Hi Yinping Yang,
I have used GH-Bladed DLLs successfuly with FAST before.
With regards to the small angle assumption warning, FAST uses a linear modal summation to determine the tower and blade deflections. Consequently, there is a small angle approximation in the blade and tower deflection formulations. The accuracy of the deflections and loads will be diminished at the time the warning was generated and possibly thereafter if the problem persists. This warning is generally triggered by one of two problems:
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Improper choice in time step. If the time step is chosen too large for a given system, the numerical solution may become unstable and the response may “blow up.” This warning message generally, then, precedes a simulation crash. This problem can be eliminated by reducing the time step.
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Very flexible blades. In this case, the numerical solution remains stable and the large deflections are “real” and are caused by high loading or a physical instability. FAST, however, cannot model very flexible systems accurately so you must question the validity of the results.
What I typically suggest is that if (2) is the problem, that you examine the time history of the blade tip deflection and check the magnitude of the deflection at the time the warning was generated. Whenever the deflection exceeds this value in the remainder of the simulation, you must be cautious in how you interpret the results. If the deflection persistently exceeds this value, you should probably not be using the FAST code; a general-purpose multibody code would be a better choice. It is also possible that the small angle approximation is only violated during the start-up transient (I’ve seen this when modeling the NREL 5-MW turbine, for example). This happens because the blades are suddenly loaded by the wind at time zero. In this case the warning should not be of concern because the start-up transient output should not be used in the post-processing of a loads analysis. This problem can also be eliminated by choosing initial blade displacements that are representative of your given wind speed (i.e., initialize the blade displacements to the values of the steady-state displacements for a given wind speed).
In your case the simulation crashes soon after the warning is generated. So, (1) may be the problem.
The error regarding FF_Interp is telling you that the blade element has passed outside of the turbulence grid in the Y (transverse) direction. This error is typically caused by the use of a grid that is too small for the given rotor or by excessive turbine deflections. What is the width of the grid compared to the rotor diameter? (It is not the number of grid points, but the height/width of the grid that are important to this error.) Perhaps the excessive turbine deflections identified by the small angle assumption warning are triggering the error?
Best regards,