Dans le contexte péri-urbain à dominance agricole de l’Ile de France, les pressions anthropiques exercées sur les rivières jouent autant sur la qualité que sur le débit des cours d'eau. Située au sud-ouest de Paris, l’Yvette draine un bassin versant de 202 km²et à la géologie homogène. Elle est alimentée directement par des stations d'épuration (STEP) et par la nappe des sables de Fontainebleau qui représente la principale source d’eau. Sur ce type de bassin, la gestion durable du système riverain repose sur la connaissance de la distribution des flux nappe – rivière et sur l’impact de cette répartition sur la qualité des cours d’eau.La dynamique des flux d’eau a été suivie grâce à l’implantation de stations hydrométriques aux points clés du réseau hydrographique (i.e. exutoires des principaux affluents, aval des confluences sur l’Yvette). Les flux chimiques ont été étudiés par analyses d’échantillons d’eau recueillis lors de campagnes de terrain effectuées en période d’étiage sur les cours d’eau. Un modèle conceptuel global, calibré à partir des chroniques de débit, a permis (i) d’estimer des variations spatiales de la recharge de la nappe (60 – 160 mm.an-1), et (ii) d’établir une répartition journalière des parts d’eau de nappe, de STEP et de ruissellement en rivière, pour la période 2001-2014. Par ailleurs, l’analyse des traceurs géochimiques (e.g. Cl-, NO3-, SO42-) et isotopiques (222Rn, δ18Oeau, δ2Heau) soutient la prédominance, aussi bien quantitative que qualitative, de la nappe sur les rivières.D’un point de vue hydrodynamique, le contexte géomorphologique homogène procure aux rivières une dynamique similaire avec des épisodes de crues très courts (de l’ordre de quelques heures) et des périodes d’étiages marqués, quelle que soit la saison. L’étude par sous-bassin a mis à jour une différence entre les bassins topographiques et les bassins d’écoulement souterrains, créant des déficits hydriques sur l’amont de certains cours d’eau (e.g. Mérantaise, Ru des Vaux) au profit d’autres (e.g. Rhodon). Le parallèle entre bassins topographique et souterrain n’a pu se faire qu’au niveau du cours principal de l’Yvette. Sur la période 2001-2014, le débit de l’Yvette provient en moyenne à 55 % de la nappe, à 38 % du ruissellement et à 8 % des STEP. En période de basses eaux, la contribution des STEP reste sensiblement identique tandis que la nappe constitue la principale alimentation des rivières (90 %), contrôlant donc leur qualité. Mais la composition chimique de ce soutien souterrain n’est pas homogène. Pour déterminer l’origine de ces disparités, un travail à plus petite échelle a été conduit sur un affluent majeur de l’Yvette (le Rhodon). La décharge de la nappe en rivière y est bimodale : 15 % arrive par voie souterraine et 85 % transite par les milieux humides en surface. Au sein des zones humides sont observées de fortes hétérogénéités dans les écoulements et leur chimie, avec des flux de subsurface totalement épurés en nitrates et du ruissellement riche en nutriments. La prédominance de des flux de surface réduit fortement le pouvoir épurateur des milieux humides, avec seulement 6 % des eaux de nappe épurées avant leur décharge en rivière. Cette faible efficacité renforce l’impact direct de la qualité de la nappe sur les rivières. Sur le bassin, la nappe est stratifiée par l’intervention de nombreux processus de recharge (infiltrations d’eau d’étang, de STEP, de zones humides de plateau). Les petits sous-bassins (< 50 km²) semblent dominés par cette stratification, ainsi que par les apports de STEP qui contribuent aux flux chimiques des rivières à hauteur de 30 à 50 %. Le cours principal de l’Yvette, dans sa partie aval, dépend des mélanges issus des confluences avec ses affluents. Les outils utilisés ici ont donc mis en avant les différences de fonctionnement des bassins selon l’échelle concernée, tout en reliant l’importance de la compréhension du système à petite échelle pour la gestion à grande échelle. / In the peri-urban context with large farm practices of the Paris region, anthropogenic pressures on streams and rivers impact both their flow and quality. Located in the southwestern of Paris, the Yvette stream drains a watershed of 202 km², in a homogeneous geological context. The stream is sustained by waste water treatment plant (WWTP) and by the Fontainebleau sands aquifer which represents the main source of water.In this context, the sustainable management of the stream and its riparian zones needs the assessment of groundwater discharge locations and chemistry, and their impact on the stream water quality.Water fluxes in the stream and in its main tributaries have been recorded at 11 stations. Dissolved elements distributions have been studied by field works in low-flows conditions. A lumped model, calibrated with stream flow hydrographs, has allowed (i) the estimation of groundwater recharge rates (60 – 160 mm.y-1) and (ii) the distribution of stream flow contributions between 2001 and 2014 (i.e. 55 % from groundwater, 38 % from runoff and 8 % from WWTP). Moreover, the use of geochemical and isotopic tracers (e.g. Cl-, NO3-, SO42-, 222Rn, δ18Owater, δ2Hwater) confirmed the large impact of groundwater on stream flow and quality.The hydrological dynamics of the Yvette stream and its tributaries are similar because of the homogeneity of the geomorphological context of the catchment. The flood events are really fast (on the order of hours) and low-flow conditions can be observed in every season. The discretization of the Yvette catchment in 11 sub-watersheds highlighted a divergence between the topographical and hydrogeological catchments. This generates a water deficit in some tributaries heads (e.g. Mérantaise, Ru des Vaux) in favor of some others (e.g. Rhodon). The matching limits of these catchments can only be made on the Yvette channel. During the 2001 – 2014 period, considering a fairly constant WWTP contribution, the groundwater discharge provides more than 90 % of the stream flow during low-flow conditions. This highlights a stream quality dominated by groundwater discharge. However, groundwater chemistry largely varies over the catchment. To study this phenomenon, a small-scale field work has been directed on the Rhodon stream, a main tributary of the Yvette River. At this scale, the groundwater discharge appeared to have two main pathways: 15 % come from the streambed while 85 % transit in wetlands before going to the stream. Within these wetlands, a large part of groundwater flows at the surface and has high levels of nutrients, whereas a small part (6 %) goes through sediments and is nutrients free. reduces The purifying power of these wetlands, significantly reduced by this distribution, is likely related to historical drainage and reinforces the direct connection between the stream and groundwater quality. On the Yvette catchment, the stratification of groundwater chemistry is due to several recharge processes (e.g. pond water infiltration, WWTP effluents infiltration, wetlands water infiltration). Small sub-watersheds (< 50 km²) seem to be dominated by (i) this stratification, and (ii) the WWTP discharges which can contribute from 30 % to 50 % to dissolved fluxes in the streams. The main channel of the Yvette stream primarily depends of the mixing of its tributaries waters. The methods developed here allowed highlighting (i) differences of catchment functioning according to the scale used, and (ii) the utmost importance of the small-scale assessment to understand/decipher and manage streams at larger scales.

To accommodate an increasing number of tourists visiting Bequia, the second largest island of Saint-Vincent and the Grenadines, the local government constructed an airport, through a major coastline land-reclamation project. However, due to the prevailing ocean current patterns in the area, an inlet created on the east side of the new airport is prone to trapping significant amounts of ocean-borne debris. This litter accumulation creates a health risk to local fishermen who clean their daily catch using water from the inlet. It is proposed to install a rubble-mound groyne structure on the eastward side of the new inlet to address this problem. The utilisation of a coastline groyne in this case is somewhat unorthodox, as the latter is normally employed to mitigate against coastal erosion. The goal of this study is to optimise the groyne design with the assistance of a 3D numerical model. The ‘Delft3D’ open-source model (WAVE and FLOW modules) was selected to examine the effects of different orientations and lengths of the proposed groyne on the movements of floating debris. Included in the initial phase of the study was a field investigation to collect certain data which were necessary for model calibration and validation. This involves the use of an Acoustic Doppler Current Profiler (ADCP) to measure local shore bathymetry and also current velocities over a range of tidal cycles.

The objective of this dissertation is the development of computer simulation-based models for the modeling of upper ionosphere, starting from the first principles. The models were validated by exact analytical benchmarks and are seen to be consistent with experimentally obtained results. This area of research has significant implications in the area of global communication. In addition, these models would lead to a better understanding of the physical processes taking place in the upper ionosphere.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20415号 / 工博第4352号 / 新制||工||1675(附属図書館) / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 山本 量一, 教授 宮原 稔, 教授 稲室 隆二 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This thesis investigates the utilization of hydrodynamic models as tools for assessing factors impacting water quality in the Ross Barnett Reservoir. The primary focus is development of a hydrodynamic model that provides transport information to subsequent application of a water quality model. Environmental Fluid Dynamics Code (EFDC) is a complex, dynamic, multi-dimensional computer model used to simulate hydrology in water bodies. The secondary focus is on data acquisition and manipulation methods for completing the hydrodynamic modeling. Monitoring was completed to create modern bathymetry of Ross Barnett Reservoir to provide accurate model cell grid representation. Temperature and dissolved oxygen profile monitoring occurred to provide data for model output comparison. The EFDC model successfully predicted lake stratification and subsequent mixing based on changes in observed meteorological conditions.

No description available.

Microplastics are tiny particles that due to their small size, durability, and widespread usage have become a huge threat to the world and the environment. Aquatic environments like rivers and oceans have faced some irreparable problems such as the extinction of various marine species. Field sampling and numerical modeling are two methods that can help researchers have a better understanding of the situations to come up with the best solutions. Machine learning methods have drawn considerable attention in most engineering fields recently, which can be used in conjunction with field sampling and numerical simulation. In this study, by generating a fine mesh and using bathymetry, water level, and discharge data, a three-dimensional hydrodynamic modeling of the domain of study was conducted using TELEMAC 3D, which is a model that was used to simulate the behavior of the Fraser River in x, y, and z directions. The results were implemented to track the movements of microplastic particles in the lower part of the Fraser River. CaMPSim-3D, which is a three-dimensional Lagrangian particle tracking model was employed to track microplastic particles. This model, in addition to calculating the horizontal location of particles, computes their vertical movements too. The release locations of microplastic particles were chosen based on the locations of the wastewater treatment plants and combined sewer overflows and in the end, nine scenarios were conducted for this study. An unsupervised branch of machine learning is clustering which helps to cluster points by relying on their different properties. The OPTICS algorithm, which is a density-based clustering algorithm, was used to find the accumulation zones of microplastic particles in the lower part of the Fraser River. It should be mentioned that in all parts available measured data and information were used for validation. The results of the clustering algorithm indicated that there are eight accumulation zones in the study area and the breakwater in the upper branch of the Fraser River is an ideal place for microplastic particles to accumulate. A reasonable agreement was obtained between the model results and measured data.

This thesis presents an analysis of the signal-to-noise ratio in liquid core anti-resonant reflecting optical waveguides (ARROWs) and the application of hydrodynamic focusing to the waveguides. These concepts are presented as a method to improve the detection capabilities of the ARROW platform. The improvements are specifically targeted at achieving single molecule detection (SMD) with the devices. To analyze the SNR of the waveguides a test platform was designed and fabricated. This test platform was then used to examine relationship between the SNR and the location of the excitation region. It was determined that the excitation region should be moved closer to the solid-core. By moving the excitation region closer to the solid-core the distance the signal was required to travel in the hollow-core was reduced. This reduction led to a decrease in optical signal loss and resulted in a more than 2x increase in the SNR. Hydrodynamic focusing in the waveguides was developed as a method to increase the consistency of detection of the devices. In hydrodynamic focusing particles in the sample are forced towards the center of the waveguide with a buffer solution. With the particles focused to the center of the channel the percentage that passed through the excitation region can be increased improving the detection consistency of the device. ARROW chips designed for hydrodynamic focusing were simulated, fabricated, and preliminary testing was performed. Initial results have shown a more than 30% increase in particle focusing.

The understanding and study of electron transport in semiconductor systems has been the instigation behind the growth of semiconductor electronics industry which has enabled technological developments that are part of our everyday lives. However, most materials exhibit diffusive electron transport where electrons scatter off disorder (impurities, phonons, defects, etc.) inevitably present in the system, and lose their momentum. Advances in material science have led to the discovery of materials which are essentially disorder-free and exhibit exceptionally high mobilities, enabling transport physics beyond diffusive transport. In this work, we explore non-diffusive transport regimes, namely, the ballistic and hydrodynamic regimes in a high-mobility two-dimensional electron system in a GaAs quantum well in a GaAs/AlGaAs heterostructure. The hydrodynamic regime exhibits collective fluid-like behavior of electrons which leads to the formation of current vortices, attributable to the dominance of electron-electron interactions in this regime. The ballistic regime occurs at low temperatures, where electron-electron interactions are weak, constraining the electrons to scatter predominantly against the device boundaries. To study these non-diffusive regimes, we fabricate mesoscopic devices with multiple point contacts on the heterostructure, and perform variable-temperature (4.1 K to 40 K) zero-field nonlocal resistance measurements at various locations in the device to map the movement of electrons. The experiments, along with interpretation using kinetic simulations, demarcate hydrodynamic and ballistic regimes and establish the dominant role of electron-electron interactions in the hydrodynamic regime. To further understand the role of electron-electron interactions, we perform nonlocal resistance measurements in the presence of magnetic field in transverse magnetic focusing geometries under variable temperature (0.39 K to 36 K). Using our experimental results and insights from the kinetic simulations, we quantify electron-electron scattering length, while also highlighting the importance of electron-electron interactions even in ballistic transport. At a more fundamental level, we reveal the presence of current vortices in both hydrodynamic and surprisingly, ballistic regimes both in the presence and absence of magnetic field. We demonstrate that even the ballistic regime can manifest negative nonlocal resistances which should not be considered as the hallmark signature of hydrodynamic regime. The work sheds a new light on both hydrodynamic and ballistic transport in high-mobility solid-state systems, highlighting the similarities between these non-diffusive regimes and at the same time providing a way of effectively demarcating them using innovative device design, measurement schemes and one-to-one modeling. The similarities stem from total electron system momentum conservation in both the hydrodynamic and ballistic regimes. The work also presents a sensitive and precise experimental technique for measuring electron-electron scattering length, which is a fundamental quantity in solid-state physics. / Doctor of Philosophy / Electrons are the charged particles that are bound around the nuclei of atoms. But sometimes in a solid material electrons break free away from the nuclei and wander around. They are then the carriers of electric current ubiquitous in our daily lives as in our homes, and in our electronic devices such as smartphones and computers. Often an analogy is made between the flow of electric current in a material and the flow of water in a stream. However, the analogy does not hold well for most materials. In most materials the motion of electrons can be thought of as balls in a pinball machine - their movement hindered and randomized by collisions with the countless defects and impurities present in the material they travel through. However, recently scientists have been able to synthesize ultraclean materials, where electrons can indeed mimic the flow of water under the right conditions. In this aptly-named hydrodynamic regime, electrons predominantly interact with each other and that leads to the formation of current whirlpools or vortices similar to those forming in water. A telling signature of this regime is a negative electrical resistance appearing near the location of the vortex. When the interactions between electrons are weak, such as at very low temperatures, electrons move along straight-line trajectories until they hit and bounce off the device edges, similar to billiard balls. This low-temperature phenomenon is called ballistic transport. In this work we reveal that measurement of negative resistance and formation of current vortices are not unique to the hydrodynamic regime but can occur in the ballistic regime as well. It is indeed counterintuitive that electrons moving like billiards balls can behave similarly to electrons flowing like water. The similarities can be traced back to a fundamental physics conservation law active in both situations, namely momentum conservation. To experimentally realize the tests, we use a very high purity semiconductor material GaAs/AlGaAs and fabricate tiny devices on the material with a cutting-edge design, capable of precisely measuring resistance at various locations along the device to map the movement of electrons. The simulations of the novel physics indeed reveal current vortices of various sizes in the ballistic regime, in agreement with the experimental data showing negative resistance. In another experiment, we apply a magnetic field, making the electrons move in circular paths. If uninterrupted, electrons complete half circles and are collected through an opening in the device, giving resistance peaks in experiments. Due to electron-electron interactions, the electrons on their circular trajectory are interrupted by other electrons which leads to a decay in resistance peaks. This decay is utilized to measure the strength of electron-electron interactions. The work has both fundamental and applied implications. The existence of whirlpools shows that the electron momentum is not lost by collisions, and that in turn means that the conduction of electrical current in these regimes is inherently efficient. This opens up avenues for electronic devices which are faster, more functional and more power efficient than present electronic devices.

Converging shocks have long been a topic of interest in theoretical fluid mechanics, and are of prime importance in inertial confinement fusion. However, tracking converging shocks in numerical schemes poses several challenges. Numerical schemes based on shock capturing inherently diffuse out shocks to multiple grid cells, making it hard to track the shock. Converging shocks are significantly harder to track, as this numerical smearing is much more significant when converging shocks approach the axis of convergence. To mitigate this problem, we transform the conservation laws to a non-inertial frame of reference in which the accelerating shock is stationary. A system of equations is derived based on the transformed conservation laws coupled to the shock speed obtained from jump conditions and a characteristic-based derivation of a relation governing shock acceleration. We solve these equations using a finite volume method. Our numerical results compare favorably with the analytical value of Guderley exponent for self-similarly converging cylindrical hydrodynamic shocks. Results for fast magnetosonic shock in MHD are also presented and compared with results from geometrical shock dynamics (GSD). Results from our shock fitting method, developed without any approximation to the original ideal magnetohydrodynamics equations, provide further credibility to GSD applied to converging fast magnetosonic shocks. This sort of shock fitting is a precursor to future multidimensional stability analysis of imploding shocks.