Hello,

I am modeling a rigid offshore floating wind turbine, and like to find AeroDynamic forces on the rotor while the rigid body has harmonic motions in the surge direction in the form of a*cos(w*t) , where "a" is the amplitude of motion, and "w" is the radial frequency of the motion.
To model a rigid turbine, in the primary.FST file, in the part "FEATURE FLAGS" I deactivated all the degrees of freedom except CompAero.
To give harmonic oscillations to the structure in the surge direction, in the file FAST.f90 in the subroutine solver, I changed the value of Q(displacement), QD(velocity),QD2(acceleration) ,QT,QDT,QD2T for the first degree of freedom (surge DOF) to a harmonic function. In details:
The subroutine solver starts with the 2 lines: "QT = Q (:,IC(1))" " QDT = QD(:,IC(1))" before these 2 lines I added the lines :
QT (1) =amp1*cos(w*(step+1)*DT)
QDT(1) = -amp1*w*sin(w*(step+1)*DT), where amp1 is the amplitude of motion in surge, and w is the radial frequency.
The next lines in the subroutine solver are related to RK4 time integration method. In each substep, I used the same expressions as above to redefine the displacement and velocity in the surge direction. In the last substep of RK4,where the final values of Q,QD are calculated I added the lines:
Q (1,IC(NMX)) =amp1*cos(w*(step+1)*DT)
QD(1,IC(NMX)) = -amp1*w*sin(w*(step+1)*DT)
The next lines in the subroutine solver are related to Adams-Bashforth predictor, in this part I added 2 lines the same as the above lines. After the predictor, there is the line "QD2(:,IC(NMX)) = QD2T" before this line I added:
QD2T(1)=-amp1*w**2*cos(w*(step+1)*DT)
In the Corrector (Adams-Moulton) part, I added the lines:
Q (1,IC(NMX)) =amp1*cos(w*(step+1)*DT)
QD(1,IC(NMX)) = -amp1*w*sin(w*(step+1)*DT)
After the corrector part there is the line: "QD2(:,IC(NMX)) = QD2T" before this line I added:
QD2T(1)=-amp1*w**2*cos(w*(step+1)*DT)
I really appreciate it if anyone could tell me whether these changes in the source code are enough to model a rigid wind turbine with harmonic oscillations in the surge direction or not.

Please help

I didn't find the right solution from the internet.

References:
https://wind.nrel.gov/forum/wind/viewtopic.php?t=831

LINK

Thank you