**1. Introduction**

Limitations in the current blade technology constitute a technological barrier that needs to be broken in order to continue the improvement in wind-energy cost. Blade manufacturing is mostly based on composite laminates, which is labor-intensive and requires highly-quali‐ fied manpower. It constitutes a bottleneck to turbine upscaling that reflects into the increas‐ ing share of the cost of the rotor, within the total cost of the turbine, as turbine size increases. Figure 1 shows a compilation of data by NREL-DOE [26] on the proportional cost of each subsystem for different sizes of wind-turbines, where the systematic increase of the rotor cost share is clearly reflected.

**Figure 1.** Evolution of the proportional cost for the different wind-turbine subsystems, as size increases (data com‐ pilation from [26]).

© 2012 Otero et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Otero et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Moreover, while the rest of the wind turbine subsystems are highly developed technological products, the blades are unique. There is no other technological application that uses such a device, so practical experience in blade manufacturing is relatively new. Blades also operate under a complex combination of fluctuating loads, and huge size differences complicate ex‐ trapolation of experimental data from the wind-tunnel to the prototype scale. Hence, com‐ puter models of fluid-structure interaction phenomena are particularly relevant to the design and optimization of wind-turbines. The wind-turbine industry is increasingly using computer models for blade structural design and for the optimization of its aerodynamics. Nevertheless, several features of the complex interaction of physical processes that charac‐ terize the coupled aeroelastic problem still exceed the capacities of existing commercial sim‐ ulation codes. Changes in structural response due to the development of new techniques in blade construction and/or the use of new materials would also represent a major factor to take into account if the development of a new prototype blade is considered.

Hence, a key factor for a breakthrough in wind-turbine technology is to reduce the uncer‐ tainties related to blade dynamics, by the improvement of the quality of numerical simula‐ tions of the fluid-structure interaction process, and by a better understanding of the underlying physics. The current state-of-the-art is to solve the aeroelastic equations in a fully non-linear coupled mode using Bernoulli or Timoshenko beam models (see [11], where a thorough coverage of the topic is presented). The goal is to provide the industry with a tool that helps them to introduce new technological solutions to improve the economics of blade design, manufacturing and transport logistics, without compromising reliability. A funda‐ mental step in that direction is the implementation of structural models capable of capturing the complex features of innovative prototype blades, so they can be tested at realistic fullscale conditions with a reasonable computational cost. To this end, we developed a *general‐ ized Timoshenko* code [27] based on a modified implementation the Variational-Asymptotic Beam Sectional technique (VABS) proposed by Hodges et al. (see [13] and references there‐ in). The ultimate goal is to combine this code with an advanced non-linear adaptive model of the unsteady flow, based on the vorticity-velocity formulation of the Navier-Stokes equa‐ tions, called the KLE model [32,33], which would offer performance advantages over the present fluid-structure solvers.

In this chapter we present a set of tools for the design and full-scale analysis of the dynam‐ ics of composite laminate wind-turbine blades. The geometric design is carried on by means of a novel interpolation technique and the behavior of the blades is then simulated under normal operational conditions. We obtained results for the displacements and rotations of the blade sections along the span, section stresses, and fundamental vibrational modes of the blades.
