Effect of turbulence on the performance of VAWTs: An experimental study in two different wind tunnels

Turbulence is one of the main characteristics of urban flows, but its effect on the performance of small VAWTs(often used in urban wind installations) has not been thoroughly researched. This experimental study focuses in testing a H-Darrieus VAWT prototype in different turbulent conditions inside the wind tunnel, in order to study the influence of turbulence intensity, integral length scales and Reynolds number. Passive grids are used to increase the wind tunnel free stream turbulence up toIu¼15%, with integral length scales of Lux=0.18 m and Reynolds numbers of ReD=300,000. The measurements are repeated in two wind tunnels of different size, which strengthens the results and helps quantify the effect of blockage on the turbine ratings. The results show power coefficient increases up to 20% from smooth (Iu¼0.5%) to turbulent (Iu=15%) flows, an effect that is enhanced at lowλbut that fades as ReD>400,000. The study of the crossed influence of ReD,Iu and Lux offers valuable data in the process of optimizing the operation of small VAWTs inside urban environments.