Le travail présenté dans ce mémoire est une contribution à l'étude numérique des systèmes tourbillonnaires hélicoïdaux qui sont émis dans le sillage des rotors (éoliennes, hélicoptères,...) et de leurs instabilités. Ici, ces écoulements sont localement modélisés par un ensemble de tourbillons à symétrie hélicoïdale. À l'aide d'un code de simulation numérique directe dédié, des solutions de base quasi-stationnaires sont obtenues pour différents systèmes tourbillonnaires. Une caractérisation précise et détaillée de ces solutions est ensuite effectuée : vitesse de rotation, taille et ellipticité du cœur, structure des champs de vitesse et de vorticité... À l'aide d'un algorithme d'Arnoldi couplé à une version linéarisée du code, on détermine les modes dominants d'instabilité ayant la même symétrie que l'écoulement de base, en fonction des paramètres du système: nombre de vortex, pas hélicoïdal, taille de cœur, nombre de Reynolds et présence d'un vortex de moyeu. En dessous d'un certain pas hélicoïdal critique, l'instabilité est dominée par un mode de déplacement global analogue au mode d’appariement d'une allé infinie de points vortex ou d'anneaux tourbillonnaires. En régime non linéaire, ce mode est à l'origine d'une dynamique complexe du système: dépassements, saute-mouton et fusion. On utilise un autre code linéarisé pour déterminer les modes instables qui brisent la symétrie hélicoïdale de l'état de base, caractérisés par une longueur suivant l'axe. À faible nombre d'onde, ces modes induisent localement des rapprochements entre portions de spires voisines. À grand nombre d'onde, on observe un autre type de mode qui déforme les cœurs tourbillonnaires via l'instabilité elliptique. / The work presented in this manuscript is a contribution to the numerical study of helical vortex systems and their instabilities, as encountered in the near wake of rotors (wind turbines, helicopters,~...). In this work, such flows are locally modelled within the framework of helical symmetry. Using a dedicated DNS code, helical quasi-stationary basic state solutions are obtained for several configurations, and accurate tools for their characterisation are developed: angular velocity, core size and ellipticity, structure of the velocity and vorticity fields... An Arnoldi algorithm is then coupled to a linearised version of the code. The dominant instability modes with the same symmetry as the base flow are extracted as a function of the system parameters: number of vortices, helical pitch, core size, Reynolds number, presence of a central hub vortex. Under a critical helical pitch, the instability is dominated by a global displacement mode analogous to the pairing mode of an infinite array of point vortices or vortex rings. In the nonlinear regime, this mode gives rise to complex dynamics: overtaking events, leapfrogging and merging. Another linearised code is then used to extract modes characterised by a wavelength along the helix, which break the helical symmetry of the base flow. At low wavenumbers, these modes induce local displacements of the vortices and bring together portions of neighbouring coils. At large wavenumbers, another type of mode is found, which deforms the vortex cores through the elliptical instability mechanism.

Objectifs : Les Récepteurs Couplés aux Protéines G (RCPG) constituent une grande famille ubiquitaire. Leur structure est caractérisée par sept hélices transmembranaires. Les déformations de ces hélices jouent un rôle majeur dans l’activation de ces récepteurs. La plupart de ces déformations sont liées à la présence de prolines conservées. Cependant, les prolines de l’hélice 2 et 5 des RCPG ne sont pas systématiquement présentes. De plus, la position de la proline dans l’hélice 2 est variable (2.58, 2.59 ou 2.60). Nous nous intéressons aux rôles des prolines des hélices 2 et 5 dans l’activation de deux RCPG : le récepteur de la thyrotropine (TSHR) et le récepteur 2 de la vasopressine (V2R). Méthodes : pour le TSHR et le V2R, nous concevons et caractérisons des mutants pour chaque position conservée de la proline dans l’hélice 2 et/ou 5, ainsi que des mutants sans proline. Résultats : Les mutants du TSHR n’ont pas le même comportement en termes d’expression, de glycosylation ou d’activité. La position la mieux tolérée, 2.59, nous permet de rapprocher le TSHR des récepteurs avec une proline en position 2.59 qui possèdent un renflement dans l’hélice 2. Pour l’hélice 5, les données expérimentales couplées à l’analyse des séquences et la modélisation moléculaire suggèrent une structure non renflée. Pour le V2R, le changement de position de la proline de l’hélice 2 est plus délétère que l’absence de proline dans cette hélice. La proline de l’hélice 5 est indispensable pour l’activité de ce récepteur. Conclusion : Les données obtenues sur le TSHR permettent de proposer un modèle avec une hélice 2 renflée et une hélice 5 non renflée et d’améliorer la modélisation de la cavité interne de ce récepteur, ce qui est essentiel pour le drug design. L’étude du V2R permet de proposer un modèle évolutif de ce récepteur et met en évidence sa spécificité par rapport à des récepteurs proches. / Objectives : Class A G-Protein-coupled receptors (GPCRs) constitute a large family of transmembrane receptors. Helical distortions play a major role in the overall fold and in the activation mechanism of these receptors. Most distortions are related to the presence of conserved proline residues. However, in helices TM2 and TM5, the presence of proline is not mandatory and the correlated mutation of these proline residues is observed in several GPCR sub-families. In addition, the position of the TM2 proline is variable (2.58 to 2.60). We are interested in the role of the TM2 and TM5 proline residues in the folding and activation mechanism of two GPCRs : the thyrotropin receptor (TSHR) and the vasopressin receptor type 2 (V2R). Methods : For both receptors, we engineered and characterized mutants with proline residues at different positions in TM2 and/or at position 5.50 in TM5, and without proline. Results : The expression, the glycolysation or the activity of TSHR mutants are differentially altered by changes in the proline pattern. The “best” mutant, TSHR P2.59, is consistent with a bulged structure for TM2. Experimental data in addition to sequences analysis and modeling suggest an unbulged structure for TM5. For V2R, the absence of proline in TM2 is better tolerated than ashift in the position. The TM5 proline is mandatory for the receptor activation. Conclusion : We suggest a model for TSHR with a bulged TM2 and an unbulged TM5. This should improve the modeling of the transmembrane cavity, which is fundamental for drug design. Our results on V2R suggest an evolutionary model for this receptor and enlighten its specificity compared to nearby receptors.

Crossing of stent graft limbs during endovascular aneurysm repair (EVAR) is often used to assist cannulation and prevent graft kinking when the aortic bifurcation is widely splayed. Little has been reported about the implications of cross-limb EVAR, especially in comparison to conventional EVAR. Using computational fluid dynamics, this work numerically examines the hemodynamic differences between these two out-of-plane stent graft configurations against a planar configuration commonly found in literature. Predicted values of displacement force, wall shear stress, and oscillatory shear index were similar between the out-of-plane configurations. The planar configuration predicted similar wall shear stress values, but significantly lower displacement forces than the out-of-plane configurations. These results suggest that the hemodynamic safety of cross-limb EVAR is comparable to conventional EVAR. However, a study of clinical outcomes may reveal reduced thrombosis incidence and long-term structural implications for the stent graft in cross-limb EVAR.

Crossing of stent graft limbs during endovascular aneurysm repair (EVAR) is often used to assist cannulation and prevent graft kinking when the aortic bifurcation is widely splayed. Little has been reported about the implications of cross-limb EVAR, especially in comparison to conventional EVAR. Using computational fluid dynamics, this work numerically examines the hemodynamic differences between these two out-of-plane stent graft configurations against a planar configuration commonly found in literature. Predicted values of displacement force, wall shear stress, and oscillatory shear index were similar between the out-of-plane configurations. The planar configuration predicted similar wall shear stress values, but significantly lower displacement forces than the out-of-plane configurations. These results suggest that the hemodynamic safety of cross-limb EVAR is comparable to conventional EVAR. However, a study of clinical outcomes may reveal reduced thrombosis incidence and long-term structural implications for the stent graft in cross-limb EVAR.

In an effort to achieve lighter and more economical space structures, a new patented steel composite member has been suggested and used in the construction of some steel roof structures. This special element has a sandwich construction composed of some strips of steel plates placed longitudinally along a helical wire core. The function of the helical core is to transfer the shear between the flange plates and increase the sectional inertia of the resulting composite member by keeping the flange plates at a desired distance from each other. Because of the lack of research, design engineers usually treat such elements as a solid member as if it has a full shear transfer between the flanges. However, a detailed analysis shows that this is not a valid assumption and leads to very unsafe results. In this context, the purpose of this study is to investigate the behavior of such members under axial compression and determine their effective sectional flexural rigidity by taking into account the shear deformations. This study applies an analytical investigation to a specific form of such elements with four flange plates placed symmetrically around a helical wire core. Five independent parameters of such a member are selected for this purpose. These are the spiral core and core wire diameters, the pitch of the spiral core, and the flange plate dimensions. Elements with varying combinations of the selected parameters are first analyzed in detail by finite element method, and some design charts are generated for the determination of the effective sectional properties to be used in the structural analysis and the buckling loads. For this purpose, an alternative closed-form approximate analytical solution is also suggested.

Novel antennas attractive to fit in the internal space of a PDA (Personal Digital Assistant) for WLAN (Wireless Local Area Network) operations are presented in this dissertation. The proposed antennas have in common good impedance bandwidth (defined by 10 dB return loss), covering the dual-band WLAN operation in the 2.4/5.2 GHz bands. Two novel designs of foam-base surface-mount antennas are proposed in Chapters 2 and 3. Surface-mountable antennas, compared with ceramic antennas, are generally low cost in fabrication and rigid in nature. Low-profile and good dual-band operation of the proposed surface-mountable antennas can be observed in Chapters 2 and 3, and in addition, a few present-day WLAN bands at 5 GHz are covered in the operating bandwidths of the proposed foam-base surface-mountable shorted monopole antenna, shown in Chapter 3. Finally, in Chapter 4, a novel planar helical antenna printed on both surfaces of a dielectric substrate is demonstrated. This patent-pending helical antenna is very suitable to print and integrate on a circuit board of a PDA device for 2.4/5.2 GHz WLAN operation.

The present study describes the flow characteristics of swirling flows induced by twisted tape inserts in circular pipes. The study is focused on the secondary flow which is investigated experimentally and with numerical models. The results are expected to improve the paper manufacturing process by identifying and removing the detrimental secondary flow. Experimental tests show for the first time the existence of two co-rotating helical vortices superimposed over the main swirling flow, downstream of twisted tapes. The close proximity of the two co-rotating vortices creates a local counter-rotating flow at the pipe centerline. The flow is analyzed using LDV measurements and high speed camera visualization with fine air bubbles seeding which confirm that the helical vortices are stable. After extracting the characteristic tangential velocity profiles of the main vortex and of the two secondary vortices, it was observed that the maximum tangential velocity of all three vortices is the same, approximately half of the bulk velocity. The winding of the helical vortices is in the swirl direction and the pitch of the helical vortices is found to be independent of the inlet velocity. The experimental findings are confirmed by numerical simulations. The numerical results show that the helical vortices originate inside the swirler and evolve from single co-rotating vortices on each side of the tape. The flow characteristics are analyzed in detail. Swirlers with multiple twists and multiple chambers are shown to have less stable secondary motion and could be employed in applications were the secondary motion is detrimental.

The present study investigates experimentally the laminar mixing and heat transfer of a range of helical ribbon and anchor impellers for both Newtonian and inelastic non-Newtonian fluids. The work also correlates the experimental data empirically in the form of dimensionless groups. In order to estimate the relative importance and the effect of all the geometrical parameters on the mixing power and heat transfer, data from the published literature sources will be utilized and combined with the results from this study. Thus, reliable empirical correlations will be obtained which are applicable over the widest range of operating conditions. The study also investigates the ablity of the various impellers to level out temerature distributions. The measurement of these temperature gradients and the impeller power requirements gives a measure of the mixing efficiency of the impeller used.

Palladium-catalyzed olefin dioxygenation is a powerful tool in the generation of complex and valuable substrates, one which may become complimentary to the well known Sharpless dihydroxylation. In this work the mechanism of this transformation is examined via reaction kinetics and Hammett studies, which corroborate a PdII/IV catalytic cycle and suggest that the rate determining step is the oxidation of PdII to PdIV. Olefin dioxygenation was also found to proceed in the presence of catalytic quantities of BF3•OEt2 or triflic acid, with stoichiometric hypervalent iodine oxidant and an acetic acid solvent. Furthermore, asymmetric variants of intramolecular palladium-catalyzed olefin dioxygenation were also investigated, which resulted in the formation of tetrahydrofuran products in up to 36% ee. Next, chelate-assisted C–H bond functionalization of indoles at the C7 position and of carbazoles at the C1 position was investigated with a variety of arylation, halogenation and oxygenation techniques. Lastly, our efforts towards the synthesis of a mono-phosphine based [5]helicene ligand via olefin metathesis and photocyclization strategies will be discussed.

Palladium-catalyzed olefin dioxygenation is a powerful tool in the generation of complex and valuable substrates, one which may become complimentary to the well known Sharpless dihydroxylation. In this work the mechanism of this transformation is examined via reaction kinetics and Hammett studies, which corroborate a PdII/IV catalytic cycle and suggest that the rate determining step is the oxidation of PdII to PdIV. Olefin dioxygenation was also found to proceed in the presence of catalytic quantities of BF3•OEt2 or triflic acid, with stoichiometric hypervalent iodine oxidant and an acetic acid solvent. Furthermore, asymmetric variants of intramolecular palladium-catalyzed olefin dioxygenation were also investigated, which resulted in the formation of tetrahydrofuran products in up to 36% ee. Next, chelate-assisted C–H bond functionalization of indoles at the C7 position and of carbazoles at the C1 position was investigated with a variety of arylation, halogenation and oxygenation techniques. Lastly, our efforts towards the synthesis of a mono-phosphine based [5]helicene ligand via olefin metathesis and photocyclization strategies will be discussed.