@phdthesis{Klein2016, author = {Klein, Marten}, title = {Inertial wave attractors, resonances, and wave excitation by libration : direct numerical simulations and theory}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-41712}, school = {BTU Cottbus - Senftenberg}, year = {2016}, abstract = {Resonance phenomena are ubiquitous in Nature. Resonance means that a system can accumulate large amounts of kinetic energy. In rotating flows inertial waves provide a mechanism for resonance by redistributing momentum, kinetic energy and helicity. In order to investigate inertial waves a Taylor-Couette system was investigated which consists of a homogeneous liquid confined between two coaxial cylinders and two rigid lids. The inner cylinder is slightly conical (frustum) to break the vertical mirror symmetry. Inertial waves were excited by two different forcing configurations: the frustum in libration and the lids together with the outer cylinder in libration. Libration means that the rotation rate of the wall is modulated with a fixed amplitude and frequency of the order of the mean rotation rate. Direct numerical simulations (DNS) were conducted with a numerical solver in terrain-following coordinates. DNS results reveal that inertial wave excitation is localised at the edges of the confinement, which is in very good agreement with recent laboratory measurements of Seelig (2014, PhD thesis, BTU Cottbus - Senftenberg). A model of the wave excitation mechanism was developed with the aid of boundary layer theory. The model suggests that a difference in the boundary layer mass flux (Ekman flux) excites the waves by driving an excess Ekman pumping velocity at the edges. The DNS results exhibit this flux difference, and the simulated kinetic energy spectra of the waves exhibit the frequency dependency predicted by the model. However, DNS results also exhibit helical vortices at the edges which are not part of the model. Conservation properties suggest that these vortices are merely a compensating phenomenon which tends to stabilise the boundary layer flow. The details of this flow, however, appear less important for the wave excitation. Response spectra of the kinetic energy, the dissipation rate, the helicity, and the quality factor were computed in order to assess resonance conditions. Simulated resonance peaks have a width of only 1/20th of the mean rotation rate. At these peaks, the kinetic energy was found to increase by a factor 10-50 even though viscous forces were still rather large (Ekman number of the order 1/100,000 with the length scale given by the mean radial gap width). The flow patterns found at those resonances were investigated and found to be in very good agreement with the spatial patterns obtained by laboratory measurements and geometric ray tracing. The DNS results suggest that there are two types of resonance in rotating flows: modes and wave attractors. In contrast to a mode, a wave attractor exhibits net focusing of wave energy and occupies a finite frequency band. DNS results show that the wave attractor resonance frequency adjusts within the frequency band which suggests that wave attractor resonances complement 'classical' mode resonances and may, thus, be relevant in various applications.}, subject = {Direct Numerical Simulations; Rotating flows; Resonance; Inertial waves; Boundary layer theory; Direkte Numerische Simulationen; Rotierende Str{\"o}mungen; Resonanz; Tr{\"a}gheitswellen; Grenzschichttheorie; Direkte numerische Simulation; Rotationsstr{\"o}mung; Tr{\"a}gheitswelle; Taylor-Couette-Str{\"o}mung}, language = {en} } @masterthesis{Niebisch2016, type = {Bachelor Thesis}, author = {Niebisch, Michael}, title = {Generierung von Niederschlagszeitreihen f{\"u}r Niederschlagsstationen in der Niederlausitz}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-41565}, school = {BTU Cottbus - Senftenberg}, year = {2016}, abstract = {Die zeitliche Variabilit{\"a}t des Niederschlages auf Skalen von Jahren bis hin zu Ereignissen ist von wesentlicher Bedeutung f{\"u}r die Untersuchung hydrologischer Prozesse. Langj{\"a}hrige Reihen von Niederschlagsdaten mit einer hohen zeitlichen Aufl{\"o}sung stehen nur selten zur Verf{\"u}gung. Eine M{\"o}glichkeit, die Einfl{\"u}sse der Variabilit{\"a}t von Niederschl{\"a}gen auf die hydrologischen Prozesse dennoch zu quantifizieren, ist es, stochastisch generierte Reihen als Eingangsdaten f{\"u}r hydrologische Modelle zu nutzen. In dieser Arbeit wurde ein stochastischer Niederschlagsgenerator der aus zwei Stufen besteht, entwickelt. Zun{\"a}chst erfolgt die Generierung von Niederschlagsereignissen durch ein Poisson-Rectangular-Pulse (PRP) Modell, anschließend werden diese Ereignisse durch eine mikrokanonische multiplikative Zufallskaskade disaggregiert. Die Methodik wurde in die frei zug{\"a}ngliche Software R implementiert.}, subject = {Niederschlagsgenerator; Niederschlag; Disaggregierung; Niederlausitz; Disaggregation; Precipitation generator; Niederlausitz}, language = {de} } @masterthesis{Krueger2015, type = {Bachelor Thesis}, author = {Kr{\"u}ger, Simone}, title = {The impact of discharge regulation on microbial sediment respiration at a land-water-interface of the river Spree}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-40496}, school = {BTU Cottbus - Senftenberg}, year = {2015}, abstract = {Flooding of dry sediments is known to trigger pulses of microbial respiration at land-water-interfaces. The regulation of discharge variability is therefore proposed to affect the respiration balance of these sites. In this study, I assessed the impact of discharge regulation on microbial respiration associated to surface sediments at a land-water-interface of the river Spree. I developed a theoretical model, based on empirical respiration data, to estimate the two-month total respiration at the study site for three discharge scenarios. The real scenario represented the actual discharge at the study site, which was regulated by the Spremberg reservoir dam. In the unregulated scenario, the regulating effect by the dam was excluded. In the extremely regulated scenario, a hypothetical constant discharge was modeled. For each scenario, the daily discharge, the corresponding flooded areas, the extent of dry or rewetted areas, and the durations of flooding or rewetting by rain at the study site were determined. Microbial respiration rates associated to surface sediments were measured with a respirometer under flooded, dry, and rewetted conditions. The model applied these rates to the respective flooded, dry or rewetted areas of the study site, to calculate the daily areal respiration. In all sediments from the land-water-interface, a distinctive respiration pulse was measured on the first day of flooding, and higher respiration rates under flooded than under dry conditions. The discharge of the unregulated scenario was characterized by a higher variability and larger flow volume than the regulated real scenario. Due to the higher total discharge, larger areas were flooded in the unregulated scenario, the total respiration from sediments under long-term flooded conditions was therefore higher. Moreover, the daily extent of flooded areas fluctuated more strongly, hence more short-term respiration pulses upon flooding were triggered than in the real regulated scenario. The calculated total two-month respiration of the unregulated scenario exceeded that of the regulated real scenario by almost 14 \%. These results suggest that discharge regulation can have a considerable negative impact on sediment-associated microbial respiration at land-water-interfaces.}, subject = {Respiration; Land-water-interface; Spree; Atmung; Bodenatmung; Flussufer}, language = {en} }