Dear Younes,
First of all, your equation for T_Aero should involve the wind speed squared.
Your equations assume some important simplifications of your FAST model:
*The rotor is rigid (so, J_Rotor is constant). That is, FlapDOF1 = FlapDOF2 = EdgeDOF in FAST.
*The support structure is rigid (so, the rotor acceleration is solely related to RotAccel. That is, most structural DOFs in FAST are disabled.
*The aerodynamic loads are computed quasi-steadily. That is, StallMod = STEADY and InfModel = EQUIL in AeroDyn.
Your equation should work with these simplifications.
More information about the drivetrain model in FAST is available in the following forum topic: Resistant moment of the rotor and of the electric generator (in this forum post, RotTorq is identified as LSShftTq; both are equivalent).
Best regards,