A numerical investigation into the influence of unsteady wind on the performance and aerodynamics of a vertical axis wind turbine

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∞ = 7 m/s were conducted and results have shown a typical performance curve prediction for this particular VAWT scale. etailed 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 λ 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 λ that is lower than λ for peak performance coefficient causes the VAWT to run in the λ 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 λ 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.