A review of the research of the turbulent flows in the Laboratory for Thermal Engineering and Energy of the Vinča Institute of Nuclear Sciences

Dr. Dejan Cvetinović, Res. Assist., Dr. Predrag Stefanović, Princ. Res. Fellow, Dr. Vukman Bakić, Princ. Res. Fellow, Dr. Simeon Oka, Retired Prof., University of Belgrade, Institute of Nuclear Sciences “Vinča”, Laboratory for Thermal Engineering and Energy

In the paper are given the most important results of the Laboratory for Thermal Engineering and Energy in the field of research of turbulent flows. Paper presents detailed overview of the history of the scientific research provided in the laboratory, from the beginning, in the mid 60s to today and the main reasons for this research began. In turbulence research, the results achieved in laboratories were from the beginning, and still are, at the global level. Particularly interesting are the investigations of the structure of the turbulence provided by the late academician Prof. Zoran Zarić, who started turbulence research in laboratories and in the former Yugoslavia. After the first period, which was mainly devoted to the research of the structure of the turbulence, since the beginning of the 80s, research is mainly oriented to the flows at high temperatures including chemical reactions and to the development and improvement of existing differential mathematical models as a modern and very efficient tool in the technological development. This research significantly contributed to the development of pulverized coal burners, plasma-chemical reactors, and, recently, focused on the mathematical modelling of three-dimensional flow in the pulverized coal fired boilers which is used for the optimization of its operating parameters and prediction of the greenhouse gases emissions. Most recent period includes experimental and numerical studies of the coherent structures in turbulent fluid jets, mathematical modelling of various turbulent thermal flow processes including two-phase turbulent flow in the multiphase heat exchangers and mathematical modelling of the atmospheric boundary layer.

Keywords: turbulent flow, high temperature flow, two-phase gas-particle flows, mathematical modelling, fundamental research, applied research, technology development.

Some examples of OpenFOAM usage in computations of turbulent flows

Dr. Aleksandar Ćoćić, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering

Several interesting cases of numerical simulations of turbulent flow is presented. For all simulations extend version (foam-extend) of OpenFOAM software is used. First case deals with the flow in simple model room, where interesting flow phenomena like unsteady flow separation and vortex formations are captured usind k-ω SST model. Obtained results shows good agreement with experimental results from the literature. Second case is devoted to the computations of flow in asymmetric planar diffuser, using various turbulence models. Third case deals with the steady simulations in industrial valves. In this case, capabilities of snappyHexMesh and cfMesh generators are tested for automated meshing of complex geometries. At the end, both steady and unsteady flow in axial and radial fans is computed using available and self-developed tools for turbomachinery in foam-extend 3.1 version of the software.

Keywords: CFD, OpenFOAM, RANS modelling.

Numerical modeling of dissolved oxygen recovery by electrolysis of water at the lake bottom

Dr. Nenad Jaćimović, Assist. Prof., University of Belgrade, Faculty of Civil Engineering, Prof. Dr. Takashi Hosoda, Kyoto University, Japan, Prof. Dr. Marko Ivetić, University of Belgrade, Faculty of Civil Engineering

Eutrofication is a natural process of an increase of primary productivity in lakes, which may lead to severe deterioration of water quality. One of available lake restoration technologies is injection of compressed air or oxygen into a hypolimnion as an artificial destratification, inducing a buoyancy driven bubble plume which, in time, attenuates water density gradient.
In this paper, a numerical model for simulation of bubble flow is presented, with consideration of gas compressibility and oxygen dissolution. In the model, three dimensional volume-averaged, two-fluid governing equations are simultaneously solved. Developed model is intended to be utilized for simulation of dissolved oxygen recovery process. Model is firstly verified by simulation of bubble flow experiments, reported in the literature, where very good quantitative and qualitative agreement between measured and simulated results is observed. Both flow configurations are tested: in which steady state is achieved, and where steady conditions can not develop.
In the second part, model is applied for simulation of conducted experiments on electrolysis of water, as a novel approach in order to provide the oxygen directly from the water. The experiment is conducted in a 90L vessel, with seven pairs of platinum plates (20 x 7 cm) placed at the bottom. After imposing electric current, increase of dissolved oxygen concentration is measured at 25, 35 and 45 cm from the vessel bottom. Finally, the developed modes is utilized to reproduce the field experiments, conducted at the lake Biwa in Japan. Overall, a good agreement between observed and modeled changes of dissolved oxygen concentrations are obtained.

Keywords: Multiphase flow, dissolved oxygen, numerical modeling.

Numerical analysis of active boundary layer control in linear cascades

Dr. Jelena Svorcan, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering

Numerical analysis of subsonic and transonic flow in linear cascades has been performed. Active boundary layer control was employed in the form of sources (jets) distributed across the upper surface of the profile. Fluid is viscous and compressible, flow is turbulent, while performed analyses are 2D. Goals of the study are: definition of an adequate numerical setting that enables sufficiently correct simulation of the problem in question, as well as evaluation of the possible increase in aerodynamic performance of the cascades. As the choice of turbulence model affects the final solution of Reynolds equations, turbulence was modelled by four different models: Spalart-Allmaras, a variant of k-ε, k-ω SST and γ-Reθ, and a comparison of obtained results is performed.

Keywords: flow in cascades, computational aerodynamics, turbulence models, active boundary layer control.

Numerical modeling of low calorific gaseous fuels combustion within porous inert media

Dr. Mirjana Stamenić, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering

The recent rise of interest in combustion of low calorific gaseous fuels has been derived, on the one hand, form the fact that fossil fuels shortfall will happen in near future, and on the other, there is constant struggle with tighten emission regulations. Low calorific gases are usually generated as side products in technological processes (i.e. blast furnace gas, waste gas from cupola and coke ovens, etc.), or can be produced in biomass gasification process or as a landfill gas and gas from mines. Common problems associated with combustion of low calorific gaseous fuels are burning stability in narrow operating range, high cost of auxiliary equipment for safe operation of burner system and operation instability mainly caused by variability of fuel content. Combustion within porous inert media (PIM) shows distinctive difference from open flame burning systems. The first important factor, which influences combustion within PIM, is high area/volume ratio implying high efficiency in heat transfer between flue gas and porous media. The other factor is intensive heat recuperation within porous media, which contributes combustion stability, possibility of operation within the wide range and stabile burning the lean fuel/air mixtures. Turbulence greatly influences intensity of heat exchange between gaseous and solid phase within porous media. Some studies on combustion within porous inert media showed that combustion turbulization is significant only in the high velocity regimes. On the other hand, due to complexity, many publications on combustion within porous inert media observe laminar flow regime while undergoing chemical reaction. However, recent awareness of the importance of treating intra-pore turbulence introduced new investigating field, where the extension of the standard k-ε model is applied. Paper presents 1-D mathematical model of low calorific gaseous fuel combustion within porous inert media, which provides universal tool for design, optimization and analysis of combustion within porous burners.

Keywords: combustion, porous media, numerical modeling, turbulent flow, gaseous fuels.

Laser Doppler velocimetry and confined flows

Prof. Dr. Jelena Ilić, University of Belgrade, Faculty of Mechanical Engineering, Dr. Slavica Ristić, Principal Research Fellow, Institute Goša, Prof. Dr. Milesa Srećković, University of Belgrade, Faculty of Electrical Engineering

Laser Doppler velocimetry (LDV) is one of a few standard testing techniques in fluid dynamics. As an absolute measurement technique (requires no calibration) and nonintrusive tool, it is readily used in laboratory conditions for measurements of turbulent flows. Its main advantage is that it makes almost no interference with turbulence structures of a flow – it doesn’t harm even small vortices. Furthermore, in its nature, along with the velocity values, LDV provides Root Mean Square (RMS) of velocity values, which represents the velocity fluctuation at the measurement point. Thus, LDV measurement promptly gives the turbulence level in that point. LDV is especially suitable in open flow research, where 2D and 3D LDV systems easily give reliable results. However, its application in confined flow measurements has some restrictions, and requires some corrections due to refraction of laser beams on wall surfaces. When the flow parameters are measured through a flat wall, only minute corrections of measurement results should be made (such as the corrections of measurement volume position). In many technical problems, flows in cylindrical tubes are of interest to be investigated. In that case, if axial velocity component is measured in points of a radial plane (a plane that contains tube axis) laser beams of LDV system act as if they pass through a flat wall, and stay in the same plane. This fact is used in measurements, presented in this paper, that were performed on water flows in glass tubes with cylindrical elements. The measurements were done by Dantec 1D LDV system in Military Technical Institute in Belgrade. The values of axial velocity components, and its turbulence levels were measured in several tubes of different shapes but with cylindrical tube base (cylindrical tube with asymmetrical widening, with one or two spherical widening, cylindrical tube with obstacle within it …). The distributions of velocity values and their turbulence levels along the tube axis and along the diameters of chosen cross-sections are graphically presented in this paper. Corrections that take into account measurement volume dislocation and the change of calibration constant, due to beam refraction, were derived and applied on measurement results, and graphically presented in this paper. The possibilities of measurement out of radial plane are analyzed in detail, and some numerical results of this analysis are presented. Especially, the impact of tube wall thickness and type of fluid on measurement volume positions and angle of deviation for radial velocity component are considered. Conditions required for reliable application of 2D LDV systems are noted.

Keywords: Laser Doppler velocimetry, flow in a cylindrical tube, turbulence level.

Experimental investigation of axial ducted fan turbulent flow by hot wire anemometry

Dr. Milan Šekularac, Teaching and Research Assistant, University of Montenegro, Faculty of Mechanical Engineering

Determination of turbulent flow field properties at the exit of a high-speed axial ducted fan is presented. Selected fan is of small scale (55mm rotor diameter) and originaly intended use is for UAV aircraft, at a operating point 27500 RPM. Hot wire anemometry technique is used with a X-type 2-sensor probe of 2.5mikron size, with a frequency range of 20 kHz. Fields of velocity components, turbulent kinetic energy and intensity, integral length scales, Reynolds stresses and other quantities are presented, at the fan duct exit cross section. The algorithm to diferentiate the turbulence from periodic mean flow is discussed, the signal filtering possibilities and future improvements in the procedure.

Keywords: axial fans, turbulence, turbomachinery.

High speed stereo PIV investigation of the NASA Common Research Model wing tip vortex

Dr. Đorđe Čantrak, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering, James Heineck, Principal Investigator, NASA Ames Research Center, Experimental Aero-Physics Branch, Moffett Field, USA, Laura Kushner, Research Engineer, Aerospace Computing, Inc., Mountain View, USA, Novica Janković, PhD student, Research Assist., University of Belgrade, Faculty of Mechanical Engineering

The NASA Common Research Model (CRM), developed as an open-source contemporary transonic supercritical wing for various studies in aerodynamics, has been studied since 2008. Aerodynamic performance data were and are still collected and serve as a good basis for CFD studies. Here is presented experimental study of the wing tip vortex, a circulatory three-dimensional motion that trails downstream from the wing. Prediction of the wing tip vortices is still a challenge for CFD codes due to significant pressure and velocity gradients. High-speed stereo particle image velocimetry (HSS PIV) measurements of the wing-tip vortex from a 3% scaled semi-span model of the CRM without nacelle and pylon are performed in a vertical cross-stream plane three tip-chords downstream of the wing tip trailing edge. CRM was tested in the 32- by 48- inch Indraft Tunnel, located in the Fluid Mechanics Laboratory (FML) at NASA Ames Research Center for three various angles of attack 0°, 2° and 4°. The wind tunnel speed was approximately the same for all measurements approximately 50 m/s. This corresponds to a chord Reynolds number 2,68*105, where the chord length of 3.2“ is considered the characteristic length. The HSS PIV system was working at the sampling rate of 2kHz. Obtained region of interest was x=220 mm and y=90 mm. Velocity fields and turbulence statistics are presented for all cases, as well as turbulence structure in the light of the invariant theory. It was shown that velocities didn't change significantly as the function of the angle of attack. Points of the total velocity minimum and streamwise vorticity maximum are obtained in the same points which denote vortex core coordinates for all angles of attack. The highest streamwise vorticity was obtained for the angle 4°, while the lowest for angle 0°. It was also shown that vorticity direction is opposite for angle 0° than for other two angles. 20,000 PIV samples were acquired at each angle-of-attack. Turbulence kinetic energy, as well all Reynolds stresses, reach maximum in the vortex core center. All possible turbulence states are studied and presented in the anisotropy invariant map. Further experimental investigations are on CRM are planned and will be continued.

Keywords: turbulence, vortex, high speed stereo PIV, CRM.



A hydraulic design procedure of axial-flow pump

Zhenhua Shen, Xiaofen Ma, Grundfos China Holding Co., Ltd., Svend Rasmunssen, Prof. Christian Brix Jacobsen, Grundfos Holding A/S, Bjerringbro, Denmark

Presented by Dušan Durković, Country Manager Serbia, Jasminka Savić, External Sales Engineer Industry, Grundfos Srbija d.o.o. (Branch Office Belgrade - Sales)

A radical decrease of the product development time has been achieved by implementation of simulation driven development on axial-flow pump design. A new automatic hydraulic design procedureof axial-flow pumps was developed. It consists of in-house developed parametric design template, Visual Basic macro, OpenFOAM and an in-house developed pump performance plot tool. In this procedure the parametric template includes propeller design and guide vane design. A mesh is generated by using OpenFOAM snappyHexMesh utility. Visual Basic macro is used to link the parametric template and OpenFOAM.
In order to validate the accuracy of predicting the hydraulic performance from this procedure, a high specific speed axial-flow pump was designed by using this procedure and an in-house optimization tool. Finally an aluminum scaled prototype was made by CNC machining. The CFD results show that BEP efficiency is equal to 83.5%, and testing shownBEP efficiency of 85%. It indicates that the hydraulic design from this procedure is both reliable and able to produce high efficiency designs.

Note: This paper is presented and published in ASME-JSME-KSME Joint Fluids Engineering Conference 2015, AJK2015-FED, July 26-31, 2015, SEOUL, KOREA, Paper No. AJKFLUIDS2015-33036, pp. 2294-2299.


Successful case studies with ANSYS CFD in the area of turbulence modelling

Evangelia Bika, Sales Engineer, SimTec Software and Services, Greece

Reynolds-Averaged Navier-Stokes turbulence models are the vast majority of industrial CFD applications and are expected to remain so for many years to come. However, the continuing increase in computer power and especially High Performance Computing (HPC), as well as new model developments in Scale-Resolving Simulation (SRS), combined with requirement for high accuracy level for certain type of applications like acoustics, fluid-structure interaction and vortex-induced cavitation, are gaining increasingly higher attention and usage. In addition, Large-Eddy Simulation, being the oldest and SRS model, is being displaced by other more “practical” SRS models (in general LES variants) with lower CPU and memory requirements, without a sacrifice in accuracy. A brief discussion is made on the characteristics and requirements of these “new” SRS methods, as well as the preferred flows to be applied in.
For both RANS and SRS models several validations are presented in a wide range of flows, through comparison with experimental and DNS data.

Keywords: Reynolds-Averaged Navier-Stokes (RANS), Scale-Resolving Simulation (SRS), Large-Eddy Simulation (LES), Detached-Eddy Simulation (DES), Scale-Adaptive Simulation (SAS).