OPUS Online Publikationen der Universität Stuttgart
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Revisiting the modelling of mixing time scales for Lagrangian filtered density function methods
No full textMixing models for multiple mapping conditioning (MMC) methods are revisited as some details of their implementation have not yet been assessed. We use simulations of scalar mixing in non-reacting homogeneous isotropic decaying turbulence (HIT) such that (1) key modelling parameters can be taken from the direct numerical simulations without incurring additional modelling uncertainties and (2) direct validation is possible. Variants of Curl’s model are studied and direct comparison is sought with the variants’ performances in the context of standard (intensive) and sparse (such as MMC) particle approaches for the modelling of the probability density function (PDF). The second aim is to show the relative importance of micro-mixing and spatial diffusion in the presence of differential diffusion. The results demonstrate that MMC approximates the correct relaxation towards Gaussian independent of the mixing model’s variant. This is different from the standard PDF approach that requires a clear spatial localization given by the computational mesh to achieve a similar outcome. This spatial localization is not needed in MMC as the MMC mixing model already employs a localization in reference space. Differential diffusion effects can, however, only be accurately predicted if not only mixing but also spatial transport accounts for the differences in the molecular diffusion term. It is insufficient to adjust the mixing time scales only and future MMC models may require adjustments for accurate prediction capabilities.Projekt DEALDeutsche Forschungsgemeinschaf
Phase transition in porous materials : effects of material parameters and deformation regime on mass conservativity
No full textPhase transition in porous materials is relevant within different engineering applications, such as freezing in saturated soil or pancake sea ice. Mathematical descriptions of such processes can be derived based on Biot’s consolidation theory or the Theory of Porous Media. Depending on parameters such as density ratio, permeability or compressibility of the solid matrix, either small or finite deformations occur. Numerical solution procedures for the general, finite deformation case, suffers from instabilities and high computational costs. Simplifications, assuming small deformations, increases stability and computational efficiency. Within this work shortcomings of simplified theories based on Biot and linearisations of the Theory of Porous Media (TPM) are systematically studied. In order to determine the interaction of the different model parameters a non-dimensional model for poro-elasticity is presented. Based on a characteristic test-case including phase-transition and consolidation, the simplified models are compared to the fully non-linear TPM, focusing on mass errors as well as the time behaviour of the solution. Taking further into account the efficiency of discretisation based on different primal variables and finite-element-spaces, a guideline for selecting an appropriate combination of model, kinematic assumption and discretisation scheme is presented.Bundesministerium für Bildung und ForschungDeutsche ForschungsgemeinschaftBundesministerium für Wirtschaft und Klimaschut
Mapping vulnerability to climate change for spatial planning in the region of Stuttgart
No full textThe Stuttgart region in southwest Germany already experiences heat stress and extreme precipitation events. According to German law, spatial planning at the municipal and regional levels has an important role in adapting to such events. However, this is a challenge to achieve alongside other demands on land use. One important resource to support adaptive planning is spatial risk analyses, which can provide justification for prioritising adaptation and information about where and how to prioritise different measures. Such maps should not just consider information on the nature of the hazards but also on the vulnerability of people and exposed areas. While in theory this has been recognised and vulnerability analysis methods have been developed, there is a significant gap in linking this research to planning practice in the German context. In this paper, we use a GIS-based method for mapping quantitative indicators of social vulnerability to heat and pluvial flooding for the region of Stuttgart. We share insights from the process of developing these maps based on the needs of spatial planning and discuss how such information can be used in planning practice. We propose solutions regarding issues such as spatial resolution, indicator selection, aggregation, and complexity; report initial feedback from planners; and make recommendations for further bridging the gap between risk and vulnerability research and planning practice.German Federal Ministry of Education and Research (BMBF
Optimization of transferable force fields based on reduced order and surrogate models
No full textClassical transferable force fields are utilized to make predictions about physical properties and microscopic processes in both pure substances and mixtures. The process of developing these force fields involves optimizing the nonbonded interactions through multidimensional optimization techniques, utilizing computationally intensive Monte Carlo and molecular dynamics simulations. Due to the high correlation of force field parameters, this optimization process can be challenging. To improve efficiency, physically-based surrogate models were utilized to expedite the optimization process. In particular, new reduced order models for transport properties based on entropy scaling were developed. Now transport properties such as viscosity, thermal conductivity and self-diffusion coefficients can be represented in the optimization of force fields through surrogate models alongside static properties, which was previously not possible. Furthermore, machine-learned models for transport properties based on linear multifidelity Gaussian processes utilized in reduced residual entropy space were developed. Here, experimental data is combined with simulation results which enables to enhance predictive capabilities and optimize force fields more effectively.Klassische transferierbare Kraftfelder werden verwendet, um Vorhersagen über physikalische Eigenschaften und mikroskopische Prozesse sowohl von reinen Substanzen als auch von Gemischen zu treffen. Bei der Entwicklung dieser Kraftfelder werden die nicht gebundenen Wechselwirkungen durch mehrdimensionale Optimierungsverfahren optimiert, wobei rechenintensive Monte-Carlo- und Molekulardynamiksimulationen zum Einsatz kommen. Aufgrund der hohen Korrelation der Kraftfeldparameter kann dieser Optimierungsprozess eine Herausforderung darstellen. Um die Effizienz zu verbessern, verwenden wir physikalische Ersatzmodelle (Surrogate Models), um den Optimierungsprozess zu beschleunigen. Insbesondere wurden neue Modelle für Transporteigenschaften entwickelt, die auf Entropieskalierung basieren. Dies ermöglicht die Berücksichtigung von Transporteigenschaften wie Viskosität, Wärmeleitfähigkeit und Selbstdiffusionskoeffizient bei der Optimierung von Kraftfeldern gleichberechtigt neben statischen Eigenschaften, was bisher nicht möglich war. Darüber hinaus werden Machine Learning Modelle für Transporteigenschaften entwickelt, die auf experimentellen Daten und Simulationsergebnissen basieren. Dies wird es uns ermöglichen, unsere Vorhersagefähigkeiten zu verbessern und Kraftfelder effektiver zu optimieren. Die Modelle basieren auf linearen Multifidelity Gauß-Prozessen, die im reduzierten residuellen Entropieraum verwendet werden
Experimental investigation of the heat transfer characteristics, operating limits, and temperature distribution of a prototypically 3 m long two-phase closed thermosyphon for spent fuel pool passive cooling
No full textThe current study investigated the heat transfer performance, operation limits, and temperature distribution of a 3 m prototype model for a large-scale straight two-phase closed thermosyphon with deionized water designed for passive cooling of nuclear spent fuel pools with a filling ratio ranging from 20% to 100% and a heat source temperature ranging from 45 to 80 °C. After the accident at the Fukushima Daiichi nuclear power plant, the need for a reliable cooling system for these pools increased, with thermosyphons emerging as a promising solution for passive cooling. The experimental procedure implemented in this study yielded a comprehensive understanding of the operation and phenomena inside a thermosyphon, providing crucial data for the validation and enhancement of numerical models that simulate relevant phenomena within nuclear power plants, including passive residual heat removal with thermosyphons. The results indicated that the optimal heat transfer performance was achieved at a filling ratio of 30%, where the thermosyphon begins operation at a heat source temperature of 45 °C. Additionally, the temperature distribution along the thermosyphon confirmed the operation limits, including partial dryout at a 20% filling ratio and geyser boiling and flooding (entrainment) limits at 75% and 100% filling ratios, respectively.Projekt DEA
Aggregate morphing of self-aligning soft active disks in semi-confined geometry
No full textWe study the dependence of alignment and confinement on the aggregate morphology of self-aligning soft disks(particles) in a planer box (two dimensional) geometry confined along y direction using Langevin dynamics simulations. We show that when the box width decreases, the aggregate wall accumulation becomes non-uniform and displays non-monotonic behaviour in terms of phase behavior and height of these aggregates with an increase in alignment strength. Additionally, we identify two distinct categories of wall aggregates: layered and non-layered structures each exhibiting distinct local structural properties. For non-layered structures, local speed of the particles stay nearly constant as we move away from the boundary, while for layered structures, it increases with distance from the boundary. Our analysis shows that active pressure difference is a useful indicator for different aggregate morphologies and the peaks in the pressure curve are indicative of the average and minimum height of the structure
Transversal compression drawing for the efficient manufacturing of high-strength sheet metal parts
No full textDer steigende Bedarf an Stahlprodukten sowie die damit verbundenen Treibhausgasemissionen und begrenzten Ressourcen stellen eine globale gesellschaftliche und ökologische Herausforderung dar. Dadurch steigen die Anforderungen an eine nachhaltige und wirtschaftliche Herstellung maßhaltiger Blechbauteile. Höchstfeste Stahlbleche ermöglichen eine Verringerung der Blechdicke, wodurch Leichtbauvorteile und eine Reduktion des Rohstoffbedarfs erzielt werden können. Allerdings führen sie bei konventionellen Umformverfahren zu einem Zielkonflikt zwischen Herstellbarkeit und Effizienz, da qualitätssteigernde Maßnahmen oft einen höheren Energie- und Materialeinsatz erfordern. Die steigenden Festigkeiten moderner Stahlbleche bringen die bisherigen Umformprozesse, Werkzeug- und Pressentechniken an ihre Grenzen, weshalb innovative Ansätze notwendig sind, um Materialverluste, Energieverbrauch und Ausschuss in der Produktion nachhaltig zu minimieren. Im Rahmen dieser Arbeit wurde mit dem Tiefdrücken ein neuartiges Blechumformverfahren zur effizienten und maßhaltigen Herstellung von Biegeformteilen aus höchstfesten Stahlblechen entwickelt und analysiert. Durch die Konsolidierung von Lösungsansätzen zur Reduktion der Rückfederung mithilfe von Spannungsüberlagerung sowie durch Maßnahmen zur Senkung des Energie- und Materialbedarfs wird mit dieser Arbeit eine neuartige Umformkinematik für Biegeformteile vorgeschlagen. Die Substitution des Blechhalters durch einen konturierten Schlitten ermöglicht eine formschlüssige, horizontale und zugleich vertikale (biaxiale) Kraftübertragung auf die Platinenkanten sowie eine Kalibrierung des Flanschbereichs am Ende des Umformvorgangs. Dies führt zu einem werkzeuggesteuerten, reproduzierbaren und progressiven Platineinlauf mit geringem Einfluss der Reibungsbedingungen, der eine Umformkinematik mit kontinuierlicher Überlagerung von Biege- und Druckspannungen durch Knick- und Rollbiegevorgänge erzeugt. Numerische FE-Simulationen und Umformversuche zeigten, dass diese Umformkinematik sowohl die Rückfederung als auch die Blechausdünnung reduziert und gleichzeitig die energieintensive, reibungskontrollierte Blechrückhaltung vermeidet. Im Vergleich zum Tiefziehen zeigt das Tiefdrückverfahren somit neben einer verbesserten Maßhaltigkeit und Effizienz eine erhöhte Prozessrobustheit sowie ein gesteigertes Restumformvermögen der Bauteile. Angesichts der heutigen Anforderungen an Produktionsprozesse von Strukturbauteilen aus hochfesten Stahlblechen in Bezug auf Qualität, Zeit, Kosten und Nachhaltigkeit kann durch den Einsatz dieser Technologie ein Beitrag zur Steigerung der Maßhaltigkeit, des Formänderungsvermögens sowie der Energieeffizienz und des Materialnutzungsgrades geleistet werden. Das Tiefdrücken bietet somit eine konkrete Alternative zur Erweiterung der Prozessgrenzen konventioneller Tiefziehverfahren für eine effiziente und maßhaltige Herstellung von Blechbauteilen
Acoustical properties of the new sandwich structures for aircraft cabin interiors with integrated vacuum insulation
No full textAny modern passenger aircraft must provide a high level of comfort for passengers who spend considerable time inside the cabin. The cabin's climate and interior noise levels contribute to this comfort. Vacuum insulation panels (VIP) have been explored as insulation materials to improve these factors due to their extremely low thermal conductivity. When integrating VIPs into the aircraft cabin's interior, it was discovered that the thermal conductivity of the entire sandwich structure was 3-6 times lower than conventional structures. This finding has generated much interest in using VIPs for aircraft cabin insulation. This article delves into the acoustic properties of these new structures featuring integrated VIPs. Tests were carried out to analyze the sound insulation capabilities of these structures. The results showed that the new interior structures exhibited promising acoustic properties.Projekt DEA
Analysis of laser-induced damage threshold of circular grating waveguide structures exposed to sub-picosecond laser radiation centered at a wavelength of 1030 nm
No full textThe laser-induced damage threshold of a grating waveguide output coupler (GWOC) exposed to laser radiation at a wavelength of 1030 nm and with a pulse duration of 500 fs was investigated. The GWOC is a combination of a sub-wavelength circular grating and a partial reflector based on a Nb2O5 and SiO2 multilayer sequence. It was designed to be used as an output coupler of a thin-disk laser cavity for the generation of beams with radial polarization. The results revealed a laser-induced damage threshold (LIDT) fluence of 0.36 J/cm² for single-pulse tests and 0.26 J/cm² for multiple-pulse conditions with up to 1000 shots. These threshold values are comparable to those of an unstructured output coupler with Nb2O5 and SiO2 coating layers, highlighting the minor influence of the grating on the LIDT.Projekt DEALEuropean Union’s Horizon 2020 research and innovation progra
Uncertainty-biased molecular dynamics for learning uniformly accurate interatomic potentials
No full textEfficiently creating a concise but comprehensive data set for training machine-learned interatomic potentials (MLIPs) is an under-explored problem. Active learning, which uses biased or unbiased molecular dynamics (MD) to generate candidate pools, aims to address this objective. Existing biased and unbiased MD-simulation methods, however, are prone to miss either rare events or extrapolative regions-areas of the configurational space where unreliable predictions are made. This work demonstrates that MD, when biased by the MLIP’s energy uncertainty, simultaneously captures extrapolative regions and rare events, which is crucial for developing uniformly accurate MLIPs. Furthermore, exploiting automatic differentiation, we enhance bias-forces-driven MD with the concept of bias stress. We employ calibrated gradient-based uncertainties to yield MLIPs with similar or, sometimes, better accuracy than ensemble-based methods at a lower computational cost. Finally, we apply uncertainty-biased MD to alanine dipeptide and MIL-53(Al), generating MLIPs that represent both configurational spaces more accurately than models trained with conventional MD.Deutsche Forschungsgemeinschaf