Simulation of flow in a simplified draft tube: Turbulence closure considerations

Numerical simulations of turbulent flow in a simplified draft tube are carried out. The main emphasis of these simulations is on the investigation of different turbulence closure models in predicting the flow behavior. Both steady and unsteady simulations are performed for axisymmetric as well as three-dimensional grid in an axisymmetric geometry, and results are compared with available experimental data. It is seen that steady simulations, performed with FLUENT and OpenFOAM, using Reynolds-Averaged Navier-Stokes (RANS) models for the axisymmetric and three-dimensional flow geometries give the same symmetric results. These steady, symmetric results underpredict the levels of turbulent kinetic energy (by at least 40%) and axial velocity (by at least 14%) near the centerline of the draft tube. No significant improvement is achieved applying various turbulence RANS models such as realizable k-ε, RNG k-ε, SST k-ω and v²-f model. Unsteady simulations are also performed using both URANS and hybrid URANS/LES (DES) turbulence closure approaches. It is seen that URANS models cannot capture the self-induced nature of the vortex rope and result in steady solutions due to steady boundary conditions while hybrid URANS/LES model can capture unsteady features of the flow. Importance of time-dependent inlet boundary conditions is discussed and it is shown that predictions are improved (at least 10%) by applying time-dependent boundary conditions which include turbulence fluctuations at inlet. Finally, results of two methods of vortex rope identification (iso-pressure surface and Δ-criterion) are compared.