Eines der größten Probleme bei der kontrollierten Kernfusion ist die immense thermische Belastung der mit dem Plasma in Berührung kommenden Materialien. Um den Energiefluß aus dem Plasma auf eine möglichst große Fläche zu verteilen, werden die betroffenen Komponenten in der Regel so angebracht, da"s das magnetische Feld annähernd parallel zur Oberfläche verläuft. Im Rahmen dieser Arbeit wurden ein spezieller drehbarer Meßkopf zur winkelabhängigen Messung des Strom- und Energieflusses entwickelt und ausführliche experimentelle Untersuchungen zur Winkelabhängigkeit der Teilchen- und Energieflüsse auf eine Fläche durchgeführt. Zum Verständnis der zu Grunde liegenden Mechanismen wird basierend auf den Gyrationsbahnen der Teilchen ein analytisches Modell entwickelt und dessen qualitative Übereinstimmung mit den experimentellen Befunden festgestellt. Die Durchführung der Experimente erfolgte am Plasmagenerator PSI-2, einem linearen Divertor-Simulator mit einem moderaten magnetischen Feld. Der Aufbau des Meßkopfes als ebene Sonde in einer isolierten Fläche enspricht dabei in etwa der einer sogenannten ,,flush-mounted probe''''. Die äußeren Maße der Sonde sind dabei vergleichbar mit dem Ionengyroradius ri}. Während die Elektronen bei den Experimenten stark magnetisiert sind, variieren die Bedingungen für die Ionen zwischen unmagnetisiert und magnetisiert je nach Ionenmasse und Magnetfeldstärke. Bei den Experimenten wurden verschiedene Größen der Plasmarandschicht als Funktionen des Winkels zwischen der Oberflächennormale der Sonde und dem Vektor des magnetischen Feldes bestimmt. / In fusion experiments, the energy flux to the target plates is an important issue. In order to spread the heat load, surfaces are usually designed to intersect magnetic field lines at very shallow angles. In the course of this work, a sensitive probe allowing simultaneous measurements of energy flux and current density as functions of a bias voltage was developed. Extensive experimental data on the particle and energy flux densities as functions of the angle between a surface and the confining magnetic field are provided. An analytical model is developed in order to reveal the physics involved; it is in good qualitative agreement with the experimental results. The experiments were conducted at the PSI-2 facility, a linear divertor simulator with moderate magnetic field strength. The probe was rotated in a spatially homogeneous plasma. The active area, a tungsten covered Peltier module, was immersed in a ceramic surface, closely resembling the geometry of a flush mounted probe. Its dimensions were comparable to the ion gyro radius ri. While the electrons were strongly magnetized, the ion conditions varied between unmagnetized and magnetized depending on the ion species. Sheath parameters were determined as functions of the angle alpha between the probe surface normal and the magnetic field.

abstract: This thesis focuses on an improved understanding of the dynamics at different length scales of wind farms in an atmospheric boundary layer (ABL) using a series of visualization studies and Fourier, wavelet based spectral analysis using high fidelity large eddy simulation (LES). For this purpose, a robust LES based neutral ABL model at very high Reynolds number has been developed using a high order spectral element method which has been validated against the previous literature. This ABL methodology has been used as a building block to drive large wind turbine arrays or wind farms residing inside the boundary layer as documented in the subsequent work. Studies conducted in the thesis involving massive periodic wind farms with neutral ABL have indicated towards the presence of large scale coherent structures that contribute to the power generated by the wind turbines via downdraft mechanisms which are also responsible for the modulation of near wall dynamics. This key idea about the modulation of large scales have seen a lot of promise in the application of flow past vertically staggered wind farms with turbines at different scales. Eventually, studies involving wind farms have been progressively evolved in a framework of inflow-outflow where the turbulent inflow is being fed from the precursor ABL using a spectral interpolation technique. This methodology has been used to enhance the understanding related to the multiscale physics of wind farm ABL interaction, where phenomenon like the growth of the inner layer, and wake impingement effects in the subsequent rows of wind turbines are important owing to the streamwise heterogeneity of the flow. Finally, the presence of realistic geophysical effects in the turbulent inflow have been investigated that influence the flow past the wind turbine arrays. Some of the geophysical effects that have been considered include the presence of the Coriolis forces as well as the temporal variation of mean wind magnitude and direction that might occur due to mesoscale dynamics. This study has been compared against field experimental results which provides an important step towards understanding the capability of the mean data driven LES methodology in predicting realistic flow structures. / Dissertation/Thesis / Turbulent flow past a large periodic wind farm / Doctoral Dissertation Mechanical Engineering 2018

Modification of land use/land cover in South Florida has posed a major challenge in the region’s eco-hydrology by shifting the surface-atmosphere water and energy balance. Although drainage and development in South Florida took place extensively between the mid- and late- 20th century, converting half of the original Everglades into agricultural and urban areas, urban expansion still accounts for a dominant mode of surface cover change in South Florida. Changes in surface cover directly affect the radiative, thermophysical and aerodynamic parameters which determine the absorption and partitioning of radiation into different components at the Earth surface. The alteration is responsible for changing the thermal structure of the surface and surface layer atmosphere, eventually modifying surface-induced convection. This dissertation is aimed at analyzing the extent and pattern of land cover change in South Florida and delineating the associated development of urban heat island (UHI), energy flux alteration, and convective rainfall modification using observed data, remotely sensed estimates, and modeled results. Urban land covers in South Florida are found to have increased by 10% from 1974 to 2011. Higher Landsat-derived land surface temperatures (LST) are observed in urban areas (LSTu-r =2.8°C) with satisfactory validation statistics for eastern stations (Nash-Sutcliffe coefficient =0.70 and R2 =0.79). Time series trends, significantly negative for diurnal temperature range (DTR= -1°C, p=0.005) and positive for lifting condensation level (LCL > 20m) reveal temporal and conspicuous urban-rural differences in nocturnal temperature (ΔTu-r = 4°C) shows spatial signatures of UHI. Spatially higher (urban: 3, forest: 0.14) and temporally increasing (urban: 1.67 to 3) Bowen’s ratios, and sensible heat fluxes exceeding net radiation in medium and high-intensity developed areas in 2010 reflect the effect of urbanization on surface energy balance. Radar reflectivity-derived surface-induced convective rainfall reveals significantly positive mean differences (thunderstorm cell density: 6/1000 km2and rain rate: 0.24 mm/hr/summer, p < 0.005) between urban and entire South Florida indicating convective enhancement by urban covers. The research fulfils its two-fold purposes: advancing the understanding of post-development hydrometeorology in South Florida and investigating the spatial and temporal impacts of land cover change on the microclimate of a subtropical city.