FloatLab: experimental testing of +20MW scaled floating wind turbines models

In this work, we present the design of a wind generator and of a wind turbine rotor to be used for lab-scale experimental testing of floating offshore wind turbines. The wind generator, measuring 5 m × 5 m, is composed of 1 m2 units that can be controlled independently, enabling the generation of complex flow features, such as unsteady wind, wind shear and different turbulent coherence patterns. We show a preliminary calibration of one unit where a near-linear relation between the fan RPM and m the resulting wind speed is found. Moreover, the addition of three coarse screens after the fan ensures a 2% turbulence intensity in the flow across the tested RPMs. The wind turbine rotor is a 1:70 Froude-scaled version of the IEA 22MW wind turbine, featuring a rotor diameter of 4 m. The Froude scaling ensures dynamic similarity in terms of gravity and inertial forces between the model and full-scale system, critical for the reproduction of the floater dynamcis. However, this breaks the Reynolds number similarity of the rotor, and of the associated aerodynamic forces. To address this, we present an innovative rotor aerodynamic design procedure aimed at reproducing two key dynamic parameters, i.e. the mean thrust and the rotor-induced aerodynamic damping, which are known to have the strongest influence on the dynamics of floating wind systems. The procedure is based on three key steps 1. selecting a low-Reynolds airfoil, in this case the SD7003 with 8.5% thickness 2. choosing a design angle of attack where the slope of the lift curve divided by the lift coefficient is close to the full scale value 3. optimizing the chord for maximum power 4. adjusting the blade pitch to match the mean thrust of the full-scale rotor across all wind speeds. The resulting rotor design achieves a close match of the distributed thrust on the blade. It also reproduces the damping characteristics of the full-scale rotor to within ±30% at below-rated conditions and down to ±10% for above-rated conditions.