This article deals with the large-eddy simulation (LES) of a complex turbulent free-surface flow in an unbaffled mixing tank reactor. The free-surface vortex generated in such a configuration is captured using a front-tracking method, while the stirrer is modelled with an immersed boundary condition technique. Comparisons of mean and fluctuating velocities show good agreement with both theory and experimental laser Doppler velocimetry measurements. The study of mean and instantaneous hydrodynamics points out several interesting features, especially coherent structures, which may have a strong impact on mixing in the reactor. Finally, Reynolds stresses analysis confirms the high anisotropy of turbulence throughout the tank. (C) 2010 Elsevier Ltd. All rights reserved.