1,720,973 research outputs found
Entropy-based moment closures for rarefied gases and plasmas
No full textThe method of moments provides a flexible mathematical framework to derive reduced-order models for the approximation of the kinetic Boltzmann equation. This thesis investigates moment equations based on the principle of entropy maximization to describe moderately rarefied gas and plasma flows in the transition regime between the collision dominated continuum and the free molecular flow regime.The maximum-entropy system has favorable mathematical features: The resulting system of partial differential equations is in conservative form, satisfies an H-theorem and is symmetric hyperbolic. However, the robust and efficient numerical solution of entropy-based moment closures is challenging: First, the maximum-entropy closure can have a singularity in the closing flux around the equilibrium distribution, rendering initial value problems with data in local thermodynamic equilibrium questionable. Second, the Hessian matrix of the Newton method used in the dual minimization problem for the Lagrange parameters is arbitrarily ill-conditioned. Third, moments of the maximum-entropy distribution function are in general not available in closed form and have to be evaluated numerically, which can result in excessive run times.The first issue can be avoided by bounding the velocity domain. Numerical examples show that enlarging the velocity domain leads to smaller sub-shocks, i.e., unphysical discontinuities in the continuous shock-structure problem. To study the effect of the singularity in the closing flux, a closed-form closure for a simplified toy model problem is considered. Numerical results demonstrate that the sub-shock in the continuous shock-structure problem can be mitigated by non-linear closures and eventually removed by a singularity in the closing flux.Several numerical examples are provided for the 35-moment system in slab geometry, showing promising results for shock-structure and Riemann problems. The use of an adaptive basis method for the dual minimization problem allows the robust simulation of strongly nonequilibrium processes.To reduce the excessive computational run times of the maximum-entropy closure, high-performance implementations of the numerical integration algorithms are developed for multi-core processors and graphics cards. Additionally, new efficient explicit and semi-implicit time-integration schemes based on a formulation in the Lagrange parameters of the dual minimization problem are presented
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
An open-source model for high-throughput flow battery cell performance predictions
No full textReferences:
1. J. Fuhrmann, D. Abdel, J. Weidner, A. Seiler, P. Farrell, and M. Liero, “VoronoiFVM.jl - Finite volume solver for coupled nonlinear partial differential equations.” Weierstrass Institute Berlin. [Online]. Available: https://github.com/j-fu/VoronoiFVM.jl
2. T. Janoschka, N. Martin, M. D. Hager, and U. S. Schubert, Angew. Chem. Int. Ed., 55, (2016)
3. G. Mourouga, R. P. Schaerer, X. Yang, T. Janoschka, T. J. Schmidt, and J. O. Schumacher, Electrochimica Acta, 415 (2022)
4. R. P. Schaerer, “RfbScFVM” (2023) [Online]. Available: https://github.com/Isomorph-Electrochemical-Cells/RfbScFVMFlow batteries are a promising technology for large-scale energy storage of intermittent renewable energy provided by photovoltaic systems and wind farms. Recently, different organic redox active species have been investigated and proposed for next-generation sustainable flow battery systems. The European research project SONAR has set the aim to develop a multiscale simulation framework from the atomistic to the techno-economic scale for assessing the suitability of redox active species for flow battery applications. To connect the lower scale atomistic predictions with key cell performance values, such as cell voltage and power density, we developed a macrohomogeneous model, named RfbScFVM, for single flow battery cells. The model solves for the spatially resolved profiles of species concentrations, electric and ionic potentials, pressure, and temperature. The transport equations are discretised with a Voronoi FV scheme [1] in the through-plane direction of the cell assembly (horizontal axis in Fig. 1). The model is flexible with respect to the number and types of redox active species and electrochemical reactions, which allows for a simple adaption of the model to other types of flow battery cells. The model has been validated for a lab-sized cell using the all-organic methyl viologen / TEMPTMA system [2] showing good agreement with experimental measurements. In contrast to more reduced models, such as simplified 0D flow cell models [3], the RfbScFVM model allows for good performance predictions also at large current densities. To facilitate the integration of the cell model into the high-throughput pipeline of SONAR, model input and output parameters are provided via JSON configuration files, which allows for a good interoperability with other models. The model has been published on GitHub under the permissive MIT license [4], allowing other research groups to use and adapt the model to their needs
Insights in MEA operation behavior from a modeling perspective I : performance and degradation modelling of MEAs
No full textA new closure mitigating the sub‐shock phenomenon in the continuous shock‐structure problem
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