David Štefan

Proper Orthogonal Decomposition of Pressure Fields in a Draft Tube Cone of the Francis (Tokke) Turbine Model

The simulations of high head Francis turbine model (Tokke) are performed for three operating conditions - Part Load, Best Efficiency Point (BEP) and Full Load using software Ansys Fluent R15 and alternatively OpenFOAM 2.2.2. For both solvers the simulations employ Realizable k-e turbulence model. The unsteady pressure pulsations of pressure signal from two monitoring points situated in the draft tube cone and one behind the guide vanes are evaluated for all three operating conditions in order to compare frequencies and amplitudes with the experimental results. The computed velocity fields are compared with the experimental ones using LDA measurements in two locations situated in the draft tube cone. The proper orthogonal decomposition (POD) is applied on a longitudinal slice through the draft tube cone. The unsteady static pressure fields are decomposed and a spatio-temporal behavior of modes is correlated with amplitude-frequency results obtained from the pressure signal in monitoring points. The main application of POD is to describe which modes are related to an interaction between rotor (turbine runner) and stator (spiral casing and guide vanes) and cause dynamic flow behavior in the draft tube. The numerically computed efficiency is correlated with the experimental one in order to verify the simulation accuracy.

Decomposition of the swirling flow field downstream of Francis turbine runner

Practical application of proper orthogonal decomposition (POD) is presented. Spatio-temporal behaviour of the coherent vortical structures in the draft tube of hydraulic turbine is studied for two partial load operating points. POD enables to identify the eigen modes, which compose the flow field and rank the modes according to their energy. Swirling flow fields are decomposed, which provides information about their streamwise and crosswise development and the energy transfer among modes. Presented methodology also assigns frequencies to the particular modes, which helps to identify the spectral properties of the flow with concrete mode shapes. Thus POD offers a complementary view to current time domain simulations or measurements.

Spatio-temporal description of the cavitating flow behavior around NACA 2412 hydrofoil

Spatio-temporal description of the cavitating flow around hydrofoil with 8 degrees incidence using proper orthogonal decomposition (POD) is presented. POD is a suitable tool, which provides information not only about the flow dynamics, but also about relevance of different flow structures. POD also enables to track energy transport within the domain and energy transfer among the eigenmodes of the flow field. Analysis documents change of the flow structure for decreasing cavitation number, which can be most likely attributed to sheet/cloud cavitation transition.

Reduced order model of draft tube flow

Swirling flow with compact coherent structures is very good candidate for proper orthogonal decomposition (POD), i.e. for decomposition into eigenmodes, which are the cornerstones of the flow field. Present paper focuses on POD of steady flows, which correspond to different operating points of Francis turbine draft tube flow. Set of eigenmodes is built using a limited number of snapshots from computational simulations. Resulting reduced order model (ROM) describes whole operating range of the draft tube. ROM enables to interpolate in between the operating points exploiting the knowledge about significance of particular eigenmodes and thus reconstruct the velocity field in any operating point within the given range. Practical example, which employs axisymmetric simulations of the draft tube flow, illustrates accuracy of ROM in regions without vortex breakdown together with need for higher resolution of the snapshot database close to location of sudden flow changes (e.g. vortex breakdown). ROM based on POD interpolation is very suitable tool for insight into flow physics of the draft tube flows (especially energy transfers in between different operating points), for supply of data for subsequent stability analysis or as an initialization database for advanced flow simulations.

Spatio-Temporal Description of the Swirling Flow in Hydraulic Turbine Draft Tube

Highly swirling ow leaves runner of the hydraulic turbine, which is operated o the best e ciency point. Instability of the swirling ow results in so called vortex rope. Rotating vortex rope is a source of pressure pulsations, causes structural loads, vibrations and noise. Proper orthogonal decomposition (POD) provides a spatio-temporal description of the draft tube swirling ow and proves that only a very limited number of the ow eigenmodes is necessary to describe the ow dynamics. Construction of the reduced order model (ROM), which is derived by projection of Navier-Stokes equation onto POD basis forms a suitable state equation for the draft tube ow control. This paper presents a review of POD application to draft tube ows and a proposal of extension to ROM and closed loop ow control.

Investigation of the Cavitation Within Venturi Tube: Influence of the Generated Vortex

The main scope of the paper is evaluation of the experimental results and comparison of the dynamics of the vortex affected cavitating flow with the dynamics of purely axial flow within the Venturi nozzle. The analysis of the high-speed records with the sampling frequency of 20,000 images per second will be presented. Records will be analyzed using the proper orthogonal decomposition (POD) and spectral analysis of the pixel intensity within the selected region of images. These analyses will be done for the wide range of regimes for the both experimental configurations (with and without the swirl generator). Analysis of the pressure pulsations and vibrations will be utilized for the verification of the results. The cavitation patterns affected by the presence of the vortex will be described and compared with the case of axial inflow. The analysis of the experimental data will be complemented with the numerical computations of the chosen regimes carried out using the OpenFoam v1606+ and its multiphase interPhaseChangeFoam solver.

Numerical and experimental investigation of the cavitating flow within Venturi tube

December 16-21 2017 Abstract Hydrodynamic cavitation represents complex physical phenomenon undesirably affecting operation as well as lifespan of many hydraulic machines from small valves to the large hydro power plants. On the other hand, the same phenomenon and its concomitants such as pressure pulsations can be exploited in many positive ways. One of them which seems to be very promising and perspective is the cavitation utilization for reduction of the microorganisms such as cyanobacteria within large bulks of water. Mutual effect of the swirl induced by the upstream mounted generator and flow constriction in convergingdiverging nozzle has been experimentally investigated. The main scope of this paper is numerical investigation complementing the experimental results. The multiphase simulations were carried out using the OpenFoam 1606+ and its interPhaseChangeFoam solver. The present study focuses on CFD results of the cavitating velocity field within the nozzle and analysis of the loss coefficient within the nozzle.

Manipulation of the swirling flow instability in hydraulic turbine diffuser by different methods of water injection

Vortex rope, which induces substantial pressure pulsations, arises in the draft tube (diffuser) of Francis turbine for off-design operating conditions. Present paper focuses on mitigation of those pulsations using active water jet injection control. Several modifications of the original Susan-Resiga’s idea were proposed. All modifications are driven by manipulation of the shear layer region, which is believed to play important role in swirling flow instability. While some of the methods provide results close to the original one, none of them works in such a wide range. Series of numerical experiments support the idea that the necessary condition for vortex rope pulsation mitigation is increasing the fluid momentum along the draft