Jure Ravnik

• Professor
• Professor (Full) at University of Maribor

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 fo...

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...

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 th...

Microfiltration is an important process in the pharmaceutical industry. Filter selection and validation is a time-consuming and expensive process. Quality by design approach is important for product safety. The article covers the instrumentalization and process control of a laboratory-scale dead-end microfiltration layout. The layout is a downscale...

A numerical model of a flue gas scrubber sump is developed with the aim of enabling optimization of the design of the sump in order to reduce energy consumption. In this model, the multiphase flow of the continuous phase, i.e., water, and the dispersed phase, i.e., air bubbles, is considered. The air that is blown in front of the agitators, as well...

In the framework of computational studies of particulate multiphase flow systems, either dilute or dense, particle–particle as well as particle–wall collisions need to be considered, which in the case of nonspherical particle shapes still presents a computational challenge. In this study, we present an efficient numerical implementation of a novel...

For the simulation of fluid flow the velocity‐vorticity formulation of the Navier‐Stokes equations can be used. Approximating the first order time derivative in these equations by finite differences, essentially, the Yukawa equation for the velocity has to be solved with an inhomogeneity governed by the vorticity. Applying the Boundary Element Meth...

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. Gi...

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...

Humans are constantly exposed to airborne pollutants such as pollen, exhaust residues, microplastics, fabrics, aerosols, or, as recently, ash particles from volcanic eruptions, which are rarely perfectly spherical. In order to reduce the impact of harmful particles or, on the contrary, to improve the targeted delivery of drugs, understanding the mo...

In the case of microscopic particles, the momentum exchange between the particle and the gas flow starts to deviate from the standard macroscopic particle case, i.e. the no-slip case, with slip flow occurring in the case of low to moderate particle Knudsen numbers. In order to derive new drag force models that are valid also in the slip flow regime...

The COVID-19 pandemic is one of the greatest challenges to humanity nowadays. COVID-19 virus can replicate in the host’s larynx region, which is in contrast to other viruses that replicate in lungs only, i.e. SARS. This is conjectured to support a fast spread of COVID-19. However, there is sparse research in this field about quantitative comparison...

The paper deals with numerical modelling of adsorption kinetics in a narrow channel of a honeycomb type adsorber. The mass transfer model consists of solving the diffusion-convection transport equation for adsorbate species in the fluid phase. The adsorbate flux to the wall is computed implicitly by using the Boundary Element Method for the solutio...

In engineering, several physical models result in inhomogeneous partial differential equations. A prototype of such an equation is the modified Helmholtz equation or also called Yukawa equation. It may result from fluid mechanics (false transient approach) or heat transfer if a semi-discretisation in time with a finite difference schema is applied....

V številnih vejah znanosti je danes možno zaslediti uporabo nano in mikro delcev. V prispevku so obravnavani magnetni delci v tekočini pod vplivom sile neuniformnega magnetnega polja. Za obravnavo omenjenega problema je bila v sklopu računalniške dinamike tekočin uporabljena metoda robnih elementov. Uporabljene enačbe temeljijo na hitrostno vrtinčn...

The paper proposes a hybrid analytical–numerical model for calculating the maximum elastic force acting on a flow-driven prolate spheroidal particle during its collision with a rigid wall. This model assumes that the maximum elastic force is a function of normal impact velocity, particle material properties, particle size, particle aspect ratio and...

In this paper, forces and torques on solid, non-spherical, orthotropic particles in Stokes flow are investigated by using a numerical approach on the basis of the Boundary Element Method. Different flow patterns around a particle are considered, taking into account the contributions of uniform, rotational and shear flows, to the force and the torqu...

In this paper, we analyse the performance of two types of heat exchangers with nanofluid as the working fluid in turbulent flow regime (Re≈ 4, 000–180, 000). Based on the experimental uncertainty of the thermophysical properties of the nanofluids, we use the Stochastic Collocation Method in combination with a deterministic simulation programme to e...

This paper deals with uncertainty quantification of transcranial electric stimulation (TES) of realistic human head model. The head model taken from Visible Human Project consists of 9 subdomains: scalp, skull, CSF, grey matter, white matter, cerebellum, ventricles, jaw and tongue. The deterministic computation of quasi-static induced electric scal...

V prispevku poročamo o izvedbi meritev svetlobnega onesnaženja na urbanem področju mest Maribor, Gornja Radgona in Senovo in rezultate predstavljamo v obliki kart. V ta namen smo uporabili namensko izdelan merilnik, ki meri osvetljenost na podlagi svetlobnega zaznavala. Rezultati kažejo, da je Maribor med obravnavanimi lokacijami najbolj onesnaženo...

Reliable and accurate day-ahead forecasting of natural gas consumption is vital for the operation of the Energy sector. Three different forecasting models are developed in this paper: The sigmoid function regression model, the feed-forward neural network, and the recurrent neural network model. The models were trained, compared, and validated using...

Predstavljeno je modeliranje abrazije delcev in kavitacijske erozije na primeru profila krila NACA 0015. Cilj tega dela je bil združiti obravnavo obeh mehanizmov, kar je v tem delu izraženo s skupno globino erozijskega profila. Za napoved kavitacijske erozije je uporabljen model Schenke-Melissaris-van Terwisga, ki temelji na prenosu potencialne en...

Since end of 2019 the COVID-19 pandemic, caused by the SARS-CoV-2 virus, is threatening humanity. Despite the fact that various scientists across the globe try to shed a light on this new respiratory disease, it is not yet fully understood. Unlike many studies on the geographical spread of the pandemic, including the study of external transmission...

To respond to the ongoing pandemic of SARS-CoV-2, this contribution deals with recently highlighted COVID-19 transmission through respiratory droplets in form of aerosols. Unlike other recent studies that focused on airborne transmission routes, this work addresses aerosol transport and deposition in a human respiratory tract. The contribution ther...

This paper deals with the boundary element (BE) approach to modelling of transcranial electric stimulation as an alternative to the widely used finite element method (FEM). The advantages of the BE approach are listed in the paper and demonstrated on a computational example. The formulation is based on the quasi-static approximation of currents and...

The development of the freeze-drying processes through the use of a combination of targeted experiments and the application of multidimensional computational models is applied increasingly in pharmaceutical practice, especially for scale-up purposes. This study deals with the analysis of uncertainties in the data on material properties and model pa...

We propose a novel approach for two-way coupled simulations of multiphase flows within an Eulerian-Lagrange framework. Lagrangian particles are commonly approximated as point sources of either energy, mass or momentum. To introduce them into the Eulerian computation of the fluid flow, an approximation of the Dirac delta function has to be made. By...

In this paper, we present a numerical study of laminar mixed convection of a nanofluid in a pipe and compare the results to experimental measurements. Mechanisms that control the behavior of nanoparticles in the base fluid and the fluid motion are not very well known. Thus, it is important to know, which mathematical model describes the nanofluid b...

We develop three reliable iterative methods for solving the nonlinear 1D, 2D and 3D second-order wave equation and compare the results with a discretization-based solver. The iterative Tamimi-Ansari method (TAM), Daftardar-Jafari the method (DJM) and the Banach Contraction Method (BCM ) are used to obtain the exact solution for linear equations. Fo...

In this paper we present the development of numerical methods for two problems in chemical engineering: adsorption of carbon dioxide into phenyl glycidyl ether and diffusion and reaction processes in a porous catalyst. Both problems are modelled by systems of two coupled nonlinear ordinary differential equations. Dirichlet and Neumann type boundary...

In this paper we present the development of numerical methods for two problems in chemical engineering: adsorption of carbon dioxide into phenyl glycidyl ether and diffusion and reaction processes in a porous catalyst. Both problems are modelled by systems of two coupled nonlinear ordinary differential equations. Dirichlet and Neumann type boundary...

In this paper, we develop a boundary-domain integral formulation of the unsteady convection-diffusion equation with variable material properties. The derivation is based on the Green’s second theorem using the fundamental solution of the modified Helmholtz equation. Several discretisation approaches are considered: the full matrix and domain-decomp...

The rapid growth of computer power enables performing of more and more complex numerical simulations. However, the need to develop efficient numerical algorithms is always present. In this study, we propose an acceleration of an algorithm to solve the nonlinear system of Navier‐Stokes equations. We present an accelerated Boundary‐Domain Integral me...

In this paper, we couple a numerical method aimed at simulation of flow and heat transfer of nanofluids with stochastic modelling of input and material parameters. In order to simulate nanofluids, an in-house numerical method was developed, based on the solution of 3D velocity–vorticity formulation of Navier–Stokes equations. A fast Boundary-Domain...

The paper proposes a unified shear-induced lift force model which divides the lift force into four lift components arising from the spin tensor, the volumetric and the deviatoric parts of the rate of deformation tensor, and the inertia effect of the Stokes drag. This unified model is successfully simplified into a shear-induced lift model for prola...

Numerical simulation of nanofluid flows is of maximum importance for a large area of applications, especially in the solar energy technology. Even though a lot of numerical studies are available in the open literature, there is still a large debate in regard to the most appropriate approach when dealing with nanofluids. Plus, a precise simulation o...

The paper reports on the development of unique standard gas consumption profiles for the end gas consumers and the preparation of a method for the implementation of the developed profiles for forecasting and preliminary gas allocation. Four years of gas consumption and temperature measurements were used to develop eight types of consumption profile...

The paper reports on the development of an advanced Lagrangian particle tracking model of sludge flocs that takes into account its nonspherical shape, the internal porosity and permeability, as well as the nonhomogenous mass distribution. The floc shapes, sizes and free settling velocities are determined based on the experimental measurement of set...

In this paper, we present a method to decrease the computational cost of the Boundary-Domain Integral Method. We focus on the solution of the velocity–vorticity formulation of the Navier–Stokes equation for incompressible 3D fluid flow. The objective is to accelerate the solution of the boundary vorticity values. In order to reduce the computationa...

The present contribution is the second part of a two-part research work presenting a generic method to extend lift force models that were originally devised for single linear shear flow to arbitrary flow conditions. The method can be applied to the computation of lift forces exerted on prolate spheroidal particles (or fibres) in arbitrary non-unifo...

The objective of this article is to develop a boundary element numerical model to solve coupled problems involving heat energy diffusion, convection and radiation in a participating medium. Radiation is modelled using two approaches for optically thick fluids: the Rosseland diffusion approximation and the P1 approximation. The governing Navier–Stok...

The aim of this study is to present a new implementation of an efficient iterative method proposed by Daftardar-Gejji and Bhalekar to solve a nonlinear initial value problem. We consider the thin film problem of a non-Newtonian fluid on a moving belt. The proposed method is based on using the Banach’s contraction principle. In order to determine th...

The aim of this paper is to develop the Daftardar-Jafari iterative method (DJM) for a mathematical model that represents the nonlinear thin film flow of a non-Newtonian third-grade fluid on a moving belt with the aim to obtain an approximate solution of high accuracy. When applying the DJM there is no need to resort to any additional techniques suc...

The paper proposes a generic method to extend lift force models that were originally devised for single linear shear flow, to arbitrary flow conditions. The method computes the lift force due to the dominant streamwise flow shear in the Stokes flow regime by implementing a series of coordinate transformations, facilitating the computation of the li...

The aim of this paper is to develop the Daftardar-Jafari iterative method (DJM) for a mathematical model that represents the nonlinear thin film flow of a non-Newtonian third-grade fluid on a moving belt with the aim to obtain an approximate solution of high accuracy. When applying the DJM there is no need to resort to any additional techniques suc...

Non-spherical particles suspended in fluid flows are subject to hydrodynamic torques generated by fluid velocity gradients. For small axisymmetric particles, the most popular formulation of hydrodynamic torques is that given by Jeffery (Proc R Soc Lond A 102:161–179, 1922), which is valid for uniform shear flow in the viscous Stokes regime. In the...

The paper reports on the development of a numerical model for the simulation of a lyophilization process in a vial. Experimental analysis is presented of lyophilization dynamics inside a single vial in a laboratory scale lyophilizer. The problems of lyophilization modelling of a mannitol water solution are covered in detail. The effects of the smal...

This contribution deals with fluid flow-particle interactions in fluid dominated two phase flows. Spherical as well as non-spherical particles in the form of are considered. In the case of ellipsoids, the hydrodynamic drag force model based on the Brenner-type resistance tensor is applied. As high shear flow regions are frequently encountered in co...

In this paper, we derive a boundary-domain integral formulation for the vorticity transport equation under the assumption that the viscosity of the fluid, through which the vorticity is transported by diffusion and convection, is spatially changing. The vorticity transport equation is a second order partial differential equation of a diffusion-conv...

In this paper, we present a fast boundary element method (BEM) algorithm for the solution of the velocity-vorticity formulation of the Navier-Stokes equations. The Navier-Stokes equations govern incompressible fluid flow, which is inherently nonlinear and when discretizised by BEM requires the discretization of the domain and calculation of domain...

The lyophilization of lactose and mannitol aqueous solutions was investigated with an emphasis on analyzing the effects of controlled nucleation, temperature of nucleation, and pore size distribution on the freeze-drying process. The experimental procedure involved the depressurization technique of controlled nucleation, in-vial temperature measure...

In this paper, the linear and nonlinear Fokker-Planck equations (FPE) are solved by a semi-analytical iterative technique. This technique was proposed by Temimi and Ansari (TAM) in 2011. It is used to obtain the exact solutions for the 1D, 2D and 3D FPE. We solve several linear and nonlinear examples to show that the method is efficient and applica...

In this paper, we present a method to decrease the computational demand of the boundary-domain integral equation based 3D flow solver. We focus on the solution of the velocity–vorticity formulation of the Navier–Stokes equation, which governs incompressible viscous fluid flow. We introduce the cross approximation into the solution of the boundary v...

In this paper, the linear and nonlinear Fokker-Planck equations (FPE) are solved by a semi-analytical iterative technique. This technique was proposed by Temimi and Ansari (TAM) in 2011. It is used to obtain the exact solutions for the 1D, 2D and 3D FPE. We solve several linear and nonlinear examples to show that the method is efficient and applica...

Purpose In this paper, the exact solutions of the Schlömilch’s integral equation and its linear and non-linear generalized formulas with application are solved by using two efficient iterative methods. The Schlömilch’s integral equations have many applications in atmospheric, terrestrial physics and ionospheric problems. They describe the density...

This paper deals primarily with the sedimentation of magnetic microparticles in the presence of an external magnetic field. Based on the analysis of raster images, taken during sedimentation process under the influence of magnetic field, the development of settling velocities occurring as a result of increasing magnetic force is measured. Basics of...

In this paper we derive a boundary-domain integral formulation for the energy transport equation under the assumption that the fluid properties, through which the energy is transported by diffusion and convection, are spatially and temporally changing. The energy transport equation is a second order partial differential equation of a diffusion-conv...

The paper reports on development of Boundary Element Method (BEM) based numerical algorithm for the numerical simulation of the freeze drying process in a vial. In the paper the problems of freeze-drying modeling are covered in detail. The BEM based algorithm is developed for the axisymmetrical geometry case using the Subdomain BEM approach. A spec...

In this paper, we will present the validation of numerical methodology for the improvement of baking performance of a forced convection oven. The baking performance will be assessed as the degree of uniformity between browning and temperature distribution in the oven. We found a linear relationship between the experimentally measured grade of brown...

Abstract Purpose – In this paper, the exact solutions of the Schlömilch’s integral equation and its linear and non-linear generalized formulas with application are solved by using two efficient iterative methods. The Schlömilch’s integral equations have many applications in atmospheric, terrestrial physics and ionospheric problems. They describe th...

The paper presents numerical results for three-dimensional double-diffusive natural convection in a cubic enclosure fully filled with fluid saturated porous media. Two opposite vertical walls of the enclosure are subjected to different values of temperature and concentration, which causes buoyant and diffusive flow in the porous media domain. Mathe...

Abstract The problem of unsteady coupled moisture, air and heat energy transport through a porous solid is studied numerically using singular boundary integral representation of the governing equations. The governing transport equations are written and solved for the continuous driving potentials, i.e. relative humidity, temperature and air pressur...

Natural convection in a differentially heated square cavity filled with nanofluid-saturated porous media is analyzed numerically using the boundary element method (BEM). The mathematical model for fluid flow through porous media is based on the Darcy–Brinkman–Forchheimer formulation; moreover, a single-phase nanofluid model was used. The coupled se...

The objective of all manufacturers and users of ovens is to achieve uniform browning of various baked foods. In recent years, manufacturers have found it difficult to achieve this, due to the rapid appearance of new trends and due to progressively shorter development times. In this paper, we present the development and validation of a time-dependen...

A numerical study of particle motion in a cubic lid driven cavity is presented. As a computational tool, a boundary element based flow solver with a Lagrangian particle tracking algorithm is derived. Flow simulations were performed using an in-house boundary element based 3D viscous flow solver. The Lagrangian particle tracking algorithm is capable...

Efficiency of cooling fins inside of a light emitting diode fog lamp is studied using computational fluid dynamics. Diffusion in heat sink, natural convection and radiation are the main principles of the simulated heat transfer. The Navier-Stokes equations were solved by the computational fluid dynamics code, including Monte Carlo radiation model a...

In this paper we have developed a hybrid LES/URANS turbulent model for a BEM based turbulent fluid flow solver. We employed the unified LES/URANS approach, where the interface between the LES and URANS regions is defined using a physical quantity, which dynamically changes during numerical simulation. The main characteristic of the unified hybrid m...

In this paper we develop a numerical method and present results of simulations of flow and heat transfer of nanofluids. We consider a heated circular and elliptical cylinder in a cooled cubic enclosure. Natural convection, which drives the flow, and heat transfer are simulated for different temperature differences and enclosure inclination angles....