Question about BeamDyn Module

I’m just getting into using OpenFAST for my research, and want to get the thrust force calculated by Beamdyn, so i want to know the differences between LSShftFxa in Elasto module/ RtAeroFxh in AeroDyn module/RootFxr in BeamDyn module.
Second, I want to replace the root motion of the beamdyn module with the tower top displacement from other software, but I can’t find the location of the code and what it contains
Can you help me? Thanks a lot!

Dear Hao.Hu,

The RtAeroFxh output from AeroDyn is the aerodynamic applied thrust force.

The LSShftFxa output from ElastoDyn is the reaction force along the low-speed shaft and includes the applied aerodynamic thrust force, as well contributions from weight / inertia of the rotor.

The RootFxr output from BeamDyn is the reaction force at the blade root along the xb axis of the IEC blade coordinate system.

The blade root motion is a module-level input to BeamDyn. The data is stored within a point mesh data structure called RootMotion in the BeamDyn registry file (Registry_BeamDyn.txt), which is allocated during initialization within SUBROUTINE Init_u() called by BD_Init() (within BeamDyn.f90), and passed as in input to the BeamDyn update states and calculate output routines (SUBROUTINEs BD_UpdateStates() and BD_CalcOutput() within BeamDyn.f90).

I hope that helps.

Best regards,

Dear Jason,
it’s so kind of you to help me.
So does the Root reaction loads in ED module equal to RootFxr in BD module? I plot them but not, they are close to each other.
And does the root motion BD got is calculated by ED module? I can’t find the output of root motion in OutlisiParameters.xlsx.
Thank you for your reply!

Dear Hao.Hu,

ElastoDyn output RootFxb* is equivalent to BeamDyn output RootFxr, i.e., both are the reaction force at the blade root along the xb axis of the IEC blade coordinate system. But I would not expect the results to match exactly between BeamDyn output RootFxr from an ElastoDyn+BeamDyn simulation and ElastoDyn output RootFxb* from an ElastoDyn-only simulation due to diferences in the underlying beam formulation for the blade structural dynamics between BeamDyn and ElastoDyn.

Yes, the root motion input to BeamDyn is derived from the hub motion output from ElastoDyn. This hub motion output is not directly available as an output in the ElastoDyn OutList, but it is a module-level output available within the ElastoDyn source code (and is included in the point mesh data structure called BladeRootMotion in the ElastoDyn registry file, ElastoDyn_Registry.txt).

Best regards,

Thank you, Jason. I think I got it.