Dear @Tongzhou.Zhang,
Regarding (1), I would guess the oscillations in blade deflections and aerodynamic loads are driven by a combination of gravity and shaft tilt, the latter of which induces skewed flow. You could avoid the former by setting Gravity = 0 and could avoid the latter by eliminating shaft tilt and disabling the tower degrees of freedom.
Regarding (2), I would expect steady-state deflection at rated wind speed (around 11.4 m/s) to be around 5.5 m. This is the deflection you are calculating, but differs from the paper you reference. But again, I’m not familiar with this paper and can’t explain their results. Moreover, the statement made in the paper about the FAST-AeroDyn results is incorrect. The structural model of old versions of FAST without BeamDyn, which became the ElastoDyn structural module of OpenFAST, is not a linear beam model. Instead, this structural formulation accounts for geometric nonlinearities, e.g. as discussed in the following forum topic: Coupled blade modes in FAST. I would generally expect ElastoDyn and BeamDyn to predict very similar blade deflections for the NREL 5-MW baseline wind turbine, except for a small amount of blade torsion that BeamDyn can capture that ElastoDyn can’t.
How does your model compare without and without flexibility, as shown in the paper?
Best regards,