A Dual-Grid Multiscale Approach to CFD-DEM Couplings for Multiphase Flow

Doctoral thesis (2018)

This thesis focuses on a novel dual-grid multiscale approach to CFD- DEM1 couplings, proposes its advantages in terms of numerical proper- ties and performance, and provides examples of engineering ... [more ▼]

This thesis focuses on a novel dual-grid multiscale approach to CFD- DEM1 couplings, proposes its advantages in terms of numerical proper- ties and performance, and provides examples of engineering applications that can benefit from it. In recent years, CFD-DEM couplings are be- coming a more and more adopted solution for the numerical simulation of particle-laden flows. In particular, couplings based on the volume av- eraging technique have become a standard for numerical simulations in chemical and process engineering. Furthermore, they are rapidly spread- ing to civil, geotechnical and mechanical applications due to their ability in dealing with arbitrarily complex mixtures of continuum and granular media. Despite the several advantages that these Eulerian-Lagrangian cou- plings provide, their rigorous application to complex scenarios is currently limited by two main factors. First, the computational traceability of the solutions can become problematic due to the lack of a general theory on the subject. In particular, grid-convergence studies for the solution of the continuous phases are often not feasible due to the averaging procedure that imposes limitations on the grid structure and refinement. Second, the parallel implementation of these numerical schemes holds important disadvantages in terms of memory consumption and inter-physics com- munication load. These disadvantages are significantly limiting the ex- tension of these approaches to large-scale scenarios. This thesis collects some of the most significant works published in the last years on a novel approach that allows solving the two above- mentioned problems, and, therefore, tackling more complex and expen- sive scenarios. I refer to this approach as dual-grid multiscale approach for CFD-DEM couplings. It consists in using two different computational grids, one for the coupling between continuum and discrete entities and one for the solution of the so-obtained continuum equations. The two grids, i.e. the two problems, are in this way resolved on two different scales. The first scale or “bulk” scale is chosen to optimize the averag- ing operation. At this length-scale, the discrete entities are considered as zero-dimensional, and interact with the fluid with local exchanges of momentum, mass, and energy. The second scale or “fluid-fine” scale is identified as the one at which a unique solution for the averaged equa- tions can be provided. In practice, this is chosen as the one at which the solution of the fluid equations becomes grid-independent. An inter-scale communication is adopted by interpolating fields from the fluid-fine scale to the bulk one and vice-versa. The theoretical description of the method is first provided with par- ticular reference to the DEM-VOF coupling. Even in its simplest version, the multiscale approach is shown to generate grid-convergent solutions and significantly higher accuracy than a standard CFD-DEM coupling. This shows how the new approach is able to overcome the first main limitation described above. Then, an optimized parallel implementation of the method is pro- posed to show how this multiscale approach can provide significant ben- efits also for what concerns the execution time. Technically, this is made possible by moving the communication cost of the coupling from the inter-physics communication that characterized the standard CFD-DEM couplings to an optimized inter-scale communication routine. This en- ables the method to overcome a major bottleneck of the parallel execution of CFD-DEM couplings and therefore the second main limitation of those schemes. Finally, the dual-grid multiscale method is applied to approach in- dustrially relevant problems that were till now out-of-reach for standard CFD-DEM couplings, proving how this technique can have direct real- case application and produce immediate benefits for practitioners willing to adopt it. [less ▲]

Dual-Use Research In Ransomware Attacks: A Discussion on Ransomware Defence Intelligence

in Proceedings of the 6th International Conference on Information Systems Security and Privacy (2020)

Previous research has shown that developers rely on public platforms and repositories to produce functional but insecure code. We looked into the matter for ransomware, enquiring whether also ransomware ... [more ▼]

Previous research has shown that developers rely on public platforms and repositories to produce functional but insecure code. We looked into the matter for ransomware, enquiring whether also ransomware engineers re-use the work of others and produce insecure code. By methodically reverse-engineering 128 malware executables, we have found that, out of 21 ransomware samples, 9 contain copy-paste code from public resources. Thanks to this finding, we managed to retrieve the decryption keys with which to nullify the ransomware attacks. From this fact, we recall critical cases of code disclosure in the recent history of ransomware and, arguing that ransomware are components in cyber-weapons, reflect on the dual-use nature of this research. We further discuss benefits and limits of using cyber-intelligence and counter-intelligence strategies that could be used against this threat. [less ▲]

Duale Studiengänge als „unerwartete“ Form der institutionellen Durchlässigkeit zwischen Berufs- und Hochschulbildung in Deutschland

in Solga, Heike; Brzinsky-Fay, Christian; Graf, Lukas (Eds.) et al Vergleiche innerhalb von Gruppen und institutionelle Gelingensbedingungen. Vielversprechende Perspektiven für die Ungleichheitsforschung (2013)

A duality for Moore groups

in Journal of Operator Theory (2013), 69(2), 571-600

A duality of locally compact groups which does not involve the Haar measure

E-print/Working paper (2012)

Le duché de Lorraine sous Ferry III. Nouvelles questions de recherche

in Annales de l'Est (2015)

Le Duché de Luxembourg sur la "route espagnole" (16e et 17e siècles). Pour une histoire sociale des armées.

in Kolnberger, Thomas; Niederkorn, Benoît (Eds.) Militärgeschichte Luxemburgs. Grundzüge einer transnationalen Entwicklung von Militär, Krieg und Gesellschaft. Histoire militaire du Luxembourg. Principales caractéristiques d'un développement transnational de l'armée, de la guerre et de la société. (2022)

DuFLOR: A parallel corpus of Italian dubbed films from 1965 to 2003

Textual, factual or bibliographical database (2018)

»Dumme Semantik« oder: Wie klug muss eine Metasprache sein? Ein Stimmungs-Bild.

in Kaus; Liebrand, Claudia (Eds.) Interpretieren nach den »Turns«. (2014)

Dung’s Argumentation is Essentially Equivalent to Classical Propositional Logic with the Peirce-Quine Dagger

in Logica Universalis (2011), 5(2), 255-318

In this paper we show that some versions of Dung’s abstract argumentation frames are equivalent to classical propositional logic. In fact, Dung’s attack relation is none other than the generalised Peirce– ... [more ▼]

In this paper we show that some versions of Dung’s abstract argumentation frames are equivalent to classical propositional logic. In fact, Dung’s attack relation is none other than the generalised Peirce– Quine dagger connective of classical logic which can generate the other connectives ¬,∧,∨,→ of classical logic. After establishing the above correspondence we offer variations of the Dung argumentation frames in parallel to variations of classical logic, such as resource logics, predicate logic, etc., etc., and create resource argumentation frames, predicate argumentation frames, etc., etc. We also offer the notion of logic proof as a geometrical walk along the nodes of a Dung network and thus we are able to offer a geometrical abstraction of the notion of inference based argumentation. Thus our paper is also a contribution to the question: “What is a logical system” in as much as it integrates logic with abstract argumentation networks [less ▲]