Ing. David Štefan, Ph.D.
E-mail:   David.Stefan@vut.cz  Pracoviště:   Energetický ústav odbor fluidního inženýrství Viktora Kaplana Zařazení:   Odborný asistent Místnost:   A1/0524

Vybrané publikace:

• ŠTEFAN, D.; RUDOLF, P.; MUNTEAN, S.; SUSAN-RESIGA, R.;:
  Proper Orthogonal Decomposition of Self-Induced Instabilities in Decelerated Swirling Flows and Their Mitigation Through Axial Water Injection, ASME
  článek v časopise ve Web of Science, Jimp
• RUDOLF, P.; ŠTEFAN, D.; SEDLÁŘ, M.; KOZÁK, J.; HABÁN, V.; HUZLÍK, R.:
  Spatio-temporal description of the cavitating flow behavior around NACA 2412 hydrofoil,
  Journal of Physics: Conference Series, pp.1-5, (2015), IOP Publishing
  článek ve sborníku ve WoS nebo Scopus
  akce: 9th International Symposium on Cavitation (CAV2015), Lausanne, 06.12.2015-10.12.2015
• JANDOUREK, P.; HABÁN, V.; HUDEC, M.; DOBŠÁKOVÁ, L.; ŠTEFAN, D.:
  Design of hydraulic recuperation unit,
  EPJ Web of Conferences, pp.347-350, (2015), EDP Sciences
  článek ve sborníku ve WoS nebo Scopus
  akce: Experimental fluid mechanics 2015, Praha, 17.11.2015-20.11.2015
• KOZÁK, J.; RUDOLF, P.; ŠTEFAN, D.; HUDEC, M.; GRÍGER, M.:
  ANALYSIS OF PRESSURE PULSATIONS OF CAVITATING FLOW IN CONVERGING-DIVERGING NOZZLE,
  Cavitation and dynamic problems - IAHRWG 2015 - Proceedings, pp.10-19, ISBN 978-961-6770-31-6, (2015)
  článek ve sborníku ve WoS nebo Scopus
  akce: IAHRWG 2015, Ljubljana, 09.09.2015-11.09.2015
• ŠTEFAN, D.; RUDOLF, P.; HUDEC, M.; HABÁN, V.:
  Comprehensive Study Of Unsteady Pressure Pulsations Induced By The Spiral Vortex Structure In A Conical Diffuser,
  Proceedings of 6th IAHR meeting of the Working Group, IAHRWG 2015, pp.331-338, ISBN 978-961-6770-31-6, (2015), Faculty of Technologies and Systems
  konferenční sborník (ne článek)
  akce: IAHRWG 2015, Ljubljana, 09.09.2015-11.09.2015
• ŠTEFAN, D.; ZUBÍK, P.; HUDEC, M.; RUDOLF, P.:
  Numerical and experimental investigation of swirling flow in a conical diffuser, EPJ Web of Conferences
  článek v časopise ve Web of Science, Jimp
  akce: Experimental Fluid Mechanics 2014, Český Krumlov, 18.11.2014-21.11.2014
• ŠTEFAN, D.; RUDOLF, P.:
  Proper Orthogonal Decomposition of Pressure Fields in a Draft Tube Cone of the Francis (Tokke) Turbine Model,
  Journal of Physics: Conference Series, pp.1-14, (2015), IOP Publishing
  článek ve sborníku ve WoS nebo Scopus
  akce: Francis 99, Trondheim, 15.12.2014-16.12.2014
• RUDOLF, P.; ŠTEFAN, D.; HUDEC, M.; KOZÁK, J.:
  Cavitation in swirling flows,
  Proceedings of Kolloquium Kavitation und Kavitations Erosion, pp.1-15, (2014), RUB Bochum
  článek ve sborníku ve WoS nebo Scopus
  akce: Kolloquium Kavitation und Kavitations Erosion, Bochum, 08.12.2014-09.12.2014
• RUDOLF, P.; ŠTEFAN, D.:
  Reduced order model of draft tube flow,
  IOP Conference Series-Earth and Environmental Science, pp.1-10, (2014), IOP Science
  článek ve sborníku ve WoS nebo Scopus
  akce: 27th IAHR Symposium on Hydraulic Machinery and Systems, Montreal, 22.09.2014-26.09.2014
• RUDOLF, P.; ŠTEFAN, D.:
  Spatio-temporal description of the swirling flow in hydraulic turbine draft tube,
  Proceedings of 18th International Seminar on Hydropower, pp.191-201, ISBN 978-3-9501937-9-4, (2014), TU Wien
  článek ve sborníku ve WoS nebo Scopus
  akce: 18th International Seminar on Hydropower Plants, Vienna, 26.11.2014-28.11.2014
• ŠTEFAN, D.; ZUBÍK, P.; HUDEC, M.; RUDOLF, P.:
  Numerical and experimental investigation of swirling flow in a conical diffuser,
  Proceedings of the International conference Experimental Fluid Mechanics 2014, pp.563-567, (2014), EDP Sciences
  článek ve sborníku ve WoS nebo Scopus
  akce: Experimental Fluid Mechanics 2014, Český Krumlov, 18.11.2014-21.11.2014
• RUDOLF, P.; ŠTEFAN, D.; POCHYLÝ, F.:
  Dynamics of swirling flows in industrial applications,
  Engineering Mechanics 2014: Book of Full Texts, pp.38-41, ISBN 978-80-214-4871-1, (2014), ACAD SCI CZECH REPUBLIC, INST THERMOMECHANICS, DOLEJSKOVA 5, PRAGUE 8, 182 00, CZECH REPUBLIC
  článek ve sborníku ve WoS nebo Scopus
  akce: Engineering Mechanics 2014, Svratka, 12.05.2014-15.05.2014
• RUDOLF, P.; HUDEC, M.; GRÍGER, M.; ŠTEFAN, D.:
  Characterization of the cavitating flow in converging-diverging nozzle based on experimental investigations,
  EPJ Web of Conferences, pp.1-6, ISBN 978-80-260-5375-0, (2014), EDP Sciences
  článek ve sborníku ve WoS nebo Scopus
  akce: Experimental fluid mechanics 2013, Kutná Hora, 19.11.2013-22.11.2013
• ŠTEFAN, D.; RUDOLF, P.; MUNTEAN, S.; SUSAN-RESIGA, R.:
  Structure of Flow Fields Downstream of Two Different Swirl Generators,
  Engineering Mechanics, Vol.20, (2014), No.5, pp.339-353, ISSN 1805-4633
  článek v časopise - ostatní, Jost
• ŠTEFAN, D.; HABÁN, V.; HUDEC, M.; VESELÝ, J.; RUDOLF, P.:
  Analysis of Pelton Turbine Jet Decay Using a Proper Orthogonal Decomposition of Experimentally Obtained Image Ensemble,
  Book of Abstract, 5th International Workshop on Cavitation and Dynamic Problems in Hydraulic Machinery, (2013), Ecole polytechnique fédérale de Lausanne, Laboratory for Hydraulic Machines
  abstrakt
• ŠTEFAN, D.; RUDOLF, P.:
  Computational Fluid Dynamic Study of the Flow Downstream of the Swirl Generator Using Large Eddy Simulation
  abstrakt
• RUDOLF, P.; ŠTEFAN, D.:
  Decomposition of the swirling flow field downstream of Francis turbine runner,
  IOP Conference Series-Earth and Environmental Science, pp.1-8, (2012), IOP Publishing Ltd.
  článek ve sborníku ve WoS nebo Scopus
  akce: 26th IAHR Symposium on Hydraulic Machinery and Systems, Beijing, 19.08.2012-23.08.2012
• ŠTEFAN, D.; RUDOLF, P.; SKOTÁK, A.; MOTYČÁK, L.:
  Energy transformation and flow topology in an elbow draft tube,
  Applied andComputational Mechanics, Vol.6, (2012), No.1, pp.93-106, ISSN 1802-680X, University of West Bohemia
  článek v časopise - ostatní, Jost
• RUDOLF, P.; POCHYLÝ, F.; ČERMÁK, L.; ŠTEFAN, D.:
  Instability of the swirling flows with/without cavitation,
  Book of Extended Abstracts, pp.1123-1124, ISBN 978-80-86246-39-0, (2012), Czech Society for Mechanics
  abstrakt
• ŠTEFAN, D.; RUDOLF, P.; MUNTEAN, S.; SUSAN-RESIGA, R.F.:
  Structure of Flow Fields Downstream of Two Different Swirll Generators,
  Engineering Mechanics 2012, Book of Extended Abstracts, pp.326-327, ISBN 978-80-86246-39-0, (2012)
  článek ve sborníku ve WoS nebo Scopus
  akce: Engineering Mechanics 2012, Svratka, 14.05.2012-17.05.2012
• RUDOLF, P.; HUDEC, M.; ZUBÍK, P.; ŠTEFAN, D.:
  Experimental measurement and numerical modeling of cavitating flow in converging-diverging nozzle,
  EPJ Web of Conferences, pp.1-8, ISBN 978-80-7372-784-0, (2012), EDP Sciences
  článek ve sborníku ve WoS nebo Scopus
  akce: Experimental Fluid Mechanics 2011, Jičín, 22.11.2011-25.11.2011

Seznam publikací na portálu VUT

Anotace nejvýznamnějších prací:

• ŠTEFAN, D.; ROSSI, M.; HUDEC, M.; RUDOLF, P.; NIGRO, A.; RENZI, M.:
  Study of the internal flow field in a pump-as-turbine (PaT): Numerical investigation, overall performance prediction model and velocity vector analysis, Elsevier
  článek v časopise ve Web of Science, Jimp
  This work presents a detailed analysis of the three-dimensional flows in a Pump-as-Turbine (PaT). The flow field is studied through URANS simulations and a velocity vector analysis. Moreover, a prediction model is used to forecast the performance of the PaT. The results of these methodologies were validated using experimental results. Flow rate and head at Best Efficiency Point (BEP) in turbine mode were 27% and 41% higher than the ones in pump mode, respectively; the mechanical efficiency was 4% lower. To give a fluid flow interpretation of this behaviour, velocity triangles in turbine mode were analytically calculated, as well as numerically evaluated. The analytically identified PaT flow rate at BEP is in agreement with CFD and measurements. Only small differences appear, which might be explained by three reasons: i) the real flow angle is influenced by the finite number of blades and their thickness, ii) the PaT leading edge has a sharp ending since the impeller is designed to operate only in pump mode and iii) tapering of the meridional cross-section from volute to impeller. While close to the BEP the mechanical efficiency is flat, a sensible drop caused by flow detachments and swirling flows was detected at part-load.
• ŠTEFAN, D.; HOUDE, S.; DESCHÊNES, C.:
  Numerical Investigation of Flow in a Runner of Low-Head Bulb Turbine and Correlation With Particle Image Velocimetry and Laser Doppler Velocimetry Measurements, The American Society of Mechanical Engineers
  článek v časopise ve Web of Science, Jimp
  It is a well-known fact and a much studied problematic that the performance of low-head hydraulic turbines is highly dependent on the runner–draft tube coupling. Around the optimal operating conditions, the efficiency of the turbine follows closely the performance of the draft tube that in turn depends on the velocity field exiting the runner. Hence, in order to predict correctly the performance of the draft tube using numerical simulations, the flow inside the runner must be simulated accurately. Using results from unique and detailed particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) measurements inside the runner channel of a bulb turbine, this paper presents an extensive study of the predictive capability of a widely used simulation methodology based on unsteady Reynolds-averaged Navier–Stokes equations with a k-epsilon closure model. The main objective was to identify the main parameters influencing the numerical predictions of the velocity field at the draft tube entrance in order to increase the accuracy of the simulated performance of the turbine. This paper relies on a comparison of simulations results with already published LDV measurements in the draft tube cone, interblade LDV, and stereoscopic PIV measurements within the runner. This paper presents a detailed discussion of numerical–experimental data correlation inside the runner channel and at the drat tube entrance. It shows that, contrary to widely circulated ideas, the near-wall predictions at the draft tube entrance is surprisingly good while the simulation accuracy inside the runner channels deteriorates from the leading to the trailing edges.
• ŠTEFAN, D.; RUDOLF, P.; MUNTEAN, S.; SUSAN-RESIGA, R.;:
  Proper Orthogonal Decomposition of Self-Induced Instabilities in Decelerated Swirling Flows and Their Mitigation Through Axial Water Injection, ASME
  článek v časopise ve Web of Science, Jimp
  The swirling flow exiting the runner of a hydraulic turbine is further decelerated in the discharge cone of the draft tube to convert the excess of dynamic pressure into static pressure. When the turbine is operated far from the best efficiency regime, particularly at part load, the decelerated swirling flow develops a self-induced instability with a precessing helical vortex and the associated severe pressure fluctuations. This phenomenon is investigated numerically in this paper, for a swirl apparatus configuration. The unsteady three-dimensional (3D) flow field is analyzed using a proper orthogonal decomposition (POD), and within this framework we examine the effectiveness of an axial jet injection for mitigating the flow instability. It is shown that a limited number of modes can be used to reconstruct the flow field. Moreover, POD enables to reveal influence of the jet injection on the individual modes of the flow and illustrates continuous suppression of the modes from higher-order modes to lower-order modes as the jet discharge increases. Application of POD offers new view for the future control effort aimed on vortex rope mitigation because spatiotemporal description of the flow is provided. Thereby, POD enables better focus of the jets or other flow control devices.
• ŠTEFAN, D.; RUDOLF, P.; SKOTÁK, A.; MOTYČÁK, L.:
  Energy transformation and flow topology in an elbow draft tube,
  Applied andComputational Mechanics, Vol.6, (2012), No.1, pp.93-106, ISSN 1802-680X, University of West Bohemia
  článek v časopise - ostatní, Jost
  Článek se zabývá výpočtovou studií energetické přeměny ve dvou geometrických konfiguracích kolenových sacích trub pro Kaplanovu turbínu. Koeficient regenerace tlaku, hydraulická účinnost a ztrátový součinitel jsou vyhodnoceny pro sérii průtoků a vírových čísel korespondujících s provozním režimem turbíny. Tyto integrální charakteristiky jsou dále porovnány s vlastnostmi lokálního proudového pole, které jsou znázorněny pomocí topologie čar povrchových proudnic. Hlavním cílem je poukázat na příčiny náhlého poklesu účinnosti kolenové sací trouby.
• ŠTEFAN, D.; RUDOLF, P.; MUNTEAN, S.; SUSAN-RESIGA, R.F.:
  Structure of Flow Fields Downstream of Two Different Swirll Generators,
  Engineering Mechanics 2012, Book of Extended Abstracts, pp.326-327, ISBN 978-80-86246-39-0, (2012)
  článek ve sborníku ve WoS nebo Scopus
  akce: Engineering Mechanics 2012, Svratka, 14.05.2012-17.05.2012
  Článek se zabývá porovnáním proudových polí generovaných dvěma odlišnými vírovými generátory. Vírové generátory jsou určeny k imitaci proudění vyskytujícího se na výstupu z oběžného kola hydraulické turbíny a ke studiu prostorově-časového chování vířivého proudění v sací troubě (t.j. výstupnímu difusoru hydraulické turbíny), kde se vlivem nestability formuje tzv. vírový cop. Nestacionární výpočty byly provedeny pro identické hodnoty Reynoldsova čísla. Výsledné profily rychlosti a vířivosti jsou posouzeny vzhledem ke struktuře vírového copu. Rozdíly ve vybuzených tlakových pulzacích jsou znázorněny v amplitudo-frekvenčním spektru statického tlaku na stěně diffusoru.