Multi-Flexible-Body Analysis for Application to Wind Turbine Control Design: September 10, 1999 -- October 31, 2003

A computational framework for aeroelastic analysis of Horizontal Axis Wind Turbines (HAWT's) is presented. The structural model is separated into multi-rigid-body and flexible-body parts. Equations for the former are derived using Kane's method; and the flexible portions are assumed to be beam-like structures, described using a mixed formulation. The equations of motion are of a relatively loworder in terms of geometrically-exact beam finite elements. The flexible and rigid subsystems are coupled with an aerodynamic model to form an aeroelastic analysis. A nonlinear, periodic, steady-state solution and a linearized transient solution about the periodic steady state are obtained. The computational framework for two-bladed, HAWT's is built using time finite elements over a half-period.The linearized ordinary differential equations have periodic coefficients in time, and a Floquet stability analysis for the linearized system is directly undertaken using periodic steady state results. Numerical results are presented for horizontal axis wind turbines including steady-state response and Floquet characteristic exponents and operating mode shapes. Effects on the dynamics of thesystem for pre-cone, rotor speed, teetering hinge lateral offset, teetering and yawing stiffness and damping, and composite blade properties are investigated. A user's guide for the computer program WTFlex is included in the appendix.