Very Large Eddy Simulations of Draft Tube Flow

Paper in proceeding, 2006

The objective of this work is to improve numerical predictions of unsteady turbulent flows in the draft tubes of hydraulic power plants. The standard two-equation turbulence models are known to have a strong damping effect on the resolved turbulence in this type of flow. In order to reduce this negative influence of the model, while retaining the usually satisfying near-wall behaviour, a dynamic filtering technique of the turbulent length and time scales is generalised, employed and evaluated. The filter limits the influence of the modeled turbulent length and time scales on the mean flow in regions where unsteadiness can potentially be resolved. The Wilcox (1988) $k-\omega$ turbulence model was chosen as basis for the investigations, and the effects of five different filter widths were examined. The original non-filtered model is also evaluated. A swirling flow through a straight axisymmetric diffuser was chosen as a test case and detailed measurements carried out by Clausen et al. were used to validate the numerical results. The influence of the filtering approach on the resolved frequencies and the time averaged solutions were analysed. It is shown that the filtering procedure gives better predictions of the time-averaged velocity field and more information on the large scale unsteadiness.