Pressure jump interface law for the Stokes-Darcy coupling: Confirmation by direct numerical simulations

It is generally accepted that the effective velocity of a viscous flow over a porous bed satisfies the Beavers-Joseph slip law. To the contrary, interface law for the effective stress has been a subject of controversy. Recently, a pressure jump interface law has been rigorously derived by Marciniak-Czochra and Mikeli\'c. In this paper, we provide a confirmation of the analytical result using direct numerical simulation of the flow at the microscopic level.Comment: 25 pages, preprin

Motion by Stopping: Rectifying Brownian Motion of Non-spherical Particles

We show that Brownian motion is spatially not symmetric for mesoscopic particles embedded in a fluid if the particle is not in thermal equilibrium and its shape is not spherical. In view of applications on molecular motors in biological cells, we sustain non-equilibrium by stopping a non-spherical particle at periodic sites along a filament. Molecular dynamics simulations in a Lennard-Jones fluid demonstrate that directed motion is possible without a ratchet potential or temperature gradients if the asymmetric non-equilibrium relaxation process is hindered by external stopping. Analytic calculations in the ideal gas limit show that motion even against a fluid drift is possible and that the direction of motion can be controlled by the shape of the particle, which is completely characterized by tensorial Minkowski functionals.Comment: 11 pages, 5 figure

Design of Numerical Methods for Simulating Models of a Solid Oxide Fuel Cell

The performance of fuel cells is significantly affected by “loss mechanisms”. This work is devoted to developing concepts for the efficient numerical computation of the diffusion polarization in the porous anode of a solid oxide fuel cell (SOFC). The following topics were covered: The first part of this work is focused on the numerical verification of coupling conditions for effective viscous flows over a porous medium. It is generally accepted that the “Beavers-Joseph-Saffman slip law” holds true for a main flow direction which is tangential to the interface. However, the interface law for the effective stress has been a subject of controversy. We provide a confirmation of the “pressure jump law”, which has been recently derived by Marciniak-Czochra and Mikelic, for a range of configurations using a direct numerical simulation of the flow at the microscopic level. The second part of this work is about the derivation of a goal-oriented, a posteriori error estimator for the finite element approximation of elliptic homogenization problems based on the “Dual Weighted Residual method” of Becker and Rannacher. In general, the solution of the macroscopic equation in the homogenized model depends on effective coefficients which in turn depend on the solutions of some additional auxiliary equations. Therefore, the accuracy of the physical goal functional is influenced by the discretization error of the macroscopic and the auxiliary solutions. By employing the error estimator developed in this work we can estimate the contribution of the discretization of each sub-problem (effective model and auxiliary problems) onto the overall error. These contributions are then balanced within a successive refinement cycle to set up an efficient discretization. Local error indicators are used to steer an adaptive mesh refinement for the macroscopic problem as well as the auxiliary problems. We demonstrate the functionality of this algorithm on some prototypical homogenization problems and on an effective model developed in this work to simulate the gas transport in the anode of an SOFC. In the latter, the diffusion polarization is the quantity of interest. For a given accuracy, the application of the local mesh refinement based on the adaptive algorithm in this context decreases the number of degrees of freedom and computation time significantly compared to the global mesh refinement

Molekulardynamik feuchter granularer Medien

Ziel dieser Arbeit ist die Untersuchung eines feuchten granularen Mediums mit Hilfe einer Molekulardynamik-Simulation. Für die Simulation wurden zwei verschiedene mikroskopische Modelle von Wasserbrücken zwischen den einzelnen Teilchen des Granulats verwendet. Beide besitzen aufgrund ihrer Hysterese eine Dissipation in der Energie. Das Medium wird durch ein von außen angelegtes Potential geschert. In Abhängigkeit der Stärke dieses Potentials zeigt es einen Phasenübergang von fest nach flüssig. Die feste und flüssige Phase wird mit Hilfe der Diffusionskonstante $D$ und der granularen Temperatur $T_{Gran}$ von einander unterschieden. Der Phasenübergang wurde gefunden und dessen Parameter in Abhängigkeit der Dichte und Form der Wasserbrücken vorläufig bestimmt. Zusätzlich wurde die Anzahl der Wasserbrücken pro Teilchen für die Simulation ermittelt und mit experimentellen Daten verglichen. In Zukunft soll ein Vergleich zwischen Computer-Tomographie Daten und der Simulation mit der Hilfe von Minkowski-Funktionalen stattfinden. Der Programm-Code der Molekulardynamik-Simulation wurde durch die Simulation eines Lennard-Jones-Fluids überprüft. Für das LJ-Fluid konnte die kritische Isotherme in Übereinstimmung mit den aus der Literatur bekannten Werten gemessen werden. Die kritischen Fluktuationen der Teilchen des LJ-Fluids wurden gemessen und mit Hilfe der Minkowski-Funktionale analysiert und qualitativ beschrieben. Mit Hilfe einer Gausschen Näherung der Minkowski-Funktionale, konnte gezeigt werden, dass für die Beschreibung dieser Fluktuationen die 2-Punkt-Korrelationsfunktion nicht ausreichend ist, sondern zusätzlich $n$-Punkt-Korrelationsfunktionen verwendet werden müssen. Für die Gausschen Näherungen konnte eine Divergenz der Minkowski Funktionale bei der kritischen Temperatur $T^*_c$ und somit ein Skalierungsverhalten nachgewiesen werden. Ein Test dieses Verhaltens steht für die vollen Minkowski-Funktionale noch aus. Auch die Bestimmung der kritischen Exponenten war aufgrund numerischer Fehler nicht möglich. Das kritischen Verhalten der Minkowski Funktionale muss aber noch für weitere Systeme, wie z.B. für das 3-D-Isingmodell, untersucht werden. Es wurden bisher auch keine Auswirkungen von finite-size-Effekten untersucht

Effects of fecal microbiota transplantation in subjects with irritable bowel syndrome are mirrored by changes in gut microbiome

Irritable bowel syndrome (IBS) is a common disorder of the lower gastrointestinal tract. The pathophysiology is far from settled, but a gut microbial dysbiosis is hypothesized to be a contributing factor. We earlier published a randomized double-blind placebo-controlled clinical trial on fecal microbiota transplantation (FMT) for IBS – the REFIT trial. The present data set describes the engraftment and includes participants from the study who received active FMT; 14 participants with effect of FMT (Effect) and 8 without (No effect). Samples were collected at baseline, after 6 and 12 months. Samples from the transplants (Donor) served as a comparator. In total 66 recipient samples and 17 donor samples were subjected to deep metagenomic sequencing, and taxonomic and functional analyses were performed. Alpha diversity measures showed a significantly increased diversity and evenness in the IBS groups compared to the donors. Taxonomic profiles showed higher relative abundance of phylum Firmicutes, and lower relative abundance of phylum Bacteroidetes, compared to donors at baseline. This profile was shifted toward the donor profile following FMT. Imputed growth rates showed that the resulting growth pattern was a conglomerate of donor and recipient activity. Thirty-four functional subclasses showed distinct differences between baseline samples and donors, most of which were shifted toward a donor-like profile after FMT. All of these changes were less pronounced in the No effect group. We conclude that FMT induces long-term changes in gut microbiota, and these changes mirror the clinical effect of the treatment. The study was registered in ClinicalTrials.gov (NCT02154867)

Colonic distribution of FMT by different enema procedures compared to colonoscopy – proof of concept study using contrast fluid

Background Fecal microbiota transplantation (FMT) has become an important treatment method in recurrent Clostridioides difficile infections and is under investigation as a treatment for several other diseases. FMT’s mechanism of action is assumed to be through alterations of the colon microbiota. FMT can be delivered by several methods, but few studies have directly compared how FMT is distributed in the colon by different methods. Specifically, the proximal distribution of FMT delivered by enema is unknown. Methods In eight participants, we administered contrast fluid (CF) with viscosity similar to an FMT in a crossover study design. First, CF was administered by colonoscopy, followed by an abdominal X-ray to visualize the CF distribution. Next, after four to eight weeks, participants were given CF, but as an enema, followed by a positioning procedure. X-rays were obtained before (enema ÷) and after (enema +) the positioning procedure. Conclusion Proportion of participants with CF in cecum were 100% after colonoscopy, 50% after enema+and 38% after enema ÷. In the transverse colon, proportions were 100% (colonoscopy), 88% (enema +) and 63% (enema ÷). There were no adverse events. Interpretation This study shows proof of concept for the distribution of FMT to proximal colon when delivered by enema. A positioning procedure after the enema slightly improves the proximal distribution. However, colonoscopy is the only method that ensures delivery to the cecum. Studies are needed to see if FMT colon distribution correlates with treatment effectiveness. Trial registration The study was retrospectively registered at ClinicalTrials.gov (NCT05121285) (16/11/2021)

Optimization potentials of the transverse flux machine over the product life cycle

This study focuses on improving the performance and reliability of a transverse flux machine (TFM) for automotive applications over the whole product life cycle. TFMs offer high torque density but present challenges in electromagnetic design, cooling, and vibration control. To address these issues, different measures like additive manufacturing, sensor integration, and optimization techniques are explored and evaluated. By incorporating sensors for real-time data collection during operation and integrating structural improvements during development, TFMs can achieve higher efficiency and reliability. This study gives an overview over several topics which have been researched in 2 projects, each of which consists of 3 participating institutions. It explores the integration of vibration sensors/actuators and temperature sensors. Additionally, additive manufacturing techniques are utilized for manufacturing of soft magnetic components to reduce eddy current losses and optimize the cooling. The findings demonstrate the potential of these approaches to enhance TFMs for automotive use, and further research is recommended to assess their durability and applicability under real-world conditions

Are Terrestrial Biosphere Models Fit for Simulating the Global Land Carbon Sink?

The Global Carbon Project estimates that the terrestrial biosphere has absorbed about one-third of anthropogenic CO$_2$ emissions during the 1959–2019 period. This sink-estimate is produced by an ensemble of terrestrial biosphere models and is consistent with the land uptake inferred from the residual of emissions and ocean uptake. The purpose of our study is to understand how well terrestrial biosphere models reproduce the processes that drive the terrestrial carbon sink. One challenge is to decide what level of agreement between model output and observation-based reference data is adequate considering that reference data are prone to uncertainties. To define such a level of agreement, we compute benchmark scores that quantify the similarity between independently derived reference data sets using multiple statistical metrics. Models are considered to perform well if their model scores reach benchmark scores. Our results show that reference data can differ considerably, causing benchmark scores to be low. Model scores are often of similar magnitude as benchmark scores, implying that model performance is reasonable given how different reference data are. While model performance is encouraging, ample potential for improvements remains, including a reduction in a positive leaf area index bias, improved representations of processes that govern soil organic carbon in high latitudes, and an assessment of causes that drive the inter-model spread of gross primary productivity in boreal regions and humid tropics. The success of future model development will increasingly depend on our capacity to reduce and account for observational uncertainties