Very large eddy simulation of the vortex rope in the draft tube of Francis turbine

Abstract

There is usually a strong swirling flow between runner and draft tube of Francis turbine at part load, which would lead to a vortex rope in draft tube. RANS and LES models are used by lots of authors to simulate this phenomenon, and the calculated results is almost similar with test observation. However, these turbulent models can’t predict the length of helical vortex rope exactly, especially the low pressure and high vorticity in the core of vortex. Very large eddy simulation, as a hybrid RANS-LES methodology, could combine the advantages of RANS and LES, which has been proved by many authors. This paper presents a VLES simulation of draft tube flow field compared with other three turbulence models. The results show that VLES model is able to effectively reduce the eddy viscosity and velocity dissipation in vortex rope field, leading to lower pressure and longer rope than other models Moreover, it is demonstrated that proper mesh is also significant for VLES to predict the cavitation vortex rope observed in model test.