Numerical simulations using RANS-based CFD have been utilised to carry out investigations on the effects of steady and unsteady wind on the performance of a wind tunnel scale VAWT. Using a validated CFD model, steady wind simulations at U-infinity = 7 m/s were conducted and results have shown a typical performance curve prediction for this particular VAWT scale. Very detailed understandings of the flow physics are discussed showing the importance of stall and flow re-attachment on the performance of the turbine with unsteady winds. The three blades of the VAWT experience very different flow regimes as they rotate during a single periodic oscillation of the wind speed. When the VAWT operates in periodically fluctuating wind conditions, overall performance slightly improves if the following are satisfied: the mean tip speed ratio is just above the lambda of the steady performance maximum, the amplitude of fluctuation is small (<10%), and the frequency of fluctuation is high (>1 Hz). Operation at a mean lambda that is lower than lambda for peak performance coefficient causes the VAWT to run in the lambda band with deep stall and vortex shedding, to the detriment of the VAWT performance coefficient. Large fluctuations in wind speed causes the VAWT to run in lambda conditions that are drag dominated, thus reducing the performance of the wind turbine. Within realistic conditions, higher frequencies of fluctuation marginally improve the performance of the VAWT. (C) 2013 Elsevier Ltd. All rights reserved.