02 Fakultät Bau- und Umweltingenieurwissenschaften
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/3
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Item Open Access Seasonal dynamics of gaseous CO2 concentrations in a karst cave correspond with aqueous concentrations in a stagnant water column(2023) Class, Holger; Keim, Leon; Schirmer, Larissa; Strauch, Bettina; Wendel, Kai; Zimmer, MartinDissolved CO2 in karst water is the key driving force of karstification. Replenishment of CO2 concentrations in karst water occurs by meteoric water that percolates through the vadose zone, where CO2 produced from microbial activity is dissolved. CO2 can thus be transported with the percolating water or in the gas phase due to ventilation in karst systems. We measured seasonally fluctuating CO2 concentrations in the air of a karst cave and their influence on aqueous CO2 concentrations in different depths of a stagnant water column. The observed data were compared to numerical simulations. The data give evidence that density-driven enhanced dissolution of gaseous CO2 at the karst water table is the driving force for a fast increase of aqueous CO2 during periods of high gaseous concentrations in the cave, whereas during periods of lower gaseous concentrations, the decline of aqueous CO2 is limited to shallow water depths in the order of 1 m. This is significant because density-driven CO2 dissolution has not been previously considered relevant for karst hydrology in the literature. Attempts at reproducing the measured aqueous CO2 concentrations with numerical modeling revealed challenges related to computational demands, discretization, and the high sensitivity of the processes to tiny density gradients.Item Open Access Experimental and simulation study on validating a numerical model for CO2 density-driven dissolution in water(2020) Class, Holger; Weishaupt, Kilian; Trötschler, OliverCarbon dioxide density-driven dissolution in a water-filled laboratory flume of the dimensions 60~cm length, 40~cm height, 1~cm thickness was visualized using a pH-sensitive color indicator. We focus on atmospheric pressure conditions, like in caves where CO2 concentrations are typically higher. Varying concentrations of carbon dioxide were applied as boundary conditions at the top of the experimental setup, leading to the onset of convective fingering at differing times. The data were used to validate a numerical model implemented in the numerical simulator Dumux. The model solves the Navier-Stokes equations for density-induced water flow with concentration-dependent fluid density and a transport equation including advective and diffusive processes for the carbon dioxide dissolved in water. The model was run in 2D, 3D, and pseudo-3D on two different grids. Without any calibration or fitting of parameters, the results of the comparison between experiment and simulation show satisfactory agreement with respect to the onset time of convective fingering as well as the number and the dynamics of the fingers. Grid refinement matters in particular in the uppermost part where fingers develop. The 2D simulations consistently overestimated the fingering dynamics. This successful validation of the model is the prequisite for employing it in situations with background flow and for a future study of karstification mechanisms related to CO2-induced fingering in caves.Item Open Access The role of retardation, attachment and detachment processes during microbial coal-bed methane production after organic amendment(2020) Emmert, Simon; Davis, Katherine; Gerlach, Robin; Class, HolgerMicrobially enhanced coal-bed methane could allow for a more sustainable method of harvesting methane from un-mineable coaldbeds. The model presented here is based on a previously validated batch model; however, this model system is based on upflow reactor columns compared to previous experiments and now includes flow, transport and reactions of amendment as well as intermediate products. The model implements filtration and retardation effects, biofilm decay, and attachment and detachment processes of microbial cells due to shear stress. The model provides additional insights into processes that cannot be easily observed in experiments. This study improves the understanding of complex and strongly interacting processes involved in microbially enhanced coal-bed methane production and provides a powerful tool able to model the entire process of enhancing methane production and transport during microbial stimulation.Item Open Access Berechnung von Temperaturfahnen im Grundwasser mit analytischen und numerischen Modellen(2022) Ohmer, Marc; Klester, Artur; Kissinger, Alexander; Mirbach, Stefan; Class, Holger; Schneider, Martin; Lindenlaub, Martin; Bauer, Michael; Liesch, Tanja; Menberg, Kathrin; Blum, PhilippMit zunehmender Anzahl an Grundwasserwärmepumpenanlagen steigt auch das Potenzial an Nutzungsüberlagerungen. Im Rahmen des wasserrechtlichen Genehmigungsverfahrens ist es daher erforderlich, u. a. die von einer Anlage ausgehenden Temperaturfahnen im Grundwasser zu berechnen. In Baden-Württemberg wurde als Erweiterung des für Anlagen ≤ 45.000 kWh/a gültigen Leitfadens des Umweltministeriums Baden-Württemberg das Thermische Online-Modell (TOM) entwickelt. Hierbei handelt es sich um ein vereinfachtes, browserbasiertes, numerisches Grundwassermodell. Ziel dieser Studie ist, die mit TOM berechneten Ausdehnungen von Temperaturfahnen durch mit analytisch und einem kalibrierten numerischen Modell berechnete Temperaturfahnen zu evaluieren. Der Vergleich mit den analytischen Berechnungsmethoden zeigt, dass diese die Ausbreitung der Temperaturfahnen in Relation zu den numerischen Modellen grundsätzlich überschätzen. Für kleine und mittlere Anlagen (< 10 l/s) wurden mit TOM vergleichbare Ergebnisse wie mit einem kalibrierten numerischen Modell berechnet. Für größere Anlagen (> 10 l/s) ist aufgrund der weiträumigeren Ausbreitung der Temperaturfelder die Simulation mit einem kalibrierten numerischen Planungsmodell zu empfehlen.