This is a very basic question, but I was going to check that the structural frequencies of my FAST model were similar to the model I am trying to reproduce, but I was surprised that the frequencies of tower and blades are not explicitly input. The frequencies and mode shapes are calculated in BModes and then the mode shapes are parameterised, using a 6-th order polynomial. But it appears the frequencies are not used by FAST. The subroutine Coeff calculates frequencies based on stiffness and mass for each degree of freedom (structural mode). This relies on a “model stiffness” having been calculated first. I will keep looking, but I suppose the only way to find out the structural frequencies would be using the linearization tool?
The mode shapes specified in the FAST input files are used as shape functions in a nonlinear beam model of the blades and tower. The shape functions are normalized by the tower-top displacement. (The DOF corresponding to each mode is the transverse tower-top displacement.) Because only two shape functions (two DOF) are used for each bending direction, it is important to specify shape functions that correspond to realistic deflections, i.e. the natural mode shapes. The shape functions are also used to form the generalized mass and stiffness matrices of the beams. You can’t specify the natural frequencies in FAST; these result from the specified mass and stiffness (generalized matrices) and geometry – not only of the beam, but also of the beam’s boundary conditions.
The natural frequencies calculated within routine Coeff() are only estimates (they don’t include the effects of the boundary conditions on the natural frequencies) and are used only for the calculation of the generalized damping term.
You can derive the natural frequencies from FAST in two ways: (1) through a FAST linearization analysis and (2) through post-processing of time-domain simulations (e.g., PSDs).