Wake to wake interaction of floating wind turbine models in free pitch motion: An eddy viscosity and mixing length approach

Experiments were performed using two model wind turbines operated in tandem with a bottom-fixed configuration and a floating configuration with both turbines allowed to freely oscillate in the streamwise direction. Wakes of both turbines were measured using stereoscopic Particle Image Velocimetry. Turbulent characteristics of the far wake of the first turbine acting as the inflow for the downwind turbine were characterized calculating the eddy viscosity and mixing length profiles from the obtained data. The influence of the far wake on the statistical properties of the near wake of the second turbine are compared between the fixed and oscillating configurations. The incoming mixing length clearly influences the Reynolds stresses and turbulence production of the near wake in the shear layer. Below, the connection between incoming mixing length and the near wake is less evident, due to the impact of the nacelle and rotation of the rotor. For the oscillating turbine, the Reynolds stresses and turbulence production in the near wake of the downwind turbine are damped. Vertical fluctuations were found to decrease though an increase in the mean vertical component. New challenges arise in the design of a floating offshore wind farms, in terms of farm layout and load estimations.