Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 37

Other spectra may also be used, but to comply with the IEC standard the high frequency asymptotic behaviour must tend to the following relationship:

(2.35)

with Λ₁ defined as above: it is a function only of height above ground but differs between edition 2 and edition 3 for heights above 30 m. Expressing this as

(2.36)

the asymptotic parameter A can then be compared for different spectra, as in Figure 2.7. This shows how the DS 472 asymptote is similar to IEC edition 2 (both Kaimal and von Karman spectra in that standard have the same asymptote), but the asymptote becomes much lower for IEC editions 3 and 4 above 30 m height and is now more comparable with Eurocode. The ESDU modified von Karman spectrum is more difficult to characterise because the asymptote now varies also with wind speed, surface roughness, and geographical latitude. Figure 2.7 shows the results for 20 m/s wind speed and 50° latitude for two different roughness lengths. The asymptote can be made to match the IEC editions 3 and 4 specification, but only by choosing a very small roughness length – even smaller to match edition 2. However, if such a low roughness length is selected, the turbulence intensity implied by the ESDU model will be much smaller than that required by the standards. It is common practice when using the ESDU model to adjust the surface roughness at each wind speed until the turbulence intensity matches the standard, although this clearly makes little physical sense. Obviously it is not possible to adjust the roughness to match both the required asymptote and the required turbulence intensity at the same time. Compared to the physical model, therefore, the standards are probably conservative, which is surely to be expected. It can also be argued that the physical model is valid for flat terrain, while many wind farms are built in complex terrain where the turbulence intensities will indeed be higher, and the length scales shorter.

Figure 2.7 Some asymptotic limits

Note also that the IEC edition 3 and 4 standards further specify that in the high frequency limit.