How to output model shapes and natural frequencies (FAST)


Does anyone know how to output wind turbine model shapes and natural frequencies from FAST? those are important in dynamic analysis, but I donot know how FAST calculate them, and how to output them.


[color=blue]I think you mean ‘Mode Shape’. I also do not know about natural frequencies from FAST, maybe Bmodes?

Dear Kuangmin Gong,

BModes is a code used to calculate the mode shapes and frequencies of individual beam elements (e.g., blade or tower). The mode shape outputs from BModes become inputs to FAST.

Once a complete FAST wind turbine model has been built, not only can you run nonlinear time-domain simulations, but you can use the FAST linearization functionality to generate linearized models of the full system about various operating points. See the “Linearization” chapter of the FAST User’s Guide for more information.

Post-processing of the FAST linearization output via eigenanalysis gives the full system mode shapes (eigenvectors) and natural frequencies/damping ratios (eigenvalues). The eigenanalysis is complicated in a wind turbine by rotor rotation (unless the rotor is parked or idling, in which case eigenanalysis can be applied directly to the FAST linearization output). If the linear model was generated for a spinning 3-bladed wind turbine, one must transform the rotating states into the fixed frame before eigenanalysis. We are developing a code MBC3 ( to do the transformation and subsequent eigenanalysis. For a spinning 2-bladed wind turbine, Floquet theory is required. See the MBC3 User’s Guide for more information.

I hope that helps.

Best regards,