Nous étudions des suspensions concentrées de particules non-colloïdales de PVC. Ces suspensions, appelées plastisols, sont utilisées principalement pour la fabrication de revêtements de sols.Elles présentent notamment un phénomène de rhéoépaississement (hausse de la viscosité en cisaillant). Nous montrons que cela provient des forces de frottement entre particules. En effet, à l’aide d’un microscope à force atomique, nous sommes pour la 1ère fois capable de relier directement la contrainte macroscopique d’apparition du rhéo-épaississement à la contrainte microscopique d’apparition de la friction solide entre particules.Nous caractérisons la viscosité de la suspension jusqu’à 100 000 s-1 et nous observons qu’une plus grande polydispersité limite le rhéoépaississement. Les différences de contraintes normales N1 et N2 sont aussi mesurées. Par ailleurs, des mesures à l’aide de rayons X ou d’ultrasons ne montrent aucune migration de particules sous cisaillement.Nous montrons également que la géométrie a un fort impact sur l’écoulement de la suspension concentrée. Un entrefer plus petit provoque une baisse de la viscosité et retarde le rhéoépaississement. Une approche non-locale permet de rationaliser les résultats.La substitution de particules de PVC par des particules de CaCO3 modifie profondément la viscosité et la densité d’empilement maximum. Nous développons alors des modèles simples pour modéliser ces effets. En outre, nous mesurons l’impact sur la rhéologie d’un éventuel surfactant à la surface des particules.Nous avons pu finalement étudier des instabilités observées en étalant ces suspensions à haute vitesse. Une instabilité de surface est d’abord observée. A plus haute vitesse, un dépôt se forme en aval sur le couteau. Nous corrélons ces instabilités avec l’apparition de différences de contraintes normales. / Highly concentrated and non-colloidal suspensions consisting of micrometric PVC particles dispersed in a liquid phase, were studied. These suspensions, called plastisol, are mostly used in vinyl flooring manufacture.A key feature of these suspensions is shear-thickening, since viscosity greatly increases as a function of the applied shear rate. This phenomenon is explained as being related to frictional forces between particles. Indeed, using an Atomic Force Microscope, we were able, for the first time, to link the macroscopic stress, at which shear-thickening appears, with the microscopic stress needed to enter a frictional regime.We then characterize the suspension viscosity up to 100 000 s-1. We observed that shear thickening is lowered with a more polydisperse powder. Large normal stress differences N1 and N2 were also measured, along with shear thickening. In addition, using both X-ray radiography and ultrasound, no particle migration in the sheared suspension could be detected.We also found that geometry plays a major role in the features of the flow of concentrated suspensions. For a smaller gap, the viscosity is lower and shear-thickening is pushed to higher shear rates. A non-local approach accounts for our experimental results.Replacing a number of PVC particles with CaCO3 particles changes both the viscosity and the maximum packing fraction quite dramatically. For both of these, we developed simple models that matched quite well with the experimental data. Moreover, we elucidate the rheological changes resulting from adding surfactant at the surface of each particle type.Finally, we investigated some instabilities observed while coating at high speed. At a moderate speed, a ribbing phenomenon appears. At a higher speed, a deposit is formed on the knife (downstream). The appearance of these instabilities correlates with normal stress differences

Au cours des dernières décennies, la théorie de la stabilité a été intensivement utilisée pour caractériser le comportement instationnaire d'écoulements. Cela a donné naissance à un grand nombre d'approches, mais malheureusement chacune d'entre elles semble présenter ses propres limitations. De plus, leurs conditions de validité sont encore très mal connues, ce qui soulève la question de la fiabilité de ce genre de méthodes dans un cas général.Cette problématique est traitée dans cette thèse en s'intéressant dans un premier temps aux approches classiques de stabilité, qui étudient l'évolution de petites perturbations autour d'une solution stationnaire -- un champ de base -- des équations de Navier-Stokes. Pour cela, le phénomène du screech -- un bruit tonal que peuvent causer les jets sous-détendus -- est étudié d'un point de vue de la stabilité linéaire. Les résultats obtenus montrent que la dynamique non-linéaire du phénomène est correctement prédite par une analyse linéaire de stabilité du champ de base. Une confrontation avec d'autres analyses similaires montre qu'un tel résultat n'est pas toujours observé. Cependant, lorsque les oscillations auto-entretenues d'un écoulement sont provoquées par un bouclage acoustique, comme c'est le cas entre autres pour le screech, l'écoulement de cavité ou encore les jets impactants, alors les non-linéarités ont une faible influence sur le phénomène de sélection de fréquence. Cela explique la capacité d'une analyse linéaire à caractériser ces écoulements, même dans le régime non-linéaire.Une autre approche, consistant à étudier la stabilité linéaire du champ moyen, a montré de bons résultats dans certaines configurations qui ne peuvent être correctement étudiées par une analyse linéaire du champ de base. Cela est justifié dans cette thèse en mettant en évidence le rôle que joue la résolvante autour du champ moyen dans la dynamique d'un écoulement. Il est montré que lorsque cet opérateur présente une forte séparation de valeurs singulières, ce qui correspond à l'existence d'un mécanisme d'instabilité fort, alors les modes de Fourier de l'écoulement sont proportionnels aux modes de résolvante dominants. Ce résultat fournit des conditions mathématiques et physiques pour l'utilisation et le sens de diverses méthodes d'analyse du champ moyen, telles qu'une analyse d'équations de stabilité parabolisées (Parabolised Stability Equations). De plus, cela permet de mettre en place un modèle de prédiction du spectre fréquentiel en tout point d'un écoulement, à partir d'une ou de quelques mesures ponctuelles et du champ moyen. L'ensemble de ces résultats est illustré et validé sur un cas de marche descendante turbulente. Enfin, cela est exploité dans un cadre expérimental, afin de reconstruire le comportement instationnaire d'un jet rond transitionnel, à partir de la seule connaissance du champ moyen et d'une mesure ponctuelle. L'étude montre que, sous certaines précautions expérimentales, la reconstruction est très précise et robuste. / Linear stability theory has been intensively used over the past decades for the characterization of unsteady flow behaviors. While the existing approaches are numerous, none has the ability to address any general flow. Moreover, clear validity conditions for these techniques are often missing, and this raises the question of their general reliability.In this thesis, this question is addressed by first considering the classical stability approach, which focuses on the evolution of small disturbances about a steady solution -- a base flow -- of the Navier-Stokes equations.To this end, the screech phenomenon -- a tonal noise that is sometimes generated by underexpanded jets -- is studied from alinear stability point of view. The results reveal that the nonlinear dynamics of this phenomenon is well-predicted by a linear base flow stability analysis. A confrontation with other similar analyses from the literature shows that such a satisfactory result is not always observed. However, when a self-sustained oscillating flow is driven by an acoustic feedback loop, as it is the case for the screech phenomenon, cavity flows and impinging jets for instance, then the nonlinearities have a weak impact on the frequency selection process, explaining the ability of a linear analysis to characterize the flow, even in the nonlinear regime.Another alternative approach, based on a linearization about the mean flow, is known to be successful in some cases where a base flow analysis fails. This observation from the literature is explained in this thesis by outlining the role of the resolvent operator, arising from a linearization about the mean flow, in the dynamics of a flow. The main finding is that if this operator displays a clear separation of singular values, which relates to the existence of one strong convective instability mechanism, then the Fourier modes areproportional to the first resolvent modes. This result provides mathematical and physical conditions for the use and meaning of several mean flow stability techniques, such as a parabolised stability equations analysis of a mean flow.Moreover, it leads to a predictive model for the frequency spectrum of a flow field at any arbitrary location, from the sole knowledge of the mean flow and the frequency spectrum at one or more points. All these findings are illustrated and validated in the case of a turbulent backward facing step flow. Finally, these results are exploited in an experimental context, for the reconstruction of the unsteady behavior of a transitional round jet, from the sole knowledge of the mean flow and one point-wise measurement. The study shows that, after following a few experimental precautions, detailed in the manuscript, the reconstruction is very accurate and robust.

Code validation and uncertainty quantification are important tasks in nuclear reactor safety analysis. Code users have to deal with large number of uncertain parameters, complex multi-physics, multi-dimensional and multi-scale phenomena. In order to make results of analysis more robust, it is important to develop and employ procedures for guiding user choices in quantification of the uncertainties.   The work aims to further develop approaches and procedures for system analysis code validation and application to practical problems of safety analysis. The work is divided into two parts.   The first part presents validation of two reactor system thermal-hydraulic (STH) codes RELAP5 and TRACE for prediction of two-phase circulation flow instability.   The goals of the first part are to: (a) develop and apply efficient methods for input calibration and STH code validation against unsteady flow experiments with two-phase circulation flow instability, and (b) examine the codes capability to predict instantaneous thermal hydraulic parameters and flow regimes during the transients.   Two approaches have been developed: a non-automated procedure based on separate treatment of uncertain input parameters (UIPs) and an automated method using genetic algorithm. Multiple measured parameters and system response quantities (SRQs) are employed in both calibration of uncertain parameters in the code input deck and validation of RELAP5 and TRACE codes. The effect of improvement in RELAP5 flow regime identification on code prediction of thermal-hydraulic parameters has been studied.   Result of the code validations demonstrates that RELAP5 and TRACE can reproduce qualitative behaviour of two-phase flow instability. However, both codes misidentified instantaneous flow regimes, and it was not possible to predict simultaneously experimental values of oscillation period and maximum inlet flow rate. The outcome suggests importance of simultaneous consideration of multiple SRQs and different test regimes for quantitative code validation.   The second part of this work addresses core degradation and relocation to the lower head of a boiling water reactor (BWR). Properties of the debris in the lower head provide initial conditions for vessel failure, melt release and ex-vessel accident progression.   The goals of the second part are to: (a) obtain a representative database of MELCOR solutions for characteristics of debris in the reactor lower plenum for different accident scenarios, and (b) develop a computationally efficient surrogate model (SM) that can be used in extensive uncertainty analysis for prediction of the debris bed characteristics.   MELCOR code coupled with genetic algorithm, random and grid sampling methods was used to generate a database of the full model solutions and to investigate in-vessel corium debris relocation in a Nordic BWR. Artificial neural networks (ANNs) with classification (grouping) of scenarios have been used for development of the SM in order to address the issue of chaotic response of the full model especially in the transition region.   The core relocation analysis shows that there are two main groups of scenarios: with relatively small (&lt;20 tons) and large (&gt;100 tons) amounts of total relocated debris in the reactor lower plenum. The domains are separated by transition regions, in which small variation of the input can result in large changes in the final mass of debris.  SMs using multiple ANNs with/without weighting between different groups effectively filter out the noise and provide a better prediction of the output cumulative distribution function, but increase the mean squared error compared to a single ANN. / Validering av datorkoder och kvantifiering av osäkerhetsfaktorer är viktiga delar vid säkerhetsanalys av kärnkraftsreaktorer. Datorkodanvändaren måste hantera ett stort antal osäkra parametrar vid beskrivningen av fysikaliska fenomen i flera dimensioner från mikro- till makroskala. För att göra analysresultaten mer robusta, är det viktigt att utveckla och tillämpa rutiner för att vägleda användaren vid kvantifiering av osäkerheter.Detta arbete syftar till att vidareutveckla metoder och förfaranden för validering av systemkoder och deras tillämpning på praktiska problem i säkerhetsanalysen. Arbetet delas in i två delar.Första delen presenterar validering av de termohydrauliska systemkoderna (STH) RELAP5 och TRACE vid analys av tvåfasinstabilitet i cirkulationsflödet.Målen för den första delen är att: (a) utveckla och tillämpa effektiva metoder för kalibrering av indatafiler och validering av STH mot flödesexperiment med tvåfas cirkulationsflödeinstabilitet och (b) granska datorkodernas förmåga att förutsäga momentana termohydrauliska parametrar och flödesregimer under transienta förlopp.Två metoder har utvecklats: en icke-automatisk procedur baserad på separat hantering av osäkra indataparametrar (UIPs) och en automatiserad metod som använder genetisk algoritm. Ett flertal uppmätta parametrar och systemresponser (SRQs) används i både kalibrering av osäkra parametrar i indatafilen och validering av RELAP5 och TRACE. Resultatet av modifikationer i hur RELAP5 identifierar olika flödesregimer, och särskilt hur detta påverkar datorkodens prediktioner av termohydrauliska parametrar, har studerats.Resultatet av valideringen visar att RELAP5 och TRACE kan återge det kvalitativa beteende av två-fas flödets instabilitet. Däremot kan ingen av koderna korrekt identifiera den momentana flödesregimen, det var därför ej möjligt att förutsäga experimentella värden på svängningsperiod och maximal inloppsflödeshastighet samtidigt. Resultatet belyser betydelsen av samtidig behandling av flera SRQs liksom olika experimentella flödesregimer för kvantitativ kodvalidering.Den andra delen av detta arbete behandlar härdnedbrytning och omfördelning till reaktortankens nedre plenumdel i en kokarvatten reaktor (BWR). Egenskaper hos härdrester i nedre plenum ger inledande förutsättningar för reaktortanksgenomsmältning, hur smältan rinner ut ur reaktortanken och händelseförloppet i reaktorinneslutningen.Målen i den andra delen är att: (a) erhålla en representativ databas över koden MELCOR:s analysresultat för egenskaperna hos härdrester i nedre plenum under olika händelseförlopp, och (b) utveckla en beräkningseffektiv surrogatsmodell som kan användas i omfattande osäkerhetsanalyser för att förutsäga partikelbäddsegenskaper.MELCOR, kopplad till en genetisk algoritm med slumpmässigt urval användes för att generera en databas av analysresultat med tillämpning på smältans omfördelning i reaktortanken i en Nordisk BWR.Analysen av hur härden omfördelas visar att det finns två huvudgrupper av scenarier: med relativt liten (&lt;20 ton) och stor (&gt; 100 ton) total mängd omfördelade härdrester i nedre plenum. Dessa domäner är åtskilda av övergångsregioner, där små variationer i indata kan resultera i stora ändringar i den slutliga partikelmassan. Flergrupps artificiella neurala nätverk med klassificering av händelseförloppet har använts för utvecklingen av en surrogatmodell för att hantera problemet med kaotiska resultat av den fullständiga modellen, särskilt i övergångsregionen. / <p>QC 20170309</p>

In-situ vapor extraction is performed by applying a pressure differential across a hydrophobic porous membrane that forms a wall of the channel as a means of reducing the local quality of flow boiling within the channel. As the local quality is reduced, the heat transfer capability can be improve while large pressure drops and flow instability can be mitigated. The present study investigates the potential of vapor extraction, by examining the characteristics and mechanisms of extraction. The physics based models for transition among extraction regimes are developed which can be used as a basis for a regime-based vapor extraction rate model. The effects of vapor extraction on flow boiling in a microscale fractal-like branching network and diverging channels are studied by using a one-dimensional numerical model based on conservation of mass and energy, along with heat transfer and pressure drop correlations. The results show the improvement in reduced pressure drop and enhanced flow stability, and show the potential of heat transfer enhancement. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from March 25, 2013 - March 25, 2014

Peripheral vascular grafts are used for the treatment of peripheral arterial disease and arteriovenous grafts for vascular access in end stage renal disease. The development of neo-intimal hyperplasia and thrombosis in the distal anastomosis remains the main reason for occlusion in that region. The local haemodynamics produced by a graft in the host vessel is believed to significantly affect endothelial function. Single spiral flow is a normal feature in medium and large sized vessels and it is induced by the anatomical structure and physiological function of the cardiovascular system. Grafts designed to generate a single spiral flow in the distal anastomosis have been introduced in clinical practice and are known as spiral grafts. In this work, spiral peripheral vascular and arteriovenous grafts were compared with conventional grafts using ultrasound and computational methods to identify their haemodynamic differences. Vascular-graft flow phantoms were developed to house the grafts in different surgical configurations. Mimicking components, with appropriate acoustic properties, were chosen to minimise ultrasound beam refraction and distortion. A dual-beam two-dimensional vector Doppler technique was developed to visualise and quantify vortical structures downstream of each graft outflow in the cross-flow direction. Vorticity mapping and measurements of circulation were acquired based on the vector Doppler data. The flow within the vascular-graft models was simulated with computed tomography based image-guided modelling for further understanding of secondary flow motions and comparison with the experimental results. The computational assessments provided a three-dimensional velocity field in the lumen of the models allowing a range of fluid dynamic parameters to be predicted. Single- or double-spiral flow patterns consisting of a dominant and a smaller vortex were detected in the outflow of the spiral grafts. A double- triple- or tetra-spiral flow pattern was found in the outflow of the conventional graft, depending on model configuration and Reynolds number. These multiple-spiral patterns were associated with increased flow stagnation, separation and instability, which are known to be detrimental for endothelial behaviour. Increased in-plane mixing and wall shear stress, which are considered atheroprotective in normal vessels, were found in the outflow of the spiral devices. The results from the experimental approach were in agreement with those from the computational approach. This study applied ultrasound and computational methods to vascular-graft phantoms in order to characterise the flow field induced by spiral and conventional peripheral vascular and arteriovenous grafts. The results suggest that spiral grafts are associated with advanced local haemodynamics that may protect endothelial function and thereby may prevent their outflow anastomosis from neo-intimal hyperplasia and thrombosis. Consequently this work supports the hypothesis that spiral grafts may decrease outflow stenosis and hence improve patency rates in patients.