Page 105 - Fluid Power Engineering

W i nd Measurement 83

Redundant anemometers are used at each height for two reasons:
1. To remove the shadow effect of the tower. If one of the
anemometers is in the shadow of the tower, then the other
anemometer can measure the correct wind speed.
2. If one anemometer fails, it does not have to be replaced im-
mediately. Lowering the tower and raising it back up is an
expensive operation in remote locations, therefore, in order
to minimize this expense, redundant sensors are used.

Wind vanes are usually positioned about 2 to 4 m below the anemome-
ters. For instance, the top wind vane may be at 58 or 56 m, and
the second wind vane may be at 28 or 26 m. This is done to mini-
mize the wake effect on the anemometer because of the wind vane.
The orientation of the wind vane should be same as the first anemome-
ter. Instruments at all other levels should have the same orientation
as the top-level instruments.
In this plot, airflow is along the x direction. From the center of
the monopole, notice the deficit in wind speed along the negative x
direction, the increase in wind speed along the negative and positive y

y

x

FIGURE 6-6 Isospeed lines around a monopole. Top view of the monopole
with flow in the horizontal plane from left to right. The values are normalized
to the upstream free flow wind speed. (From International Electrotechnical
Commission (IEC). Wind Turbines – Part 12-1: Power Performance
Measurements of Electricity Producing Wind Turbines, 2005. IEC 61400-12-1,
with permission.)