Environmental Engineering Reference
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t = 0.5/ n max
,t 0 £ 0.5/ n max
(8-25a)
t = 0.5/t 0
, 0.5/ n max < t 0 < 0.5/ n min
(8-25b)
t = 0.5/ n min
,t 0 ³ 0.5/ n min
(8-25c)
If it is desired to utilize a discrete gust approach for fatigue load calculations, it is
necessary to estimate the number of times that the turbine will be exposed to gusts
exceeding a given magnitude. A model for estimating the number of times per hour,
averaged over a year, that the gust speed exceeds a prescribed value is given in Frost et al.
[1978] and Frost and Turner [1982]. This estimate is based on the well-known “number
of crossings” model of Rice [1944, 1945] and the assumption of a Weibull distribution of
steady wind speeds during a year. Validation of this model with test data is limited.
A number of studies on gust statistics and other gust characteristics have been carried
out in an effort to shed more light on gust phenomena and to validate discrete gust models
References in addition to the sources mentioned earlier are Ramsdell [1978], Cliff and
Fichtl [1978], and Huang and Fichtl [1979]. The general approach used in these studies is
to spatially-average gusts by band-pass filtering of the data. Various high- and low-pass
filter combinations are reported throughout these works. Doran and Powell [1980] provide
some of the more complete statistics, including the following:
--
likelihood of occurrence of gusts of a given magnitude regardless of duration;
--
likelihood of occurrence of gusts of a given duration regardless of magnitude;
--
statistical relationships between gust magnitude and gust duration (Fig. 8-22).
Figure 8-22. Typical statistical relationships between gust magnitude and duration.
[Doran and Powell 1980]
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