Wind Energy

y/h
2.5
2
1.5
1
0.5
0
7 Models for Computer Simulation of Wind Flow 41
q2 /(2u 2
Ref)=0.05
−2 −1 0 1
x/h
2 3
Fig. 7.1. Turbulent kinetic energy over a forested hill. Wind tunnel data (dashed
line, solid line); Svensson [6] (dashed line), Liu [4] (solid line)
y/h
2
1.5
1
0.5
x/h=0.02
0
0 2
0.25 0.55
1.2
2.6 6.5 13.2
0 2
0 2
0 2
k (m/s) 2
0 2
0 2
0 2
21.8
0 2
Fig. 7.2. Turbulence kinetic energy along a forest clearing. Wind tunnel data (�);
Svensson [6] (dashed line), Liu [4] (solid line), v2-f model (dotted line)
layer type of flow above the trees. The results by the two models in [4, 6]
(7.2 and 7.3) are virtually identical, and downstream of the hill, 2–5 tree
heights above the top of the trees, the difference between wind tunnel data
and model results is too high. The experimental and computer result differ
by a factor of 10, suggesting hidden aspects in the experimental data that
are beyond model capabilities. For instance the presence of large unsteady
flow structures originating from the trees at hill top, which would have gone
unnoticed by a conventional averaging of the LDA measurements.
Flow over a modelled flat forest
The effect of the two alternative formulations as in [6] and [4], is most obvious
in the case of the wind flow along a clearing, Fig. 7.2. Model [6] yields turbulence
values which are too high everywhere, as a result of insufficient turbulence
dissipation along the forest upstream of the clearing. The two laboratory
models (sinusoidal hill and flat clearing) differ also in terms of the forest density,
which may evidence the differences between the model approaches.