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Stirred tanks are widely used in process industries for blending of singleand/or multiple-phase fluids, which often involves mass and/or heat transfer, chemical reactions, etc. Deep knowledge of flow field in the stirred tank is necessary to carry out these unit operations efficiently. As a result, extensive investigations of the flow in stirred tanks have been conducted over the past several decades. However, this does not mean that we have completely understood the hydrodynamics in stirred tanks. Generally speaking, the fluid flow in a stirred tank is highly complicated. It is more complex when the free-surface deformation is taken into account. Up to now, most published studies on the hydrodynamics in stirred tanks have been carried out in baffled stirred tanks. In this case, the liquid surface at the top of the stirred tank was commonly assumed to be flat (see for example, Alcamo et al., 2005; Armenante et al., 1997; Dong et al., 1994; Shekhar and Jayanti, 2002).1–4 But in fact, the liquid surfaces in baffled stirred tanks are wavy. There is surface macro-swell, i.e., elevation/depression of the liquid surface with time (Jahoda et al., 2011).5 In some process industries, such as animal cell cultures, micromixing (Aloi and Cherry, 1996; Rousseaux et al., 2001; Assirelli et al., 2008)6–8, the use of unbaffled stirred tanks may be desirable. In such cases, surface vortices are formed and the liquid surface can no longer be treated as flat. Accordingly, the free surface deformation must be modelled to obtain a more accurate result. The volume of fluid (VOF) method has been widely used for the modeling of fluid flow in stirred tanks. The first application was performed by Serra et al. (2001).9 They simulated the flows with wavy free-surface in a fully baffled stirred tank. Haque et al. (2006 and 2011)10–11 for the first time used an Eulerian-Eulerian multiphase flow model coupled with VOF to determine the gas-liquid interface in unbaffled vessels. The predicted liquid surface profiles, the mean flow characteristics and the turbulent kinetic energy were generally well predicted. However, some differences between the numerical predictions and experimental results could still be observed. They attributed this to the weaknesses of the RANS turbulence models they had used. Glover and Fitzpatrick (2007)12 and Torré et al. (2007)13 used the same approach for the modelling of vortex in an unbaffled stirred tank. They also pointed out Free Surface Turbulent Flow in an Unbaffled Stirred Tank: Detached Eddy Simulation and VOF Study