Validation of an IDDES-type turbulence model and application to a Francis pump turbine flow simulation in comparison with experimental results

Abstract

In this work we demonstrate the superiority of the IDDES turbulence model approach compared to the RANS formulation at the application of a Francis pump turbine flow simulation at part load conditions. At this operating point strongly swirling flow occurs in the draft tube, which leads to the vortex rope phenomenon, a rotating vortex around the stagnant region in the centre of the diffuser flow leading to a local low pressure zone.This hybrid RANS-LES turbulence model approach – based on the SST turbulence model – was implemented in OpenFOAM®-1.6-ext open source code and successfully validated for different test cases, like the decay of isotropic turbulence, periodic channel flow, flow over an abrupt expansion and a periodic hill flow. With these test cases constants were calibrated, a good prediction of boundary layer flow was proven and the ability to predict the flow correctly for numerically more difficult flow configurations.The flow in the pump turbine draft tube was analysed with laser measurements (LDA and PIV) and wall pressure sensors, as well as torque, hydraulic head and discharge measurements. All measurements serve as validation for the flow simulations. The results of the flow simulations with the IDDES turbulence model fit quite well with the measurements for the velocity components and especially for the second order statistics. This means that the turbulence is well resolved. The shape of the vortex rope is better predicted, leading to better conformity with the measurements, even for high frequencies. The hybrid turbulence model resolves the inertial range of turbulence.