Is Bladed’s linearisation appropriate for the analysis of coupled modes, and control design for 2-bladed rotors? Does the theory behind the linearisation take the periodicity of the 2 bladed turbine into account?
Bladed has been widely used for the analysis of 2-blade turbines (see public examples below)
The MBC transformation is applied for 2-blade turbines, resulting in the correct coupling between rotor and tower modes for a given azimuth. But the analysis should be repeated at different azimuth angles.
For a 2-blade turbine, it’s not possible to remove all of the periodicity to produce a linear time invariant system, so technically the coupled modes should be determined by Floquet analysis rather than a conventional eigen analysis. However, Bladed uses eigen analysis for this process, giving reasonable results that have been applied successfully to perform control design of 2-blade turbines.
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Example applications in public domain:
Control design for 20MW reference 2-blade turbine here
Implementation of individual pitch control on NREL CART2 turbine here
PhD thesis on control design for 2-blade turbines here
Floquet recommendation example:
Paper showing advantage of Floquet analysis over Eigen analysis here