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TEE-Based Distributed Ledgers and Their Resilience
Get PDFResilience is the ability of a (distributed) system to withstand any stressful situation without imposing massive restrictions and, above all, without long-term consequences. Permissioned distributed ledgers based on state machine replication (SMR) offer a promising approach to achieving high resilience and fairness in federated systems. SMR provides a fault-tolerant service for clients by relying on all replicas being in a consistent state. The consistent state is achieved through a consensus algorithm, typically an atomic broadcast, that decides on a total order of client requests. In the Byzantine fault model, replicas are assumed to be potentially malicious; a Byzantine fault-tolerant (BFT) protocol withstands a fixed share of malicious actors. Classic BFT SMR protocols require replicas and multiple rounds of communication to withstand faulty replicas, making the implementation complex and limiting achievable throughput and increasing latency. Trusted Execution Environments (TEEs) allow to implement SMR in the so-called hybrid fault model in which replicas are assumed to be potentially Byzantine but the TEE is restricted to only fail by crashing. In the hybrid fault model, SMR requires less communication and can be implemented with a fault tolerance of replicas. While many proposals aim to optimize BFT SMR by using TEEs, they still rely on a so-called leader that coordinates the agreement process among the replicas. The leader is known to be a bottleneck and, if it fails, the system has to recover from the failure and elect a new leader. The additional coordination required to elect a new leader can cause significant performance degradation, limiting the achieved resilience. Asynchronous protocols based on directed acyclic graphs (DAGs) eliminate the reliance on distinguished replicas by allowing all replicas to participate equally in the agreement process. While asynchronous approaches and the hybrid fault model independently contribute to increasing the resilience of BFT SMR systems, their combination has largely been unexplored. This dissertation aims to fill this gap by answering the following research question:
What is the achievable performance and resilience of DAG-based, hybrid fault-tolerant state machine replication and under which preconditions can the leaderless nature be safely exploited to maximize throughput?
We proceed in three steps to enhance the resilience and performance of BFT SMR systems and to identify potential trade-offs that arise from the assumption of TEEs and asynchrony in BFT SMR. First, we investigate the fit of TEE-based SMR for consortium-operated applications using the example of Mobility-as-a-Service ticketing systems. We propose an SMR application that uses TEEs to protect sensitive customer and mobility provider data while limiting possibilities for fraud by both customers and mobility providers, and ensuring correct billing. We find that as long as secure multiparty computation is not competitive in terms of performance, TEE-based SMR can provide significant advantages in terms of efficiency and resilience while providing reasonable confidentiality guarantees. We describe the characteristics of the Mobility-as-a-Service use case and identify similar use cases from other domains, e.g., central bank digital currencies, allowing us to conclude that our findings generalize.
In the second step, we establish the foundation for a comprehensive analysis by proposing and proving TEE-Rider, the first hybrid fault-tolerant, asynchronous, and DAG-based atomic broadcast protocol. TEE-Rider builds upon the DAG-Rider protocol family and an optimized, DAG-aware, and TEE-based causal order broadcast we propose and prove. We then identify fundamental issues that arise from the combination of TEEs and asynchrony in BFT SMR. These are the impossibility of a fault-tolerant setup and the impossibility of garbage collection. Furthermore, we prove that for partially synchronous, TEE-based reliable broadcast it is impossible to reinitialize a TEE after a crash without relying on the participation of all replicas. We conclude the theoretical contributions with the proposal of the NxBFT SMR framework. Following an assumption-algorithm co-design, NxBFT is built upon TEE-Rider for the "Not eXactly Byzantine" (NxB) operating model to maximize throughput without sacrificing resilience. Moreover, NxBFT leverages SMR state transfer to circumvent the limitations imposed by TEEs and asynchrony and provides, under the assumption of partial synchrony, garbage collection, recovery, and reconfiguration.
Finally, we contribute an extensive empirical evaluation. To this end, we develop the ABCperf evaluation framework focusing on the fair and straightforward comparison of fault-tolerant SMR and agreement protocols. We investigate the performance characteristics of NxBFT and find that cryptographic operations for signature creation and verification are the main bottleneck. We compare the performance of NxBFT with the state-of-the-art leader-based, hybrid fault-tolerant protocols MinBFT and Chained-Damysus and investigate the impact of the SMR client model (BFT vs. NxB), payload sizes, network sizes, network latencies, and crash faults. While all algorithms can benefit from the NxB client model, NxBFT achieves the highest throughput in all scenarios with up to requests per second. All algorithms show an improvement of the end-to-end latency when using the BFT instead of the NxB client model. When small latencies are required, MinBFT and Damysus are at an advantage with Damysus showing competitive throughput and impressively low latencies for small deployments. In contrast to leader-based approaches, NxBFT\u27s performance is almost not impacted when actual crash faults occur
HOGraspFlow: Exploring Vision-based Generative Grasp Synthesis with Hand-Object Priors and Taxonomy Awareness
Get PDFWe propose Hand-Object (HO)GraspFlow, an affordance-centric approach that retargets a single RGB with hand-object interaction (HOI) into multi-modal executable parallel jaw grasps without explicit geometric priors on target objects. Building on foundation models for hand reconstruction and vision, we synthesize SE(3) grasp poses with denoising flow matching (FM), conditioned on the following three complementary cues: RGB foundation features as visual semantics, HOI contact reconstruction, and taxonomy-aware prior on grasp types. Our approach demonstrates high fidelity in grasp synthesis without explicit HOI contact input or object geometry, while maintaining strong contact and taxonomy recognition. Another controlled comparison shows that HOGraspFlow consistently outperforms diffusion-based variants (HOGraspDiff), achieving high distributional fidelity and more stable optimization in SE(3). We demonstrate a reliable, object-agnostic grasp synthesis from human demonstrations in real-world experiments, where an average success rate of over 83% is achieved
Design study of the MONIKA ORC-Turbine and comparison with experimental results
Get PDFThis study presents the development, verification, and validation of a MATLAB code for conducting a mean-line analysis of a four-stage Organic Rankine Cycle (ORC) turbine with a power output of around 100 kW, operated with supercritical propane at the Karlsruhe Institute of Technology (KIT). Typically, a mean-line analysis is performed for an envisaged design point during the initial phase of turbomachinery design. In this study, it is used to analyse the performance and efficiency of a turbine at different operating conditions. Accuracy checks were carried out to validate the applied loss correlations. The ORC turbine was installed in the MONIKA test facility, a modular geothermal power plant, which was tested here using a simulated heat source instead. Propane at a design pressure of 5.5 MPa and a temperature of 390 K, undergoes expansion to an outlet pressure of 1.1 MPa. During the tests, however, the test conditions deviated significantly from the original design, and nitrogen leakage into the propane cycle could not be prevented from the turbine sealing system. The mean-line analysis confirms that the developed code can still predict the ORC turbine\u27s performance and attributes with reasonable accuracy. Additionally, the software can analyse the impact of different design alterations on the thermodynamic properties of the fluid at the inlet and outlet, as well as on the performance of the turbine
Processing Optimization of the New Steel Grade 45SiCrV9Ni for Modern Leaf Springs in Battery Electric Vehicles
Get PDFThe optimization of battery electric vehicles requires advanced high-strength steels that combine ductility and toughness, enabling lightweight leaf spring constructions with improved performance. This study investigates processing optimization by comparing the newly developed 45SiCrV9Ni, previously identified as promising for stress peening and fatigue, with the conventional 51CrV4 as a benchmark. Dilatometric, mechanical, and microstructural analyses were conducted in as-supplied and heat-treated conditions. Both steels show excellent high-temperature ductility, making them suitable for hot forming under similar conditions. However, 45SiCrV9Ni requires a higher temperature for homog-enized austenitization. After tempering, it consistently exhibits superior hardness and toughness compared to 51CrV4. Importantly, its ductility remains nearly constant over a wide tempering temperature range, allowing lower ones to be chosen without compromising strength or toughness, offering additional energy-saving possibilities. These results highlight the potential of 45SiCrV9Ni for leaf spring applications
SSZ‐13‐Zeolith mit Isolierten Co‐Zentren als ein Effizientes und Beständiges Katalysatorsystem für die Nichtoxidative Ethandehydrierung
Get PDFDie nichtoxidative Ethandehydrierung (EDH) ist eine attraktive Methode für die zielgerichtete Herstellung von Ethen. Es bleibt jedoch schwierig, mit Katalysatoren auf Basis von in der Erdkruste reichlich vorkommenden Metallen eine hohe Aktivität und insbesondere eine hohe Beständigkeit zu erreichen. In dieser Arbeit stellen wir das Co/SSZ-13-System mit ausschließlich zweiwertigen Kobaltionen (Co2+) vor, welches die oben genannten Anforderungen erfüllt. Mithilfe komplementärer Charakterisierungsmethoden konnten zwei Co2+-Spezies nachgewiesen werden: Co2+–Z2 in den sechsgliedrigen Ringfenstern und [Co(OH)]+-Z in den achtgliedrigen Ringfenstern, wobei Z für ein geladenes Zentrum des Zeolithgerüsts steht. Eine quantitative Korrelation zwischen der Ethenbildungsgeschwindigkeit und der Zentrenbesetzung bestätigt Co2+ Z2 als die aktive Spezies. Die strukturelle und elektronische Stabilität dieser wurde mittels In-situ-Röntgenabsorptionsspektroskopie unter Hochtemperaturreaktionsbedingungen bestätigt. Der optimierte 0.9Co/SSZ-13-Katalysator(0.9Co) war über 200 Dehydrierungs-/oxidativen Regenerationszyklen bei 600–650 °C besonders beständig und arbeitete 150 Stunden mit industriell relevanter Produktivität. In dieser Hinsicht übertrifft der Katalysator alle zuvor entwickelten Katalysatoren, einschließlich solcher, die Platin als aktive Komponente enthalten. Die diskutierten Ergebnisse legen die Ursachen der EDH-Aktivität/Beständigkeit des Co/SSZ-13-Systems auf atomarer Ebene offen und unterstreichen die zentrale Rolle der Position der Metallzentren im Zeolithgerüst für die Entwicklung von hochaktiven, selektiven und beständigen Katalysatoren für die zielgerichtete Ethenproduktion
CsPbCl₃:Yb³⁺ nanocrystals: Adverse effects of colloidally stable ytterbium-rich reaction by-products on luminescent down-conversion performance
Get PDFMethodology for the Electromagnetic Design of Superconducting Accelerator Magnets based on the Integrated Product Engineering Model - iPeM = Methodik für den elektromagnetischen Entwurf von supraleitenden Beschleunigermagneten auf der Grundlage des integrierten Produktentstehungsmodells - iPeM
Get PDFIn dieser Arbeit wird eine Methodik für den Entwurf der elektromagnetischen Aspekte von supraleitenden Beschleunigermagneten vorgestellt. Diese Methodik basiert auf dem integrated Product engineering Model (iPeM) und beschreibt die formalisierten Entwurfsschritte während des Simulationsprozesses eines solchen Magneten. Zur Implementierung der Methodik in aktuelle Simulationsabläufe wird ein Software-Tool, der sogenannte Magnet Model Management Layer (3ML), konzipiert und implementiert. Das Dissertationsprojekt wird am CERN durchgeführt, welches im Zentrum der Entwicklung supraleitender Beschleunigermagnete steht, um die Infrastruktur für internationale HEP-Experimente bereitzustellen. Die Dissertation hat das Ziel, die Lücke der fehlenden organisationsweiten, ganzheitlichen Methodiken Entwicklungsunterstützung in diesem Bereich im Allgemeinen und am CERN im Besonderen zu schließen.
Entwicklungsherausforderungen in der Domäne, dem CERN als Organisation und in der CERN TE-MSC Gruppe werden identifiziert und quantifiziert. Diese Herausforderungen und ihr Einfluss aufeinander werden visualisiert und Erfolgskriterien für zukünftige Entwicklungsprojekte in diesem Bereich abgeleitet. Wissensmanagement und -transfer werden als kritische Herausforderung im Entwicklungsprozess für supraleitende Magnete identifiziert. Basierend auf diesen Erkenntnissen werden die Methodik, der Tool-Support und die Integration durchgeführt. Eine Methodik für den Entwurf elektromagnetischer Magnete wird vorgestellt. Diese Methodik steht im Einklang mit den bestehenden Systems Engineering (SE) und Product Management (PM) Standards am CERN. Ein Tool-Support wird entwickelt und im Rahmen eines Validierungstests eingeführt, um den die Methodik den Anwendern zu vermitteln. Mit dem Fokus auf Wissensmanagement wurde der Magnet Model Management Layer (3ML) mit einer Webschnittstelle implementiert, die es allen Benutzern ermöglicht Simulationsmodelle zu organisieren und Referenzsystemelemente zu identifizieren, ohne ein spezielles Simulationswerkzeug öffnen zu müssen. Das Routine for the Optimization of magnet X-sections, Inverse field calculation and coil End design (ROXIE) Programm wird als Beispiel verwendet, um die Integration von 3ML in reale Simulations-Arbeitsabläufe zu demonstrieren. Es werden fortgeschrittene Modellierungsanwendungsfälle erläutert, die durch die neue Software ermöglicht werden.
Der Ausblick zeigt das Verbesserungspotenzial von 3ML auf und motiviert zu weiteren Tests und Validierungsstudien
A coupled land use change-ecohydrological model for multi-seasonal arid agricultural systems: an Egyptian case study
Get PDFModelling interactions between climate, water, crops, and human decision-making requires coupling of biophysical and socioeconomic processes to model outcomes and explore potential futures. This study presents a novel coupled model of land-use change and ecohydrological processes in arid agricultural systems. The model links SWAT+, which simulates ecohydrological processes, including crop growth and irrigation water use, with CRAFTY, an agent-based framework that allocates land according to agent characteristics and resource conditions. Egypt is used as a case study where climate and socioeconomic stressors constrain agricultural production. The coupling captures how shifts in potential yields, driven by elevated CO and warming, shape land-use change. Crop yields vary by crop and scenario, with the largest gains – and declines – under high-emission futures, while water use efficiency consistently improves, especially at higher CO concentrations. Relying on open global datasets, the model provides a transferable approach for exploring climate adaptation in data-scarce, water-limited regions
Characteristics of Intense Multi‐Day Wet Spells Over West Africa
Get PDFThis study examines the characteristics of intense multi-day wet spells in West Africa, which have significant socioeconomic impacts but remain understudied, using rain gauge and satellite-based rainfall data from 1982 to 2020. A gridded dataset, developed from the Karlsruhe African Surface Station-Database (KASS-D) using the SPHEREMAP interpolation method with data from 239 rain-gauging stations was compared with GPCC, CHIRPS and IMERG datasets. Wet days (rainfall > 1.0 mm) were aggregated into 3-day and 5-day events, with the 90th percentile threshold identifying intense wet spells. The quantile distribution of wet-day rainfall amounts in KASS-D aligns closely with GPCC, CHIRPS and IMERG datasets, although discrepancies occur at the tails of the distribution. KASS-D and the arithmetic mean of station data show similar quantile distributions when grid pixels contain 1–4 stations, but KASS-D values increase with more than four stations per pixel. The analysis reveals a latitudinal variation in wet spell frequency, with 5-day events often extending from 3-day events, peaking in June and September along the Guinea Coast and in August in other climatic zones. Over the course of the study period, the frequency of intense wet spells increased significantly, particularly in KASS-D, highlighting the added value of dense rain gauge networks in creating reanalysis and improving satellite rainfall products. Spatially large-sized intense events, along with average and maximum sizes, have increased, while small and medium-sized events have declined significantly (both in GPCC and CHIRPS), leading to a greater contribution of 3-day and 5-day wet spells to annual rainfall. The Guinea Coast experienced the highest increase in percentage contribution, while the Sahel saw the lowest. This study underscores the rising intensity, size and contribution of multi-day wet spells to annual rainfall in West Africa, emphasising the need for high-quality rain gauge networks and a deeper understanding of the dynamics driving these events