Effects of freestream turbulence on the wake growth rate of a model wind turbine and a porous disc

Abstract

This study presents the results of an experimental investigation that focuses on quantifying the differences between the spreading rates of a model wind turbine wake and a porous disc wake at different freestream turbulence intensity levels. Two-dimensional two-component particle image velocimetry (2D2C PIV) measurements are performed within the wakes of a model wind turbine and a porous disc (up to 7D downstream) of the same diameter and a matching thrust coefficient. The wind turbine is operated at a Tip Speed Ratio (TSR) of 2 in order to have matching thrust coefficient conditions for a consistent wake comparison. The results show that the mean wake flow field (both near and far wake) is significantly different for the wind turbine compared to the porous disc even if they are operating at similar, high or low, freestream turbulence levels. The wake of the wind turbine recovers much faster than that of a porous disc with a matching thrust coefficient especially in the far wake region at both low and high freestream turbulence levels. On the other hand, the data shows that the far wake of the turbine operating at low freestream turbulence is very similar to that of the disc operating at high freestream turbulence. This suggests caution and stresses the importance in choosing the freestream turbulence intensity level when using porous discs to represent wind turbines in wind tunnel studies.