Verification of BModes: Rotary Beam and Tower Modal Analysis Code; Preprint

This paper describes verification of BModes, a finite-element code developed to provide coupled modes for the blades and tower of a wind turbine. The blades, which may be rotating or non-rotating, and the towers, whether onshore or offshore, are modeled using specialized 15-dof beam finite elements. Both blade and tower models allow a tip attachment, which is assumed to be rigid body with sixmoments of inertia, and a mass centroid that may be offset from the blade or tower axis. Examples of tip attachments are aerodynamic brakes for blades and nacelle-rotor subassembly for towers. BModes modeling allows for tower supports including tension wires, floating platforms, and monopiles on elastic foundations. Coupled modes (implying coupling of flap, lag, axial, and torsional motions) arerequired for modeling major flexible components in a modal-based, aeroelastic code such as FAST1. These are also required for validation of turbine models using experimental data, modal-based fatigue analysis, controls design, and understanding aeroelastic-stability behavior of turbines. Verification studies began with uniform tower models, with and without tip inertia, and progressed torealistic towers. For the floating turbine, we accounted for the effects of hydrodynamic inertia, hydrostatic restoring, and mooring lines stiffness. For the monopole-supported tower, we accounted for distributed hydrodynamic mass on the submerged part of the tower and for distributed foundation stiffness. Finally, we verified a model of a blade carrying tip mass and rotating at different speeds(verifications of other blade models, rotating or non-rotating, have been reported in another paper.) Verifications were performed by comparing BModes-generated modes with analytical results, if available, or with MSC.ADAMS results. All results in general show excellent agreement.