A prerequisite of the new method is a more thorough understanding of how the wind turbine modifies the flow as it approaches the rotor. To increase our understanding, this WP will use state-of-the-art computational methods to derive engineering models, in order to provide a model to quantify the blockage effect of the rotor on the inflow. Modelling will be made on several turbine rotor designs with different control algorithms, to establish a general and robust understanding of the induction factor at different distances from the rotor over the rotor disc area.
Task 1.1 Modelling the blocking effect of the turbine rotor in flat terrain
The wind approaching a turbine is slowed down by the pressure thrust of the blades and can no longer be considered as free, i.e unaltered by the turbine. The optimal distance from the rotor for the wind speed measurements will be determined: very close to the rotor the wind field is strongly affected by the blades, while further from the rotor the wind field is more uniform. The approaching wind will be analysed with computational fluid dynamics (CFD) solver Ellipsys3D in combination with the advanced aeroelastic simulation tool HAWC2. The impact of the turbine on the free wind will be calculated and compared with full scale measurements provided by the first measurement campaign (see WP3).
Task 1.2 Modelling the impact of the terrain on the wind field close to the rotor
The terrain effects will be calculated by the CFD solver EllipSys3D using a high resolution two-equation RANS model, that includes stability effects combined with the rotor blocking effect. Realistically varying wind fields are achievable when terrain and stability effects are accurately simulated. Having validated the model based on the experimental results from the second measurement campaign (see WP3), the steady wind approaching the rotor will be analysed with a series of CFD actuator disc simulations under different terrain orography and different steady wind shears, veers and atmospheric stability conditions.
Task 1.3 Method to retrieve the free wind speed from the near flow field
A simple model for the axial induction, which slows down the wind, will be developed to estimate the free wind speed based measurements close to the rotor. To derive the model, simplified inviscid potential flow will be investigated. The wind field directly in front of the rotor will be approximated by a potential field from a dipole source. The strength of the dipole can either be estimated based on the turbine thrust or from multiple wind field measurements at different rotor distances. Having determined the dipole strength, the rotor induced velocity field can be calculated using the Biot-Savart law and the free wind estimated.