The aerodynamic characteristics of the NREL 5 MW rotor have been examined using a Reynolds-averaged Navier–Stokes method, OVERFLOW2. A comprehensive off-body grid independence study has been performed. A strong dependence on the size of the near-body wake grid has been found. Rapid diffusion of the wake appears to generate an overprediction of power and thrust. A large, continuous near-wake grid at a minimum of two rotor diameters downstream of the rotor appears to be necessary for accurate predictions of near-body forces. The NREL 5 MW rotor demonstrates significant inboard flow separation up to 30% of span. This separation appears to be highly three dimensional, with a significant amount of radial flow increasing the size of the separated region outboard. A simple, continuous full-chord fence was applied at the maximum chord location of the blade, within the region of separation. This non-optimized device reduced the boundary-layer cross-flow and resulting separation. The fence increased energy capture by nearly 1% at a wind speed of 8 m s−1 and slightly increased blade loading over the length of the span. Copyright © 2011 John Wiley & Sons, Ltd.