Natural Modal Computation of Wind Turbine Blades Considering Structural Multi-Degree-of-Freedom Coupling

• • Shear deformation reduces second-order flapwise and edgewise natural frequencies by approximately 4.7% and 1.0%, respectively, which must be accounted for in fatigue life prediction to avoid underestimating dynamic stresses in large flexible blades. • • Bend-twist coupling decreases bending frequencies and increases torsional frequencies, with higher modes showing greater sensitivity: third-order edgewise frequency drops by 3.1% and second-order torsional frequency rises by 1.6%, directly impacting aeroelastic stability margins and flutter boundaries. • • Flap-lag coupling significantly alters edgewise modal characteristics more than flapwise ones, and is the dominant mechanism causing coupled bending mode shapes; this necessitates coupled-mode analysis for accurate load estimation in edgewise-dominated fatigue scenarios. • • Axial-bending coupling has the smallest effect among the three coupling types, suggesting that for preliminary design, axial-bending terms can be neglected without substantial loss of accuracy, reducing computational cost by up to 15% in modal solvers.