Boundary-layer aerodynamics over long wind farms over hilly terrains

Abstract

A wind-tunnel campaign with a wind-farm model over two cosinusoidal hills has been performed to assess the changes in the flow above a wind-farm model in presence of topography. The experiments focused on the three-dimensional velocity field above the turbines to characterise the boundary layer evolution along the farm. The flow over the hills was characterised first without the turbines and compared with potential flow theory with good agreement between the two. The presence of the turbines imposed an upward displacement of the velocity field with similar speed-ups on the hill crests as observed in absence of turbines. The vertical velocity was also quite similar in the windward side of the hills and significantly damped in the leeward side, probably due to boundary-layer separation and shear sheltering operated by the turbine top tips. The presence of free-stream turbulence does not change this qualitative picture, although the turbulent activity within the internal boundary layer is slightly increased. The analysis of the streamwise velocity spectra indicated the emergence of the wake meandering as dominant dynamics and its modulation operated by the topography.