No description available.

Dans la crise énergétique mondiale actuelle, la demande de production d'énergie reposant sur des sources renouvelables est en hausse. Les biocarburants offrent une option intéressante pour cette transition énergétique. Les intérêts dans le développement d'une troisième génération de biocarburants produits à partir de microalgues et de cyanobactéries ont nettement augmenté. Ainsi, la conception et la construction des systèmes de culture qui fonctionnent dans des conditions optimales sont nécessaires pour bénéficier au maximum du contenu énergétique de ces espèces. Les photobioréacteurs (PBRs) sont des systèmes qui offrent un bon contrôle sur les conditions de culture et de la croissance.Les effets du stress hydrodynamique sur la croissance ainsi que sur la motilité de deux espèces de microorganismes, la cyanobactérie Synechocystis et la microalgue Chlamydomonas reinhardtii, sont étudiés dans deux types de PBR: cuve agitée (agitation mécanique) et airlift tubulaire (agitation par des bulles ascendantes). Les résultats ont montré que Synechocystis est très résistant au cisaillement; la variation de son taux de croissance exponentiel est limitée à la décomposition des colonies cellulaires, alors que sa capacité porteuse semble augmenter avec le cisaillement jusqu'à une valeur maximale. D'autre part, C. reinhardtii se montre plus sensible; son taux de croissance exponentiel augmente avec l'intensité du cisaillement, alors que sa capacité porteuse semble être moins affectée. Un modèle logistique comportant deux paramètres de croissance, le taux de croissance exponentiel et la capacité porteuse, est proposé pour décrire la croissance avec le temps. En suivant une approche de systémiques dynamiques, il a été expérimentalement montré que le taux de croissance instantanée et le taux de croissance per capita tendent vers zéro et oscillent autour d'un point fixe stable où la densité de la population atteint la capacité maximale du système.Un autre aspect de ce travail est d'étudier la motilité des deux microorganismes au cours de leurs cycles de croissance lorsqu’ils sont soumis aux différents niveaux de contrainte de cisaillement. La vitesse moyenne de nage est déterminée pendant la croissance pour des différentes valeurs de cisaillement. Les résultats ont montré que la motilité de C. reinhardtii suit trois phases différentes; une phase ascendante qui commence au milieu de la phase exponentielle de croissance, une phase descendante et enfin une phase amortie au cours de la phase stationnaire de croissance. Il a été montré que l'agitation augmente l'amplitude de la vitesse moyenne de nage et qu’elle avance la motilité cellulaire. En outre, une intensité de cisaillement élevée a conduit à un amortissement plus rapide de la vitesse moyenne vers sa valeur finale en phase stationnaire de croissance. Pour Synechocystis, la motilité n'a pas suivi une tendance claire avec le temps. Cependant, il semble que la vitesse maximale se produit toujours au milieu de la phase exponentielle de croissance. / Under the current global energy crisis, the demand of energy production relying on renewable sources has become a global need. Biofuels offer a transition towards a world of renewable energy supply and production. The interests in developing a third generation of biofuels produced from microalgae and cyanobacteria have clearly increased. Thus, the design and construction of a convenient culturing system under optimal conditions is necessary to benefit from the energy content of these species. Photobioreactors (PBRs) offer a good control over culture conditions and growth.This work investigates the effects of shear stress, generated by stirring in agitated PBRs and bubbling in airlift PBRs, on the growth and motility of two species of microorganisms, the cyanobacterium Synechocystis and the microalgae Chlamydomonas reinhardtii. The results show that Synechocystis is highly resistant to shear stress; the variation in exponential growth rate is limited to the breakdown of cellular colonies, while the carrying capacity appears to increase as a function of shear stress up to a maximal value. On the other hand, C. reinhardtii shows to be more sensitive; the exponential growth rate increases with shear stress intensity, while the carrying capacity seems to be less affected. A logistic growth model featuring two growth parameters, the exponential growth rate and the carrying capacity, is proposed to describe the growth with time. From a point of view of dynamical system approach, it is experimentally shown that the population’s instantaneous growth rate and the growth rate per capita tends to zero and converges around a stable fixed point once the population’s density reaches the carrying capacity of the growth system.Another aspect of this work is studying the motility of the two microorganisms during their growth cycle when different levels of shear stress are applied on them. The average swimming velocity is determined as a function of growth cycle for different shear stress intensities. The results show that the motility of C. reinhardtii follows three different phases; a rising phase starting in the middle of the exponential growth phase, a decay phase and finally a damped phase during the stationary growth phase. It is shown that agitation increases the magnitude of the average velocity and advances the cellular motility. Besides, high intensity in the applied shear stress led to an increase in the damping of the average velocity implying a quicker decay to the limit value at the end of the growth. For Synechocystis, the average velocity did not follow a defined pattern with time. However, it seems that the peak of the velocity occurs always in the middle of exponential phase.

Poster exhibited at GPSC Student Showcase, February 24th, 2016, University of Arizona.

No description available.

Factor VIII (FVIII) is an essential blood-clotting protein and mutations in the FVIII gene are the cause of hemophilia A, a severe inherited bleeding disorder. FVIII synthesis has been observed in discreet endothelial sub-populations including liver sinusoidal endothelial cells and in selected microvascular beds. The mechanistic basis for this differential expression is unknown. Differences in shear stress are believed to play an important role in determining endothelial heterogeneity. In this study, we have evaluated the effect of various shear stress conditions on FVIII expression in blood outgrowth endothelial progenitor cells (BOECs) with an in vitro flow system. Under static conditions, BOECs do not express FVIII. In contrast, after exposure to laminar shear stress for 48 hrs, a significant increase in FVIII expression was documented by qRT-PCR, regardless of the magnitude of shear stress studied (1, 5, 15 and 30 dynes/cm2). To determine the effect of prolonged shear stress, laminar flow was applied over 120 hrs and FVIII mRNA levels returned to static levels. Induction of gene expression by laminar shear stress followed by repression after longer durations is common to other pro-coagulant genes induced by non-laminar or oscillatory flow (eg. tissue factor). BOECs exposed to 15 dyne/cm2 of shear stress, oscillating every 0.5 sec for 120 hrs, had FVIII mRNA levels 4.7-fold that of cells in static conditions. This was significantly higher than FVIII expression in BOECs exposed to 15 dyne/cm2 of laminar shear stress for the same duration. Expression of KLF2, a transcription factor that suppresses endothelial pro-coagulant gene expression under laminar shear stress, was significantly reduced in BOECs exposed to oscillatory as opposed to laminar shear stress. Finally, in BOECs exposed to oscillatory shear stress, FVIII protein was synthesized and co-localizes with its carrier protein VWF in Weibel-Palade bodies. These studies show that shear stress is a significant regulator of FVIII expression in BOECs, that FVIII expression is inversely correlated with that of KLF2, and that FVIII protein co-localizes with VWF in these cells. / Thesis (Master, Pathology & Molecular Medicine) -- Queen's University, 2013-03-04 17:00:27.994

No description available.

As scouring around foundations is the most common cause of bridge failures, one of the most pressing questions of this research is to determine whether or not it is possible to predict the critical shear stress of different soil types using only soil property information. This report shows that it is possible to predict critical shear stress and determines the soil properties that are required to predict the critical shear stress based on soils from Georgia. Multiple methods to predict soil erodibility categories are developed based on the amount of soil information available to the researcher. The report shows how the methods to predict soil erodibility can be integrated with HYRISK, a scour risk assessment tool. In particular, the probabilities of bridge failures and expected economic losses are calculated for approximately 40 bridges in Georgia; soil erodibility characteristics for these bridges are calculated using the methods developed in this thesis. The goal of this thesis is to provide a faster and more cost-effective approach to calculate critical shear stress ranges likely to be encountered at a bridge foundation. Implementation of theses methodologies will help balance funding for new and existing bridges while simultaneously ensuring safe bridge foundation and minimizing economic consequences associated with overbuilding a bridge and/or having to retrofit or replace a bridge that has scour damage due to underbuilding it to withstand a major storm event.

Braiding is a mesmerizing phenomenon since flow and sediment transport interact and are able to change the morphology of a channel in a rapid and complex fashion. Conventional two-dimensional morphodynamic models estimate bedload distribution using shear stress distribution. However, it is unclear if the use of such shear stress distributions is relevant or applicable for all situations when using two-dimensional morphodynamic modelling. This thesis strives to investigate whether shear stress distributions are useful to predict bedload transport pathways. This study focuses upon prediction of bedload transport pathways using a morphodynamic model (Delft3D) of an anabranch of the Rees River (New Zealand). Observed bedload transport pathways were compared to modelled bedload transport pathways in an attempt to validate the predictive ability of the model. Results show that there is a significant correlation between predicted bedload transport pathways and the apparent bedload transport pathways derived from the field measurements. Furthermore, bedload transport predictions were in good agreement with observed data in areas where the model’s predictions of high shear stress were comparable to field observations. However, substantial bedload transport predictions in low shear stress areas were not adequately captured by the model, suggesting that the observed pathways were not due to high shear stress, but rather to other sediment supply sources.

<p> A high-pressure nitrogen-driven viscometer has been used to study the melt fracture of polystyrene. The polystyrene samples used differed in molecular weight and molecular weight distribution. The weight average molecular weight (Mw) ranged from 97,200 to 1.8 x 10^6 and the distribution breadth (Mw/Mn) from 1.06 to 9.21. Results obtained indicate that the critical shear stress varies linearly with 1/Mw, increases slightly with temperature and is independent of the polydispersity of polymers. This type of behaviour is satisfactorily explained in terms of Graessley's entanglement theory.</p> / Thesis / Master of Engineering (MEngr)

Friction Stir Welding (FSW) process development is very costly, and it is still experimental. A predictive model would optimize the weld by changing parameters and obtaining results that reflect the physical process. Friction is the primary adjustable parameter in FSW modeling. Currently, friction model selection is not physic-based. It is based on what is available and contributes to the best fit between the model and experimental data. The research objective is to characterize the interface tool/base material by studying the effect of tool friction coefficient and thermal properties. This is accomplished by changing welding parameters such as force, rpm, and temperature and studying the effects on dependent variables that contribute to the shear stress produced by friction. The study's findings challenge traditional friction concepts by revealing how the rapid engagement of a tool with the base material significantly reduces the impact of sliding friction. Instead, the observed friction primarily depends on the resistance of the shear layer to the tool's motion. This resistance, in turn, is chiefly influenced by the interface temperature, a factor strongly impacted by the thermal diffusivity of the tool material. Remarkably, thermal diffusivity holds the most influence (49.3%) on interface temperature. The interface of the tool material is characterized by a shear stress equation integrating pressure, RPM, thermal diffusivity, and interface temperature. Additionally, the investigation highlights the critical role of heat extraction, where materials with higher thermal diffusivity exhibit distinct outcomes: heightened torque, reduced surface temperature, minimized layer volumes, and shorter operation times.