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Districting: Multi-Period and Stochastic Approaches
Get PDFDistricting involves grouping smaller areas into larger ones or dividing a larger area into multiple subareas. This thesis examines districting with a focus on multi-period dynamics and uncertainty - two aspects that have received limited attention in existing models since the field’s beginning in 1965. The consideration of these two factors not only better reflects reality, but - as shown in this thesis - taking them into account can also lead to better decisions.
A deterministic model is first proposed to serve as a basis, ensuring feasible solutions. The first major contribution of the thesis then consists of an in-depth examination of the multi-period setting, where time-dependent reassignments are allowed and a savings parameter is introduced to balance compactness and reassignments. Computational analyses are conducted to identify when the multi-period model provides advantages over static approaches. This is achieved through detailed computational experiments that perform linear correlation analysis at different levels of granularity in order to provide a comprehensive assessment of the relation between demand fluctuations and values of multi-period solutions.
The next part of the thesis then addresses the topic of districting under uncertainty in detail. To this end, we adapt a two-stage stochastic model. Modifications are made to the definition of the average allowed demand in each district, which is now scenario-dependent. The analysis focuses on performance indicators, including the value of the stochastic solution and the expected value of perfect information. The conditions under which stochastic modeling is beneficial, particularly with demand fluctuations between scenarios, are evaluated.
In the final methodological part, both uncertainty and multi-periodicity are combined in a multi-stage stochastic districting model, supported by a relax-and-fix heuristic. The model is tested on various instances, including larger instances, to assess its applicability and performance with respect to cost, demand variability, and the number of territorial units and periods.
The methodology proposed in this thesis is applied in a comprehensive case study of home healthcare services in Karlsruhe. This realistic case demonstrates the benefits of the developed models in addressing districting challenges, highlighting their potential impact on decision-making. The case study begins with a deterministic single-period setting, followed by an exploration of a multi-period framework. It then transitions to a stochastic setting before combining both time and uncertainty factors. The case study offers valuable insights into the practical application of the proposed districting models in real-world settings, along with important managerial implications
The aftermath of the pandemic: how the COVID-19 pandemic affected physical activity, fitness, health, and body fat in first-year students in Norway
Get PDFPublic health measures to limit the spread of COVID-19 included restricting physical activity (PA). Here we described the impact of pandemic restrictions and reduction in PA on physical fitness and health and body composition amongst first-year students, and the associations to body fat and total PA at the end of their first year. “On your own feet” is a longitudinal study exploring changes in lifestyle habits amongst first-year students. Questionnaires for assessment of perceived restriction, PA behaviour and fitness and health were administered at the start and end of the first year at university. Body composition (bioelectrical impedance analysis) and total PA (Actigraph®) were recorded at both time-points. In multivariable models we identified factors associated to body fat and total PA. We included 150 students aged 18-22 years, 53% of whom reported restrictions and 34% a reduction in PA due to the COVID-19 pandemic. Students reporting restrictions had comparable fitness, health, body composition and PA level at baseline and follow-up, compared to those without restrictions. Students with reduced PA less often reported “good” fitness (30% vs. 56%, p < 0.001) and health (54% vs. 70%, p = 0.046) and had higher mean body fat percentage (27% vs. 23%, p = 0.009) and lower total PA (314 vs. 420 cpm, p < 0.001) at baseline, compared to those without reduction in PA. At follow-up, they less often reported “good” physical fitness (26% vs. 54%, p = 0.005), while body composition and total PA were comparable. We concluded that students who report pandemic reduction in PA may need targeted interventions to improve fitness
Energy efficiency of single-pass electrodialysis and nanofiltration/reverse osmosis for brackish water desalination: An experimental comparison
Get PDFFrom Points to Field Scale: A Decade of Soil‐Moisture Monitoring in a German Deciduous Forest (2014–2024)
Get PDFLong-term, spatially representative soil-moisture records are critical for characterising ecosystem responses to water availability. We present a decade-long (2014–2024) dataset of continuous soil-moisture observations from distributed in situ networks and cosmic-ray neutron sensing (CRNS) across a 1 ha temperate deciduous forest in Germany. Spatial sensor coverage varied over time and challenged the derivation of a consistent spatial average due to the persistence of soil moisture patterns. We therefore implemented a semi-automatic workflow that (i) identifies reference periods via a bootstrap-based minimum required number of sensors (MRNS) and (ii) maps point measurements to the field-scale distribution using empirical CDF transformation. The resulting record provides a coherent long-term signal suitable for ecohydrological analyses and validation of remote-sensing products. Since any decade-scale monitoring will encounter sensor losses and replacements, we emphasise the critical role of robust data integration techniques to ensure the reliability of extended soil moisture datasets
Modulare thermische Modellierung und Unsicherheitsanalyse von Getriebemotor-Baukästen
Get PDFAntriebssysteme sind im Betrieb stark durch thermische Randbedingungen, insbesondere Temperaturgrenzwerte, eingeschränkt. In der Projektierung werden thermische Betrachtungen meist durch die separate Analyse einzelner Antriebskomponenten durchgeführt. Eine ganzheitliche thermische Modellierung des Gesamtsystems ermöglicht umfassendere und präzisere Aussagen über das Systemverhalten.
In dieser Arbeit wird ein Baukastensystem entwickelt, das den modularen Aufbau des Gesamtsystems aus einzelnen Komponenten ermöglicht. Es werden Modelle für die verschiedenen Antriebskomponenten erstellt und ein Algorithmus entwickelt, der diese Einzelkomponenten automatisiert zu einem Gesamtsystem zusammenführt. Der Fokus der Modellbildung liegt auf der präzisen Abbildung des makroskopischen Systemverhaltens, wobei Unsicherheiten durch Fertigungsschwankungen, Messfehler und physikalisch vereinfachte Modelle berücksichtigt werden.
Um die Verluste von Modellen in verschiedenen Betriebsbereichen genau zu bestimmen, wird eine automatisierte Methode zur Messung von Wirkungsgraden entwickelt. Diese Methode ermöglicht eine präzise Aufteilung in einzelne Verlustkomponenten und deren Unsicherheiten, indem die Temperaturen während des Messvorgangs in einem engen Temperaturband gehalten werden. Der entwickelte Messablauf gewährleistet eine stabile Temperatur, was zu genaueren und zuverlässigeren Ergebnissen führt.
Zur Validierung der Modelle werden Messungen unterschiedlicher Lastprofile für einen Solomotor und einen Getriebemotor durchgeführt. Ein Vergleich der Messergebnisse mit den Vorhersagen des thermischen Modells, einschließlich der berechneten Unsicherheiten im Temperaturverlauf, zeigt, dass die Messergebnisse innerhalb des berechneten Konfidenzintervalls liegen
An Automated Approach to Generating Card-Based Cryptographic Protocols
Get PDFCard-based cryptographic protocols provide a simple and illustrative way of performing multi-party computation without computers, but instead use just a set of playing cards. A lot of research has been done on finding minimal protocols, with respect to the number of cards or the number of protocol steps, for various functions. To automate the process of finding new card-based protocols, Koch, Schrempp, and Kirsten (2021) employed the technique of software bounded model checking for a symbolic program that implements the basic actions and states. The bounded model checker is then used to synthesize a secure protocol by automatically generating a bounded (symbolic) program run, or, if there exists no such run, prove impossibility within the given bounds.
In this thesis, we evaluate and extend the above technique for a generalization to more boolean functions, for an introduction of modularity so that (more) complex protocols can be found more efficiently, and finally for using bitwise datatype encodings so that finding protocols can be done more efficiently. From the increased efficiency and more universal applicability, we were able to extend the scope of the automated approach for generating card-based protocols to further impossibility proofs and various more protocols, for which some of them had already been found manually (but not formally verified) within literature
A micromechanical investigation of plasticity in ordered NbMoCrTiAl and disordered TaNbHfZrTi refractory compositionally complex alloys at room temperature
Get PDFRefractory compositionally complex alloys (RCCAs) are known for their exceptional high-temperature resistance. However, their inherent brittleness at room temperature limits broader practical applications. To explore the effects of microstructure and loading conditions on their deformation behavior, micromechanical experiments, including microbending and micropillar compression tests, were performed on two representative RCCAs: equimolar NbMoCrTiAl (ordered B2 crystal structure) and TaNbHfZrTi (disordered A2 crystal structure). Both alloys demonstrated significant plastic deformation, with strains exceeding 40% at room temperature. Despite prior reports of limited ductility in NbMoCrTiAl at the millimeter scale, our micropillar compression tests on single-crystalline pillars oriented along 100 and 110 reveal substantial plasticity. The dominant deformation mechanisms in NbMoCrTiAl were identified as crystallographic slip and cross-slip of screw dislocations. By contrast, TaNbHfZrTi exhibited a broader range of mechanisms, including screw dislocation slip and a high density of non-screw dislocations, accompanied by kink band formation and activation of high-order slip planes, which collectively contribute to its remarkable ductility among the highest reported for body-centered cubic RCCAs. The atomic size mismatch inherent in compositionally complex alloys enhances dislocation mobility, while the random distribution of elements promotes the formation of edge segments, further improving ductility. These findings highlight the critical role of microstructural characteristics in tailoring the deformation behavior of RCCAs for room-temperature applications
Effect of Ionomer-to-Carbon Ratio on PEMFC Carbon Corrosion: An Electrochemical Study
Get PDFThis study investigates the effect of the ionomer-to-carbon (I/C) weight ratio in polymer electrolyte membrane fuel cell (PEMFC) cathode catalyst layers (CCLs) on carbon corrosion during high-potential accelerated stress tests (ASTs). Membrane electrode assemblies (MEAs) with I/C ratios of 0.5, 0.85, and 1.2 were analyzed using polarization curves, cyclic voltammetry, limiting current measurements, and electrochemical impedance spectroscopy. Impedance data analysis, based on distribution of relaxation times and transmission line modeling, showed that higher I/C ratios (0.85 and 1.2) exhibit superior beginning-of-life (BoL) performance due to lower ionic resistance in the CCL. However, the MEA with a lower I/C ratio (0.5) exhibited a performance improvement of up to 35% during initial AST cycles and enhanced carbon corrosion resistance. Compared to BoL, performance improved significantly due to a 28%–46% reduction in charge transfer resistance and a 91% reduction in ionic resistance. These findings emphasize the trade-off between BoL performance and long-term durability when determining the optimal I/C ratio. They also underscore the need for further investigation into how the I/C ratio influences CCL structure and electrochemistry. Optimizing the I/C ratio has the potential to substantially improve PEMFC electrode performance and durability, guiding the design of more resilient catalyst layers
Testing lepton non-unitarity with the next generation of (Germanium-based) CENS reactor experiments
Get PDFCoherent elastic neutrino-nucleus scattering (CENS) has been experimentally confirmed using neutrinos from pion decay at rest, solar neutrinos and reactor antineutrinos. Future CENS experiments will foreseeable lead to precision measurements which will be a powerful tool to search for new physics beyond the Standard Model. In this work, we investigate possible deviations from unitarity in the leptonic mixing matrix that controls the propagation of active neutrinos. Such deviations may originate from the mixing with additional gauge singlet fermions and depending on their mass scale and mixing, the resulting phenomenology can differ substantially. We explore two well-motivated regimes: the \textit{seesaw limit}, where the new fermions are heavy and kinematically inaccessible, leading to effective deviations from unitarity in the active sector; and the \textit{light sterile limit}, where they are light enough to be produced and participate in neutrino propagation and scattering processes. We show how these scenarios modify both CENS and elastic neutrino--electron scattering (ES), and we present the corresponding sensitivity projections for a future CENS reactor experiment obtained by upscaling the CONUS+ experiment, which reported the first observation of reactor CENS. We identify the leading experimental systematics relevant for such an upscaling and demonstrate the resulting capability to probe TeV-scale new physics. Our results highlight the strong potential of CENS to test the structure of the lepton sector and to search for physics beyond the Standard Model
Application of the homogeneous relaxation model for flash boiling under sub-atmospheric pressures
Get PDFFlashing two-phase flows under sub-atmospheric outlet conditions in a converging–diverging nozzle are investigated using the Homogeneous Relaxation Model (HRM) within a two-phase mixture flow framework. The main objectives of this study are to conduct an in-depth investigation of low-temperature, low-pressure flash evaporation, which is essential in flash-based wastewater purification and power generation systems that utilize low-grade waste heat as an energy source, and to support the improvement of a proof-of-concept experimental setup currently being established in our laboratory through the findings of this study.
The numerical results demonstrate that the mathematical model accurately reproduces pressure and void fraction distributions reported in the literature. It also captures key flashing features—including pressure undershoots, vapor generation delays, and pressure recovery—through the relaxation-time formulation. The results indicate that flashing flow in a converging–diverging nozzle is characterized by a sharp pressure drop near the throat, followed by rapid vapor generation and partial pressure recovery in the diverging section. This behaviour is primarily governed by nozzle geometry and the large disparity in specific volumes between the liquid and vapor phases. Vapor generation increases markedly at higher inlet pressures and temperatures, driven by the greater availability of superheat energy. The simulations further reveal that the mass flow rate is highly sensitive to inlet conditions: elevated inlet temperatures intensify vapor generation and consequently reduce mass flow rate, whereas achieving both high vapor production and high mass flow rates requires sufficiently high inlet pressures. The model also predicts shorter flash-delay distances at higher pressures, indicating an earlier onset of phase change, while longer delays occur at elevated temperatures due to increased metastability. Additionally, pressure undershoots become more pronounced with higher inlet temperatures, whereas their dependence on inlet pressure is negligible. It is found that, under fixed inlet conditions, lower sub-atmospheric back pressures enhance steam generation and promote pressure recovery after the nozzle throat, while simultaneously reducing the mass flow rate and the magnitude of pressure undershoots