Wake interactions between floating offshore wind turbines: A wind tunnel study

Floating offshore wind turbines (FOWTs) experience 6-degree-of-freedom motion impacting their performance, fatigue loading, and wakes leading to complex interactions between FOWT arrays. For this study, we tested two downscaled models of DTU 10 MW wind turbines with a rotor diameter (D) of 40 cm installed in tandem at the UT Dallas Boundary Layer and Subsonic Wind Tunnel (BLAST) using multi-hole pressure probes, 6-DOF force sensors, and electric generators. The upstream FOWT is installed on a mechatronic emulator reproducing typical turbine motion during offshore operations, specifically imposing sway motion with different non-dimensional amplitude (A/D) from 0 to 0.16, and Strouhal number (St) from 0 to 0.35. FOWT operational tip-speed ratios (TSRs) were set to sub-optimal (4.8), optimal (5.2), and super-optimal (7.5) values, with a downstream turbine located at a distance of 5D or 10D operating at the optimal TSR. Results show that high St and A/D enhance wake recovery, particularly at higher TSRs, increasing power capture and thrust loading of the downstream turbine, causing lateral wake expansion and vertical shrinkage, thus resulting in an elliptical mean wake shape. Increased St and A/D also increase Reynolds stresses and turbulent kinetic energy in the wake.