow. The results of this experiment are compared to the theory of Ericksen and Leslie through a one dimensional dynamic model that provides simulated director profiles and corresponding simulated conoscopic images. Good agreement between the data and simulation is observed, whereby the director is seen to rotate to become parallel to the flow direction whilst exhibiting no net tilt distortion at all flow rates. The presence of small surface pretilt from a rubbed planar aligning polyimide layer and its effect on director rotation is also examined for cells that are rubbed in both the parallel and anti-parallel directions. The result observed is a striking difference in the mean director rotation when initially aligned close to normal to the direction of flow. The results of these experiments are also compared to the theory of Ericksen and Leslie through the one dimensional dynamic model. Good agreement is seen, highlighting the dramatic effect that a small amount of surface pretilt can have on the overall director orientation, whilst also demonstrating the need for caution when assuming that rubbed conventional alignment techniques provide true planar orientation. Two methods for producing intermediate or large pretilt angles at liquid crystal align- ment surfaces are also examined. Here, two recipes involving the commercial polyimides Nissan SE-1211, Nissan SE-130 and Nissan SE-4811 are experimentally investigated, with results showing the ability to tune the director pretilt angle as a function of the rubbing strength used to align the sample. The results also show an interesting depen- dance on the material upon which the aligning layer is deposited for the recipe involving Nissan SE-1211. Here, vastly different pretilt angles are observed for cells constructed with glass and indium tin oxide (ITO) layers. Finally, the large pretilt angles produced from the recipes mentioned above are also used to fabricate pressure driven flow cells exhibiting large pretilt angles on both sur- faces, constraining the director to align in a splayed state. When aligned parallel to the flow direction, experiments examining the valve-like nature of the director profile suggest that a preferential flow direction exists in what here is termed the ‘diode cell’. Measurements of the pressure gradient required to achieve a constant volumetric flow rate through the cell are compared for flow in both directions relative to the splayed di- rector profile. A striking difference is observed for flow ‘with’ the splay and ‘against’ the splay, leading to the realisation of a cell exhibiting a preferential flow direction through surface treatment. Again, results are compared to the theory of Ericksen and Leslie through the one dimensional dynamic model, showing good agreement.

Magnetic Resonance Imaging, MRI, flow mapping experiments were designed, developed and tested to investigate flow patterns within deformed drops of non-Newtonian polydimethylsiloxane PDMS relaxing to equilibrium shape. The drops have a density of approximately 0.97 g/ml and were made neutrally buoyant in a Newtonian solution of deuterium oxide and deuterated methanol to match the same density. The latter was adjustable depending on the position the drop was required to be in within the solution (e.g. high up or at the bottom). The drops were deformed using gravitational, rotational and reciprocating forces using specially designed electro-mechanical kits. The drops were typically 5mm average diameter and typical velocities were 1mm/s. Drops have a range of viscosities that fall between 1000cSt and 10000cSt and average molecular weights of 28000g/mole and 35000g/mole respectively. They were tested with and without the addition of three different surfactant (i.e. Octaethylene glycol monododecyl ether C28H58O8) concentrations. Three MRI flow data sets were obtained in the form of velocity maps. The first and second sets were acquired just after drop deformation. The relaxation was recorded in the required plane using a Fast Low Angle Shot (FLASH) flow acquisition program. For each drop a movie of ten flow patterns images, with an average of 3 s to 9s in between each image was created to monitor the relaxation process. The results were compared to learn about the effect of viscosity and surfactant concentration on drops' behaviour. The third data set was acquired during the deformation of drops, using a reciprocating force. The deformation was recorded using a gated Pulsed Gradients Spin-Echo (PGSE) flow measurement acquisition program. For each drop flow patterns were formed at six different positions of the reciprocal drop deformation cycle. Both velocity acquisition programs were based on a Phase Contrast PC velocity measurement concept. These resultant flow patterns allowed us to look non-destructively at the internal flow of deformed drops, showing flow activities in real time. These constructed flow patterns demonstrated a wide range of responses, which substantially contributed to drop deformation studies and provided access to unique and rich information that until now was not accessible. The method offers data which will ultimately contribute to the on going theoretical efforts to understand and predict, hence control and improve, the drop's material properties world-wide.

We study the flow of a dilute polymer solution in a wavy channel under steady-state flow conditions by employing the non-equilibrium thermodynamics two-fluid model (Mavrantzas-Beris, 1992), allowing for the coupling between polymer concentration and polymer stresses. The resulting highly complex system of partial differential equations describing inhomogeneous transport phenomena in the fluid are solved with an efficient implementation of the mixed finite-element method. We present numerical results for polymer concentration, stress, velocity and fluxes of polymer as a function of the non-dimensional parameters of the problem (the Deborah number , the Peclet number , the Reynolds number , the ratio of the solvent viscosity to the total fluid viscosity , and the constriction ratio of the channel width ). We find that the constricted part of the wall is depleted of polymer, when the polymer diffusion length-scale, expressed by the ratio of / , increases. The migration is more pronounced for macromolecules characterized by longer relaxation times, and takes place towards the expanding part of the channel or towards the centerplane. Migration is also enhanced by the width variability of the channel: the more corrugated the channel, the stronger the transfer of polymer to the centerplane. This increases the spatial extent of polymer depletion near the wall or induces a zone of sharp variation in polymer stress and concentration, which moves away from the channel wall, especially in lower polymer concentration. The development of a polymer-depleted layer smooths out the boundary layer which is known to arise with Boger fluids at the walls of such corrugated channels or tubes and gives rise to an “apparent” slip in the constricted section of the wall and to a very low value of the drag force on the wall. When and where boundary layers arise, they scale as (1/De) for the stresses and as (De⁄Pe)^(1⁄3) for the concentration. / --

Cette thèse est une entreprise commune de VINCI Technologies et du laboratoire CNRS CORIA. De nombreux injecteurs comportent une zone de mélange interne dans laquelle les phases liquides et gazeuses sont toutes deux présentes dans une proportion significative. Par conséquent, cette zone appartient à la catégorie des écoulements diphasiques denses. Pour simuler la dispersion du liquide et caractériser le spray de ces injecteurs, des modèles appropriés sont nécessaires. Les points clés de cette approche sont la dispersion du liquide qui peut être associé au flux liquide turbulent et la quantité de surface liquide-gaz. En particulier, ce manuscrit rapporte, d’une part le développement théorique des modèles de la famille ELSA et, d’autre part, les approximations industrielles correspondantes. Le solveur proposé bascule dynamiquement du spray ICM au spray de sous- maille, à travers le concept ELSA et grâce à l’indicateur basé sur la résolution (IRQ). D’autre part, une fois la zone diluée se forme, l’approche ELSA est couplée à la méthode d’écoulement multiphase, qui vise à déterminer la distribution du spray à l’aide de l’équation WBE. Cette dernière équation est résolue avec une méthode hybride Euler-Lagrange. Le but est de résoudre l’équation WBE avec une approche stochastique Lagrangienne, tout en préservant la compatibilité avec la description Eulerienne de l’écoulement diphasique, basée sur ELSA, pour tirer parti des deux approches. Finalement, ces approches développées ont été utilisées pour des applications industrielles montrant leur robustesse et leur capacité à aider dans le processus de développement de nouveaux injecteurs. / This PhD is a joint venture between VINCI Technologies and the CNRS Laboratory CORIA. For its application, VINCI Technologies, developed mainly oil-related equipments and in particular injection/atomization systems. Some of these injectors are characterized by a very big geometrical dimensions (several meters long), that leads to very high Reynolds and Weber number. In addition, many injectors incorporate an internal mixing zone, in which liquid and gas phases are both present in a significant proportion. Consequently, this zone belongs to the dense two-phase flow category. To simulate the liquid dispersion and to characterize the spray formation special from these injectors, appropriate models are required. On its side, the CORIA team, has developed a suitable approach, so-called ELSA, based on the pioneering work of Borghi and Vallet [1, 2]. Key points of this approach are the liquid dispersion that can be associated to the turbulent liquid flux and the amount of liquid-gas surface that can be used to determine eventually the Sauter mean diameter (SMD) of the spray. During this PhD, the applications proposed by V INCI Technologies, have promoted a review of a large part of the multiphase flow approaches to find the more appropriate for each case. This has been the opportunity to clarify the range of application of each approach, and therefore stress the necessity to develop coupled approaches, in order to cover the proposed application in the most suitable way. In particular, this manuscript reports, in one hand, the theoretical development of the ELSA family models, and on the other hand, the corresponding industrial approximations. Since ELSA approaches are originally developed for RANS simulation of the dense zone, it has been extended to LES description. The link of this approach to the DNS¡ ICM approach, has been studied with a special care. The resulting proposed solver, switches dynamically from ICM to subgrid spray, through the ELSA concept, and thanks to resolution based indicator (IRQ). On the opposite side, once the dispersed spray is formed, the ELSA approach is coupled to multiphase flow method, that aims to determine the spray distribution through the WBE equation. This later equation, is solved with an original hybrid Euler-Lagrange method. The purpose is to solve the WBE equation with a Lagrange stochastic approach, and at the same time, preserving the compatibility to the Euler description of two-phase flow, based on ELSA, to benefit from both approaches. This coupled approach has been tested against academic experimental data coming from ECN research initiative, a combined DNS and experimental measurement of dispersed spray on a Diesel jet, and under an air-blast atomizer numerical test case, for which the mean liquid volume fraction has been measured. Eventually, these developed approaches have been applied to industrial application showing there robustness and their capacity to help in the process of design development of new injectors.

Cette thèse concerne l’aérodynamique de pales d'extrémité transsonique. Ces pales sont conçues pour maximiser le rendement en croisière, tout en générant la traction requise au décollage. Elles ont des profils fins et peu cambrés, travaillant à forte incidence au décollage, ce qui peut entraîner l’apparition d’un tourbillon de bord d’attaque (TBA). Or ce TBA présente des similitudes avec les tourbillons d’apex d’aile Delta, connus pour leur capacité à générer de la portance tourbillonnaire.Cette étude consiste à examiner l’intérêt du TBA pour les performances aérodynamiques.La démarche a consisté dans un premier temps à caractériser la topologie du TBA sur une maquette représentative d’une pale d’ Open Rotor, à l'aide d'essais PIV résolus en temps et de calculs RANS k-omega SST, et à évaluer la capacité de la simulation RANS à reproduire les caractéristiques d’intérêt pour cette étude. Un algorithme a été développé afin d'estimer la contribution de ce TBA à la portance à partir du champ de pression pariétal RANS.Afin d'expliciter l'influence des paramètres géométriques et de fonctionnement de la pale sur la portance tourbillonnaire, un modèle 1D de la portance tourbillonnaire a été développé puis couplé à la méthode de l'élément de pale.Les premières comparaison de géométries à iso-traction ont montré que la portance tourbillonnaire permet de générer la traction requise au décollage avec une surface alaire plus faible. Ces résultats ouvrent de nouvelles perspectives pour la conception de géométries avec un meilleur rendement en croisière. / This thesis deals with the aerodynamic properties of propeller blades. Those blades are designed to maximise cruise efficiency, while achieving the target thrust at take-off. Their thin, low-cambered profiles must work at high incidence at take-off, which may give rise to a leading-edge vortex (LEV).The topology of this LEV looks similar to Delta wing LEVs, which are known to generate vortex lift.the aim of this study is to explore the probable impact of the LEV on lift at take-off in order to reconsider propeller blade designs. The approach first consisted in caracterising the LEV topology on a model blade representative of an Open Rotor front blade, using both Time-Resolved PIV and RANS k-omega SST calculations. The comparison between both methods demonstrated the ability of RANS calculations to reproduce the LEV characteristics of interest to this study.Then, the LEV contribution to lift was evaluated thanks to an algorithm developed to estimate vortex lift contribution from RANS wall pressure fields.In order to explicit the influence of the blade's geometrical and functioning parameters on vortex lift, a 1D vortex lift model was developed and coupled to the Blade Element Momentum Theory.The first blade geometry comparative studies at iso-thrust showed that vortex lift enables to generate target thrust at take-off with a lower blade surface. This opens new perspectives for the design of blade geometries with enhanced cruise efficiency.