PReaCH: A Fast Lightweight Reachability Index using Pruning and Contraction Hierarchies

We develop the data structure PReaCH (for Pruned Reachability Contraction Hierarchies) which supports reachability queries in a directed graph, i.e., it supports queries that ask whether two nodes in the graph are connected by a directed path. PReaCH adapts the contraction hierarchy speedup techniques for shortest path queries to the reachability setting. The resulting approach is surprisingly simple and guarantees linear space and near linear preprocessing time. Orthogonally to that, we improve existing pruning techniques for the search by gathering more information from a single DFS-traversal of the graph. PReaCH-indices significantly outperform previous data structures with comparable preprocessing cost. Methods with faster queries need significantly more preprocessing time in particular for the most difficult instances

Theory and Implementation of Software Bounded Model Checking

This thesis provides a detailed overview of the theory of software bounded model checking (SBMC) and its implementation in LLBMC, which is based on the LLVM compiler framework. The whole process from a C program to an SMT formula is described in detail. Furthermore, a theory of dynamic memory allocation is introduced which allows modelling C\u27s memory model with high precision. Finally, it is shown that LLBMC\u27s approach to software bounded model checking performs well compared to competing tools

Dynamic Procedure for Filtered Gyrokinetic Simulations

Large Eddy Simulations (LES) of gyrokinetic plasma turbulence are investigated as interesting candidates to decrease the computational cost. A dynamic procedure is implemented in the GENE code, allowing for dynamic optimization of the free parameters of the LES models (setting the amplitudes of dissipative terms). Employing such LES methods, one recovers the free energy and heat flux spectra obtained from highly resolved Direct Numerical Simulations (DNS). Systematic comparisons are performed for different values of the temperature gradient and magnetic shear, parameters which are of prime importance in Ion Temperature Gradient (ITG) driven turbulence. Moreover, the degree of anisotropy of the problem, that can vary with parameters, can be adapted dynamically by the method that shows Gyrokinetic Large Eddy Simulation (GyroLES) to be a serious candidate to reduce numerical cost of gyrokinetic solvers.Comment: 10 pages, 10 figures, submitted to Physics of Plasma

Gyrokinetic simulation of multimode plasma turbulence

Durch Mikroturbulenz verursachter Wärme- und Teilchentransport in magnetisch eingeschlossenen Hochtemperaturplasmen ist eines der drängendsten Probleme der Fusionsforschung.In dieser Arbeit werden die gyrokinetischen Gleichungen, die magnetisierte Plasmen bei fusionsrelevanten Parametern beschreiben, für allgemeine magnetische Geometrien unter Berücksichtigung von Stößen präsentiert, weiterhin werden Aspekte der numerischen Implementierung in den massiv parallelen Plasmaturbulenz-Code GENE diskutiert. Nichthermitescher Entartungen in lineare Modenübergängen werden mit Hilfe eines Eigenwertlösers untersucht. Statistische Untersuchungen der ExB-Nichtlinearität im Fall reiner Trapped Electron Mode (TEM) Turbulenz zeigen, dass diese durch einen Diffusionsterm approximiert werden kann, was ein schon bekanntes quasilineares Transportmodell stützt. Es werden Übergänge zwischen TEM- und Ion Temperature Gradient (ITG) Turbulenz untersucht, eine Koexistenz führt zu interessanten Effekten beim Teilchentransport. Abschließend werden verschiedene Aspekte der ITG-Turbulenz im Stellarator W7-X mit adiabatischen und kinetischen Elektronen diskutiert

Performance of conventional and additive manufactured austenitic stainless steels under gaseous hydrogen environment using in-situ hollow specimen technique

Please click Additional Files below to see the full abstrac

From Lampoldshausen to Orbit: DLR Spin-off GreenDelta and the Development Status of Green Propellant Thrusters Based on H\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e and N\u3csub\u3e2\u3c/sub\u3eO

The German Aerospace Center\u27s Institute of Space Propulsion in Lampoldshausen has more than a decade of experience in green propellant research and green propulsion hardware development. In the frame of internal research projects as well as ESA and third-party projects DLR employees gained a deep and extensive knowledge of propulsion hardware. Based on this knowledge, thrusters and propulsion hardware were developed in-house and the TRL was increased step by step. Currently, the two most promising technologies are: the HyNOx bipropellant technology, based on nitrous oxide and hydrocarbon fuels, as well as the hypergolic HIP_11 technology. The HyNOx bipropellant offers a high Isp, non-toxic components, self-pressurized propulsion systems, easy handling and very low cost. HIP_11 is a patented, hypergolic combination based on hydrogen peroxide and ionic liquid fuels, which offers a comparable Isp, significantly reduced costs, and easy to handle propellants. To commercialize the two propulsion technologies, a DLR spin-off called GreenDelta will be founded in summer 2023. The preparation of the spin-off is currently funded by the Helmholtz Association and DLR. This paper gives an overview on the development of the two technologies and their development status. First thrusters from GreenDelta will be commercially available at Q4 2023

All-electron periodic G0W0G_0W_0 implementation with numerical atomic orbital basis functions: algorithm and benchmarks

We present an all-electron, periodic {\GnWn} implementation within the numerical atomic orbital (NAO) basis framework. A localized variant of the resolution-of-the-identity (RI) approximation is employed to significantly reduce the computational cost of evaluating and storing the two-electron Coulomb repulsion integrals. We demonstrate that the error arising from localized RI approximation can be reduced to an insignificant level by enhancing the set of auxiliary basis functions, used to expand the products of two single-particle NAOs. An efficient algorithm is introduced to deal with the Coulomb singularity in the Brillouin zone sampling that is suitable for the NAO framework. We perform systematic convergence tests and identify a set of computational parameters, which can serve as the default choice for most practical purposes. Benchmark calculations are carried out for a set of prototypical semiconductors and insulators, and compared to independent reference values obtained from an independent $G_0W_0$ implementation based on linearized augmented plane waves (LAPW) plus high-energy localized orbitals (HLOs) basis set, as well as experimental results. With a moderate (FHI-aims \textit{tier} 2) NAO basis set, our $G_0W_0$ calculations produce band gaps that typically lie in between the standard LAPW and the LAPW+HLO results. Complementing \textit{tier} 2 with highly localized Slater-type orbitals (STOs), we find that the obtained band gaps show an overall convergence towards the LAPW+HLO results. The algorithms and techniques developed in this work pave the way for efficient implementations of correlated methods within the NAO framework.Comment: 23 pages, 7 figure