*3.2. Experimental Airfoil Polars*

In order to emphasize the impact of turbulence on the polar curves of the airfoil an additional experimental campaign was performed in the wind tunnel of the FLOW (Fluid Dynamic and Thermodynamics) research group at the Vrije Universiteit Brussel (Belgium). This boundary layer wind tunnel has a length of 11 m and a test section with 1.04 m of height and 2 m width. Its maximum wind speed is 20 m/s and its base turbulence level is 0.5%. As in the previous experiments, the turbulence was increased by using wooden grids and the wind speed was set in order to match the mean Reynolds numbers experienced by the VAWT blades over a rotation. In this case only two flow conditions were tested: smooth flow (*Iu* = 0.5%) and high turbulence (*Iu* = 9.5%).

The blade tested was a NACA0018 adapted to the virtual camber effect that the blade experiences while rotating in VAWTs of high *c*/*R* ratio (equal to 0.2 in the present case), as it is described in [36]. A 20 cm chord model was built by laser cutting and gluing together wooden aerofoil sections. The airfoil was mounted on an aerodynamic balance to record the forces acting on it, and endplates where added to avoid 3-D effects. The setup can be observed in Figure 5.

**Figure 5.** Airfoil model mounted inside the FLOW wind tunnel, with the turbulence grid installed in the background.
