Dear JaeSang,
The structural time step (DT) required in FAST is dictated by numerical stability requirements. The structural time step of 0.005 s is suitable for the WindPACT 1.5-MW turbine with all relevant degrees-of-freedom (DOF) enabled. You cannot increase this time step without disabling relevant DOFs. For more information, see the following forum topic for more information: FAST Integrator, step size & precision.
If you wish, you can always output results at a lower resolution then the structural time step via input DecFact in FAST v7 or DT_Out in FAST v8.
I’m not sure what you are referring to when you say the time step is modifying the wind field. If the wind data and FAST time step are not equivalent, the wind data will be linearly interpolated.
I’m not sure what you mean when you say that “Test13 is not physically correct” and I don’t know what corrections you want.
Best regards,