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2.6.3 Turbulence intensity

The turbulence intensity in the neutral atmosphere clearly depends on the surface roughness. For the longitudinal component, the standard deviation au is approximately constant with height, so the turbulence intensity decreases with height. More precisely, the relationship au « 2.5u* may be used to calculate the standard deviation, with the friction velocity u* calculated as in the previous section. More recent work (ESDU, 1985) suggests a variation given by:

where

This approximates to au = 2.5u* close to the ground, but gives larger values at greater heights. The longitudinal turbulence intensity is then

The lateral (v) and vertical (w) turbulence intensities are given (ESDU, 1985) by a v T A ^ „„ if xz\

Note that specific values of turbulence intensity for use in design calculations are prescribed in some of the standards used for wind turbine design calculations, and these may not always correspond with the above expressions. For example, the Danish standard (DS472,1992) specifies

with Iv = 0.8Iu and Iw = 0.5Iu. The IEC standard (IEC, 1999) gives

where I15 = 0.18 for 'higher turbulence sites' and 0.16 for 'lower turbulence sites', with corresponding values of a of 2 and 3 respectively. For the lateral and vertical components, a choice is allowed: either Iv = 0.8Iu and Iw = 0.5Iu, or an isotropic model with Iu = Iv = Iw. The Germanischer Lloyd rules (GL, 1993) simply specify 20 percent turbulence intensity. Figure 2.4 shows example longitudinal turbulence intensities for the GL, IEC and Danish standards. The values for the Danish standard are given for 50 m height with roughness lengths of 0.3 and 0.03 m respectively.

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