Synchronised WindScanner Field Measurements of the Induction Zone Between Two Closely Spaced Wind Turbines

Abstract. Field measurements of the flow interaction between the near-wake of an upstream wind turbine and the induction zone of a downstream turbine are scarce. Measuring and characterising these flow features in wind farms for various operational states can be used to evaluate flow models and design control systems at the windfarm level. In this paper, we present induction zone measurements of a utility-scale 3.5 MW turbine with a rotor diameter of 126 m in a two-turbine wind farm operating under waked and un-waked conditions. The measurements were conducted with two synchronised continuous-wave WindScanner lidars that could resolve longitudinal and lateral velocities by dual-Doppler reconstruction. An error analysis was performed to quantify the uncertainty in measuring complex flow situations with two WindScanners by simulating the measurement setup, WindScanners sensing characteristics, and inflow conditions in a Large-Eddy Simulation. The flow evolution in the induction zone of the downstream turbine was characterised by performing horizontal planar dual-Doppler scans at the hub height for four different inflow cases, varying from undisturbed inflow to full and partial wake scenarios. The measurements revealed more evidence of horizontal asymmetry of the induction zone owing to vertical wind shear under undisturbed inflow conditions. Evaluation of the engineering models of the undisturbed induction zone showed good agreement along the rotor axis. In the full wake case, the measurements indicated a deceleration of the upstream turbine wake due to the downstream turbine induction zone as a result of the very short turbine spacing. We observed that the downstream turbine induction zone during wake steering depended on the direction of the wake steering while the lateral movement of the deflected wake could be measured.