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Povzetek: The morphology-adaptive multifield two-fluid model MultiMorph focuses on the reliable and robust simulation of interfacial multiphase flows in industrial applications. The idea is to combine the Volume-of-Fluid approach with the Euler-Euler approach to model both large and small scale interfacial structures efficiently. The choice of the local representation of interfacial structures, such as bubbles or droplets, by either the first or the second of the aforementioned basic method strongly depends on the ratio of the length scale of the interface feature to the grid spacing. In case the computational grid gets too coarse to locally resolve an interfacial structure, a morphology transfer is required. Such a transfer process allows to convert resolved fluid into non-resolved one, i.e. changing from a continuous description to a disperse one. A formulation for such a numerically motivated morphology transfer process is presented. It is validated with a case of a two-dimensional single rising bubble on a grid with gradually varying cell size. The model is then applied to two further cases: an oil–water phase inversion and a water jet plunging into a free water surface. Hereby, functionality, robustness and feasibility of the proposed morphology transfer mechanism are demonstrated. This work contributes to a hybrid modelling approach for the simulation of two-phase flows adapting the numerical representation depending on local flow morphology and on available computational resources
Objavljeno v DiRROS: 28.03.2025; Ogledov: 35; Prenosov: 5
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Ključne besede: manjšine, Hrvaška, slovenščina, raba jezika, jezikovna politika, učenje jezika, šolstvo
Objavljeno v DiRROS: 28.03.2025; Ogledov: 27; Prenosov: 5
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Povzetek: Pisauridae are a global and heterogeneous assemblage of spider genera with diverse morphologies and lifestyles. So far, the monophyly of Pisauridae and the inclusion of fishing spiders (Dolomedes) in this family have not been thoroughly tested. Here, we amend the systematics and classification of these lineages within a UCE phylogenomic framework and through a detailed morphological reappraisal. For estimations of their evolutionary age, we perform and compare outcomes from two divergence estimation approaches, an a posteriori likelihood, and an a priori Bayesian. Phylogenies reject the monophyly of both Pisauridae and Dolomedes: (1) Focal Clade I groups true Pisauridae genera including Pisaura; (2) Focal Clade II contains Blandinia and is sister to Trechaleidae and Lycosidae; (3) Focal Clade III groups Dolomedes, Megadolomedes, and Ornodolomedes, and is sister to Blandinia, Trechaleidae, and Lycosidae. We therefore propose to delimit Pisauridae by removing Dolomedidae rank resurrected (including Dolomedes, Bradystichus, Megadolomedes, Caledomedes, Mangromedes, Ornodolomedes, and Tasmomedes) and Blandinia incertae sedis. Likelihood and Bayesian time calibration approaches yield comparable divergence estimations: Pisauridae origin is estimated at 29–40Ma; Blandinia 21–34Ma; Dolomedidae 10–17Ma; Dolomedes 9–16Ma. Reconstructions suggest that the evolution of terrestrial and web-building lifestyles from semi-aquatic ancestors in Pisauridae coincided with cooling and drying climates during the mid-Miocene, but this was not the case in the few recent cases of terrestrialization in Dolomedes species. This historic reconstruction illustrates how climatic changes, or rapid radiation, can drive lifestyle diversification.
Ključne besede: classification, climate change, divergence time estimation, Dolomedes, fishing spiders, lifestyle evolution, MCMCtree, RelTime
Objavljeno v DiRROS: 28.03.2025; Ogledov: 30; Prenosov: 34
Celotno besedilo (5,95 MB)
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Povzetek: Sintered Nd-Fe-B-type permanent magnets are normally manufactured using a conventional powder-metallurgy approach. The limitations of such high-temperature, pressureless sintering methods include restricted control over the microstructure due to grain growth and constraints on the magnet’s geometry. The modern spark-plasma sintering (SPS) technique employs Joule heating and external pressure to offer lower consolidation temperatures and faster heating cycles compared to conventional approaches. As a result, the SPS has the potential for the rapid, low-temperature, net-shape manufacture of magnets. However, the nonequilibrium conditions associated with the SPS can lead to poor grain-boundary wetting and the formation of the soft-magnetic α-iron phase in samples prepared from anisotropic, microcrystalline Nd-Fe-B powders produced by standard procedures, i.e., strip casting, hydrogen decrepitation, and jet-milling. This study revealed that the absence of the Nd-rich grain-boundary film is related to the presence of hydrogen. Degassing the Nd-Fe-B powder before applying the SPS improved the distribution of the grain-boundary phase. Moreover, reducing the electrical currents in the sample during the SPS prevented the decomposition of the RE2Fe14B matrix, ensuring a favorable phase composition. Compared to magnets conventionally sintered at 1070 °C, the mean grain diameter of the SPS samples prepared at ≈880 °C was reduced by ≈33%. This reduction decreased the temperature coefficient of coercivity from -0.65 to -0.58%/°C. The coercivity increase measured for the SPS samples amounted to more than 15%, which was attributed to the combined effect of smaller grain size and reduced texture.
Ključne besede: spark-plasma sintering, microstructure refinement
Objavljeno v DiRROS: 28.03.2025; Ogledov: 29; Prenosov: 14
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Povzetek: The green transition has spiked demand for high-performance sintered Nd-Fe-B permanent magnets, necessitating advanced powder consolidation technologies to enhance production efficiency. This study explores the rapid sintering methodology for an Nd-Fe-B powder using a radiation-assisted sintering approach. The case study material is an industrially used powder, prepared through strip-casting, hydrogen decrepitation, and jet milling, with a mean particle size of 5.5 µm. The powder is sintered to full density in a modified Spark Plasma Sintering furnace, achieving pressureless conditions and eliminating electrical currents in the sample to preserve grain alignment and prevent decomposition of the hard-magnetic phase. Fully-dense samples are obtained with heating rates ranging from 10 to 200 °C/min and up to 5 minutes of dwell time at 1100 °C. Rapid heating results in grain size and microstructure comparable to conventionally sintered magnets prepared from the same powder, without compromising magnetic performance after post-sinter annealing at 520 °C for 120 minutes. This sintering method contributes to a novel strategy for optimizing magnet production by utilizing efficient thermal-radiation heat transfer. The combination of rapid heating and pressureless sintering drastically reduces heat-up and dwell times, providing a fundamental advantage over slow conventional sintering.
Ključne besede: rapid sintering, finite-element modeling
Objavljeno v DiRROS: 28.03.2025; Ogledov: 27; Prenosov: 8
Celotno besedilo (2,89 MB)