Wake control through tip-speed ratio adaptation

Abstract. Offshore wind farms can generate wake losses between 10 and 20 % even after layout optimization. By altering the turbine operational parameters, it is possible to reduce internal wake effects. A wake-control approach by modifying the operational tip-speed ratio is presented here using LES high-fidelity modelling with DTU 10 MW reference wind turbines. Single- and two-turbine simulations for tip-speed ratios ranging from 5 to 12 are assessed for wake losses and improved power production. Simulations were conducted with non-turbulent inflow, as subtle rotor- and wake aerodynamic effects are difficult to identify in turbulent flow.

Single-turbine simulation results show that the wake development is strongly influenced by the operational tip-speed ratio. At a tip-speed ratio of 8, a stronger wake with increased turbulence and a relatively short recovery distance is observed. Tip-speed ratios greater than 8 create a more turbulent near wake, increased mixing, and the shortest recovery distance.

The tip-speed ratio influences not only the magnitude of turbulence in the wake, but also the axial position where the wake becomes fully turbulent. With increasing tip-speed ratio, the point of a fully turbulent wake state moves upwind towards the rotor, enhancing turbulent mixing and reducing wake recovery distance. At high tip-speed ratios wake turbulence dissipates faster, and downwind turbines are not exposed to increased turbulence loads. At a tip-speed ratio of 10, the minimum wind speed at 6 rotor diameters downwind is enhanced by 50 % compared to the optimal operational tip-speed ratio of 8.

An increase in net power production is observed by operating the upstream turbine at a higher tip-speed ratios compared to the downwind turbine operating at the tip-speed ratio of 8. The net power production increases up to 10 %.

These results demonstrate the potential of varying tip-speed ratio to control wake development to maximize net power production of turbine arrays. Furthermore, turbulence-induced loads can be modified with this control strategy. These proof-of-concept simulations show the interesting potential of tuning the operational tip-speed ratio for wake control.