Dear OpenFAST users,
I am currently trying to compute the vibration modes of the IEA15RWT blade using BeamDyn.
Specifically, I use the linearization feature of OpenFAST (v3.0.0) interfaced with BeamDyn. In order to compute invacuo blade modes, all degrees of freedom other than those associated with the blades have been disabled, gravity is set to zero, as well as initial conditions. The rotor is not rotating. The fully populated stiffness matrices given in the GitHub repo are used, material damping is set to zero. A timemarching simulation is run for 5 seconds (dt=0.002) and rhoinf=0 or 1, then linearization is performed. Eventually, eigenvalues are computed.
The BeamDyn solver parameters are the following:
True Echo
False QuasiStaticInit
[0.0 or 1.0] rhoinf
2 quadrature
â€śDEFAULTâ€ť refine
â€śDEFAULTâ€ť n_fact
â€śDEFAULTâ€ť DTBeam
â€śDEFAULTâ€ť load_retries
30 NRMax
â€śDEFAULTâ€ť stop_tol
False tngt_stf_fd
False tngt_stf_comp
â€śDEFAULTâ€ť tngt_stf_pert
â€śDEFAULTâ€ť tngt_stf_difftol
True RotStates
1 member_total
31 kp_total
[variing] order_elem
I have observed several behaviours that I find difficult to interpret:

It seems that the natural frequencies do not seem to converge when the order_elem parameter is increased (see the graphs below for parameters rhoinf=0 and rhoinf=1, where dashed lines report the frequencies found by Rinker et al. TORQUE 2020).
It is also not very clear to me how order_elem should be set with respect to member_total (kept fixed to 1 in my investigations). 
Even if the blades have no material damping, the damping of the modes is not very small (except when order_elem=1) as visible above. Interestingly, when the geometry of the blade is set as being straight (by setting kp_xr to zero), then damping returns to small values (on the order of 1e12).

When nonzero material damping is considered, sometimes the timemarching solution crashes (error message: FAST_Solution:FAST_AdvanceStates:B2:BD_GA2:BD_DynamicSolutionGA2:Solution does not converge after the maximum number of iterations)
As there have been some bug reports recently regarding BeamDyn (e.g. github.com/OpenFAST/openfast/issues/823), Iâ€™m not sure what could be the trace of a bug and what could be on my side a misunderstanding of the recommended settings for using BeamDyn.
Thank you in advance for any clarification!
Cheers,
JeanLou