Wind Energy

49
Modelling Wind Turbine Wakes with
a Porosity Concept
Sandrine Aubrun
49.1 Introduction
The wind quality of a site is neither controllable nor improvable by wind energy
specialists. The wind turbines arrangement in a wind farm is however
designed by them before the implantation of a wind farm, in order to minimize
turbine interactions. It is therefore of great interest to correctly assess these
interactions. Since the field measurements are rare and difficult to interpret,
wind turbine wakes and their interactions are usually treated with numerical
models. An alternative to this is the physical modelling in wind tunnels.
Indeed, its degree of modelling is lower than for numerical approaches.
Wind turbines are not plain obstacles and can be considered as porous
elements, extracting kinetic energy from the flow, distorting streamlines and
generating turbulence. The use of a porosity-drag approach, inspired from
concepts developed to model the urban of forest canopy flows (numerical [1]
and physical [2, 3]) may be well adapted to simulate the wind turbine wake.
The present project is to model in a wind tunnel at a geometric scale of
1:400 wind turbines with metallic mesh discs to replicate the actuator discs.
A parametric study of the flow field downstream of the wind turbine model
has been performed in a homogeneous and turbulent approaching flow upon
the disc size, the porosity level, the mesh size. Then the interaction between
several porous discs located in a modelled atmospheric boundary layer was
studied in order to model an offshore located wind farm.
49.2 Experimental Set-up
The wind turbines were modelled according to the actuator disc concept. The
actuator disc extracts kinetic energy, generating a spreading of the stream
tube, a velocity deficit downstream of the disc and an appearance of sheargenerated
turbulence. A porous disc produces exactly the same features [2].
Discs of 100 mm, 200 mm and 300 mm of diameter (D), made of metallic mesh,