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Roughness and Wind Shear
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High above ground level, at a height of about 1 kilometre, the wind is hardly influenced by the surface of the earth at all. In the lower layers of the atmosphere, however, wind speeds are affected by the friction against the surface of the earth. In the wind industry one distinguishes between the roughness of the terrain, the influence from obstacles, and the influence from the terrain contours, which is also called the orography of the area. We shall be dealing with orography, when we investigate so called speed up effects, i.e. tunnel effects and hill effects, later. Roughness Roughness Classes and Roughness Lengths |
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Sheep are a wind turbine's best friend. In this picture from Akaroa Spit, New Zealand, the sheep keep the roughness of the landscape down through their grazing. Photograph © 1998 Soren Krohn |
In the wind industry,
people usually refer to roughness classes or roughness
lengths, when they evaluate wind conditions in a landscape.
A high roughness class of 3 to 4 refers to landscapes with many
trees and buildings, while a sea surface is in roughness class
0. Concrete runways in airports are in roughness class 0.5. The same applies to the flat, open landscape to the left which has been grazed by sheep. The proper definition of roughness classes and roughness lengths may be found in the Reference Manual. The term roughness length is really the distance above ground level where the wind speed theoretically should be zero. |
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This graph was plotted with the wind
speed calculator on the next page. It shows you how wind
speeds vary in roughness class 2 (agricultural land with some
houses and sheltering hedgerows with some 500 m intervals), if
we assume that the wind is blowing at 10 m/s at a height of 100
metres. |
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© Copyright 1999 Soren Krohn. All rights reserved. Updated 6 August 2000 http://www.windpower.org/tour/wres/shear.htm |