Small angle assumption violated in SUBROUTINE SmllRotTrans()

Dear Jason,

I’m trying to use model of the NREL 5-MW baseline turbine atop the OC3-monopile in FAST v7 to calculate the aerodynamic loads acting on wind turbine in parked state, but assume that the blades are pitched-out. The value of BlPitch and BlPitchF were set 0 and also set TpitManS and TpitManE to 0 so the blades are pitched-out during the entire simulation. Is that correct? And FAST aborted with a warning “Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large blade deflection”. I’m curious about how to solve this problem.

Another problem is that I’m using the routine of coupled linear springs foundation model written by J. Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies to simulate the wind turbine response, and FAST also aborted with an error “Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large tower deflection”.


Best regards,
Kevin

Dear Kevin,

Regarding the parked condition, are you following my guidance for modeling idling rotors in FAST v7 in my post dated Mar 30, 2010 in the following forum topic: http://forums.nrel.gov/t/extreme-events/219/1? To model a parked rotor (where the generator is kept from rotating), you should further disable the generator DOF in FAST’s primary input file (GenDOF = FALSE).

Regarding the coupled springs model, based on the error, I would guess your model has gone numerically unstable. To simulate with the CS foundation, which has higher-frequency modes than the rigid foundation model, you’ll need to reduce the time step (DT) relative to the model with the rigid foundation to keep the model numerically stable.

Best regards,

Dear Jason,

With your help, I have solved first problem. For the second problem, I have downloaded the BModes including CS_Monopile.bmi and CS_Monopile_tower_sec.dat, but I don’t know how to use them, and how to modify Tower’s input file and Blade’s input file? Should I modify the source file FAST_Mods.f90 and other files? Thanks in advance.

Best regards,
Kevin

Dear Kevin,

The FAST v7 model of the NREL 5-MW baseline turbine atop the OC3-monopile with coupled springs (CS) foundation has been provided on the forum before, requiring:

Are you using these input files, source code, and time step?

Best regards,

Dear Jason,

I did the simulation with the updated input file, NRELOffshrBsline5MW_Tower_Monopile_CS.dat and NRELOffshrBsline5MW_Platform_Monopile_CS.dat, that you provided, and the time step DT=0.00125s was also set and modified the user-specified platform SUBROUTINE UserPtfmLd_CS.f90, but the fast still aborted.

Another problem is that when I am doing recompilation of FAST following the draft of “Instructions for Compiling FAST using IVF for Windows”, an error is displayed as “FAST.obj : error LNK2019: Unresolved external symbol _USERTWRLD referenced in function _FASTSUBS_mp_RTHS; FAST_iwin32.exe : fatal error LNK1120: 1 unresolved externals”, and it makes me confusd.

Best regards,
Kevin

Dear Kevin,

So, were you able to recompile FAST v7 with the UserPtfmLd_CS.f90 source file that I provided? From your error messages, I would guess not.

Your first error message implies to me that the CS foundation is not being modeled (my guess is you are seeing large platform motions as a result of a lack of coupled springs). In your second error message, I don’t see that the object file UserPtfmLd_CS.f90 is in the list. Your second error message also refers to SUBROUTINE() UserTwrLd, which in FAST v7 is located at the bottom of the HydroCalc.f90 source file. Does this routine exist in that file (or has it been commented out)?

Best regards,

Dear Jason,

Thanks for your help, now it goes well.

Best regards,
Kevin