An oscillating freestream over a stationary S809 airfoil is simulated numerically using ANSYS Fluent 12.1. For comparison the laminar-turbulent transition is simulated with the realizable k-epsilon and SST k-omega models, and finally the SST k-omega model is chosen. Where possible, the resultant aerodynamic coefficients presenting dynamic stall phenomena are compared with aerodynamic coefficients from existing experimental and semi-empirical data for a pitch oscillating S809 airfoil. The simulation was extended for several Reynolds numbers, 10(4), 10(5) and 10(6), for a large range of the reduced frequency, from 0.026 to 18. The level of the dependency of the results on the reduced frequency is examined based on wake velocity profiles, vorticity contours and aerodynamic coefficients. Leading edge erosion, which is a common problem for in-service wind turbine blades, is simulated based on the range of erosion found in turbine operating conditions. For the eroded airfoil, the lift decrease, causing a severe impact on wind turbine performance, was investigated in detail. (C) 2011 Elsevier Ltd. All rights reserved.