Validation of the Actuator Disk Model for wind turbine wakes calculations.

Numerical simulation of wind turbine wakes is a fundamental tool for the optimization of the exploitation of the wind renewable energy potential especially in off-shore conditions or in coastal areas. Usually analytical models are used for on-shore wind farm with success and poor calculation load; anyway for off-shore cases CFD (Computational Fluid Dynamics) models are recently discovered to be able to reproduce the wakes interactions in a more reliable way. In present work an actuator disc model was implemented in order to simulate the wakes for a small coastal wind farm. CFD numerical simulations were performed using WindSim 5.0 to reproduce the interaction between the main wind flow and the wakes induced by the actuator discs for the prevailing direction sectors. A particular attention was addressed to the boundary conditions: since generally the domain is rectangular the inlet and outlet conditions must be different for orthogonal and non-orthogonal cases. The CFD code PHOENICS works using an orthogonal structured cartesian grid so that only for orthogonal sectors the disc can be aligned face to the wind. This problem can be overcome rotating the model: for coastal areas where a small influence of the terrain is also to be considered such technique can give an improvement of the results. In the present work the numerical model was validated using real production data from a small wind farm in the western coastal region of Finland; the numerical wind speed profiles were verified using anemometer data from a mast placed near the turbines. Results demonstrate that, despite its simplicity, the actuator disc model can give very useful information when developing a wind farm in off-shore or coastal areas. This work was developed within a cooperation between the Department of Industrial Engineering of the University of Perugia and VTT, the technical research centre of Finland.