Ability of URANS approach in prediction of unsteady turbulent flows in an unbaffled stirred tank

Three-dimensional, unsteady Navier-Stokes equations are numerically solved to investigate the turbulent flows in a stirred vessel. The computational domain consists of an unbaffled, cylindrical vessel with a pitched-blade turbine impeller. An Eulerian-Eulerian multiphase flow model is applied to determine the shape of the free-surface vortex core. This numerical method is validated by comparing its results with laser Doppler velocimetry measurements in terms of velocity distribution and turbulence kinetic energy profiles at different positions. In the present study, URANS approach with a hybrid zonal turbulence model, k−ω SST and SST-SAS, is used to predict the unsteady pressure and velocity fluctuations within the vessel. Pressure and inward-outward radial velocity waves are generated by the impeller rotation and are captured in the time/space domain close to the impeller trailing edge. The pressure and velocity spectra are computed to characterize the blade passing frequency as the main source of unsteadiness in the turbulent flow within the vessel. The results indicate that the current URANS approach with a proper turbulence model and well-resolved grids can be used as a predictive tool for the flow field and large turbulence scales in the stirred tanks.