11. Numerical modelling of dendritic solidification based on phase field formulation and adaptive meshless solution procedure : dissertationTadej Dobravec, 2021, doktorska disertacija Opis: The main aim of the dissertation is to develop a novel numerical approach for an accurate and computationally efficient modelling of dendritic solidification, which is one of the most commonly observed phenomena in the industrial casting of the metallic alloys. The size and the morphology of dendritic structures as well as the distribution of the solute within them critically effect the mechanical and the electro-chemical properties of the solidified material. The numerical modelling of dendritic solidification can be applied for an in-depth understanding and optimisation of the casting process under various solidification conditions and chemical compositions of the alloy under consideration.
The dendritic solidification of pure materials and dilute multi-component alloys is considered in the dissertation. The phase field formulation is applied for the modelling of dendritic solidification. The formulation is based on the introduction of the continuous phase field variable that is constant in the bulk of the solid and liquid phases. The phase field variable has a smooth transition from the value denoting the solid phase to the value denoting the liquid phase at the solid-liquid interface over the characteristic interface thickness. A phase field model yields a system of coupled non-linear parabolic partial differential equations that govern the evolution of the phase field and other thermodynamic variables.
The meshless radial basis function-generated finite-differences (RBF-FD) method is used for the spatial discretisation of the system of partial differential equations. The forward Euler scheme is applied for the temporal discretisation. Fifth-degree polyharmonic splines are used as the shape functions in the RBF-FD method. A second-order accurate RBF-FD method is achieved by augmenting the shape functions with monomials up to the second degree.
The adaptive solution procedure is developed in order to speed-up the calculations. The procedure is based on the quadtree domain decomposition of a rectangular computational domain into rectangular computational sub-domains of different sizes. Each quadtree sub-domain has its own regular or scattered distribution of computational nodes in which the RBF-FD method and the forward Euler scheme apply for the discretisation of the system of partial differential equations. The adaptive solution procedure dynamically ensures the prescribed highest density of the computational nodes at the solid-liquid interface and the lowest-possible density in the bulk of the solid and liquid phases. The adaptive time-stepping is employed to further speed-up the calculations. The stable time step in the forward Euler scheme depends on the density of the computational nodes; hence, different time steps can be used in quadtree sub-domains with different node densities.
The main originality of the present work is the use of the RBF-FD method for the thorough analysis of the impact of the type of the node distribution and the size of a local sub-domain to the accuracy when the phase field modelling of dendritic solidification for arbitrary preferential growth directions is considered. It is shown how the use of the scattered node distribution reduces the undesirable mesh-induced anisotropy effects, present when the partial differential equations are discretisied on a regular node distribution. The main advantage of the RBF-FD method for the phase field modelling of dendritic growth is the simple discretisation of the partial differential equations on the scattered node distributions. The RBF-FD method is, for the first time, used in combination with the spatial-temporal adaptive solution procedure based on the quadtree domain decomposition. The adaptive solution procedure successfully speeds-up the calculations; however, the advantages of the use of the scattered node distribution are partly compromised due to the impact of regularity in the quadtree domain decomposition.
Ključne besede: dendritic solidification, phase field method, meshless methods, RBF-FD, adaptive solution procedure
Objavljeno v RUNG: 07.04.2021; Ogledov: 3095; Prenosov: 134
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12. Expanded art for a social technology : counting craters in dialog between human and machine2020, radijska ali televizijska oddaja, podkast, intervju, novinarska konferenca Ključne besede: expanded installations, mixed media, internet, privacy, decentralized infrastructures, workshop method, transgenerational, personal data
Objavljeno v RUNG: 09.02.2021; Ogledov: 2283; Prenosov: 20
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13. Multi-wavelength aerosol absorption coefficient measurements: instrument inter-comparison and results of source and source-component modellingVera Bernardoni, Luca Ferrero, Francesca Soldan, Sara Valentin, Mario Massabò, Griša Močnik, Asta Gregorič, Marco Cataldi A., Ezio Bolzacchini, Paolo Prati, Gianluigi Valli, Roberta Vecchi, 2019, objavljeni povzetek znanstvenega prispevka na konferenci Ključne besede: aerosol absorption, method intercomparison
Objavljeno v RUNG: 17.07.2019; Ogledov: 3507; Prenosov: 0
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14. Modelling of Macrosegregation of a Low-Frequency Electromagnetic Direct Chill Casting by a Meshless MethodVanja Hatić, 2019, doktorska disertacija Opis: The main aim of the dissertation is to develop a meshless model that describes the solidification and macrosegregation phenomena during the direct chill casting (DCC) of aluminium alloys under the influence of a low-frequency electromagnetic field. Macrosegregation is an undesired consequence of alloy solidification. It represents one of the major casting defects and substantially reduces the quality of the finished product. On the other hand, low-frequency electromagnetic casting (LFEC) is a process that promises to increase greatly the product quality, including the reduction of macrosegregation. The modelling of both processes is of tremendous importance to the metallurgical industry, due to the high costs of experiments during production.
The volume-averaging formulation is used for the modelling of the solid-liquid interaction. The conservation equations for mass, energy, momentum, and species are used to model the solidification of aluminium-alloy billets in axysimmetry. The electromagnetic-induction equation is coupled with the melt flow. It is used to calculate the magnetic vector potential and the Lorentz force. The Lorentz force is time-averaged and included in the momentum-conservation equation, which intensifies the melt flow. The effect of Joule heating is neglected in the energy conservation due to its insignificant contribution. The semi-continuous casting process is modelled with the Eulerian approach. This implies that the global computational domain is fixed in space. The inflow of the liquid melt is assumed at the top boundary and the outflow of the solid metal is assumed at the bottom. It is assumed that the whole mushy area is a rigid porous media, which is modelled with the Darcy law. The Kozeny-Carman relation is used for the permeability definition. The incompressible mass conservation is ensured by the pressure correction, which is performed with the fractional step method. The conservation equations and the induction equation are posed in the cylindrical coordinate system. A linearised eutectic binary phase diagram is used to predict the solute redistribution in the solid and liquid phases. The micro model uses the lever rule to determine the temperature and the liquid fraction field from the transport equations.
The partial differential equations are solved with the meshless-diffuse-approximate method (DAM). The DAM uses weighted least squares to determine a locally smooth approximation from a discrete set of data. The second-order polynomials are used as the trial functions, while the Gaussian function is used as the weight function. The method is localised by defining a smooth approximation for each computational node separately. This is performed by associating each node with a unique local neighbourhood, which is used for the minimisation. There are 14 nodes included in the local subdomains for the DCC and LFEC simulations. The stability of the advective term is achieved with a shift of the Gaussian weight in the upwind direction. This approach is called the adaptive upwind weight function and is used in the DAM for the first time. The Explicit-Euler scheme is used for temporal discretisation.
The use of a meshless method and the automatic node-arrangement generation makes it possible to investigate the complicated flow structures, which are formed in geometrically complex inflow conditions in a straightforward way. A realistic inflow geometry and mould can therefore be included in the model. The number of computational nodes is increased in the mushy zone and decreased in the solid phase, due to the optimisation of the computational time and memory. The computational node arrangement is automatically adapted with time, as the position of the mushy zone is changed in shape and position.
Ključne besede: low-frequency electromagnetic casting, direct chill casting, macrosegregation, electromagnetic stirring, aluminium alloys, meshless methods, diffuse-approximate method, multiphysics model, solidification
Objavljeno v RUNG: 25.04.2019; Ogledov: 4004; Prenosov: 148
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15. Synthesis of a Cu/ZnO Nanocomposite by Electroless Plating for the Catalytic Conversion of CO2 to MethanolMaja Pori, Iztok Arčon, Damjan Lašič Jurković, Marjan Marinšek, Goran Dražić, Blaž Likozar, Zorica Crnjak Orel, 2019, izvirni znanstveni članek Opis: The process of methanol synthesis based on the hydrogenation of CO2
was investigated over binary Cu/ZnO catalyst materials,
prepared by applying a novel electroless plating fabrication method. The activity of the produced catalytic samples
was determined at temperature range between 200 and 300 °C and the feedstock conversion data were supplemented with
a detailed microstructure analysis using high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction
(XRD) and Cu and Zn K-edge, X-ray absorption near-edge structure (XANES) measurements and extended X-ray
absorption fine-structure (EXAFS) measurements. It was confirmed that the disorder in the Cu crystallites created unique
geometrical situations, which acted as the additional reactive centres for the adsorption of the reactant molecule species.
Copper and zinc structural synergy (spill-over) was also demonstrated as being crucial for the carbon dioxide’s activation.
EXAFS and XANES results provide strong evidence for surface alloying between copper and zinc and thus the present results
demonstrate new approach applicable for explaining metal–support interactions.
Ključne besede: EXAFS, CuZn alloy, Spillover mechanism, CO2 valorization, Electroless deposition method, Heterogeneous
catalysis
Objavljeno v RUNG: 12.04.2019; Ogledov: 3650; Prenosov: 0
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16. A NEW TOOL TO EVALUATE CORTISOL CONCENTRATIONS IN ANIMAL HAIRAntonella Comin, Tanja Peric, Mara Colzani, Francesco De Vecchi, Cristina Bergamin, Alberto Prandi, 2018, objavljeni povzetek znanstvenega prispevka na konferenci Opis: The study of cortisol in hair requires the use of a highly sensitive assay. To date, hair cortisol concentrations (HCC) have been evaluated with different analytical methods as Radio Immune Assay (RIA), Enzyme Immunossay (EIA), Enzyme Linked Immunosorbent Assay (ELISA), ChemiLuminescent Immunoassay (CLIA) and High Performance Liquid
Chromatography with Mass Spectrometry (HPLC/MS) or Liquid Chromatography with Tandem Mass Spectrometry (LC-MS/MS). Amplified luminescent proximity homogeneous assay (AlphaLISA) is an ELISA-like, nonradioactive technology first reported in 1994 [1]. This technology allows the quantitative detection of molecules of interest in a light-induced chemiluminescence immunoassay using a microplate without wash step. The aim of the study was to compare RIA with AlphaLISA method and examine the sensitivity in evaluating cortisol concentrations in animal hair of the Cortisol AlphaLISA kit (PerkinElmer, USA),
originally suggested for the analysis of buffer and serum samples. The study has been carried out on 4 samples of calf, 4 samples of foal and 8 samples of sheep hair. After washing with isopropanol, two extracts were obtained from each hair sample. One extract has been evaluated for HCC by RIA as previously described [2,3]. The second extract has been reconstituted with the buffer provided by the Cortisol AlphaLISA Kit and HCC evaluated by the use of this commercially available kit. The preliminary results indicate that the Cortisol AlphaLISA Kit is capable to detect cortisol also in hair samples with high sensitivity, with a detection limit of 17 pg/ml. Intra- and inter-assay coefficients of variation (CVs) were 4.0% and 9.0%, respectively. Cortisol concentrations ranged between 1.31 and 16.94 pg/mg, 6.30 and 57.65 pg/mg, and 2.16 and 45.08 in calf, foal and sheep hair, respectively. The HCC obtained by Cortisol AlphaLISA Kit and RIA showed a good correlation (r=0.79, p<0.01).
Considering the possibility to use a low amount of extracted sample, its no-washing procedure and the performances showed, we can conclude that the Cortisol AlphaLISA Kit can be considered an excellent tool to evaluate cortisol concentrations also in hair derived from animal species.
Ključne besede: hair, cortisol, analyses, method, AlphaLISA, Amplified luminescent proximity homogeneous assay
Objavljeno v RUNG: 09.04.2019; Ogledov: 3452; Prenosov: 0
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