Recent Submissions
Colmation: Unravelling physical interactions of surface and subsurface processes
(Stuttgart : Universität Stuttgart, Institute for Modelling Hydraulic and Environmental Systems, 2024) Koca, Kaan; Haun, Stefan; Wieprecht, Silke; Noack, Markus
Colmation, the infiltration and accumulation of fine sediment in gravel riverbeds, is a natural process in riverine ecosystems. However, when excessive amounts of fine sediments are transported into rivers due to human activities (e.g., intensive agriculture, mining), they can substantially clog the pores of the riverbed, reduce its hydraulic conductivity, usually leading to detrimental impacts on water quality and ecological health. Despite extensive research on colmation, considerable knowledge gaps exist regarding the spatio-temporal dynamics and interactions between near-bed and interstitial flow and processes governing colmation. This is mainly due to the lack of measurement methods that can be utilized at pore scale without disturbing the natural environment. To this end, we developed a novel smart sensor capable of monitoring and measuring sediment infiltration and deposition processes within the pores of the gravel bed. The developed sensor was compared to the industry standard gamma-ray computer tomography (Gamma CT), exhibiting good agreement across a range of infiltrating particle sizes, from sand to fine gravel. Flume experiments further demonstrated the reliability of the smart sensor in acquiring spatially-distributed information on sediment deposition dynamics at high temporal resolution and with reproducible results. While persistent technical malfunctions hindered the acquisition of interstitial flow measurements using an endoscopic PIV system, the developed sensor alone provides valuable insights into sediment accumulation processes, making it a promising tool for engineers, geomorphologists, and ecologists. A potential combination of our sensor with pore-scale velocity measurements and/or eddy-resolving simulations can be considered in the future to elucidate the interactions between local flow fields and progressive pore occlusion by fine sediments. In this respect, our contribution does not only fill a critical gap in our ability to non-destructively monitor sediment deposition process in the interstitial pore space, but also offers the potential for supporting development of more realistic, high-resolution numerical models, which are essential for understanding subsurface-surface interactions at larger scales and finally coming up with sustainable management strategies.
Swirl flow stability : thermodynamic analysis and experiments
(2024) Maršík, František; Trávníček, Zdeněk; Weigand, Bernhard; Seibold, Florian; Antošová, Zuzana
The current paper presents a theoretical analysis of swirl flow stability, both inside a tube (vortex tube) and in a free annular swirl flow. The starting concept is the study of the evolution of velocity and temperature fluctuations. Methods of non-equilibrium thermodynamics are used to describe the magnitude of fluctuations and their properties. The important role of the total enthalpy follows from a variational analysis. Moreover, the thermodynamic criterion of the stability is formulated using the total enthalpy, and compared with experiments, numerical results and classical Rayleigh theory support its applicability. It was shown that the solid body vortex is at the margin of stability, which is experimentally observed. Analogously, the potential vortex is by the thermodynamic criterion stable; however, by the Rayleigh criteria it is on the onset of stability. The classical Taylor experiment of flow between two rotating cylinders is analysed from the point of view of this criterion. These results are underlined by swirl tube experiments at the Institute of Aerospace Thermodynamics at Stuttgart University and the annular nozzle experiments performed in the Institute of Thermomechanics CAS in Prague. Both independent experiments confirm the transformation of the initial annular vortex into a stable potential-type vortex. The results of this theory can also be used to explain the exceptional stability of tropical cyclones.
A polygon model of the functional base-of-support improves the accuracy of balance analysis
(2025) Millard, Matthew; Sloot, Lizeth H.
Mathematical balance models have the potential to identify people at risk of falling before an injury occurs. However, most balance models depend on a model of the base-of-support (BOS) of the feet to calculate how well someone is balancing. Here we evaluate the functional base-of-support (fBOS): the convex polygon on the bottom of the foot that can support a large fraction of the body's weight. First, we develop a geometric model of the fBOS by measuring the center-of-pressure (COP) and kinematic data of the feet of 27 younger adults instructed to move their body mass in large loops without taking a step. We extract a planar convex polygon that contains the COP data. Finally, we compare the area of this fBOS model to a marker-based BOS model before evaluating if the fBOS differs across four everyday conditions: footwear, stance-width, foot dominance, and during single and double stance. We found that the fBOS is much smaller (23% the size) than a marker-based BOS model. Our analysis suggests that using the fBOS, rather than a marker-based BOS, can improve the accuracy of the margin-of-stability by 20% of foot width and 16% of the length. In addition, we found that the fBOS area does not differ across footwear (p=0.88), stance-width (p=0.88), and foot dominance (p=0.68), but during single stance the fBOS is 17% (p=0.0003) larger than during double stance. So that others can use and extend our work, we have made the models, example data and code publicly available.
The size of the functional base of support decreases with age
(2025) Sloot, Lizeth H.; Gerhardy, Thomas; Mombaur, Katja; Millard, Matthew
Falls occur more often as we age. To identify people at risk of falling, balance analysis requires an accurate base-of-support model. We previously developed a functional base-of-support (fBOS) model for standing young adults and showed that its area is smaller than the footprint area. Our fBOS model is a polygon that contains centre-of-pressure trajectories recorded as standing participants move their bodies in the largest possible loop while keeping their feet flat on the ground. Here we assess how the size of the fBOS area changes with age by comparing 38 younger (YA), 14 middle-aged (MA), and 34 older adults (OA).
The fBOS area is smaller in older adults: OA area is 58% of the YA area (p<0.001), and 59% of the MA area (p=0.001), with no difference between YA and MA. The reduction in fBOS area among the OA is primarily caused by a reduction in the length of the fBOS. In addition, among older adults smaller fBOS areas correlated with a lower score on the Short Physical Performance Battery (τ=0.28, p=0.04), a reduced walking speed (τ=0.25, p=0.04), and a higher frailty level (p=0.09). So that others can extend our work, we have made our fBOS models available online.
A systematic DNS approach to isolate wall-curvature effects in spatially developing boundary layers
(2024) Appelbaum, Jason; Kloker, Markus; Wenzel, Christoph
A methodology to numerically assess wall-curvature effects in boundary layers is introduced. Wall curvature, which directly induces streamline curvature, is associated with several changes in boundary-layer flow. By necessity, a local radial pressure gradient emerges to balance mean flow turning. Moreover, a streamwise (wall-tangential) pressure gradient can appear for configurations with non-constant wall curvature or a particular freestream condition; zero pressure gradient is a special case. In laminar concave flow, the Görtler instability and the associated Taylor-Görtler vortices destabilize the flow and promote laminar-turbulent transition, whereas in the fully turbulent regime, unsteady coherent structures formed by the centrifugal instability mechanism dramatically redistribute turbulent shear stress. One difficulty of assessing centrifugal effects on boundary layers is that they often appear simultaneously with other phenomena, such as a streamwise pressure gradient, making their individual evaluation often ambiguous. For numerical studies of transitional and turbulent boundary layers, it is therefore beneficial to understand the interactive nature of such coupled effects for generic configurations. A methodology to do so is presented, and is verified using the case of a subsonic, compressible turbulent boundary layer. Four direct numerical simulations have been computed, forming a 2×2matrix of turbulent boundary-layer states; namely with and without concave wall curvature, each having a zero and a non-zero streamwise-pressure-gradient realization. The setup and accompanying procedures to determine appropriate boundary conditions are discussed, and the methodology is evaluated through analysis of the mean flow fields. Differences in mean flow properties such as wall shear stress and boundary-layer thickness due to either streamwise pressure gradient or wall curvature are shown to be remarkably independent of one another.
Supercritical melt flow in high-speed laser welding and its interdependence with the geometry of the keyhole and the melt pool
(2024) Reinheimer, Eveline N.; Berger, Peter; Hagenlocher, Christian; Weber, Rudolf; Graf, Thomas
The advent of undercuts and humping limits the applicable speed of deep-penetration laser welding. Recent findings additionally show that a significant change of the keyhole’s shape is associated with the occurrence of undercuts. Considering that undercuts and humping are melt flow–induced defects, this leads to the question of how the geometry of the keyhole and the melt pool influence the melt flow and vice versa. In this work, the Froude number was used to characterize the melt flow around a keyhole. X-ray images of the keyhole and cross-sections of the weld were therefore used to determine the geometrical boundaries of the melt flow, to estimate the average melt velocity around the keyhole, and finally determine its Froude number. The flow around a cylindrically shaped keyhole was found to always be subcritical, whereas supercritical melt flow was observed around the elongated keyholes that are formed at higher welding speed. The findings may be interpreted in the sense that the elongation of the keyhole is a consequence of a supercritical stream of the melt flowing underneath and around the keyhole. This perception is consistent with the long-known experience that humping may be avoided by reducing the flow speed of the melt by widening the melt pool surrounding the keyhole (e.g., by means of beam shaping) and suggest a new explanation for the elongation of the keyhole at increased welding speed.
Optimum blue light exposure : a means to increase cell-specific productivity in Chinese hamster ovary cells
(2024) Föller, Stefanie; Regett, Niklas; Lataster, Levin; Radziwill, Gerald; Takors, Ralf
Research for biopharmaceutical production processes with mammalian cells steadily aims to enhance the cell-specific productivity as a means for optimizing total productivities of bioreactors. Whereas current technologies such as pH, temperature, and osmolality shift require modifications of the cultivation medium, the use of optogenetic switches in recombinant producer cells might be a promising contact-free alternative. However, the proper application of optogenetically engineered cells requires a detailed understanding of basic cellular responses of cells that do not yet contain the optogenetic switches. The knowhow of ideal light exposure to enable the optimum use of related approaches is missing so far. Consequently, the current study set out to find optimum conditions for IgG1 producing Chinese hamster ovary (CHO) cells which were exposed to blue LED light. Growth characteristics, cell-specific productivity using enzyme-linked immunosorbent assay, as well as cell cycle distribution using flow cytometry were analyzed. Whereas too harsh light exposure causes detrimental growth effects that could be compensated with antioxidants, a surprising boost of cell-specific productivity by 57% occurred at optimum high light doses. The increase coincided with an increased number of cells in the G1 phase of the cell cycle after 72 h of illumination. The results present a promising new approach to boost biopharmaceutical productivity of mammalian cells simply by proper light exposure without any further optogenetic engineering.
Algebraic conditions for conformal superintegrability in arbitrary dimension
(2024) Kress, Jonathan; Schöbel, Konrad; Vollmer, Andreas
We consider second order (maximally) conformally superintegrable systems and explain how the definition of such a system on a (pseudo-)Riemannian manifold gives rise to a conformally invariant interpretation of superintegrability. Conformal equivalence in this context is a natural extension of the classical (linear) Stäckel transform, originating from the Maupertuis-Jacobi principle. We extend our recently developed algebraic geometric approach for the classification of second order superintegrable systems in arbitrarily high dimension to conformally superintegrable systems, which are presented via conformal scale choices of second order superintegrable systems defined within a conformal geometry. For superintegrable systems on constant curvature spaces, we find that the conformal scales of Stäckel equivalent systems arise from eigenfunctions of the Laplacian and that their equivalence is characterised by a conformal density of weight two. Our approach yields an algebraic equation that governs the classification under conformal equivalence for a prolific class of second order conformally superintegrable systems. This class contains all non-degenerate examples known to date, and is given by a simple algebraic constraint of degree two on a general harmonic cubic form. In this way the yet unsolved classification problem is put into the reach of algebraic geometry and geometric invariant theory. In particular, no obstruction exists in dimension three, and thus the known classification of conformally superintegrable systems is reobtained in the guise of an unrestricted univariate sextic. In higher dimensions, the obstruction is new and has never been revealed by traditional approaches.
Abschlussbericht RABus (Phase 1) : Reallabor für den Automatisierten Busbetrieb im ÖPNV in der Stadt und auf dem Land
(Stuttgart : Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart (FKFS), 2025) Weinrich, Ulrike; Linnig, Magdalena; Kufky, Jonas; Grafl, Katherina; Pekyürek, Abdurrahman; Kagerbauer, Martin; Klinkhardt, Christian; Kandler, Kim; Wirkert, Sebastian; Banholzer , Matthias; Kaes , Christian; Hölbing , Enrico; Scheu , Jonathan; Frischmann , Michael
Im Projekt „RABus - Reallabor für den Automatisierten Busbetrieb im ÖPNV in der Stadt und auf dem Land“ wurden von 2020 bis 2023 die technischen, organisatorischen und gesellschaftlichen Voraussetzungen für den Einsatz automatisierter Kleinbusse im öffentlichen Nahverkehr untersucht. Ziel war es, automatisierte Fahrzeuge mit einem angestrebten Funktionsumfang nach SAE-Level 4 unter realen Bedingungen zu entwickeln, zu erproben und in bestehende Verkehrssysteme zu integrieren.
In zwei Reallaboren - Friedrichshafen und Mannheim - wurden dafür unterschiedliche städtische Strukturen betrachtet. Das Projekt umfasste neben der technischen Entwicklung und Erprobung auch infrastrukturelle Anpassungen, rechtliche Bewertungen, Maßnahmen zur Akzeptanzförderung sowie eine wissenschaftliche Begleitforschung zu verkehrlichen Wirkungen und Nutzerverhalten.
Der vorliegende Bericht dokumentiert die zentralen Erkenntnisse aus Phase 1 und bildet damit die Grundlage für die weitere Projektarbeit.
Wie wir dort gelandet sind, wo wir jetzt wegmüssen : Geschichte und Zukunft des deutschen Wissenschaftssystems
(2024) Bahr, Amrei; Eichhorn, Kristin; Kubon, Sebastian