In this study, the hydrodynamics and turbulence in a square tank stirred with a hydrofoil impeller, a Lightnin A310, is investigated using a large eddy particle image velocimetry (LE PIV) approach. The particle image velocimetry data are used as the large scale part of a large eddy simulation and the small scales are modelled assuming that the turbulent kinetic energy production is limited to the large scale structures, the turbulent energy dissipation is confined to the small scale structures and the transfer of energy takes place in the inertial sub-range. The small scale turbulence was modelled by direct calculation of the turbulent stress tensor using filtered particle image velocimetry data. The spatial distribution of the calculated dissipation rate tensors showed good agreement with previous work. The macro instabilities of the flow structure were investigated by means of spectral analysis. Low frequency phenomena separated from the mean flow were detected. The cause of these could partly be explained by the circulation time for the tank, which corresponded to the low frequency phenomena found at 0.03NHz, where N is the rotational speed. (c) 2005 Elsevier Ltd. All rights reserved.