Highly Resolved Hybrid RANS-LES Simulations of Different Operating Points of a Model Kaplan Turbine

Double regulated Kaplan turbines with adjustable guide vanes and runner blades offer a high degree of flexibility and good efficiency for a wide range of operating points. However, this also leads to a complex geometry and flow guidance with, for example, vortices of different size and strength. The flow in a draft tube is especially challenging to simulate mainly due to flow phenomena like swirl, separation and strong adverse pressure gradient and a strong dependency on the upstream flow conditions. The calculation of performance is often inaccurate with standard simulation approaches (i.e. RANS turbulence models, a coarse mesh and large time step size) and can lead to wrong predictions of the operating range. To reveal occurring flow phenomena and physical effects scale resolving hybrid RANS-LES simulations for three different operating points, one on-cam and two off-cam, on a block structured mesh of about 400 million hexahedral elements of a double regulated five blade model Kaplan turbine are carried out. In this paper first results of the ongoing simulations are presented. The major part of the simulation domain is running in LES mode and seems to be properly resolved. The validation of simulation results with experimental data shows good agreement in the global results, i.e. total head and power, and a good visual agreement with three-dimensional PIV measurements of the velocity. For the standard RANS simulations the results are inconsistent and the agreement with the measurement depends on the investigated operating point.