Page 57 - Fluid Power Engineering

Properties of W ind 35

Roughness Roughness
Description Class Length, m Shear
Open sea 0 0.0001–0.003 0.08
Open terrain with a smooth 0.5 0.0024 0.11
surface, like concrete runway,
mowed grass
Open agricultural area without 1 0.03 0.15
fences and hedgerows and
very scattered buildings. Only
softly rounded hills
Agricultural land with some 1.5 0.055 0.17
houses and 8-m-tall sheltering
hedgerows with a distance of
approx. 1250 m
Agricultural land with some 2 0.1 0.19
houses and 8-m-tall sheltering
hedgerows with a distance of
approx. 500 m
Agricultural land with many 2.5 0.2 0.21
houses, shrubs and plants, or
8-m tall sheltering hedgerows
with a distance of approx.
250 m
Villages, small towns, 3 0.4 0.25
agricultural land with many or
tall sheltering hedgerows,
forests, and very rough and
uneven terrain
Larger cities with tall buildings 3.5 0.8 0.31
Very large cities with tall 4 1.6 0.39
buildings and skyscrapers

Source: Nielsen, Per. WindPRO 2.5 Users Guide. EMD International, Aalborg, Denmark,
2006.

TABLE 3-5 Description of Roughness Classes, Roughness Length, and Wind Shear

has a logarithmic variation. Table 3-5 describes classes of roughness,
roughness length, and shear.
Often, the following approximations are used for shear. However,
these approximations can lead to significant error in energy prediction
and, therefore, must be used with caution.
4
1. Approximation 1: Shear = 1/7 = 0.14. This is the most widely
used value when wind speed is available at single height. If
extrapolating 10-m data, this value may underestimate wind