Modeling of Swirling Flow in a Conical Diffuser

Abstract

This work aims at getting a better understanding of the turbulent swirling flow in a conical diffuser, which represents a highly simplified draft tube of a water turbine. The numerical 3D (U)RANS investigations are quantitatively compared to experimental data. Qualitative comparison with experimental visualizations and computations of similar flows are also made, and strong similarities to confined swirling flow have been found. Converging/diverging smearlines at the walls reveal a very complex boundary layer and counter-rotating vortex structures are found at the diffuser exit. The solutions to the Reynolds averaged Navier-Stokes equations for the studied cases are asymmetric. The asymmetry of the mean flow so lution is originating from instable properties of the symmetric mode, and the disturbance that triggers the instability is proven to be imperfections of the CAD -geometry. There are some discrepancies regarding the agreements with experimental data, partly reminiscent of the nature of the ($k-\omega$) turbulence model that was used in this work. The origins of turbulent anisotropy are theoretically examined as well as the weaknesses of the Boussinesq assumption, which constitutes an important part of the chosen turbulence model. Also included are a discussion concerning the origin of turbulent anisotropy and aspects of the filtered (LES) and the averaged (RANS) equations.