The design of the main structure of a wind turbine blade is optimized aiming at
the improvement of the overall dynamic performance. Three optimization strategies
are developed and tested. The first fundamental one is based on minimizing the
total structural mass of the blade spar under frequency and strength constraints.
The second and third strategies are concerned with the reduction of the overall
vibration level by either minimizing a frequency-placement index or maximizing
the natural frequencies and placing them at their target values to avoid large
amplitudes and resonance occurrence. Design variables include cross-sectional
dimensions and material properties along the spanwise direction of the blade spar.
The optimization problem is formulated as a nonlinear constrained problem solved
by sequential quadratic programming (SQP) technique. Two specific layup configurations,
namely, circumferentially asymmetric stiffness (CAS) and circumferentially
uniform stiffness (CUS), are analyzed. Exact analytical methods are applied
to calculate the natural modes of vibration of a composite, thin-walled, tapered
blade spar. The influence of coupling on the vibration modes is identified, and the
functional behavior of the frequencies with the lamination parameters is thoroughly
investigated and discussed. Finite element modeling using NX Nastran solver is
performed in order to validate the analytical results. As a case study, optimized
blade spar designs of a 750-kW horizontal axis wind turbine are given. The attained
solutions show that the approach used in this study enhances the dynamic
characteristics of the optimized spar structures as compared with a known baseline
design of the wind turbine blade. |
