Relation between the turbulent intensity on the crest of a ridge and wind direction relative to the ridge line - Part 1: Field experiments

Turbulence affects the efficiency of power generation and the life time of wind turbines. Many wind turbines have been constructed on the crest of ridges or hills. Therefore, predicting turbulence as well as average wind speed is one of the subjects for the wind power generation. On the crest of a 2-D ridge, the wind speed is larger than upstream at the same height above the ground as a result of the stream convergence occurring when wind blows perpendicular to the ridge line. And on the crest of a ridge, wind speed is larger when the prevailing wind blows perpendicular to the ridge line as compared to that blowing parallel to it. However, the relation between the turbulence intensity on the crest and the prevailing wind direction to the ridge line has not been studied. Therefore we studied this subject because it is important for siting wind energy conversion systems.

The average wind speed and turbulence data obtained from two field experiments were used for the analysis. One of the sites was at the eastern ridge of Mt. Hyonosen (794 m in altitude) and the other site was at the northern ridge of Mt. Taiko (600 m in altitude) in Japan. The measurements were performed for a year. The wind speed was measured continuously on a mast at 20 m and 30 m high above the ground.

Because the measurements were performed by using 3-cup anemometers, we studied whether these 3-cup anemometers could observe the turbulence generated by the topography and surface obstacles. The response of a 3-cup anemometer (NRG#40) to wind speed fluctuations was investigated by comparing it with sonic data in Himeji city.

The results are summarized as follows:

(1) Two field experiments

The directional profile of turbulent intensity shows that on the crest of the ridge, the turbulent intensity is larger in wind direction blowing parallel to the ridge line as compared to that blowing perpendicular to it. Comparing the turbulent intensity measured at the two heights, the turbulence intensity at the height of 20 m varies more with the angles relative to the ridge line as compare to that at a height of 30 m.

When wind speed U becomes larger than 6 m/s, the turbulent intensity in the horizontal direction became smaller and converges to a constant value, because the atmospheric stability is neutral under strong wind conditions.

(2) Response experiments of a 3-cup anemometer

For U > 5 m/s, the 3- cup anemometer (NRG#40) responds to wind speed fluctuations of frequencies lower than 0.3 Hz. The 3-cup anemometer observed more than 85 % of turbulence intensity in the horizontal direction at 53 m high above the ground for U>3 m/s. Therefore, the 3-cup anemometer is capable of measuring the most of turbulence energy caused by surface roughness and complex topography in windy conditions of U > 5 m/s at 20 m or 30 m high above the ground.

The apparent turbulent intensity measured by a 3-cup anemometer increased for low wind speed less than 3 m/s. This occurred because the 3-cup anemometer repeatedly stopped and rotated when the wind speed becomes less than 1m/s.