
Assessment of Various Converging Inlet Nozzles for Wind Tunnel Using CFD
Kosiak, Pavlo ; Radnic, Tomáš ; Mamula, Milan ; Hála, Jindřich ; Šimurda, David ; Luxa, Martin
A new calibration tunnel intended for pressure probe calibration is being designed at the Aerodynamic Laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences (IT CAS). A critical part of the design is the converging nozzle since it can substantially affect the resulting flow field in the test section. Three types of nozzles were chosen for the CFD investigation: twosine, Vitoshinski, and Vagt. For each type of nozzle, two configurations were considered: long (645 mm) and short (353mm). Flow of the viscous, compressible fluid through all the variants was simulated using Ansys CFX commercial software. Results proved the Vagt nozzle to have the most uniform velocity profile and zero curvature at its end making it the most suitable for the intended use.


Investigation of Transonic Flow through Linear Cascade with Single Blade Incidence Angle Offset
Fürst, J. ; Musil, Josef ; Šimurda, David
The contribution deals with numerical and experimental investigation of the effect of incidence angle offset in a twodimensional section of a flat linear blade cascade in a highspeed wind tunnel. The aim of the work is to complement ongoing research of quasistationary approximation of aerodynamic flutter by examination of setups leading to transonic flow regimes. The numerical simulations were realized by finitevolume, inhouse code developed on top of the opensource software package OpenFOAM. The experiments were conducted in correspondence with the setting of numerical simulations. The comparison of experimental and numerical data is presented on the isentropic Mach number distributions at various locations in the blade cascade. The description of transonic flow structures in the vicinity of blades is also provided.


Effect of Cylinder Roughness on Strouhal Number
Yanovych, Vitalii ; Duda, D. ; Uruba, Václav ; Procházka, Pavel P. ; Antoš, Pavel
The main goal of this paper is to establish a better understanding of the relationship between a Strouhal number and surface roughness. Hotwire anemometry was used to evaluations the characteristics of turbulent flow behind circular cylinders with different relative roughness 0% (smooth surface) 0.83%, 1.67%, 3.33%, and 6.67%. At the experimental investigation, the Reynolds number based on the cylinder diameter was 5 × 103 < Red < 2 × 104. The obtained data showed that the Strouhal number decreased with increasing roughness. While, the dissipation rate decreases, and the value of the Kolmogorov and Taylor microscales increases. Also, spectral analysis of streamwise velocity fluctuations allowed us to estimate the location of the vortexshedding frequency which at growing roughness tends to reduce.


Simple Rheoscopic Flows Used in Teaching Fluid Mechanics
Duda, D. ; Yanovych, Vitalii ; Uruba, Václav
Four simple demonstration experiments are presented. They are used as a support in the teaching of Fluid Mechanics I (a compulsory lecture at the University of West Bohemia in Pilsen). All mentioned experiments use the rheoscopic fluid obtained as a water solution of mica powder to visualize the flow in a esthetic way, which, as we hope, has a potential to attract students attention. The experiments are: demonstration of Bernoulli equation in a widening and narrowing channel, TaylorCouette flow, the effect of viscosity to the scales and decay of turbulence, and the Galilean transformation inside an axial compressor.

 

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.
