Tower design and analysis - Wind Power Generation and Wind Turbine Design - page 537

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Time (s)

Figure 7: Displacement time-history at the top of the tower with blade interaction.

In the approach by Murtagh et al. [30], the coupled system equation of motion
is primarily cast in the frequency domain via Fourier transform. This allows the
coupling of the tower and the blades. The time domain along-wind response of the
coupled assembly is ultimately obtained by inverse Fourier transform. There are a
number of merits behind this type of approach. The technique is relatively simple,
especially compared with a more computationally expensive fi nite element formu-
lation. The approach may be used in a preliminary quantitative design, which may
subsequently be validated by a more rigorous analysis. The dynamic properties of
the coupled system are available using the dynamic properties of each of the two
sub-systems, which is an extension of the substructure synthesis approach.

3 Design of tower

A complete dynamic analysis of the tower taking into account the effect of the rota-
tion of the blades (rotors) and the nacelle mounted at the top is necessary for ensuring
the safety and operational serviceability. However, such a detailed dynamic analysis
may be time consuming and rigorous at a preliminary design stage when the initial
confi guration has to be chosen based on the design forces and displacements. Hence,
for an initial assessment it may be more attractive to use an approximate simplifi ed
approach while taking account of the stochasticity in the wind loading (and hence in
the response of the tower) and the rotor-tower interaction.
Gust response factor (GRF) approach is a simple technique used by struc-
tural engineers in the along-wind design of fl exible structures and incorporates
the stochastic and dynamic effects. This technique is now well developed due
to the contributions of Davenport [38] and Velozzi and Cohen [39]. GRF is the
ratio of the maximum or peak response quantity to the mean response quantity.