Error when running scaled wind turbine model in OpenFASTv3.0.0

[niay000]

Dear All,
I hope this message finds you well.

I am trying to run OpenFAST for a scaled wind turbine model (model-scale wind tunnel experiment).
In this setup, the tests are conducted under 50-g gravity, so gravity is scaled accordingly (×50), and the time is scaled down to 1/50.
When the model-scale dimensions are converted back to the actual full-scale turbine size for calculation,
OpenFAST runs successfully; however, running the simulation at the model-scale dimensions produces the following error.

FAST_Solution:FAST_AdvanceStates:SolveOption2c_Inp2AD_SrvD:InflowWind_CalcOutput:CalcOutput:IfW_FF
Wind_CalcOutput [position=(1.9735, 0.94676, 1.3323) in wind-file coordinates]: FF wind array
boundaries violated. Grid too small in Z direction (Z=1.3323 m is above the grid).

All the input files (ElastoDyn, AeroDyn, InflowWind, case.fst) were scaled consistently with the model size.

What I Tried
In the past, when encountering similar errors, I was able to avoid them by using a much smaller analysis time step (DT).
This time, however, even with extremely small and practically infeasible DT values, the error still occurs.
Additionally, I tried significantly expanding the BTS file grid to cover a wider spatial range, but the error was still not avoided.

BTS File Information
The .bts file was generated using PyConTurb, with the following properties:
• Time steps: 18,192
• Spatial points: 1323
• Time range: 0.0 – 3.6382 s, with Δt = 0.0002 s
• Velocity ranges:
o u component: 5.44 – 5.99 m/s
o w component: 0.0054 – 0.0059 m/s
o No NaN values
• Grid dimensions:
o k points: 3 (u, v, w)
o y points: 21
o z points: 21
• Coordinate ranges:
o Hub height: 0.529 m
o Y: −0.469 → +0.469 m
o Z: 0.06 → 0.998 m
o X: 0.0 → 0.0 m (constant)
• Example samples:
o Y: [−0.469, −0.4221, −0.3752, …, 0.469]
o Z: [0.06, 0.06, …, 0.998]
o X: [0.0, 0.0, …, 0.0]
o
The model-scale setup is defined as follows:
• Water depth: 0.1 m
• Blade length: 0.115 m
• Blade chord: 0.06 m
• Hub height: 0.529 m
• Transition piece: 0.06 m
•
Questions:
・Is it possible to perform a simulation analysis of a model-scale wind turbine under such a gravity field?
・Do you have any advice for resolving these errors?

Please find attached the files I am using. For the purpose of attachment, I have changed their extensions to .txt.
I may be misunderstanding something fundamental, so please let me know if I should provide more details.

Thank you very much for your time and assistance.
Best regards,

casefst.txt
caselog.txt
inflowwind.txt
servodyn.txt
subdyn.txt
aerodyn.txt
aerodyn_blade.txt
elastodyn.txt
elastodyn_blade.txt
elastodyn_tower.txt
expfoil_plate_coords.txt
expfoil_plate.txt
hydrodyn.txt

[jjonkman]

Dear @niay000,

I haven't spent much time reviewing your files, but the position stated in the error is well outside your stated TurbSim domain size, in both the Y and Z directions. If you specified your turbine geometry correctly, it sounds like your TurbSim domain size is sufficient for the undeflected geometry, but the deflection is very large. Are you receiving warnings about large blade or tower deflection? If so, I would assume your model is numerically unstable for some reason, e.g., due to incorrect scaling of mass/stiffness or too large of a time step.

Best regards,

[jjonkman]

Dear @niay000,

Can you clarify: Are you receiving the large blade deflection warning for a simplified model without platform DOFs in ElastoDyn enabled, without wind/aerodynamics enabled, and only a tower-top force applied through StC?

Best regards,

[niay000]

Dear Dr.Jonkman,

Thank you very much for your message. As you indicated, I have disabled the platform DOFs in the ElastoDyn file, and I have also set CompAero and CompInflow to disabled in the .fst file. The only enabled modules are CompElasto and CompServo.
Under these conditions, when running the model where the tower-top force is applied through StC in ServoDyn so that the force acts only on the nacelle, I am receiving the large blade deflection warning.
I am wondering whether this may indicate an issue with the blade or tower design itself. I would be grateful for any guidance you could provide.

Best regards,

[jjonkman]

Dear @niay000,

Thanks for confirming. The large blade-deflection warning will not in itself stop a simulation; are you receiving an error message before this warning and the end of the simulation? What do the blade and tower deflection look like for this case? Have you double checked that you are properly scaling the blade structural properties?

Best regards,

[niay000]

Dear Dr. Jonkman,

Thank you for your guidance.

I understand that the blade-deflection warning itself does not stop the simulation.
In practice, the computation has not stopped, but after the warning appears, the simulation does not advance even after running for several days.

For reference, the full warning message is as follows:

FAST_Solution:FAST_UpdateStates:FAST_AdvanceStates:ED_ABM4:ED_AB4:ED_RK4:ED_CalcContStateDeriv:Set
CoordSy:Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large blade
deflection (ElastoDyn SetCoordSy). The solution may be inaccurate. Simulation continuing, but
future warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 3.00000E-05 s
ED_CalcContStateDeriv:SetCoordSy:Small angle assumption violated in SUBROUTINE SmllRotTrans() due
to a large blade deflection (ElastoDyn SetCoordSy). The solution may be inaccurate. Simulation
continuing, but future warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 3.00000E-05 s

Following your advice, I also checked the BModes-related values to ensure that the blade and tower deflections are correctly scaled.
No discrepancies or problematic points were found.

For your reference, I have attached the files related to the blade and tower deflections.

If there are any additional points I should check or suggestions you could provide, I would greatly appreciate your guidance.

Thank you very much for your time and support.　

Best regards,
Expowt_blade.txt
Expowt_tower.txt
tower_sec_props.txt
blade_sec_props.txt
elastodyn_blade.txt
elastodyn_tower.txt

[jjonkman]

Dear @niay000,

I haven't checked your input files because you haven't explained what scaling laws you are applying to mass, stiffness, and geometry.

If I understand correctly, you are receiving the large blade deflection warning a couple of time steps into the simulation; is that correct? Presumably the simulation runs as expected with the platform DOFs disabled and the tower DOFs enabled? Which blade DOFs have you enabled? Can you isolate the issue to a specific blade mode or does enabling any mode trigger the warning?

Best regards,