Page 55 - Wind Energy Handbook

GUST WIND SPEEDS 29

with ç u as in Equation (2.35).
The standard also states that this may also be used with the von Karman model,
as an approximation to Equation (2.34). However, the standard does not specify the
coherence of the other two components to be used in conjunction with the Kaimal
model.
The three turbulence components are usually assumed to be independent of one
another. This is a reasonable assumption, although in practice Reynolds stresses
may result in a small correlation between the longitudinal and vertical components
near to the ground.
Clearly there are significant discrepancies between the various recommended
spectra and coherence functions. Also these wind models are applicable to flat sites,
and there is only limited understanding of the way in which turbulence character-
istics change over hills and in complex terrain. Given the important effect of
turbulence characteristics on wind turbine loading and performance, this is clearly
an area in which there is scope for further research.

2.7 Gust Wind Speeds

It is often useful to know the maximum gust speed which can be expected to occur
in any given time interval. This is usually represented by a gust factor G, which is
the ratio of the gust wind speed to the hourly mean wind speed. G is obviously a
function of the turbulence intensity, and it also clearly depends on the duration of
the gust – thus the gust factor for a 1 s gust will be larger than for a 3 s gust, since
every 3 s gust has within it a higher 1 s gust.
While it is possible to derive expressions for gust factors starting from the

1.7

1.6

1.5
20% turbulence
Gust factor, G 1.4 15% turbulence

1.3

10% turbulence
1.2

1.1

1
1 h 10 min 5 min 2 min 1 min 30 s 10 s 5 s 3 s 1 s
Gust duration
Figure 2.8 Gust Factors Calculated from Equation (2.40)