Effects of the time step on AoA for FAST v8.16

Wind & Water Rotor Aerodynamics
1

Hi All,

I ran two cases of FAST simulations on NREL 5MW at the rated configuration ( -5 tilt, -2.5 pre-cone, etc.). The wind condition is nonuniform wind with the power-law profile, 10 m/s at 100 m. All the DoFs in the ElastoDyn are disabled so that the rotor constantly rotate at 12 rpm. The BeamDyn uses the reference blade input file and has 7 nodes. The AeroDyn blade input file is also from the reference file.

The AeroDyn setting is the following,

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One of the cases has time step, 0.01 sec and another one has time step 0.005 sec. The AoA responses at the Blade 1 tip from these 2 cases have about 0.2 deg differences. A full-time history plot is below

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I am wondering that is it the smaller time-step yields more accurate result? or for the wind turbine aerodynamics, that 0.2 deg differences do NOT have significant effects. Is there a way to explain why different time-step causes such variation? I am guessing its the AeroDyn-BeamDyn coupling.

Thanks

2

Dear Daniel,

Normally the simulation time step should be set so as to get a converged solution (and to maintain numerical stability). While both of your solutions look numerically stable, it doesn’t appear that the 0.01-s simulation is converged. Is the solution converged with the 0.005-s time step i.e. does decreasing the time step further change the result?

FYI – Test26 in the FAST CertTest is a model of the NREL 5-MW turbine using BeamDyn to model the blade structural dynamics. This model was set up to have a time step of 0.001 s, of course, the time step could be increased when you disable all structural DOFs in ElastoDyn, as you’ve done.

Best regards,