importance of structural tailoring to increase dynamic performance and to reduce the control effort.

Armanios et al. (1990) explored the benefits of tailoring the macro- as well as the micro-structure; that is, they examined altering ply stacking sequence, fiber orientation, and blend of material plies, to contain and resist damage in flexible structures. One of their purposes was to demonstrate that damage tolerance can be designed into a structure. It was shown that for a generic damaged ply model, which includes microcracking, delamination, and fiber fracture and their interaction, damage modes alter the stillnesses of the structural component at the damage site. Therefore, redistribution of local stiffnesses could be used to enhance toughness.

Rehfield and Atilgan (1987) showed that in closed-cell blades additional couplings arise, other than those designed in, which must be accounted for. These "parasitic" couplings are extension shear (accompanies bending twist) and bending shear (accompanies extension twist). With these additional couplings the analysis will predict structural stillnesses smaller (more flexible) than without them.

In light of the above, structural, elastic, and aeroelastic tailoring concepts are promising for active/passive control of flexible structures. Therefore, development of an interdisciplinary analysis methodology for design, optimization, and control of structures would be useful. Haftka and Kamat (1989) developed an approach for simultaneous nonlinear analysis and optimization of structures. This starting analysis shows how integrated interdisciplinary approaches can lead to more understanding as well as computational benefits.

Various wind turbine configurations have been studied and tried, such as horizontal axis wind turbines (HAWTs) versus vertical axis wind turbines (VAWTs), stall- versus pitch-controlled turbines, fixed- versus free-yaw turbines, up- versus down-wind rotors, constant versus variable rpm, high-solidity slow running versus low-solidity fast running, stiff versus flexible, and different manners of overspeed protection. No doubt there are other configurations not yet tried (Watson, 1989) or even conceived. A few attempts have been made to study the gains that could be made by elastic tailoring (e.g., Karaolis et al., 1987). These studies did not lead, however, to significant changes in the way wind turbines are designed.

There are many questions to be answered concerning the design of wind turbines. Cost is one of the primary considerations. Some very inexpensive methods for building blades exist, but will they produce practical blades as far as performance is concerned? For example, it is known that pultrusion can produce blades at a fraction of the cost of present blades. One restriction of such blades, however, is that they must be spanwise uniform. Given the present structural design of HAWTs (i.e., without elastic coupling), this is known to be an energy-inefficient design for aerodynamic reasons. No research has been done, however, to indicate whether such losses could be compensated for by elastic tailoring, thus making pultrusion a practical manufacturing technique. The position of the committee is that the impact of future research on modeling and the design process will be negligible unless configuration, aerodynamic structural design, and materials properties issues are considered together.

LOAD CHARACTERIZATION

To successfully design a wind turbine blade structure, it is necessary that a representative model of the system loads be developed. While not necessarily being able to accurately predict these loads, this model should predict realistic peak-to-peak stress levels and oscillatory frequencies. The mechanisms for these stress reversals are reasonably well understood. Blades are exposed to relatively high levels of turbulence and random gusts in the air and, in some configurations, tower shadow effects. These, along with steady and quasi-periodic aerodynamic loads due to steady components of wind and vertical wind shear, coupled with steady centrifugal forces and periodic gravitational forces (in HAWTs) from the rotation create distributed loads along the blades. These distributed loads, in turn, have steady, periodic, and random components.



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