 

Octreegenerated virtual mesh for improved contact resolution in CFDDem coupling
Studeník, O. ; Kotouč Šourek, M. ; Isoz, Martin
The present work is focused on improving the efficiency of a computational fluid dynamics (CFD) – discrete element method (DEM) solver allowing for computations with nonspherical solids. In general, the combination of CFD and DEM allows for simulations of freely moving solid particles within a computational domain containing fluid. The standard approach of CFDDEM solvers is to approximate solid bodies by spheres, the geometry of which can be fully defined via its radius and center position. Consequently, the standard DEM contact models are based on an overlap depth between particles, which can be easily evaluated for a spheresphere contact. However, for a contact between two nonspherical particles, the overlap depth cannot be used and has to be replaced by the more general overlap volume. The precision of the overlap volume computation is (i) crucial for the correct evaluation of contact forces, and (ii) directly dependent on the computational mesh resolution. Still, the contact volume evaluation in DEM for arbitrarily shaped bodies is usually by at least one order of magnitude more demanding on the mesh resolution than the CFD. In order to improve the computational efficiency of our CFDDEM solver, we introduce the concept of an OCTREEbased virtual mesh, in which the DEM spatial discretization is adaptively refined while the CFD mesh remains unchanged.


Hybrid fictitious domainimmersed boundary method in CFDbased topology optimization
Kubíčková, Lucie ; Isoz, Martin
Advances in technological development, especially in 3D printing, allow engineers to design components with almost arbitrary shape and connectivity. Consequently, more and more attention is being directed towards a highlyspecialized applicationdriven component design based on topology optimization (TO). In the present work, we propose a methodology enabling TO of components in contact with flowing fluids. In particular, the optimization itself is based on multiobjective evolutionary algorithms (MOEAs) with the component geometry encoded using a binary representation. The optimization criteria are evaluated via computational fluid dynamics (CFD). The main novelty of the proposed TO framework lies in its robustness and effectiveness achieved by utilizing a single computational mesh for all the tested designs and projecting the specific components shapes onto it by the means of an immersed boundary method. The new methodology capabilities are illustrated on a shape optimization of a diffuser equipped as a part of an ejector. The optimization goal was to increase the ejector energy efficiency. The newly proposed methodology was able to identify a design by roughly 9 % more efficient than an alternative one found utilizing a previously published and less general optimization approach.


Shifted proper orthogonal decomposition and artificial neural networks for timecontinuous reduced order models of transportdominated systems
Kovárnová, A. ; Krah, P. ; Reiss, J. ; Isoz, Martin
Transportdominated systems are pervasive in both industrial and scientific applications. However, they provide a challenge for common modebased model order reduction (MOR) approaches, as they often require a large number of linear modes to obtain a sufficiently accurate reduced order model (ROM). In this work, we utilize the shifted proper orthogonal decomposition (sPOD), a methodology tailored for MOR of transportdominated systems, and combine it with an interpolation based on artificial neural networks (ANN) to obtain a timecontinuous ROM usable in engineering practice. The resulting MOR framework is purely datadriven, i.e., it does not require any information on the full order model (FOM) structure, which extends its applicability. On the other hand, compared to the standard projectionbased approaches to MOR, the dimensionality reduction utilizing sPOD and ANN is significantly more computationally expensive since it requires a solution of highdimensional optimization problems.


Estimating rheological properties of suspensions formed of arbitrarilyshaped particles via CDFDem
Kotouč Šourek, M. ; Isoz, Martin
In recent years, new methods combining computational fluid dynamics (CFD) and discrete element method (DEM) have been intensively studied. Usually, these methods are focused on simulations of spherical particles. Nevertheless, this is inadequate for a simulation of a common suspension, the rheology of which is affected by particle shapes. In this work, we leverage the capabilities of an inhouse developed CFDDEM solver to simulate suspensions formed of arbitrarilyshaped particles. Specifically, we simulate a rheological measurement to estimate the suspension viscosity. The CFDDEM estimates are in very good agreement with available experimental data and correlations proving the new solver capabilities regarding firstprinciplesbased simulations of complex nonNewtonian suspension behaviour. The practical potential of suspension simulation is illustrated in a numerical study of the washcoating process in the preparation of a catalytic filter for automotive exhaust gas aftertreatment.


A parallel algorithm for fluxbased bounded scalar Redistribution
Isoz, Martin ; Plachá, M.
Let us assume a bounded scalar function ? : Q = I × ? ? ?0, 1?, I ? R, ? ? R3, where Q is an open bounded domain and its discrete counterpart ?h defined on a computational mesh Qh = Ih × ?h. The problem of redistribution of ?h over ?h ensuring the scalar boundedness while maintaining the invariance of R ?h ?h dV is surprisingly frequent within the field of computational fluid dynamics (CFD). The present contribution is motivated by the case arising from coupling Lagrangian particle tracking and particle deposition within ? h with Eulerian CFD computation. We propose an algorithm for ?h redistribution that is (i) based on fluxes over the computational cells faces, i.e. suitable for finite volume (FV) computations, (ii) localized, meaning that a cell ?h P with ?hP > 1 affects only its closest neighbors with ?h < 1, and (iii) designed for parallel computations leveraging the standard domain decomposition methods.


Developing a coupled CFD solver for mass, momentum and heat transport in catalytic filters
Hlavatý, Tomáš ; Isoz, Martin ; Kočí, P.
Using catalytic filters (CF) in automotive exhaust gas aftertreatment decreases the system heat losses and facilitates the CF regeneration. On the other hand, the CF overall performance is strongly dependent on the catalytic material distribution within it. In the present work, we aim to provide a computational framework to study the dependence of the CF characteristics, i.e. the pressure loss and the conversion of gaseous pollutants, on the catalyst distribution. Previously, we built an isothermal computational fluid dynamics (CFD) model of the flow and conversion of gaseous pollutants inside the CF. However, the reactions occurring inside the CF are exothermic and the assumption of constant temperature proved to be too restricting for reallife applications of the developed isothermal CFD model. Thus, in this work, we extend the framework by the enthalpy balance, which requires combining all the transport equations (mass, momentum and enthalpy) in a single solver. The new and more general solver provides results in good agreement with a well established (1+1)D channel model calibrated on experimental data. Furthermore, it allows studying more complex devicescale geometries of laboratory CF samples.


Experimental validation of granular flow kinetic theory under turbulent flow conditions
Haidl, Jan ; Chára, Zdeněk ; Matoušek, Václav
The mixed classical and extended kinetic theory of granular ows is used for modeling the characteristics of particleswater turbulent sheet ow. The opensource solver sedFoam v3.1 is used for the 1D and 2D ow simulations. The simulation results are compared to the experimental data measured in the open channel. After that, the simulation parameters are optimized to achieve the best possible agreement between the simulation and the experimental results. The unsatisfactory performance of the KT models and the observed simulation instabilities are discussed.

 

Numerical assessment of stratification influence in simple algebraic turbulence model
Uhlíř, V. ; Bodnár, Tomáš ; Caggio, Matteo
This paper presents rst few results obtained using a newly developed test code aimed at validation and crosscomparison of turbulence models to be applied in environmental flows. A simple code based on nite di erence discretization is constructed to solve steady flows of incompresible nonhomogeneous (variable denstity) fluids. For the rst tests a simple algebraic turbulence model was implemented, containing stability function depending on the stratification via the gradient Richardson number. Numerical tests were performed in order to explore the capabilities of the new code and to get some insight into its behavior under di erent stratification. The twodimensional simulations were performed using immersed boundary method for the flow over low smooth hill. The resulting flow fields are compared for selected Richarson numbers ranging from stable up to unstable strati cation conditions.
