Ing. David Štefan, Ph.D.
E-mail:   David.Stefan@vut.cz  Dept.:   Energy Institute Victor Kaplan Dept. of Fluid Engineering Position:   Assistant Professor Room:   A1/0524

Publications:

• 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
  journal article in Web of Science
• 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
  conference paper
  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
  conference paper
  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)
  conference paper
  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
  conference proceedings
  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,
  EFM14 - Experimental Fluid Mechanics 2014, pp.1-5, (2015), EPJ Web of Conferences
  conference paper
  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
  conference paper
  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
  conference paper
  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
  conference paper
  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
  conference paper
  akce: 18th International Seminar on Hydropower Plants, Vienna, 26.11.2014-28.11.2014
• 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
  conference paper
  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
  conference paper
  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
  journal article - other
• 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
  abstract
• ŠTEFAN, D.; RUDOLF, P.:
  Computational Fluid Dynamic Study of the Flow Downstream of the Swirl Generator Using Large Eddy Simulation
  abstract
• 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.
  conference paper
  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
  journal article - other
• 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
  abstract
• 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)
  conference paper
  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
  conference paper
  akce: Experimental Fluid Mechanics 2011, Jičín, 22.11.2011-25.11.2011

List of publications at Portal BUT

Abstracts of most important papers:

• 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
  journal article in Web of Science
  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
  journal article in Web of Science
  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
  journal article in Web of Science
  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
  journal article - other
  Paper presents a computational study of energy transformation in two geometrical configurations of Kaplan turbine elbow draft tube. Pressure recovery, hydraulic efficiency and loss coefficient are evaluated for a series of flow rates and swirl numbers corresponding to operating regimes of the turbine. These integral characteristics are then correlated with local flow field properties identified by extraction of topological features. Main focus is to find the reasons for hydraulic efficiency drop of the elbow draft tube.
• 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)
  conference paper
  akce: Engineering Mechanics 2012, Svratka, 14.05.2012-17.05.2012
  This paper discusses the comparison of the flow fields downstream of two different swirl generators. Both swirl generators are used to imitate the flow at the exit of the hydraulic turbine runner and study spatio-temporal behaviour of the swirling flow in the draft tube (i.e. outlet diffuser part of the hydraulic turbine), which undergoes breakdown into vortex rope. Unsteady CFD computations are carried out for identical Reynolds number. Resulting velocity and vorticity profiles are correlated with the structure of the vortex rope. Difference in excited pressure pulsations is illustrated on amplitudefrequency spectra of static wall pressure.