free decay test

[sffgd]

Hello everyone, When I do free decay in openfast, I get an error as shown in the figure, and the parameters are modified as follows: ①Compinflow=0 ②Elastodyn-ptfmsurge=30 ③Hydrodyn-wavemod=0 hydrodyn-wavemod=0 If there is a solution to the trouble to tell me, thank you very much！

[sffgd]

The error is as follows： Warning: SkewedWakeCorrection encountered a large value of chi (177.49 deg), so the yaw correction will be limited, This warning will not be repeated though the condition may persist. See the AD15 chi output charnels, and consider turning off the Pitt/Peters skew model (setSkewlod=1) if this condition persists.

FAST_Solution:FAST_AdvanceStates:ED_ABI4:ED_AB4:ED_RK4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large blade deflection (ElastoDyn SetCoordSy).The solution may be inaccurate. Simlation continuing, but future warnings from Sm11RotTrans() will be suppressed. Additional debugging message from SUBROUTINE SmllRotTrans(): 0.5 s

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 19, blade 1):UA_UpdateStates:UA_UpdateDiscOtherState:ComputeKelvinChain:Mach number exceeds 0. 3. Theory isinvalid, This warning will not be repeated though the condition may persist:BEMT_UpdateStates(node 45, blade2):UA_UpdateStates:UA UpdateDiscOtherState:ComputeKelvinChain:Mach number exceeds 1.0. Equations cannot be evaluated.

OpenFAST encountered an error at simulation time 0.5 of 1000 seconds. Simulation error 1eve1: FATAL ERROR

Aborting OpenFAST

[andrew-platt]

Dear @sffgd, what turbine are you modeling? Is a 30 m surge reasonable, or would that cause large pitching motion as the mooring lines snap the turbine back towards equilibrium? From the error messages, I suspect your surge is far too large and is causing the turbine to move very violently in the first few timesteps.

[jjonkman]

Dear @sffgd,

I would also add that with CompInflow = 0, the air will be still. As such, I would suggest either disabling AeroDyn altogether (to eliminate aerodynamic loads, CompAero = 0) or if you want still-air aerodynamics in AeroDyn (CompAero = 2), to disable the wake/induction model (WakeMod = 0) and the unsteady airfoil aerodynamics model (AFAeroMod = 1) in AeroDyn because these models are only valid for normal operation. I would also disable ServoDyn altogether (CompServo = 0) or at least disable the ROSCO controller in ServoDyn (CompServo = 1 with PCMode = VSContrl = 0), which is only valid for normal operation.

Best regards,

[sffgd]

Dear @andrew-platt, The case I use is the IEA-15MW wind turbine，The free attenuation test based on NREL(Definition of the UMaine VolturnUS-S Reference Platform Developed for the IEA Wind 15-Megawatt Offshore Reference Wind Turbine),surge = 30m looks reasonable.

[sffgd]

@jjonkman,

Thanks for your answer ！When I setting CompInflow=0, CompAero=0 and ComServo=0 , I get the following error, which looks like there is a problem with the mooring system. This seemed to have something to do with the controller, so I replaced it with an OC3 controller with the help of others, and it worked successfully. But I have a doubt: the platform six degrees of freedom natural frequencies obtained with different controllers should be very close, right?

[jjonkman]

Dear @sffgd,

I'm not really sure I understand enough about what you are changing to comment. You mention both disabling the controller and swapping the controller.

It appears to me that your model is unstable for some reason, which the first error triggered by MoorDyn, that doesn't mean that MoorDyn is the cause of the instability.

Again, I would not expect that you'd have the controller enabled for a floater free-decay simulation.

Best regards