A scaling methodology for the Hybrid-Lambda Rotor – Characterization and validation in wind tunnel experiments

Abstract. The Hybrid-Lambda Rotor is an aerodynamic rotor concept that enables very low-specific-rating offshore wind turbines in order to increase the power output in light winds and to limit the loads on the very long and slender rotor blades in strong winds. In this paper, the rotor concept is scaled to wind tunnel size and validated under reproducible inflow conditions. The objectives are to derive a scaling methodology, to investigate the influence of the steep gradients of axial induction along the blade span and to characterize the wake of the Hybrid-Lambda Rotor in wind tunnel experiments. The scaling objectives are to match the axial induction distribution and to incorporate the change in the angle of attack distribution when switching between the light-wind and strong-wind operating mode. The derived model rotor with a diameter of 1.8 m is experimentally investigated and compared to a conventional model wind turbine in the large turbulent wind tunnel in Oldenburg under tailored inflow conditions produced with an active grid. A two-dimensional Laser-Doppler-Anemometer is used to measure the axial induction in the rotor plane and the wake is characterised by means of a hot-wire rig. The measurement data is supplemented with free-vortex-wake simulations on both rotors. The results demonstrate that switching the operating modes with the characteristic change in the angle of attack distribution, works similar for the model and the full-scale turbine. The strong gradients of axial induction along the blade span lead to complex three-dimensional flow structures such as an increased radial flow component in the rotor plane. The low-induction design of the outer part of the rotor reduces the load overshoots in gust events compared to the conventional model turbine. The wake characterization reveals an outer annulus with reduced wake deficits, an additional shear layer and vortex system and overall reduced wake deficits over a wide range of wind speeds below rated power. The derived results help to understand the unique flow patterns that are introduced by the Hybrid-Lambda Rotor and provide a valuable complementary data set to the simulations on the full-scale rotor.