Improving a Composite Wind Turbine Blade Considering Minimizing the Prospect of Flutter with the Method of Modal Analysis Technique

A preliminary method has been provided in designing composite blades, with requirement to meet the ultimate- and buckling-strength criteria. This method lies in examining the sections alone and focuses on getting the number of plies required to meet the criteria mentioned before. In recent years, with the concept of load alleviation and twist-coupled, the coupling between degrees of freedom, such as flapwise bending and twist, has become a matter of importance to modern twist-coupled blades. Attempts at capturing the mechanical properties and dynamic behaviors by inspecting the sections alone will inevitably neglect important three- dimensional effect such as warping and shear deformation. In this situation, dynamic behavior will not be correctly predicted. In this paper, a blade built with the preliminary method was rebuilt by Ansys. As one of modes-coupled instabilities, pitch-flap flutter was considered for improving the blade to behave a more stable property with the method of modal analysis. We focused on the ratio of the blade´s 1st torsional to 2nd flap-wise natural frequency, due to it´s especially significance to minimize the prospect of flutter. Two different ways (sweep the blade along the span or named ´pre- sweep´ and change the direction of primary load-bearing fibers) implemented for a passively adaptive blade design was also studied here for their modal characteristics. Additional studies are performed for blades to have some degree of ´pre-curve´ to increase the blade-tower clearance. This study will focus on determine these parameters´ sensitivity to this ratio, which have a significant meaning to design a twist-coupled (adaptive) blade.