Very large eddy simulation of swirling flow in the Dellenback abrupt expansion tube

Abstract

There is usually a fluid dynamic phenomenon which involves a strong swirling flow between runner and draft tube of Francis turbines at part load condition. It is characterized by highly unsteady, large scale vortices, intense turbulence production, etc. The adverse pressure gradient in the draft tube cone can lead to vortex breakdown, which is recognized now as the main cause of hydro plant’s instabilities. Therefore, it is very significant to predict the swirling flow field exactly in design period. However, the popular Reynolds-Averaged Navier-Stokes (RANS) or large eddy simulation (LES) method also has some shortcomings in CFD analysis, such as inaccurate large structures resolution or restrictive grid requirement. Very large eddy simulation(VLES), as a hybrid RANS/LES methodology, could combine the advantages of different turbulence approaches of RANS, LES, which has been proved by many authors. This paper presents a VLES case-study, based on the experimental studies of the swirling flow in the abrupt expansion by Dellenback et al. It is validated that VLES and LES model are much more accurate than the RANS models compared with test data. The standard k-ε and k-ω models are unable to accurately model the effect of the large-scale unsteadiness, while VLES almost has similar ability as LES in resolving that problem. In comparison with LES, it is demonstrated that VLES can give satisfactory convergence with a coarse mesh.