Aerodynamics of Wind Turbines
Adding Wind Speeds and Directions (Wind Velocities)
Bicycle side cartoon The wind which hits the rotor blades of a wind turbine will not come from the direction in which the wind is blowing in the landscape, i.e. from the front of the turbine. This is because the rotor blades themselves are moving.
To understand this, consider the picture of a bicycle which is equipped with a blue banner (or a wind vane) to indicate the direction of the wind: If we have completely calm weather, and the bicycles moves forwards, with, say, 7 metres per second (14 knots), the bicycle will be moving through the air at 7 metres per second. On the bicycle we can measure a wind speed of 7 metres per second relative to the bicycle. The banner will point straight backwards, because the wind will come directly from the front of the bicycle.
Bicycle top cartoon
Now, let us look at the bicycle again directly from above, and let us assume that the bicycle moves forward at a constant speed of, once again, 7 metres per second. If the wind is blowing directly from the right, also at 7 metres per second, the banner will clearly be blown partly to the left, at a 45 degree angle relative to the bicycle. With a bit less wind, e.g. 5 metres per second, the banner will be blown less to the left, and the angle will be some 35 degrees. As you can see from the picture, the direction of the wind, the resulting wind as measured from the bicycle, will change whenever the speed of the wind changes.
What about the wind speed measured from the bicycle?
The wind is, so to speak, blowing at a rate of 7 metres per second from the front and 5 to 7 metres per second from the right. If you know a bit of geometry or trigonometry you can work out that the wind speed measured on the bicycle will be between 8.6 and 9.9 metres per second.
Enough about changing wind directions, now what about the wind turbine rotor?
© Copyright 1997-2003 Danish Wind Industry Association
Updated 1 June 2003
http://www.windpower.org/en/tour/wtrb/aerodyn2.htm
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