In this thesis, I examine the variation in plant species richness along gradients of productivity and disturbance in grasslands and forest habitats in southern Sweden, and I compare the documented patterns with theoretical predictions. Moreover, I evaluate the relative importance of habitat quality and habitat configuration for the occurrence of field layer species in deciduous forests. Finally, I present a new method for the determination of the regional species pool. To examine regional and local variation in plant species richness, I gathered data on species composition in plots of different size (0.001 - 1000 m2) in three vegetation types (deciduous forests, dry grasslands and coastal meadows) in four regions of southern Sweden (Öland, Gotland, Småland and Uppland). As predicted by the species pool hypothesis, differences in small-scale species richness of deciduous forests and dry grasslands were correlated with differences in the size of the regional species pool. Moreover, among plots large-scale diversity was predictive of small-scale diversity. Species diversity showed a hump-shaped relationship with productivity in forests, and was related to environmental heterogeneity and the size of the 'habitat-specific' species pool. In the two types of grassland examined, grazed sites were richer in species than abandoned sites. Moreover, both species composition and the representation of plants with different life-history characteristics differed between grazed and abandoned sites. As predicted by the intermediate disturbance hypothesis, species richness was highest at intermediate levels of grazing in coastal meadows. However, all the above patterns were scale-dependent, and not observed at all plot sizes. The occurrence of field layer species in deciduous forests was more strongly related to habitat quality (mainly soil factors) than to habitat configuration (forest area and isolation). Across species, low seed production, clonal reproduction and habitat specificity were negatively associated with isolation.

Actuellement, la flexibilité et le stockage de l'énergie sont parmi les principaux défis de l'industrie de l'énergie. Les stations de transfert d'énergie par pompage (STEP), en utilisant des turbines-pompes réversibles, comptent parmi les solutions les plus rentables pour répondre à ces besoins. Pour assurer un réglage rapide du réseau électrique, les turbines-pompes sont sujettes à de rapides changements entre modes pompage et turbinage. Elles sont souvent exposées à un fonctionnement prolongé dans des conditions hors nominal. Pour assurer la stabilité du réseau, la zone d'exploitation continue de turbines-pompes réversibles doit être libre de toute instabilité hydraulique. Deux sources principales d'instabilités en mode pompage peuvent limiter la plage de fonctionnement continu. Il s'agit de la présence de cavitation et de décollement tournant, tous deux survenant à charge partielle. La cavitation peut conduire à des vibrations, des pertes de performance et parfois même à l'érosion de la turbine-pompe. En outre, en raison de décollements tournants (apparition et décomposition périodique de zones de recirculation dans les régions du distributeur), la machine peut être exposée à un changement incontrôlable entre les points de fonctionnement, avec une modification de charge et une baisse significative des performances. Les deux phénomènes sont très complexes, tri-dimensionnels et délicats à étudier. Surtout le phénomène de décollement tournant dans les turbines-pompes est peu abordé dans la littérature. Le premier objectif de l'étude du doctorat présenté a été d'utiliser un code numérique, testé au laboratoire, et de développer une méthodologie de calcul pour permettre la prévision des phénomènes à charge partielle. L'étude a été faite sur une géométrie à échelle réduite d'une turbine-pompe de haute chute. Des calculs numériques ont été effectués en utilisant le code FINE/Turbo avec le modèle de cavitation barotrope qui a été développé au laboratoire. L'analyse des écoulements cavitants a été faite pour des débits et de niveaux de cavitation différents. Les principales analyses portent sur des valeurs naissantes de cavitation, des courbes de chute et sur le prédiction des formes de cavitation pour différents débits et valeurs de NPSH. Une attention particulière a été portée sur l'interaction entre les formes de cavitation à l'entrée de la roue et la baisse de performance (zone de feston), causée par le décollement tournant qui apparaît dans la région du distributeur. Les résultats numériques ont montré un bon accord avec les données expérimentales disponibles. La deuxième partie de la thèse a concerné la prédiction et l'analyse de décollements tournants. Des simulations ont été utilisées pour prédire les régions d'exploitation stables et instables de la machine. La méthodologie mentionnée pourrait fournir des résultats globaux précis pour différents points de fonctionnement avec un faible coût de calcul. Afin d'obtenir des informations détaillées sur les écoulements instables, des simulations instationnaires plus précises ont été réalisées. L'analyse locale des écoulements a permis la description des mécanismes gouvernant le phénomène de décollement tournant. Les analyses permettent l'étude du nombre, de l'intensité et des fréquences de rotation des cellules tournants. En outre, les calculs instationnaires donnent une très bonne prédiction de la performance de la turbine-pompe. L'approche proposée est fiable, robuste et précise. La méthodologie de calcul proposée peut être utilisée sur plusieurs géométries de turbine-pompe (ou pompe centrifuge), pour une large gamme de débits et de géométries de directrices. Les simulations proposées peuvent être utilisées à l'échelle industrielle pour étudier les effets de géométrie, d'angles d'ouverture de directrices ou de l'influence du jeu entre la roue et le distributeur afin de réduire ou même éliminer les effets négatifs des décollements tournants. / Flexibility and energy storage seem to be the main challenges of the energy industry at the present time. Pumped Storage Power Plants (PSP), using reversible pump-turbines, are among the most cost-efficient solutions to answer these needs. To provide a rapid adjustment to the electrical grid, pump-turbines are subjects of quick switching between pumping and generating modes and to extended operation under off-design conditions. To maintain the stability of the grid, the continuous operating area of reversible pump-turbines must be free of hydraulic instabilities. Two main sources of pumping mode instabilities are the presence of the cavitation and the rotating stall, both occurring at the part load. Presence of cavitation can lead into vibrations, loss of performance and sometimes erosion. Moreover, due to rotating stall that can be observed as periodic occurrence and decay of recirculation zones in the distributor regions, the machine can be exposed to uncontrollable shift between the operating points with the significant discharge modification and the drop of the efficiency. Both phenomena are very complex, three-dimensional and demanding for the investigation. Especially rotating stall in the pump-turbines is poorly addressed in the literature. First objective of the presented PhD study has been to develop the cost-efficient numerical methodology in order to enable the accurate prediction and analysis of the off-design part load phenomena. The investigations have been made on the reduce-scaled high head pump-turbine design (nq = 27rpm) provided by Alstom Hydro. Steady and unsteady numerical calculations have been performed using code FINE/Turbo with barotropic cavitation model implemented and developed before in the laboratory. Some of the numerical results have been compared to the experimental data. Cavitating flow analysis has been made for various flow rates and wide range of cavitation levels. Flow investigation has been focused on the cavitation influence on the flow behavior and on the performance of the machine. Main analyses include incipient cavitation values, head drop curves and cavitation forms prediction for wide ranges of flow rates and NPSH values. Special attention has been put on the interaction between cavitation forms and the performance drop (hump zone) caused by the rotating stall. Cavitation results showed good agreement with the provided experimental data. Second part of the thesis has been focused on the prediction and analysis of the rotating stall flow patterns. Computationally fast steady simulations has been presented and used to predict stable and unstable operating regions. The analyses have been done on 4 different guide vanes openings and 2 guide vanes geometries. In order to get detailed information about the unsteady flow patterns related to the rotating stall, more exact unsteady simulations have been performed. Local flow study has been done to describe in details the governing mechanisms of the rotating stall. The analyses enable the investigations of the rotating stall frequencies, number of stalled cells and the intensity of the rotating stall. Moreover, the unsteady calculations give very good prediction of the pump-turbine performance for both, stable and unstable operating regions. Numerical results give very good qualitative and quantitative agreement with the available experimental data. The approach appears to be very reliable, robust and precise. Even though the numerical results (rotating stall frequencies, number of cells...) on the actual geometry should be confirmed experimentally, author believes that the methodology could be used on any other pump-turbine (or centrifugal pump) geometry. Moreover, the simulations can be used industrially to study the effects of the guide vanes geometries, guide vanes opening angles and influence of the gap between the impeller and the distributor in order to reduce or even eliminate the negative effects of the rotating stall.

No description available.

Annotation This thesis is part of Project of the Grant Agency of the Czech Republic No. P504/11/1151 (Role of plants in the greenhouse gas budget of a sedge fen) aims at estimating the turnover of aboveground biomass of Carex acuta L. in a flooded part of the Wet Meadows near Třeboň. The biomass turnover was estimated using both destructive and non-destructive types of measurement and was assessed separately for generative and vegetative tillers, which markedly differ in their life cycles. Vegetative tillers last throughout the vegetation season. Generative tillers have a fast development at the beginning of the vegetatin season but die out soon after seed development. They disapper from the stand in mid-July. Vegetative shoots reached their maximum aboveground biomass at the end of June. The average dry weight of all leaves produced so far was 1,51 g per tiller and the average dry weight of all aboveground biomass produced per tiller was 2,28 g. The turnover coefficient was 1,39 year-1 and 1,01 year-1 for leaves and whole tillers, respectively. Also generative shoots reached their maximum aboveground biomass at the end of June. The average dry weight of aboveground biomass produced so far was 1,95 g per tiller. The turnover coefficient was 1, 38 year-1and 1,10 year-1.

In this thesis I hope to provide a clear and concise introduction to Feigenbaum scaling accessible to undergraduate students. This is accompanied by a description of how to obtain numerical results by various means. A more intricate approach drawing from renormalization theory as well as a short consideration of some of the topological properties will also be presented. I was furthermore trying to put great emphasis on diagrams throughout the text to make the contents more comprehensible and intuitive.