Analysis of array spacing on tidal stream turbine farm performance using Large-Eddy Simulation

Design of efficient tidal arrays relies on the adopted spacing between turbines and their mutual interplay. Turbines affected by wake shadowing operate in harsher flow conditions, such as higher turbulence levels or lower incident velocity, which leads to reduced performance and larger extreme and fatigue loading. To extend the knowledge about turbine-to-turbine interplay in tidal arrays, high-fidelity numerical simulations using a Large-Eddy Simulation-Actuator Line Method (LES-ALM) are carried out to quantify the impact of row spacing. The developed Digital Offshore FArm Simulator (DOFAS) validates well with experimental data in terms of flow statistics and hydrodynamic coefficients, which demonstrate its adequacy to resolve the complex fluid-turbines interaction. In the cases with spacing of four and eight diameters between the rows, the lack of wake recovery has a detrimental effect on back-row turbines whose efficiency dramatically drops compared to those in the front-row. The LES-ALM captured the low-frequency wake meandering phenomenon responsible for uneven periodic loading on back-row turbines. The devices placed in the front-row suffer the largest thrust loads, blade-root bending moments and support structure moments, whilst the outermost back-row turbines experience the largest tower yaw moments due to their simultaneous exposure to low-momentum turbulent wakes and high-velocity free-stream flow. Finally, damage equivalent loads estimated by the LES-ALM are maximum for the front-row turbines except the tower yaw moment which is maximum on the outermost back-row turbines.