Interaction of Freestream Turbulence with Vortex Structures in Vertical Axis Turbines

We investigate the performance of an H-type vertical axis turbine in a turbulent freestream, focusing on the interaction of the turbulence with the vortical structures created by the turbine. Emphasis is placed on understanding the combined effects of turbulence intensity in the freestream and the turbulence integral scale relative to the generated vortices. A series of regular grids, creating turbulence intensity levels ranging from 3 % to 12 %, and turbulent integral scales that are comparable in size to the leading edge vortex, are placed upstream of the turbine. Turbulence is found to generally increase the performance of the turbine by roughly 30% for tip-speed ratios below the optimal case in a laminar flow. Freestream turbulence was found to generally weaken the leading edge vortex, but also increased the standard deviation of the total forces on the blade.