This article presents a numerical method for the simulation of turbulent flow around the nacelle of a horizontal axis wind turbine. The flow field around the turbine and nacelle is described by the Reynolds-averaged Navier–Stokes equations. The k–ϵ model has been chosen for closure of the time-averaged turbulent flow equations. The rotor disc is modelled using the actuator disc concept. The main purpose of this article is to assess the impacts of the variation of some operational parameters (e.g. blade pitch angle changes) and atmospheric turbulence upon the relationship between wind speed measured near the nacelle and freestream wind speed established for an isolated turbine. Simulation results were compared with experimental data (from a typical stall-controlled, commercially available wind turbine rated higher than 600 kW). In general, good qualitative agreements have been found that validate the proposed method. It has been shown that a level of accuracy sufficient for use in power performance testing can be obtained only when a proper aerodynamic analysis of the inboard non-lifting cylindrical sections of the blade is included. Furthermore, the numerical method has proven to be a useful tool for locating nacelle anemometers. Copyright © 2005 John Wiley & Sons, Ltd.