Albert Ruprecht

Very Large Eddy Simulation for the Prediction of Unsteady Vortex Motion

A new turbulence model for Very Large Eddy Simulation, based on the extended k-e model of Chen and Kim is developed and presented in this paper. Introducing an adaptive filtering technique, the model can distinguish between numerically resolved and unresolved parts of flow. It is applied to the simulation of unstable vortex motion in a pipe trifurcation. This flow phenomenon cannot be predicted with classical RANS methods and commonly used turbulence models. Using the VLES method with the new turbulence model, the phenomenon is well predicted and the results agree reasonably well with measurement data.

Simulation of the Water Hammer in a Hydro Power Plant Caused by Draft Tube Surge

The system oscillations of a water power plant caused by the draft tube flow in part load are investigated. A coupled simulation of a one-dimensional water hammer analysis and a three-dimensional flow calculation of the draft tube vortex rope is applied. With this approach the excitations of the oscillations, frequencies and amplitudes, have not to be estimated but are obtained from the simulation. This allows an accurate prediction of the system oscillations caused by draft tube surge.Copyright

Das Wasserkraftpotenzial in Deutschland

Deutschland hat sich zum Ziel gesetzt, 2030 einen Deckungsanteil der erneuerbaren Energien am gesamten Stromverbrauch von 45 % zu erreichen. Derzeit stellen sie insgesamt einen Anteil von etwa 14 % bereit, die Wasserkraft hat daran einen Anteil von etwa 25 %. Im Auftrag des Bundesministerium fur Umwelt, Naturschutz und Reaktorsicherheit (BMU) wurde auf der Grundlage einer fur ganz Deutschland einheitlichen Methode mit Hilfe des Linienpotenzials das zusatzlich ausbaubare Wasserkraftpotenzial als Ausgangsbasis fur eine deutsche Ausbaustrategie ermittelt.

Flow Simulation of Francis Turbines Using Hybrid RANS-LES Turbulence Models

The operation of Francis turbines in part load condition causes high pressure fluctuations and dynamic loads in the turbine as well as high flow losses in the draft tube. Owing to the co-rotating velocity distribution at the runner blade trailing edge a low pressure zone arises in the hub region finally leading to a rotating vortex rope in the draft tube. The goal of this study is to reach a quantitatively better numerical prediction of the flow at part load and to evaluate the necessary numerical depth with respect to effort and benefit. The focal point of the investigations is on flow simulations of part load conditions using two different kinds of hybrid RANS-LES turbulence model approaches: Scale Adaptive Simulation (SAS) and Improved Delayed Detached Eddy Simulation (IDDES). The flow simulations are done for a high specific speed Francis turbine and a low specific speed Francis pump turbine. The mainly dominating flow phenomenon, the rotating vortex rope, with a frequency of around one third of the runner frequency, requires long physical time and therefore long computational time using many processors.

Investigation of Francis Turbine Part Load Instabilities using Flow Simulations with a Hybrid RANS-LES Turbulence Model

The operation of Francis turbines in part load condition causes high pressure fluctuations and dynamic loads in the turbine as well as high flow losses in the draft tube. Owing to the co-rotating velocity distribution at the runner blade trailing edge a low pressure zone arises in the hub region finally leading to a rotating vortex rope in the draft tube. A better understanding and a more accurate prediction of this phenomenon can help in the design process of a Francis turbine. The goal of this study is to reach a quantitatively better numerical prediction of the flow at part load and to evaluate the necessary numerical depth with respect to effort and benefit. As standard practice, simulation results are obtained for the steady state approach with SST turbulence modelling. Those results are contrasted with transient simulations applying a SST as well as a SAS (Scale Adaptive Simulation) turbulence model. The structure of the SAS model is such, that it is able to resolve the turbulent flow behaviour in more detail. The investigations contain a comparison of the flow losses in different turbine components. A detailed flow evaluation is done in the cone and the diffuser of the draft tube. The different numerical approaches show a different representation of the vortex rope phenomenon indicating differences in pressure pulsations at different geometric positions in the entire turbine. Finally, the turbulent flow structures in the draft tube are illustrated with several evaluation methods, such as turbulent eddy viscosity, velocity invariant and turbulent kinetic energy spectra.

Flow Simulation of a Francis Turbine Using the SAS Turbulence Model

The operation of Francis turbines in part load conditions causes high fluctuations and dynamic loads in the turbine and the efficiency of the draft tube decreases strongly. For a better understanding of the various instantaneous phenomena, flow simulations of the complete Francis turbine are done with particular focus on the draft tube flow.The SAS (Scale Adaptive Simulation) model is used as turbulence model. Depending on the grid resolution and the time step size, this model allows the resolution of smaller turbulent structures compared to RANS turbulence models.Flow phenomena in a Francis turbine require long physical time to convect through the machine. Therefore, long computational time with many processors is necessary to conduct such a flow simulation.

Turbulence Resolving Flow Simulations of a Francis Turbine with a Commercial CFD Code

Transient flow simulations of a Francis turbine in part load conditions using hybrid RANS-LES turbulence models are presented. In the draft tube a rotating low pressure zone—the vortex rope phenomenon—arises, which leads to very complex flow phenomena. A detailed resolution of the flow in space and time is leading to large computational effort.

Fluid-Structure Interaction in Turbine Simulation

In this article, two examples of fluid-structure interaction (FSI) are examined. Furthermore, the issues of turbulence and the impact of turbulence models on the accuracy of Karman vortices is evaluated. For this purpose, an adaptive turbulence model is described, introduced and validated. For the FSI simulations, a partitioned scheme is introduced and implemented. Since the main application considered here is hydraulic machinery, the issue of added mass effects and unstable coupling is addressed and discussed. The scheme is validated with a benchmark application established recently. Finally, the first results obtained for the FSI coupling of a tidal current turbine runner blade under fluid loads are described. For the simulations, the established in-house CFD-Code FENFLOSS is used. For the coupling, the commercial software MpCCI is applied. ABAQUS and CalculiX are used for the solution of the structural part. Simulations are performed on a cluster and a NEC SX-8 vector computer.

Simulation of Vortex Instability in a Pipe Trifurcation

The vortex instability in a spherical pipe trifurcation is investigated by applying a Very Large Eddy Simulation (VLES). For this approach an new adaptive turbulence model based on an extended version of the k-e model is used. Applying a classical Reynolds-averaged Navier-Stokes-Simulation with the standard k-e model is not able to forecast the vortex instability. However the prescribed VLES method is capable to predict this flow phenomenon. The obtained results show a reasonable agreement with measurements in a model test.Copyright

Ermittlung des Wasserkraftpotenzials an Wasserkraftanlagenstandorten mit einer Leistung über 1 MW in Deutschland

Die Wasserkraft gilt in der Bundesrepublik Deutschland als relativ gut ausgebaut, da an den meisten potenziellen Standorten mit einer Leistung >1 MW bereits Wasserkraftwerke installiert sind. Die vorliegende Untersuchung zeigt jedoch, dass alleine an den bestehenden Laufwasserkraftwerken dieser Leistungsgruppe noch ein bedeutendes Ausbaupotenzial vorhanden ist. Dieses Potenzial kann insbesondere durch Modernisierung und Ausbau dieser bestehenden Kraftwerksstandorte genutzt werden.