Dear Luiz,
The errors you seeing are typically caused—not by a problem with the wind input file—but by a numerical instability in the FAST or AeroDyn calculations. Warnings regarding a “small angle approximation violation,” warnings regarding “supersonic blades,” a simulation crash, or very large deflections that occur in the time series near the start of a simulation are good signs of a numerical instability.
The coupled FAST with AeroDyn code uses two time steps. The time step in FAST, DT, controls the integration of the structural equations of motion. The time step in AeroDyn, DTAero, controls the integration of the aerodynamic equations. A good rule of thumb for choosing the AeroDyn time step is to have at least 200 time steps per revolution; for variable-speed turbines, I’d use the fastest likely rotor speed for this calculation. When choosing the FAST time step, a good rule of thumb is that DT should be set less than or equal to one over ten times the highest full system natural frequency. That is, in equation form: DT <= 1 / ( 10 * ). The full system natural frequencies can be found by running a FAST linearization analysis (AnalMode = 2) about the initial conditions. See the Linearization chapter of the FAST User’s Guide for more information.
Best regards,