Boundary-layer evolution over wind farms

The recent growth of wind energy has boosted the development of many large wind farms where the clustering of the turbines helps reducing installation costs. However, the ambient turbulence is expected to grow substantially due to the turbines presence, and the turbines cannot be considered as individuals but have a global effect as a whole. Kinetic energy transfer from the region above the turbines is expected indeed to play a key role to replenish the energy content of the flow passing the turbines rotor.

In the present work, the flow over a large wind farm (composed by several freely-rotating wind-turbine models) is studied through a wind-tunnel campaign with hot-wire anemometry. 152 turbines were mounted with 19 subsequent rows, allowing to go beyond the adjustment region of the internal boundary layer located in the first rows. All the three velocity components were measured, providing a good characterisation of the Reynolds-stress tensor. The balance of advection, turbine thrust force and turbulence rules such a growth according to an integrated form of the Navier-Stokes equations. The possibility of a self-similar evolution of the velocity profile and of the Reynolds stresses will be further investigated.