Wind Energy

266 S. Aubrun
Table 49.1. Metallic meshes properties
Mesh number Mesh size Wire diameter Porosity
(mm) (mm) level (%)
1 5 1.3 60
2 3.75 0.9 65
3 5.6 1.4 65
4 5.6 1.12 70
5 7.1 1.8 65
6 5 1.8 55
7 7.1 + 3.75 1.8 + 0.9 ≤55
were used. Seven different meshes were tested (see Table 49.1). The discs were
fixed to the floor with a vertical cylinder of diameter 0.05D, which is in agreement
with standard wind tubine mast diameters.
Measurements were performed in the “Malavard” wind tunnel of the Laboratoire
de Mécanique et d’Energétique. It is a close-circuit type with a test
section of 2 m wide, 2 m high and 5 m long. A turbulence grid made of square
tubes is located at the entrance of the test section. Its grid cell size is 100 mm
and the tube cross-section is 25 mm. It generates a homogeneous and isotropic
turbulence with a turbulence intensity of 3.5–4.5% in the used part of the test
section. The first set of measurements is focused on homogeneous freestream
conditions. In that case the porous disc is located in the middle of the test
section. For the second set of experiments, the freestream conditions were representative
of the offshore atmospheric boundary layer. The boundary layer
which was developing on the wind tunnel floor (made of fairly rough wooden
plates) in presence of the turbulence grid had the characteristics of the neutral
offshore atmospheric boundary layer at a geometric scale of 1:400. The roughness
length is z0 = 3 mm in full scale, the power law exponent is α =0.11.
The streamwise turbulence intensity at z = 20 m in full scale is Iu =7.3%. In
that case, only the 100 mm-diameter discs made of mesh 6, combined with a
1D high mast, were used. At the chosen geometric scale, these discs replicate
40 m-diameter wind turbines.
The three components of velocity were measured with a Dantec triple hotfiber
probe connected to the Dantec StreamLine system. The probe was fixed
on a 3D automated traverse.
49.3 Results for Homogeneous Freestream Conditions
A parametrical study was performed in a homogeneous freestream flow in order
to define the influence of the disc size, the porosity level, the mesh cell
size and the wire size on the flow. Mean velocity and turbulence intensity
were measured for each case along the horizontal cross-section of the disc.