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The lattice Boltzmann method is a modern approach to simulate fluid flow. In its original formulation, it is restricted to regular grids, second-order discretizations, and a unity CFL number. This paper describes our new off-lattice Boltzmann solver NATriuM, an extensible and parallel C++ code to perform lattice Boltzmann simulations on irregular grids. NATriuM also allows high-order spatial discretizations and non-unity CFL numbers to be used. We demonstrate how these features can efficiently decrease the number of grid points required in a simulation and thus reduce the computational time, compared to the standard lattice Boltzmann method. We detail the implementation of a recently proposed semi-Lagrangian lattice Boltzmann method and prove its efficiency in comparisons to other state-of-the-art off-lattice Boltzmann schemes.
Aufgrund der weltweiten Bemühungen, den CO2- und Schadstoffausstoß zu reduzieren, sind die Automobilhersteller veranlasst, die Entwicklung effizienter Fahrzeuge verstärkt voranzutreiben. Die gestiegenen Anforderungen machen sich besonders an den Entwicklungstrends der letzten Jahre bemerkbar. Neben Maßnahmen, um den Kraftstoffverbrauch konventioneller Fahrzeuge zu reduzieren, wie z.B. Downsizing, Start/Stopp-Automatiken oder Disconnect-Systemen bei Allradfahrzeugen, gewannen elektrifizierte Fahrzeugkonzepte wie Hybrid- und Elektrofahrzeuge stark an Bedeutung.
On the basis of the worldwide efforts to reduce CO2 and pollutant emissions, automobile manufacturers are being induced to pursue the development of more efficient vehicles with greater vigour. It is precisely the increased requirements for high-efficiency power transmission components that will allow suppliers of drivetrain components and systems as GKN to achieve a better position on the market by offering a drive technology with optimized efficiency levels.
Aufgrund der weltweiten Bemühungen, den CO2- und Schadstoffausstoß zu reduzieren, sind die Automobilhersteller veranlasst, die Entwicklung effizienter Fahrzeuge verstärkt voranzutreiben. Die gestiegenen Wirkungsgradanforderungen leistungsübertragender Komponenten ermöglichen es gerade Zulieferern von Antriebsstrangkomponenten und -systemen wie GKN, sich durch wirkungsgradoptimierte Antriebstechnik besser auf dem Markt zu positionieren.
Der vorliegende Übersichtsartikel berichtet über Fortschritte in der molekularen Modellierung und Simulation mittels massiv-paralleler Hoch- und Höchstleistungsrechner (HPC). Im SkaSim-Projekt arbeiteten dazu Partner aus der HPC-Community mit Anwendern aus Wissenschaft und Industrie zusammen. Ziel dabei war es mittels HPC-Methoden die Vorhersage von thermodynamischen Stoffdaten in Bezug auf Effizienz, Qualität und Zuverlässigkeit weiter zu optimieren. In diesem Zusammenhang wurden verschiedene Themen bearbeitet: Atomistische Simulation der homogenen Gasblasenbildung, Oberflächenspannung klassischer Fluide und ionischer Flüssigkeiten, multikriterielle Optimierung molekularer Modelle, Weiterentwicklung der Simulationscodes ls1 mardyn und ms2, atomistische Simulation von Gastrennprozessen, molekulare Membran-Strukturgeneratoren, Transportwiderstände und gemischtypenspezifische Bewertung prädiktiver Stoffdatenmodelle.
Wo Laborexperimente zu aufwendig, zu teuer, zu langsam oder zu gefährlich oder Stoffeigenschaften gar nicht erst experimentell zugänglich sind, können Computersimulationen von Atomen und Molekülen diese ersetzen oder ergänzen. Sie ermöglichen dadurch Reduktion von Kosten, Entwicklungszeit und Materialeinsatz. Die für diese Simulationen benötigten Molekülmodelle beinhalten zahlreiche Parameter, die der Simulant einstellen oder auswählen muss. Eine passende Parametrierung ist nur bei entsprechenden Kenntnissen über die Auswirkungen der Parameter auf die zu berechnenden Größen und Eigenschaften möglich. Eine Gruppe von Standardparametern in molekularen Simulationen sind die Partialladungen der einzelnen Atome innerhalb eines Moleküls. Die räumliche Ladungsverteilung innerhalb des Moleküls wird durch Punktladungen auf den Atomzentren angenähert. Für diese Annäherung existieren diverse Ansätze für verschiedene Molekülklassen und Anwendungen. In diesem Teilprojekt des Promotionsvorhabens wurde systematisch der Einfluss der Wahl des Partialladungssatzes auf potentielle Energien und ausgewählte makroskopische Eigenschaften aus Molekulardynamik-Simulationen evaluiert. Es konnte gezeigt werden, dass insbesondere bei stark polaren Molekülen die Auswahl des geeigneten Partialladungssatzes entscheidenden Einfluss auf die Simulationsergebnisse hat und daher nicht naiv, sondern nur ganz gezielt getroffen werden darf.
Transition point prediction in a multicomponent lattice Boltzmann model: Forcing scheme dependencies
(2018)
Pseudopotential-based lattice Boltzmann models are widely used for numerical simulations of multiphase flows. In the special case of multicomponent systems, the overall dynamics are characterized by the conservation equations for mass and momentum as well as an additional advection diffusion equation for each component. In the present study, we investigate how the latter is affected by the forcing scheme, i.e., by the way the underlying interparticle forces are incorporated into the lattice Boltzmann equation. By comparing two model formulations for pure multicomponent systems, namely the standard model [X. Shan and G. D. Doolen, J. Stat. Phys. 81, 379 (1995)] and the explicit forcing model [M. L. Porter et al., Phys. Rev. E 86, 036701 (2012)], we reveal that the diffusion characteristics drastically change. We derive a generalized, potential function-dependent expression for the transition point from the miscible to the immiscible regime and demonstrate that it is shifted between the models. The theoretical predictions for both the transition point and the mutual diffusion coefficient are validated in simulations of static droplets and decaying sinusoidal concentration waves, respectively. To show the universality of our analysis, two common and one new potential function are investigated. As the shift in the diffusion characteristics directly affects the interfacial properties, we additionally show that phenomena related to the interfacial tension such as the modeling of contact angles are influenced as well.
Ressourceneffiziente Optimierung von Hohlkörpern aus Kunststoff mittels Multiskalensimulation
(2017)
Die mechanischen Eigenschaften von extrusionsblasgeformten Kunststoffhohlkörpern hängen wesentlich von den vom Verarbeitungsprozess beeinflussten Materialeigenschaften ab. Ziel der dargestellten Untersuchung ist, prozessabhängige Materialkennwerte in Simulationsprogrammen zu berücksichtigen und damit deren Vorhersagegenauigkeit zu erhöhen. Hierzu ist die Schaffung einer Schnittstelle zwischen Prozess- und Bauteilsimulation notwendig. Darüber hinaus wird vorgestellt, wie Simulationen auf Mikroebene (molekulardynamische Simulationen) genutzt werden können, um Materialkennwerte ohne die Durchführung eines Realexperiments zu ermitteln.
In the pursuit to study the parameterization problem of molecular models with a broad perspective, this paper is focused on an isolated aspect: It is investigated, by which algorithms parameters can be best optimized simultaneously to different types of target data (experimental or theoretical) over a range of temperatures with the lowest number of iteration steps. As an example, nitrogen is regarded, where the intermolecular interactions are well described by the quadrupolar two-center Lennard-Jones model that has four state-independent parameters. The target data comprise experimental values for saturated liquid density, enthalpy of vaporization, and vapor pressure. For the purpose of testing algorithms, molecular simulations are entirely replaced by fit functions of vapor–liquid equilibrium (VLE) properties from the literature to assess efficiently the diverse numerical optimization algorithms investigated, being state-of-the-art gradient-based methods with very good convergency qualities. Additionally, artificial noise was superimposed onto the VLE fit results to evaluate the numerical optimization algorithms so that the calculation of molecular simulation data was mimicked. Large differences in the behavior of the individual optimization algorithms are found and some are identified to be capable to handle noisy function values.
The need for predicting physical compound properties supplementing experimental data is considerable. Nowadays a wide range of classical simulation techniques is available for computing a multitude of such properties with acceptable effort. We here give a field report about our approach, which was to fit an initial model to a single point in the phase diagram. By way of accessing commonly available experimental values we developed a compound-specific force field via simplex optimization. For predicting the desired properties of the novel model we did engage classical equilibrium as well as reverse non-equilibrium molecular dynamics in combination with Monte Carlo methods and report here the performance of these methods in detail for the example compound ethylene oxide. We find that the new model describes the experimentally observed behavior of the test compound ethylene oxide (EO) very well in the molecular dynamics section. However, when applying the simplex optimized model to the Monte Carlo section, the limits of transferability become apparent.
Ionic liquids are highly relevant for industrial applications as they stand out due to their special chemical and physical features, e.g. low vapor pressure, low melting point or extraordinary solution properties. The goal of this work is to study the capability of the three ionic liquids [C2MIM][NTf2], [C12MIM][NTf2] and [C2MIM][EtSO4] to diffuse through a POPC membrane (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). To achieve this, we used molecular simulation techniques, which on the one hand give insight into specific domains of the membrane and on the other hand compute partition coefficients and free energy profiles of solutes in lipid membranes, which cannot be measured by labor experiments. To be as accurate as possible we parameterized a new united atom force field for the ionic liquid of type 1-alkyl-3-methylimidazoliumethylsulfate [CnMIM][EtSO4] with n = 1,2,4,6,8. Like the other IL force field for [CnMIM][NTf2] (see Köddermann et al., ChemPhysChem 14, 3368–3374, 2013) used in this work, the new one was derived to reproduce experimental densities and self-diffusion coefficients. The new force field reproduces the experimental data extremely well. Using this force field, the influences of cation and anion exchanges as well as the variation of the chain length on the free energy could be analyzed. We performed umbrella-sampling to characterize the free energy profile of one ion pair, accompanied by a second one, in solution, at the membrane interface, and inside the membrane. In the outlook we present our intention to parameterize force fields in a systematic and user-friendly way. We will use the combination of two optimization toolkits, developed at SCAI: The global optimization toolkit CoSMoS and the local optimization techniques implemented in the software package GROW.
GROW: A gradient-based optimization workflow for the automated development of molecular models
(2010)
The concept, issues of implementation and file formats of the GRadient-based Optimization Workflow for the Automated Development of Molecular Models ‘GROW’ (version 1.0) software tool are described. It enables users to perform automated optimizations of force field parameters for atomistic molecular simulations by an iterative, gradient-based optimization workflow. The modularly constructed tool consists of a main control script, specific implementations and secondary control scripts for each numerical algorithm, as well as analysis scripts. Taken together, this machinery is able to automatically optimize force fields and it is extensible by developers with regard to further optimization algorithms and simulation tools. Results on nitrogen are briefly reported as a proof of concept.
The Fraunhofer Institute for Algorithms and Scientific Computing (SCAI) has developed a software tool for the automated parameterization of force fields for molecular simulations using efficient gradient-based algorithms. This tool, combined with well-established simulation techniques, can quantitatively determine many physicochemical properties for given compounds.
Formula Student is known worldwide as a design contest for engineering students, in which they train technical skills and engineering thinking by developing and manufacturing a single-seated race car every year. The efficient transfer of highly specialized and professional knowledge about physics and manufacturing has to be ensured every year, as the members turn into alumni. This requires much more than only technical skills.
In this contribution, we want to present how the Bonn-Rhine-Sieg University of Applied Sciences supports its Formula Student team in order to foster and exploit its great potentials with a systematic approach, under the supervision of its team faculty advisor. We show how senior students learn how to teach their fellow students in their highly specialized skills in a so called “Race Academy”. This aims at the evolution of teaching content, and the art of teaching itself, by systematically involving peers in the teaching process.
Experimental data on the interfacial tension of ionic liquids in CO2 and CH4 atmospheres at elevated pressures (up to 20 MPa and 353 K) are presented and discussed. In addition, molecular modeling is utilized to describe the thermophysical properties under process-relevant conditions. Molecular modeling has the potential to predict findings in order to avoid costly experiments in the future and to explain the principal behavior of the whole system in terms of simulated concentration profiles. The interfacial tension is recognized to be an important quantity in a number of processes, e.g., for describing multiphase flow. By dissolving within the liquid phase, gases reduce the interfacial tension, which in turn is closely related to the phase behavior. It is shown that the experimentally determined interfacial tension, which decreases from values of 50 mN·m–1 under atmospheric conditions down to 10 mN·m–1 in CO2 but still above 30 mN·m–1 in CH4 at 10 MPa, is appropriately reflected by molecular dynamics (MD) simulations. The obtained data are analyzed in view of literature data and by using experimentally determined pressure-dependent densities and solubilities of CH4 and CO2 within ionic liquids. The results form part of a database for the ongoing development of MD simulations.
The paper describes methods for calculating chemical equilibria based on a constrained Gibbs free energy minimization. The methods allow the treatment of multicomponent systems with multiple phases, including gaseous phases, condensed phases, and stoichiometric phases. A special aspect is the detection and treatment of miscibility gaps. The underlying mathematical problem is described in detail together with the algorithmic approach for its solution. Results are presented for some test cases, including the computation of phase diagrams for ternary systems.
In this contribution, various sales forecast models for the German automobile market are developed and tested. Our most important criteria for the assessment of these models are the quality of the prediction as well as an easy explicability. Yearly, quarterly and monthly data for newly registered automobiles from 1992 to 2007 serve as the basis for the tests of these models. The time series model used consists of additive components: trend, seasonal, calendar and error component.
