2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 317 -
velocity at the reference height with respect to the
ground. For the two slopes studied, two extrema of axial
velocity were reported: a minimum, corresponding to
deceleration at the start of the escarpment, and a
maximum, corresponding to acceleration at the crest. At
the midpoint of the slope, axial velocity was essentially
independent of the slopes, corresponding to an increase in
axial velocity of 9%. Since this position corresponds to a
speed-up that was independent of terrain slope, the wind
turbine was therefore positioned at this location in order
to highlight the effect of a vertical flow angle on nacelle
anemometry.
Fig. 4. Escarpment with no turbine: axial velocity for hub height
position for two escarpments.
Turbulence intensity for the various escarpments is
presented in Fig. 5 for a number of axial positions along
the domain. At the hub height and the middle of the
escarpment, where the wind turbine would be located,
turbulence intensity was approximately 11% and almost
independent of the escarpment slope: between the 11%
and 20% slopes, turbulence intensity increased by a mere
1.2%. Overall, turbulence intensity tended to increase
along with slope. The greatest variation was found close
to the ground, particularly around the beginning of the
escarpment. Turbulence was dissipated along the flow
downstream. This tendency towards computed turbulence
along escarpments was confirmed in an experimental
study conducted by Bowen [11].
Fig. 5. Escarpment with no turbine: turbulence intensity profiles for
various axial positions.
The axial velocity at the location of the nacelle
anemometer for the escarpment with no turbine is shown
in Fig. 6a. As discussed above, speed-up was the same
for the two escarpments studied. The inclination of flow
for the domain with no turbine at the nacelle
anemometer's position, as shown in Fig. 6b, varied
greatly, clearly depending upon ground topography.
Between the flat terrain, where flow was horizontal, and
the 20% escarpment, the vertical flow angle increased
from 0 to 6 degrees.
Fig. 6. Escarpment with no turbine: (a) axial velocity and (b) vertical
flow angle at the nacelle anemometer position.
Terrain with wind turbine
The introduction of a non-operating turbine into the
domain had the effect of accelerating the flow at the
anemometer position (Fig. 7a), mainly due to the shape
and curvature of the upper wall of the nacelle. It should
also be noted that in the presence of the nacelle, the axial
velocity at the anemometer position depended on the
ground's topography and tended to decrease as the ground
slope increased: since the anemometer was positioned
close to the rear of the nacelle, its wake had a significant
impact on this region.
Introducing the operating turbine tended to slow the
flow (Fig. 7b), and the impact of the escarpment slope on
the axial velocity at the anemometer position was reduced
with respect to the non-operating case. The trend
remained the same, however, with a decrease in speed as
terrain slope increased. A practical result for this turbine
can be derived from Fig.7. For the operating turbine,
there was one anemometer height position (1.15
H
anemo
)
where axial velocity was almost independent of the
terrain slope. For the non-operating turbine, this height
was slightly greater around 1.3
H
anemo
.
The vertical flow angle, as shown in Fig.8a, increased
significantly with the introduction of the nacelle and as
the slope of the escarpment increased. Introducing the
nacelle into the case of the flat terrain caused the vertical
flow angle to vary from zero to 8 degrees at the
anemometer position. For the 20% escarpment, the
increase was approximately 12 degrees. On the other
hand, with the presence of the nacelle and proceeding
from flat terrain to an escarpment of 20% increased the
vertical flow angle by about 4 degrees. The introduction
of the rotor also contributed to a further increase in flow
angle (Fig. 8b).
