Abstract
Increasing use of wind energy over the years results in more and larger clustered wind farms. It is therefore fundamental to have an in-depth knowledge of wind-turbine wakes, and especially a better understanding of the well-known but less understood wake-meandering phenomenon which causes the wake to move as a whole in both horizontal and vertical directions as it is convected downstream. This oscillatory motion of the wake is crucial for loading on downstream turbines because it increases fatigue loads and in particular yaw loads. In order to address this phenomenon, experimental investigations were carried out in an atmospheric-boundary-layer wind tunnel using a 3 × 3 scaled wind farm composed of three-bladed rotating wind-turbine models subject to a neutral atmospheric boundary layer (ABL) corresponding to a slightly rough terrain, i.e. to offshore conditions. Particle Image Velocimetry (PIV) measurements were performed in a horizontal plane, at hub height, in the wake of the three wind turbines in the wind-farm centreline. From the PIV velocity fields obtained, the wake-centrelines were determined and a spectral analysis was performed to obtain the characteristics of the wake-meandering phenomenon. In addition, Hot-Wire Anemometry (HWA) measurements were performed in the wakes of the same wind turbines to validate the PIV results. The spectral analysis performed with the spatial and temporal signals obtained from PIV and HWA measurements respectively, led to Strouhal numbers St = fD/Uhub ≃ 0.20 − 0.22.
Highlights
Between the years 2000 and 2015, the cumulative wind-power installations in the European Union increased from 12.9 GW to 141.6 GW [1]
The normalised instantaneous horizontal wind speed fields obtained from Particle Image Velocimetry (PIV) measurements at hub height (Fig. 7) reveal the presence of wake meandering for the three wind turbines constituting the mid row of the wind farm
PIV measurements were performed in a horizontal plane, at hub height, in the wake of the three turbines constituting the mid row of the scaled wind farm
Summary
Between the years 2000 and 2015, the cumulative wind-power installations in the European Union increased from 12.9 GW to 141.6 GW [1]. It is fundamental to have an in-depth knowledge of windturbine wakes, and especially a better understanding of the well-known but less understood wake-meandering phenomenon Because this phenomenon causes the wake to be swept in and out of the rotor disk of downstream turbines, it is crucial for loading on these turbines since it might considerably increase fatigue loads and in particular yaw loads. In their study performed with a two-bladed rotating wind-turbine scale model submitted to a uniform free-stream, Medici and Alfredsson [3] argued that the low-frequency vortex shedding might be responsible for the wake-meandering motion.
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