OPUS Online Publikationen der Universität Stuttgart
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Nonequilibrium thermodynamics of quantum coherence beyond linear response
No full textQuantum thermodynamics allows for the interconversion of quantum coherence and mechanical work. Quantum coherence is thus a potential physical resource for quantum machines. However, formulating a general nonequilibrium thermodynamics of quantum coherence has turned out to be challenging. In particular, precise conditions under which coherence is beneficial to or, on the contrary, detrimental for work extraction from a system have remained elusive. We here develop a generic dynamic-Bayesian-network approach to the far-from-equilibrium thermodynamics of coherence. We concretely derive generalized fluctuation relations and a maximum-work theorem that fully account for quantum coherence at all times, for both closed and open dynamics. We obtain criteria for successful coherence-to-work conversion, and identify a nonequilibrium regime where maximum work extraction is increased by quantum coherence for fast processes beyond linear response.Deutsche Forschungsgemeinschaft (German Research Foundation)Projekt DEA
Energy density optimization in laser-based powder bed fusion of nano-modified PA12 powder feedstocks
No full textAdditive manufacturing (AM) by powder bed fusion using a laser beam (PBF-LB) is often considered as the process of choice for industrial applications. Small changes in process parameters and intrinsic or extrinsic material parameters can significantly affect the final as-built part properties in this highly complex AM process. Choosing a machine configuration with optimal process parameters can be time-consuming, especially when new powders, i.e., nano-modified feedstocks or batches, are used. Even if a set of parameters works for one PBF-LB machine, the same parameters may produce unsatisfactory results on another machine model. Dimensionless parameters can be beneficial in simplifying complex phenomena. In this study, a semi-analytical approach based on printing monolayers was semi-blind tested by five laboratories on different PBF-LB machines. Virgin polyamide 12 (PA12), silver, and carbon black nano-particle-modified PA12 powders were tested. Two dimensionless numbers were used to describe the process. The first number describes the energy conversion dimensionless. The second dimensionless number proposes a minimum energy input demand for full densification. Both numbers combine process parameters (e.g., laser power, scan speed) and material properties (latent heat and solid density), while energy conversion number is considered by experimental results (monolayer thickness). Results indicate how nanoparticles influence thermal conductivity and energy absorption. Suggested surface energy densities based on the dimensionless numbers were compared with mechanical properties. The prediction of the highest overall mechanical values (tensile strength and elongation at break) matched the best mechanical properties. The monolayer approach presents an experimental simple method for predicting suitable machine settings and narrowing the process window in an efficient and material-conserving way.Projekt DEALDeutsche ForschungsgemeinschaftFraunhofer-Institut für Produktionstechnik und Automatisierung IP
Valorisation of food industry waste into high-performance biochar for environmental applications
No full textOrganic waste from the food industry can be thermochemically converted into valuable products, supporting the circular economy and reducing environmental and socio-economic impacts. This study explores the valorisation of food industry residues, such as rapeseed cake, maize cobs, and walnut shells, through slow pyrolysis at 600 °C under an inert atmosphere. The studied feedstocks were lignocellulosic materials with approximately 45% carbon content, but had different fibre contents, which subsequently affected the properties of the biochars. The highest char yield was observed for rapeseed cake (26%), followed by walnut shells (24%) and maize cobs (22%). Elemental analysis revealed that the carbon content in the biochars of maize cobs and walnut shells exceeded 80%, with a particularly significant surface area (356 m2/g) noted for the walnut shells, and very low for the rapeseed cake and maize cobs, respectively. Only after the chars underwent physical (steam activation at 850 °C) and chemical activation (using H3PO4 and ZnCl2), resulting in a substantial increase in surface area, exceeding 300 m2/g for rapeseed cake and maize cobs and c.a. 550 m2/g for walnut shell biochar. These biochars effectively removed organic (phenol) and inorganic (Pb2+) pollutants from aqueous solutions (100% removal of Pb2+ and 82% removal of phenol for biochar of walnut shells) and additionally did not exhibit acute toxicity in Lemna minor tests, confirming their environmental safety. The work aligns with SDG 6 (Clean Water and Sanitation) by enabling low-cost pollutant removal, SDG 12 (Responsible Consumption and Production) by upcycling food waste, SDG 13 (Climate Action) through carbon sequestration and emissions mitigation, and SDG 15 (Life on Land) by offering materials that improve soil health and support circular economy principles. This research demonstrates how engineered biochar can serve as a multipurpose environmental tool, directly supporting global sustainability targets.University of KrakowNational Science Centre, Polan
Resilience as a professional competence : a new way towards healthy teachers?
No full textConsidering teachers’ poor state of health, we conducted a literature review to investigate, whether resilience, as a professional competence, has a health-promoting effect on teachers. Re-directing the seemingly inevitable trajectory from stress to illness towards health, the effects should be reflected in the improvement of teachers’ negative strain ratios. Hence, our review investigated the effect of resilience and specific resilience resources regarding teachers’ risk of burnout, stress perception and coping, general well-being, attrition rates, and effectiveness. Our results indicate that resilience supports teachers’ health, by consistently showing a positive impact on the investigated aspects. Furthermore, our catalog of identified resilience resources supports the concept of teacher-specific resilience. As effective programs for promoting teachers’ resilience are lacking, this informs new intervention approaches to promote teachers’ health.Projekt DEA
Hierarchical collectives for HPX
No full textHigh-Performance Computing (HPC) is the act of using many concurrently running computational units to solve complex and resource intensive calculative tasks faster than singular Computer. These Tasks include the fourier Transformation, Computational Fluid Dynamics and many more. HPC uses HPC-Clusters, an accumulation of many Computing Nodes, to independently calculate different parts of these tasks and then gathering the results. For these tasks this improves performance immensely. A critical component of many HPC frameworks is the Message Passing Interface (MPI), which provides standardized methods for inter-process communication across distributed systems. The first communcation mechanism of the MPI Standard is the point-to-point Communication, which allows for singular processes to send data to each other. On a higher level, and at the center of our thesis, are the collective communication operations, which transmit data in a group of processes. The MPI Standard and its communication models are implemented by several libraries. Our thesis focuses on OpenMPI, a widely used open-source implementation of MPI, that continuously optimizes performance and scalability, making it the de facto choice for collective communication operations in large-scale operations.
A Library with Focus on Parallelism and Concurrency is the STEllAR Group’s HPX for C++. It seeks to address limitations of MPI and other traditional parallelization methods by leveraging techniques such as message-driven computation and constraint-based synchronization. While OpenMPI has introduced numerous optimizations to improve scalability, HPX’s current collective operations still rely on a single root node for message distribution, leading to performance bottlenecks. This thesis proposes a hierarchical communicator for HPX that distributes communication workload across multiple processes, reducing overhead and improving scalability. We created a benchmark to compare our hierarchical communicator to the naive HPX communicator and various OpenMPI Algorithms. Our results show a proportional performance increase across all tested collective operations on HPX, with the hierarchical communicator achieving a speedup of up to 32 for reduce and broadcast operations and 16 for gather and scatter on 256 processes. Compared to OpenMPI’s state-of-the-art implementations for gather, scatter, and broadcast our approach shows a speedup of 1.2 and 1.5 for 128 and 256 processes respectively.High-Performance Computing (HPC) ist der Einsatz vieler gleichzeitig laufender Recheneinheiten, um komplexe und ressourcenintensive Rechenaufgaben schneller zu lösen als ein einzelner Computer. Jeweilige Aufgaben können dabei die Fouriertransformation, computergestützte Strömungsmechanik und viele weitere sein. HPC verwendet HPC-Cluster, eine Ansammlung vieler Rechenknoten, um verschiedene Teile dieser Aufgaben unabhängig voneinander zu berechnen und die Ergebnisse zusammenzuführen. Für solche Aufgaben steigert dies die Leistung signifikant. Eine entscheidende Komponente vieler HPC-Frameworks ist das Message Passing Interface (MPI), welches standardisierte Methoden der Prozesskommunikation plattformübergreifend für die üblichen Systeme zur Verfügung stellt. Der erste Kommunikationsmechanismus des MPI-Standards ist die Punkt-zu-Punkt-Kommunikation. Diese erlaubt es, einzelne Prozesse untereinander zu verschicken. Auf einem höheren Niveau, und im Kern dieser These, sind die kollektiven Operationen, welche Daten in einer Gruppe von Prozessen übermittelt. Der MPI Standard und ihre Kommunikationsmodelle sind in etlichen Bibliotheken bereits implementiert. Unsere These fokussiert sich auf OpenMPI, eine weit verbreitete Open-Source Implementation des MPI, die kontinuierlich Leistungs- und Skalierbarkeit optimiert. Somit ist sie die ideale Wahl für kollektive Operationen für umfangreiche Operationen.
Eine Bibliothek mit Schwerpunkt auf Parallelität und Gleichzeitigkeit ist das HPX für C++ der STEllAR-Gruppe. Diese strebt an die Grenzen der MPI und andere traditionellen Parallelisierungsmethoden indem es Techniken wie die Nachrichten gesteuerte Berechnung und die Einschränkung basierte Synchronisierung zu nutzen. Während OpenMPI längst mehrere Optimierungen der Skalierbarkeit vorgestellt hat, kann HPX für die Nachrichtenverteilung nur auf einem Rechenknoten angewiesen sein . Dies führt letztendlich zu Leistungsengpässe. Unsere These schlägt einen hierarchischen Kommunikator für HPX vor. Dabei soll der Kommunikationsaufwand über mehrere Prozesse verteilt werden, folgend auch Overhead reduziert werden, während die Skalierbarkeit gesteigert wird. Wir haben Benchmarks erstellt, um unseren hierarchischen Kommunikator zum originalen HPX Kommunikator als auch diverse OpenMPI-Algorithmen zu vergleichen. Unsere Ergebnisse zeigen einen proportionalen Leistungsanstieg bei allen getesteten kollektiven Operationen auf HPX, wobei 256 Prozessen eine Beschleunigung von bis zu 32 für Reduktions- und Broadcast-Operationen und 16 für Gathering und Scattering erreicht. Verglichen mit den aktuellen OpenMPI-Versionen für Gather, Scatter und Broadcast zeigte unser Ansatz eine Beschleunigung von 1,2 und 1,5 für jeweils 128 und 256 Prozesse
Ferrofluid reaction wheel development and in-orbit verification
No full textIn contemporary satellite systems, the Attitude and Orbit Control System (AOCS) manages internal torque generation primarily through Reaction Wheels (RW) and Control Moment Gyros (CMG), which use mechanically mounted rotating disks to control orientation without expelling mass. Unlike magnetorquers, which interact with Earth’s magnetic field, or thruster-based Reaction Control Systems (RCS), which generate external forces by expelling propellant, RW and CMG systems rely solely on momentum exchange within the spacecraft. While state-of-the-art RWs are highly reliable and have demonstrated exceptional performance over decades of operation, their design still presents inherent challenges, such as wear, nonlinear friction effects, and tribological degradation of contact surfaces. These challenges are critical in space, where repairs are impractical and/or resource-intense. Consequently, engineers have devoted significant effort to developing robust and reliable mechanical reaction wheels. This paper explores an innovative proof-of-concept design based on a fluid-magnetic system utilizing ferrofluids and permanent magnets. This study aims to address limitations of traditional RWs by eliminating mechanical interfaces susceptible to wear and tear and replacing them with a low friction ferrofluidic bearing. Ferrofluid-based system concepts can offer a longer life due to reduced wear and tear, lower production costs by requiring less exotic materials and tolerances, self-center within the provided magnetic potential field and can therefore exhibit reduced vibration behavior. The Ferrowheel experiment, flown as part of the FARGO mission ( Überflieger 2 competition of the space agency within DLR) in March and April 2023, demonstrated the feasibility of ferrofluidic bearings for attitude control in ISS microgravity. These results contribute to exploration of innovative reaction wheel technologies, highlighting the potential of fluid-based systems for applications requiring enhanced robustness and reduced mechanical wear.Projekt DEALGerman space agency within DL
Asymmetrische Auftriebserzeugung an Hubschraubern zur lokalen Entlastung der durch Dynamic Stall gefährdeten Rotorbereiche
No full textHelikopter erfahren im Vorwärtsflug stark asymmetrische Anströmbedingungen am Hauptrotor. Während auf der vorlaufenden Seite Transsonikeffekte zu einem erheblichen Widerstandszuwachs, stark wechselnden Lasten und hohen Lärmemissionen führen, kann auf der rücklaufenden Seite nur mit sehr großen Blattanstellwinkeln genügend Auftrieb generiert werden und es droht der Dynamic Stall. Um dieses Problem zu lösen, werden bei Compoundhubschraubern zusätzliche Tragflächen und/oder Vortriebserzeuger am Rumpf installiert, wodurch der Rotor homogen entlastet wird. Tatsächlich nötig ist eine Entlastung aber nur in den stallgefährdeten Rotorbereichen der rücklaufenden Seite. Alle anderen Regionen des Rotors können den benötigten Auftrieb problemlos generieren und werden durch eine Entlastung teilweise sogar negativ beeinflusst.
Im Rahmen dieser Arbeit wird daher die gezielt asymmetrische Auftriebserzeugung untersucht, um eine lokal begrenzte und präzise abgestimmte Entlastung der kritischen Rotorbereiche zu erreichen. Dadurch lassen sich bei geringstmöglichem Mehraufwand gegenüber konventionellen Helikoptern deutlich verbesserte Rotorzustände und damit nennenswerte Steigerungen von Flugleistungen und Effizienz im Reiseflug erzielen.
Um die Effekte asymmetrischer Auftriebserzeugung auf den Rotor, die Steuereingaben und den Hubschrauber als Gesamtsystem besser verstehen und unter realen Flugbedingungen untersuchen zu können, wurde ein UAV-Versuchshubschrauber aufgebaut und mit einem Vierachs-Autopilotensystem ausgestattet, das es ermöglicht, vollautomatische Flüge nach GPS-Wegpunkten durchzuführen und so wiederholbare Flug- und Messzustände zu erzeugen.
Den Kern der Arbeit stellt die Untersuchung der Phasenverschiebung dar. Diese wird ausgehend von starren Gyroskopen erläutert und dann auf reale Rotorsysteme unterschiedlicher Typen übertragen. Durch Schwebeflugversuche mit asymmetrischer Gewichtsverteilung konnte eine bisher unbekannte, reduzierte Phasenverschiebung entdeckt werden, die im Fall statischer Situationen wirksam wird, bei denen die Piloteneinsteuerung rein zum Ausgleich asymmetrisch einwirkender Kräfte dient. In dynamischen Fällen, die in einer Rumpfrotation resultieren, bleibt die normale Phasenverschiebung bestehen. Um die Hintergründe der reduzierten Phasenverschiebung besser zu verstehen, wurde ein Simulationstool programmiert, mit dessen Hilfe das Verhalten der einzelnen Rotortypen als Reaktion auf zyklische und kollektive Steuereingaben in statischen und dynamischen Situationen detailliert untersucht wird.
Das speziell für Drehflügler entwickelte Aerodynamik- und Aeroelastik-Simulationsprogramm CAMRAD II wird genutzt, um eine Konfigurationsanalyse verschiedenster Anordnungen zur asymmetrischen Rotorentlastung an einem leichten, zweimotorigen Mehrzweckhubschrauber im Großmaßstab in Bezug auf mögliche Effizienzgewinne durchzuführen. Für die Konfiguration „Sidewing2“ ergibt sich in Kombination mit einer Drehzahlreduktion um 8 % eine Leistungseinsparung von 17,2 % im Reiseflug bei 120 kts sowie eine Reduktion des Schalldruckpegels um ca. 3,3 dB. Der Rotorzustand ist dabei als gleichwertig zum Referenzfall anzusehen, was bedeutet, dass trotz der Drehzahlsenkung keine erhöhte Stallgefahr entsteht und die Flugenveloppe unverändert erhalten bleibt.
Eine praktische Anwendung asymmetrischer Rotorentlastung zur Optimierung des Reiseflugs von Hubschraubern hinsichtlich ihrer Flugleistungen, Effizienz oder Geräuschemission wird als sinnvoll erachtet
On quaternionic bisectional curvature
No full textIn this article we study the concept of quaternionic bisectional curvature introduced by Chow and Yang in (J Differ Geom 29(2):361–372, 1989) for quaternion-Kähler manifolds. We show that non-negative quaternionic bisectional curvature is only realized for the quaternionic projective space HPn. We also show that all symmetric quaternion-Kähler manifolds different from HPnadmit quaternionic lines of negative quaternionic bisectional curvature. In particular this implies that non-negative sectional curvature does not imply non-negative quaternionic bisectional curvature. Moreover we give a new and rather short proof of a classification result by A. Gray on compact Kähler manifolds of non-negative sectional curvature.Projekt DEA
Enhancing heat transfer at low temperatures by laser functionalization of the inner surface of metal pipes
No full textThe latent heat transfer during vapour condensation in the condenser section of passive heat transport devices such as the two-phase closed thermosiphon is limited by film condensation. Dropwise condensation provides an increase of the heat transfer coefficient by up to one order of magnitude and can be achieved with a water-repellant surface. The inner surface of pipes made from stainless steel was functionalized by laser surface texturing with ultrashort laser pulses and subsequent storage in a liquid containing long-chained hydrocarbons. The pipes were separated into half-pipes by wire eroding to enable laser texturing of the inner surface, and were then joined by electron beam welding after laser texturing. As a result, superhydrophobic and water-repellent surfaces with a contact angle of 153° were obtained on the inner surface of the pipes with a length of up to 1 m. The functionalized pipes were used in the condenser section of a two-phase closed thermosiphon to demonstrate a heat transfer rate of 0.92 kW at 45 °C, which is approximately three times the heat transfer rate of 0.31 kW of a smooth reference pipe.Projekt DEALBundesministerium für Wirtschaft und KlimaschutzBundesministerium für Umwelt, Naturschutz, nukleare Sicherheit und Verbraucherschut
On using machine learning algorithms for motorcycle collision detection
No full textGlobally, motorcycles attract vast and varied users. However, since the rate of severe injury and fatality in motorcycle accidents far exceeds that of passenger car accidents, efforts have been directed towards increasing passive safety systems. Impact simulations show that the risk of severe injury or death in the event of a motorcycle-to-car impact can be greatly reduced if the motorcycle is equipped with passive safety measures such as airbags and seat belts. For the passive safety systems to be activated, a collision must be detected within milliseconds for a wide variety of impact configurations, but under no circumstances may it be falsely triggered. For the challenge of reliably detecting impending collisions, this paper presents an investigation towards the applicability of machine learning algorithms. First, a series of simulations of accidents and driving operation is introduced to collect data to train machine learning classification models. Their performance is henceforth assessed and compared via multiple representative and application-oriented criteria.Projekt DEALDeutsche Forschungsgemeinschaf