Suchergebnisse

Abstract. Avalanches and subaqueous debris flows are two cases of a wide range of natural hazards that have been previously modeled with non-Newtonian fluid mechanics approximating the interplay of forces associated with gravity flows of granular and solid–liquid mixtures. The complex behaviors of such flows at unsteady flow initiation (i.e., destruction of structural jamming) and flow stalling (restructuralization) imply that the representative viscosity–stress relationships should include hysteresis: there is no reason to expect the timescale of microstructure destruction is the same as the timescale of restructuralization. The non-Newtonian Herschel–Bulkley relationship that has been previously used in such models implies complete reversibility of the stress–strain relationship and thus cannot correctly represent unsteady phases. In contrast, a thixotropic non-Newtonian model allows representation of initial structural jamming and aging effects that provide hysteresis in the stress–strain relationship. In this study, a thixotropic model and a Herschel–Bulkley model are compared to each other and to prior laboratory experiments that are representative of an avalanche and a subaqueous debris flow. A numerical solver using a multi-material level-set method is applied to track multiple interfaces simultaneously in the simulations. The numerical results are validated with analytical solutions and available experimental data using parameters selected based on the experimental setup and without post hoc calibration. The thixotropic (time-dependent) fluid model shows reasonable agreement with all the experimental data. For most of the experimental conditions, the Herschel–Bulkley (time-independent) model results were similar to the thixotropic model, a critical exception being conditions with a high yield stress where the Herschel–Bulkley model did not initiate flow. These results indicate that the thixotropic relationship is promising for modeling unsteady phases of debris flows and avalanches, but there is a need for better understanding of the correct material parameters and parameters for the initial structural jamming and characteristic time of aging, which requires more detailed experimental data than presently available.

"Future War and the Defence of Europe offers a major new analysis of how peace and security can be maintained in Europe: a continent that has suffered two cataclysmic conflicts since 1914. Taking as its starting point the COVID-19 pandemic and way it will inevitably accelerate some key global dynamics already in play, the book goes on to weave history, strategy, policy, and technology into a compelling analytical narrative. It lays out in forensic detail the scale of the challenge Europeans and their allies face if Europe's peace is to be upheld in a transformative century. The book upends foundational assumptions about how Europe's defence is organised, the role of a fast-changing transatlantic relationship, NATO, the EU, and their constituent nation-states. At the heart of the book is a radical vision of a technology-enabling future European defence, built around a new kind of Atlantic Alliance, an innovative strategic public-private partnership, and the future hyper-electronic European force, E-Force, it must spawn. Europeans should be under no illusion: unless they do far more for their own defence, and very differently, all that they now take for granted could be lost in the maze of hybrid war, cyber war, and hyper war they must face."--Publisher's description

Es geht um die Überlebensfrage schlechthin: Wie kann der Frieden in Europa dauerhaft gesichert werden? Die vorliegende Analyse dreier herausragender Sicherheitsexperten führt uns die enormen Herausforderungen vor Augen, denen sich Europa und seine Verbündeten stellen müssen. Nach einer umfassenden Bestandsaufnahme präsentieren die Autoren ihre radikale Vision einer europäischen Verteidigungsstrategie: ein neuartiges atlantisches Bündnis, flankiert von innovativen strategischen Kooperationen zwischen Staat und Privatwirtschaft mit dem Ziel, eine E-Force als hochtechnisierte europäische Armee aufzubauen. Klar ist: Cyber-Krieg, Hybrid-Krieg und "Hyperwar" sind reale Bedrohungen - und für die Europäer ist es höchste Zeit, sehr viel mehr für die eigene Verteidigung tun. (Verlagswerbung)