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1.
A coupled multilevel vial lyophilization model for the pressure coupling in a freeze dryer
Blaž Kamenik, Jure Ravnik, Timi Gomboc, Matej Zadravec, Matjaž Hriberšek, 2025, izvirni znanstveni članek

Opis: With computational modeling of lyophilization in vials, the pressure coupling between the sublimation front and the drying chamber has traditionally been calculated using a simplified mass transfer resistance model in the form of a model, which takes into account the headspace and the stopper in a simplified way. In developing a 3D CFD-based digital twin of lyophilization in vials, a need arises for a mass flow rate-dependent vial headspace/stopper model, as it enables a more accurate calculation of the pressure conditions above the shelf as well as pressure conditions directly at the sublimation front, the latter directly affecting the sublimation mass transfer rate as well as the temperature inside the product, which is crucial for determining the risk of product collapse. The local pressure variations at a shelf level affect the heat transfer conditions due to heat conduction in the low pressure environment of the drying chamber. In the present work the development of a coupled multilevel vial lyophilization model for the freeze-drying of vials is reported, with the time-dependent 1D heat and mass transfer model at the vial level coupled with the time-dependent 3D low-pressure CFD model of the flow of the water vapor–air mixture in the drying chamber heated by the shelves. A direct pressure coupling between the sublimation front and the drying chamber space in form of vial type specific headspace/stopper resistance model is implemented. The developed multilevel lyophilization model is used to study the pressure build-up above the shelf and the headspace of the vial and its influence on the product temperature at the bottom of the vial using simulations carried out for different chamber pressures (6 Pa and 22 Pa), shelf temperatures (−20 oC and +10 oC) and vial types (10R and 15R). By implementing previously developed vial headspace/stopper pressure resistance models, the computational results show that the pressure build-up above the shelf and vial headspace significantly affect the product temperature at the bottom of the vial, especially at low chamber pressures ( Pa) and small gap sizes between the rubber stopper and the shelf above it. The increased pressure outside the vial leads also to higher heat transfer by conduction, which is particularly pronounced at the central shelf positions and within smaller shelf gaps. These results underline the importance of using a coupled multilevel model when analyzing the relationship between the local pressure variations above the shelf and their direct influence on product drying conditions, further improving the predictive capabilities of CFD based multilevel lyophilization models, especially with respect to detecting the product collapse temperature.
Ključne besede: freeze-drying, conjugate heat and mass transfer, computational fluid dynamics, multi-scale modeling
Objavljeno v DKUM: 17.06.2025; Ogledov: 0; Prenosov: 4
.pdf Celotno besedilo (4,01 MB)
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2.
Advancing nanofluid numerical modelling: A novel Euler–Lagrange method with experimental validation
Nejc Vovk, Blaž Kamenik, Elif Begum Elcioglu, Erdem Ozyurt, Ziya Haktan Karadeniz, Alpaslan Turgut, Jure Ravnik, 2025, izvirni znanstveni članek

Opis: We present a novel approach to numerical modelling of thermal nanofluids based on the Euler–Lagrange method. This approach overcomes the challenge of extremely fine temporal discretization, which previous Euler–Lagrange nanofluid numerical models struggled to address, while also avoiding the need for too many Lagrangian nanoparticles. A numerical uncertainty assessment method is adapted for the proposed approach. The model is validated with a simple verification case and applied to simulate a closed natural circulation loop heat exchanger operating with heating power ranging from 10 W to 50 W and nanoparticle volume fractions of 0.5% to 2%, using an Al2O3–water nanofluid. Results are compared with experimental temperature measurements and an Euler–Euler implementation of the same nanofluid. The model is also applied to simulate the natural convection inside a vertical enclosure, studied experimentally by other authors. The proposed novel approach demonstrates agreement with both experimental data and the Euler–Euler implementation, effectively capturing the overall behaviour of nanofluids. We establish, that the interplay of multiple transport phenomena, that occur in nanofluid operated devices, can be difficult to completely reproduce numerically within the framework of current modelling assumptions.
Ključne besede: Euler–Lagrange nanofluid modelling, numerical uncertainty assessment, natural convection loop simulation, nanoparticle concentration analysis, nanofluid heat transfer
Objavljeno v DKUM: 17.06.2025; Ogledov: 0; Prenosov: 3
.pdf Celotno besedilo (3,33 MB)
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3.
Ellipsoidal soft micro-particles suspended in dilute viscous flow
Jana Wedel, Matjaž Hriberšek, Jure Ravnik, Paul Steinmann, 2025, izvirni znanstveni članek

Opis: Soft particles in viscous flows are prevalent both in nature and in various industrial applications. Notable examples include biological cells such as blood cells and bacteria as well as hydrogels and vesicles. To model these intriguing particles, we present an extension of our recent, efficient, and versatile pseudo-rigid body approach, originally developed for initially spherical soft particles suspended in arbitrary macroscale viscous flows. The novel extension allows modeling the barycenter and shape dynamics of soft initially non-spherical, i.e. ellipsoidal particles by introducing a novel shape and orientation tensor. We consider soft, micrometer-sized, ellipsoidal particles deforming affinely. To this end, we combine affine deformations (as inherent to a pseudo-rigid body) and the Jeffery-Roscoe model to analytically determine the traction exerted on a soft ellipsoidal particle suspended locally in a creeping flow at the particle scale. Without loss of generality, we assume nonlinear hyperelastic material behavior for the particles considered. The novel extension of our recent numerical approach for soft particles demonstrates that the deformation and motion of the particles can be accurately reproduced also for ellipsoidal particles and captures results from the literature, however, at drastically reduced computational costs. Furthermore, we identify both the tumbling and trembling dynamic regime for soft ellipsoidal particles suspended in simple shear flow again capturing results from the literature. Our extended approach is first validated using experimental and numerical studies from the literature for quasi-rigid as well as soft particles, followed by a comparison of the effects of particle deformability for some well-known fluid flow cases, such as laminar pipe flow, lid-driven cavity flow, and a simplified bifurcation. We find that taking particle deformability into account leads to notable deviations in the particle trajectory compared to rigid particles, with increased deviations for higher initial particle aspect ratio. Furthermore, we demonstrate that our approach can track a statistically relevant number of soft particles in complex flow situations.
Ključne besede: soft particles, Lagrangian particle tracking, pseudo-rigid bodies, point-particle method
Objavljeno v DKUM: 19.05.2025; Ogledov: 0; Prenosov: 2
.pdf Celotno besedilo (4,82 MB)
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4.
Modeliranje prenosa sedimenta v akumulacijskem jezeru Melje : diplomsko delo
Primož Kranjc, 2024, diplomsko delo

Opis: 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.
Ključne besede: prenos sedimenta, HEC-RAS, oblikovanje podatkov, 2D modeliranje toka, vzdrževanje akumulacijskega jezera
Objavljeno v DKUM: 11.04.2025; Ogledov: 0; Prenosov: 13
.pdf Celotno besedilo (6,79 MB)

5.
Energy intake models for intermittent operation of dead-end microfiltration filling line
Jure Ravnik, Gorazd Bombek, Aleš Hribernik, Timi Gomboc, Matej Zadravec, Aleks Kapun, Grega Hrovat, Jure Gradišek, Matjaž Hriberšek, 2022, izvirni znanstveni članek

Opis: 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.
Ključne besede: membrane filtration, water hammer effect, membrane energy intake, filter clogging
Objavljeno v DKUM: 28.03.2025; Ogledov: 0; Prenosov: 1
.pdf Celotno besedilo (4,01 MB)
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6.
Euler–Euler numerical model for transport phenomena modeling in a natural circulation loop operated by nanofluids
Blaž Kamenik, Nejc Vovk, Elif Begum Elcioglu, Firat Sezgin, Erdem Ozyurt, Ziya Haktan Karadeniz, Alpaslan Turgut, Jure Ravnik, 2025, izvirni znanstveni članek

Opis: 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.
Ključne besede: CFD · Euler–Euler, multiphase fow, nanofluid, natural circulation loop, thermophoresis
Objavljeno v DKUM: 12.03.2025; Ogledov: 0; Prenosov: 5
.pdf Celotno besedilo (2,88 MB)

7.
Razvoj modula za izračun temperaturne obremenitve prve stene v fuzijskem reaktorju : diplomsko delo
Jakob Kokovnik, 2024, diplomsko delo

Opis: 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.
Ključne besede: fuzijski reaktor, prenos toplote, openfoam, hladilni tokovi, prva stena, programiranje
Objavljeno v DKUM: 06.03.2025; Ogledov: 0; Prenosov: 25
.pdf Celotno besedilo (6,35 MB)

8.
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, izvirni znanstveni članek

Opis: 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
Ključne besede: porous media, nanofluids, natural convection, boundary element method
Objavljeno v DKUM: 10.01.2025; Ogledov: 0; Prenosov: 13
.pdf Celotno besedilo (7,40 MB)
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9.
Fluid flow simulation with an ▫$ℋ^2$▫ -accelerated boundary-domain integral method
Jan Tibaut, Jure Ravnik, M Schanz, 2024, izvirni znanstveni članek

Opis: 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.
Ključne besede: boundary-domain integral method, velocity–vorticity, adaptive cross approximation, modified helmholtz equation, Yukawa potential, fast multipole method, ℋ-structure
Objavljeno v DKUM: 28.11.2024; Ogledov: 0; Prenosov: 13
.pdf Celotno besedilo (3,06 MB)
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10.
A novel pseudo-rigid body approach to the non-linear dynamics of soft micro-particles in dilute viscous flow
Jana Wedel, Matjaž Hriberšek, Jure Ravnik, Paul Steinmann, 2024, izvirni znanstveni članek

Opis: 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.
Ključne besede: soft particles, pseudo-rigid bodies, Barycenter and shape dynamics, Lagrangian particle tracking
Objavljeno v DKUM: 19.09.2024; Ogledov: 0; Prenosov: 19
.pdf Celotno besedilo (2,18 MB)
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