Dear Subin,
Correct, disabling the YawDOF disables yaw acceleration in the FAST model. If the yaw motions are “sizeable,” this will not lead to accurate calculations of the yaw loads. And without more information, it is hard to know what is causing the “ringing” in your model.
A recent topic on this forum addresses a similar question: Structural loads at pitch and yaw actuator. In my responses to that topic, I describe how YawSpr and YawDamp can be based on a desired frequency and damping of the yaw actuator. See the topic linked above and the FAST User’s Guide for more information.
Best regards,