This guide explains data preparation and execution with BModes, a finite-element code that provides dynamically coupled modes for a beam. The beam can be a rotating or non-rotating rotor blade or a tower, and it can have arbitrary distribution of structural properties and geometry along its length. A coupled mode implies presence of coupled flexural, axial, and torsion motions in a natural mode of vibration. Knowledge of flap-lag-torsion-axial coupled modes is crucial to several applications. Examples are: accurate modeling of major flexible components for modal-based aeroelastic codes such as FAST (Fatigue, Aerodynamics, Structures, and Turbulence), validation of flexible component models using experimental data, modal-based fatigue analysis, and interpretation of aeroelastic-stability behavior of turbines. Our plan is to eventually integrate BModes with FAST to provide tower and blade modes as rotor speed and blade pitch control settings change during a simulation. Obtaining rotating-blade coupled modes accurately, however, is one of the most challenging tasks in the dynamics field, and only a few codes address this. BModes is perhaps the most accurate because it uses a finite-element approach in conjunction with analytical linearization and a special finite-element assembly that captures Coriolis and centrifugal effects. The finite-element method (FEM) is based on a 15- degree of freedom (dof) element with three internal and two boundary nodes. A salient feature of BModes is its potential to handle a complex range of boundary conditions. BModes requires specification of rotor speed, blade geometry, precone, pitch control setting, and structural properties distribution along the blade as inputs. The structural properties are specified in terms of the flap bending, lag bending, torsion, and axial stiffness; mass; section moments of inertia; and chordwise offsets of section shear center, tension center, and center-of-mass along the blade. These properties may be obtained from experimental tests, blade manufacturer, or a properties-extractor code, such as PreComp or BPE. The current version of BModes can handle only metallic or isotropic composite blades. With the aforementioned inputs, the code uses FEM followed by an eigenanalysis to compute mode shapes and frequencies. This guide provides step-by-step instructions on how to prepare input files (specify blade geometry, section properties, and finite-element discretization), how to execute the code, and how to interpret the outputs. The code runs fast (usually in a few seconds).
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