1. Modeliranje prenosa sedimenta v akumulacijskem jezeru MeljePrimož Kranjc, 2024, undergraduate thesis Abstract: Nestanovitnost dežja pojasnjuje neenakomerno porazdelitev količine vodnih virov skozi vse leto. Z gradnjo jezov in akumulacijskih jezer želimo izravnati to ne enakomerno porazdelitev vodnih virov in s tem povečati zagotovljenost vode. Akumulacijska jezera se najpogosteje izkoriščajo kot skladišča pitne vode ali shrambo hidroelektrične energije. Takšen jez ne predstavlja ovire le vodi v reki ampak tudi sedimentu, ki ga reka prenaša dol vodno. Sediment se začne kopičiti v rezervoarju in s časom zniža kapaciteto shranjene vode v rezervoarju in s tem škodi glavni vlogi rezervoarja; shranjevanju vode. Sedimentacija je glavni problem vzdrževanja akumulacijskih jezer saj ga je treba redno odstranjevati. Odstranjevanje sedimenta iz akumulacijskega jezera ni enostavno, razmisliti moramo o tem kje ga bomo odstranili, v kolikšni količini (vsega ali le delno) in v kakšnih časovnih razmakih je odstranjevanje najbolj ekonomično. Diplomska naloga se navezuje na sedimentacijo v akumulacijskem jezeru Melje. Z izdelavo hidravličnega modela v programski opremi HEC-RAS ugotavljamo vpliv usedlin na tok v reki Dravi in oblikujemo smernice za odstranjevanja sedimenta v rezervoarju. Simulacije so pokazale, da se na tem območji najbolj splača prečrpavanje sedimenta v kanal. Potrebno bo dodatna študija za potrditev sklepa.
Keywords: prenos sedimenta, HEC-RAS, oblikovanje podatkov, 2D modeliranje toka, vzdrževanje akumulacijskega jezera
Published in DKUM: 11.04.2025; Views: 0; Downloads: 5
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2. Energy intake models for intermittent operation of dead-end microfiltration filling lineJure Ravnik, Gorazd Bombek, Aleš Hribernik, Timi Gomboc, Matej Zadravec, Aleks Kapun, Grega Hrovat, Jure Gradišek, Matjaž Hriberšek, 2022, original scientific article Abstract: In filling lines equipped with membrane separation devices in the form of filters energy,
consumption is only one of the important working parameters, the other being sustainable filter
performance in terms of separation efficiency. As the filling line is typically equipped with a valve,
intermittent operation of the filter is an important form of its use. Whereas the overall energy
consumption of the filtration process is governed by the continuous operation mode, the intermittent
mode, characterised by opening/closing of the valve, contributes most to problems of filter failure,
i.e., the breakthrough of filtered particles through the membrane. A model for determination of the
energy intake of a microfiltration membrane during the opening and closing of a valve is presented
in this work. The model is based on computational analysis of the pressure wave signals recorded
during the opening/closing of the valve using Fourier transform, and expressed in a nondimensional
filter area specific energy intake form. The model is applied to a case of constant pressure dead-end
microfiltration with three filter types: a single membrane filter, a stacked filter and a pleated filter
with filtration surface areas ranging from 17.7 cm2
to 2000 cm2. Both clean filters, as well as partially clogged filter cases are taken into account. Second order polynomial models of the energy intake are developed and evaluated based on extensive analysis of the experimental data. The analysis of energy intake results show that the largest energy intake was observed for the clean filter case. When membrane fouling occurs at the constant flow rate values it leads to larger energy intake, however,
due to a decreasing specific flow rate during fouling these values do not exceed the clean filter case.
Keywords: membrane filtration, water hammer effect, membrane energy intake, filter clogging
Published in DKUM: 28.03.2025; Views: 0; Downloads: 1
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3. Euler–Euler numerical model for transport phenomena modeling in a natural circulation loop operated by nanofluidsBlaž Kamenik, Nejc Vovk, Elif Begum Elcioglu, Firat Sezgin, Erdem Ozyurt, Ziya Haktan Karadeniz, Alpaslan Turgut, Jure Ravnik, 2025, original scientific article Abstract: This paper explores a computational approach to model multiphase heat transfer and fuid fow in a natural circulation loop utilizing nanofuids. We propose and implement an Euler–Euler framework in a CFD environment, incorporating an innovative boundary condition to preserve mass conservation during thermophoretic particle fux. The model’s accuracy is verifed through a one-dimensional example, by comparing results against both an Euler–Lagrange model and an in-house fnite volume solution. Experimental validation is conducted with aluminum oxide nanofuids at varying nanoparticle concentrations. We prepared the nanofuids and measured their thermophysical properties up to 60◦C. We assess the thermal performance of the nanofuid in natural circulation loop at diferent heating powers via experiment and numerical simulations. The fndings reveal that the heat transfer enhancement ofered by the nanofuid is modest, with minimal diferences observed between the proposed Euler–Euler approach and a simpler single-phase model. The results underscore that while the Euler–Euler model ofers detailed particle–fuid interactions, its practical thermal advantage is limited in this context.
Keywords: CFD · Euler–Euler, multiphase fow, nanofluid, natural circulation loop, thermophoresis
Published in DKUM: 12.03.2025; Views: 0; Downloads: 5
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4. Razvoj modula za izračun temperaturne obremenitve prve stene v fuzijskem reaktorju : diplomsko deloJakob Kokovnik, 2024, undergraduate thesis Abstract: V diplomski nalogi smo razvili modul za simulacijo prenosa toplote skozi prvo steno fuzijskega reaktorja na podlagi odprtokodne programske opreme OpenFOAM. Prav tako smo oblikovali grafični vmesnik, ki bo omogočil enostavno in hitro prilagajanje parametrov simulacije. Težava leži znotraj velikih toplotnih obremenitvah katere moramo natančno opisati. Za boljši opis toplotnih obremenitev smo naredili skript kateri že omreženo površino plazme razdeli na posamezne elemente katerim nato, določimo središča ter na njih interpoliramo potrebne vrednosti. Pridobili smo sliko temperaturnih obremenitev na površini ter jo primerjali z že izračunanimi 2D rešitvami predhodnih raziskav.
Keywords: fuzijski reaktor, prenos toplote, openfoam, hladilni tokovi, prva stena, programiranje
Published in DKUM: 06.03.2025; Views: 0; Downloads: 21
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5. Computational modeling of natural convection in nanofluid-saturated porous media: an investigation into heat transfer phenomena : Janja Kramer Stajnko ... [et al.]Janja Kramer Stajnko, Jure Ravnik, Renata Jecl, Matjaž P. Nekrep, 2024, original scientific article Abstract: A numerical study was carried out to analyze the phenomenon of natural convection in a porous medium saturated with nanofluid. In the study, the boundary element method was used for computational modeling. The fluid flow through a porous matrix is described using the Darcy– Brinkman–Forchheimer momentum equation. In addition, a mathematical model for nanofluids was used, which follows a single-phase approach and assumes that the nanoparticles within a fluid can be treated as an independent fluid with effective properties. A combination of single- and sub-domain boundary element methods was used to solve the relevant set of partial differential equations. The method was originally developed for pure flow scenarios, but also proves to be effective in the context of fluid flow through porous media. The results are calculated for the case of two- and three-dimensional square cavities. In addition to various values of dimensionless control parameters, including the porous Rayleigh number (Rap), Darcy number (Da), porosity (ϕ) and nanoparticle volume fractions (φ), the effects of the inclination angle of the cavity on the overall heat transfer (expressed by the Nusselt number (Nu)) and fluid flow characteristics were investigated. The results indicate a pronounced dependence of the overall heat transfer on the introduction of nanoparticles and inclination angle. The heat transfer in a two-dimensional cavity is increased for higher values of Darcy number in the conduction flow regime, while it is suppressed for lower values of Darcy number in the Darcy flow regime. In the case of a three-dimensional cavity, increasing the volume fraction of nanoparticles leads to a decrease in heat transfer, and furthermore, increasing the inclination angle of the cavity considerably weakens the buoyancy flow
Keywords: porous media, nanofluids, natural convection, boundary element method
Published in DKUM: 10.01.2025; Views: 0; Downloads: 9
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6. Fluid flow simulation with an ▫$ℋ^2$▫ -accelerated boundary-domain integral methodJan Tibaut, Jure Ravnik, M Schanz, 2024, original scientific article Abstract: The development of new numerical methods for fluid flow simulations is challenging but such tools may help to understand flow problems better. Here, the Boundary-Domain Integral Method is applied to simulate laminar fluid flow governed by a dimensionless velocity–vorticity formulation of the Navier–Stokes equation. The Reynolds number is chosen in all examples small enough to ensure laminar flow conditions. The false transient approach is utilized to improve stability. As all boundary element methods, the Boundary-Domain Integral Method has a quadratic complexity. Here, the ℋ2 -methodology is applied to obtain an almost linear complexity. This acceleration technique is not only applied to the boundary only part but more important to the domain related part of the formulation. The application of the ℋ2 -methodology does not allow to use the rigid body method to determine the singular integrals and the integral free term as done until now. It is shown how to apply the technique of Guigiani and Gigante to handle the strongly singular integrals in this application. Further, a parametric study shows the influence of the introduced approximation parameters. For this purpose the example of a lid driven cavity is utilized. The second example demonstrates the performance of the proposed method by simulating the Hagen–Poiseuille flow in a pipe. The third example considers the flow around a rigid cylinder to show the behavior of the method for an unstructured grid. All examples show that the proposed method results in an almost linear complexity as the mathematical analysis promisses.
Keywords: boundary-domain integral method, velocity–vorticity, adaptive cross approximation, modified helmholtz equation, Yukawa potential, fast multipole method, ℋ-structure
Published in DKUM: 28.11.2024; Views: 0; Downloads: 10
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7. A novel pseudo-rigid body approach to the non-linear dynamics of soft micro-particles in dilute viscous flowJana Wedel, Matjaž Hriberšek, Jure Ravnik, Paul Steinmann, 2024, original scientific article Abstract: We propose a novel, demonstrably effective, utmost versatile and computationally highly efficient
pseudo-rigid body approach for tracking the barycenter and shape dynamics of soft, i.e. nonlinearly deformable micro-particles dilutely suspended in viscous flow. Pseudo-rigid bodies are
characterized by affine deformation and thus represent a first-order extension to the kinematics
of rigid bodies. Soft particles in viscous flow are ubiquitous in nature and sciences, prominent
examples, among others, are cells, vesicles or bacteria. Typically, soft particles deform severely
due to the mechanical loads exerted by the fluid flow. Since the shape dynamics of a soft particle -
a terminology that shall here also include its orientation dynamics - also affects its barycenter
dynamics, the resulting particle trajectory as a consequence is markedly altered as compared to
a rigid particle. Here, we consider soft micro-particles of initially spherical shape that affinely
deform into an ellipsoidal shape. These kinematic conditions are commensurate with i) the affine
deformation assumption inherent to a pseudo-rigid body and ii) the celebrated Jeffery-Roscoe
model for the traction exerted on an ellipsoidal particle due to creeping flow conditions around
the particle. Without loss of generality, we here focus on non-linear hyperelastic particles for the
sake of demonstration. Our novel numerical approach proves to accurately capture the particular
deformation pattern of soft particles in viscous flow, such as for example tank-treading, thereby
being completely general regarding the flow conditions at the macro-scale and, as an option, the
constitutive behavior of the particle. Moreover, our computational method is highly efficient and
allows straightforward integration into established Lagrangian tracking algorithms as employed
for the point-particle approach to track rigid particles in dilute viscous flow.
Keywords: soft particles, pseudo-rigid bodies, Barycenter and shape dynamics, Lagrangian particle tracking
Published in DKUM: 19.09.2024; Views: 0; Downloads: 16
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8. A coupled point particle two-phase heat and mass transfer model for dispersed flows based on Boundary Element MethodsTimi Gomboc, Matej Zadravec, Jure Ravnik, Matjaž Hriberšek, 2024, published scientific conference contribution Abstract: In dispersed multiphase flow processes we encounter coupled heat, mass and momentum transfer between the disoersed and the continuous phase. In the context of the subdomain Boundary Domain Integral Method (BDIM) solution of the Navier-Stokes equations a two-way coupling model is presented based on the use of the elliptic fundamental solution and the Dirac delta function properties which leads to accurate evaluation of the heat and mass point particle source impacts on the continuous (air) phase. In addition, the two-phase flow case under consideration is extended to the case of porous spherical particle drying with internal moving drying front, which is solved by the Boundary Element Method (BEM).
Keywords: heat transfer, mass transfer, Boundary Element Methods
Published in DKUM: 01.07.2024; Views: 113; Downloads: 18
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9. Analiza uporabe nanotekočin v toplotnih prenosnikih : magistrsko deloAndrej Špiler, 2024, master's thesis Abstract: V magistrskem delu je obravnavana simulacija toplotnega prenosnika v krožni zanki, v katerem je nanotekočina. Nanotekočina je stabilna suspenzija majhnih delcev v nosilni tekočini. V magistrskem delu je nosilni medij voda, v kateri so nanodelci Al2O3. Simulacija se je izvedla v programskem paketu Ansys Fluent. Geometrija toplotnega prenosnika je bila narejena po toplotnem prenosniku, uporabljenem pri eksperimentu, ki sta ga izvedla Cobanoglu in Haktan Karadeniz v delu ''Effect of nanofluic thermophysical propertis on the performance prediciton of single-phase natural ciculation loops'' [1]. Za opis snovnih lastnosti nanotekočin, ki so temperaturno in koncentracijsko odvisne, se je uporabil UDF (User Defined Function). Gibanje in obnašanje nanodelcev v tekočini se je v programski paket Ansys Fluent vstavilo kot UDS (User defined Scalar). Rezultati cevi premera 4,75 mm so se primerjali z rezultati iz eksperimenta [1] pri toplotnih močeh 10, 30 in 50 W. Naknadno so se naredile simulacije za premere 3, 4, 5 in 6 mm ter pregledal se je izkoristek glede na večanje koncentracije nanodelcev. Rezultati so pokazali, da se z višanjem koncentracije nanodelcev veča izkoristek toplotnega prenosnika in manjša hitrost tekočine.
Keywords: prenos toplote, toplotni prenosniki, nanodelci, nanotekočine, Ansys Fluent, UDF, UDS
Published in DKUM: 03.04.2024; Views: 253; Downloads: 48
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10. Numerical simulations of the flow and aerosol dispersion in a violent expiratory event : Outcomes of the “2022 International Computational Fluid Dynamics Challenge on violent expiratory eventsJordi Pallares, Alexandre Fabregat Tomas, Akim Lavrinenko, Hadifathul Akmal bin Norshamsudin, Gabor Janiga, David Frederick Fletcher, Kiao Inthavong, Marina Zasimova, Vladimir Ris, Nikolay Ivanov, Robert Castilla, Pedro Javier Gamez-Montero, Gustavo Raush, Hadrien Calmet, Daniel Mira, Jana Wedel, Mitja Štrakl, Jure Ravnik, Douglas Hector Fontes, Francisco José De Souza, Cristian Marchioli, Salvatore Cito, 2023, original scientific article Abstract: This paper presents and discusses the results of the “2022 International Computational Fluid Dynamics Challenge on violent expiratory events” aimed at assessing the ability of different computational codes and turbulence models to reproduce the flow generated by a rapid prototypical exhalation and the dispersion of the aerosol cloud it produces. Given a common flow configuration, a total of 7 research teams from different countries have performed a total of 11 numerical simulations of the flow dispersion by solving the Unsteady Reynolds Averaged Navier–Stokes (URANS) or using the Large-Eddy Simulations (LES) or hybrid (URANS-LES) techniques. The results of each team have been compared with each other and assessed against a Direct Numerical Simulation (DNS) of the exact same flow. The DNS results are used as reference solution to determine the deviation of each modeling approach. The dispersion of both evaporative and non-evaporative particle clouds has been considered in 12 simulations using URANS and LES. Most of the models predict reasonably well the shape and the horizontal and vertical ranges of the buoyant thermal cloud generated by the warm exhalation into an initially quiescent colder ambient. However, the vertical turbulent mixing is generally underpredicted, especially by the URANS-based simulations, independently of the specific turbulence model used (and only to a lesser extent by LES). In comparison to DNS, both approaches are found to overpredict the horizontal range covered by the small particle cloud that tends to remain afloat within the thermal cloud well after the flow injection has ceased.
Keywords: numerical simulations, computational fluid dynamics
Published in DKUM: 28.03.2024; Views: 456; Downloads: 461
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