Civil Engineering Reference
In-Depth Information
FIGURE 4-5
Old multiblade windmill and modern three-blade wind turbine. (Source: Vestas Wind Systems,
Denmark.)
hour (knots). In other countries, it is reported in kilometers per hour or some
times in meters per second.
The wind direction is measured with an instrument called the weather
vane. It has a broad, flat blade attached to a spoke pivoted at one end. The
wind impinging on the blade turns the spoke and lines up the blades in the
wind direction. The wind direction is indicated by an arrow fastened to the
spoke, or by an electric meter remotely controlled by the weather vane. The
wind direction is often indicated in terms of a 360 degrees circular scale. On
such scale, 0° indicates the north, 90° indicates the east, 180° indicates the
south, and 270° indicates the west directions.
An optical sensor developed at the Georgia Institute of Technology may
soon replace the conventional anemometer and improve the measurement
accuracy. The mechanical anemometer can register readings at a single loca-
tion where it is actually placed. A complex array of traditional anemometers
is needed to monitor the wind speed over a large area such as in a wind
farm. The new optical sensor is able to measure average crosswind speeds
and directions over a long distance and is more accurate than the mechanical
anemometer.
Figure 4-6
depicts the sensor's layout and the working princi-
ple. The sensor is mounted on a large telescope and a helium neon laser of
about 50 millimeters diameter. It projects a beam of light on to a target about
100 meters away. The target is made of the type of retroflective material used
on road signs. The sensor uses a laser beam degradation phenomenon known
as the residual turbulent scintillation effect. The telescope collects laser light
reflected from the target and sends it through a unique optical path in the
instrument.
