Environmental Engineering Reference
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Figure 5-33. Development of a VAWT streamtube model. (a) Plan view of an idealized
blade path (b) Resulting circulation in the wake (c) VAWT streamtube
These observations lead to the following equations for the induced velocities:
a = a r - a f
(5-48a)
a r
a f
= 2 - G r
G f
(5-48b)
where
a f , a r
= front and rear axial induction factors, respectively
In order to determine the three local induced velocities ( a , a f and a r ), a third equation is re-
quired. Let us use the momentum theorem and equate the windwise momentum change to
the mean downwind blade force, which results in
B W
4 p U 2
(5-48c)
a (1 - a ) =
G f - G r
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