Wind tunnel simulation of the wind conditions inside bidimensional forest clear-cuts. Application to wind turbine siting

The flow field within bidimensional forest clear-cuts, oriented perpendicular to the wind direction, has been modelled in a wind tunnel, studying the effect of porosity and length of the leading forest and clear-cut. A simple foam model provides an effective simulation of the forest canopy flow, following similar turbulence characteristics found in other wind tunnel and field tests. Particle image velocimetry (PIV) measurements provide a clear picture of the displacement of the boundary layer above the porous canopy and the development of a cavity shear layer at the height of the forest. This shear layer originates high velocity gradients, turbulence intensities and wind gusts. The study of extreme winds is carried out using extreme value analysis on the PIV fields. The results are applied to study the technical feasibility of wind turbine siting in forests following the IEC61400-1 norm. With respect to the forest-free conditions, the turbulence intensity is increased by 4-9%, the wind energy is reduced by 5-20% and the extreme velocities are 15-30% higher. As a rule of thumb, the clearance between the rotor and the ground should be at least two times the forest height to avoid the cavity shear layer.