This thesis systematically evaluates wind farm power efficiency under cyclic yaw control (CYC) across various layouts and inflow conditions via extensive wind tunnel testing. The core mechanism governing the efficacy of CYC in wind farm power improvement is investigated. Inspired by the kinematic similarity between yawing turbines and floating offshore wind turbines (FOWTs) in motions, the work expands to a second part on the power performance and wake characteristics of a FOWT under prescribed platform motions.
The first part of the thesis on CYC includes four studies. The first study, on a single and three-turbine farm, found that CYC accelerates wake recovery, leaving more power available downstream and thus increasing wind farm power production. However, the power of the wind farm does not increase monotonically with the yaw frequency (or Strouhal number) or amplitude. Intriguingly, CYC can even reduce turbulence intensity in the core wake region compared to the baseline.
The second study broadened the scope to different farm layouts and inflow turbulence intensities, also evaluating power fluctuations (a key grid power quality metric). Results show the optimal Strouhal number depends strongly on farm length rather than inflow turbulence intensity and spanwise spacing. The percentage of power gain due to CYC decreases as inflow turbulence increases, highlighting its key role in wind farm control. Notably, within a certain Strouhal range, CYC simultaneously increases mean power production and reduces power fluctuations.
The third study provided the mechanistic insight: periodic wake meandering induced by CYC is the primary driver for power improvement. The periodic wake dynamics sustain throughout the farm at low Strouhal numbers but are suppressed at high frequencies. Inflow conditions, especially turbulence intensity and length scales, can significantly affect the evolution of these CYC-generated wake dynamics. CYC can also alter interactions between wind farm wakes and the atmospheric boundary layer flow.
The fourth study validated an analytical approach based on phase-averaged wake analysis to predict the wake of a wind turbine under CYC. The model accounts for the impacts of CYC on wake velocity and deflection. It is validated experimentally for moderate Strouhal numbers, providing a tool for dynamic wake control scenarios.
The second part expands the scope of this thesis to FOWTs. Three studies on prescribed pitch, surge, and roll motions (at moderate Strouhal numbers) reveal distinct impacts on different motions. Pitch motion influences power and enhances wake recovery, with its generated wake structures affecting downstream turbines. In contrast, surge motion has minimal effect on power or wake characteristics but primarily impacts turbine power fluctuations. Roll motion accelerates wake recovery, increases turbulence intensity, and enhances lateral meandering. Under strong dynamics, roll can also affect thrust and cause wake de
