discrepancy between FAST and AeroDyn outputs?

I am running a fairly simple model based on the 5MW sample: rigid blade, no induced velocity, 0 twist, 0 wind, 0 shaft tilt, and 0 precone. I took the steady state tangential force values from the AeroDyn output file for each aero segment and multiplied each by the RNodes value in the input file to calculate the rotor torque. This value did not match the value of the rotor torque output in the FAST output file. As a check, I summed all of the normal forces for the aerodynamic segments and checked this value against the value of the rotor thrust output by FAST. This matched. Can you tell me why the rotor torque output by FAST does not match the value I hand calculated from the AeroDyn outputs?

Dear Chris,

How different are two torques that you are comparing?

Is the drivetrain or generator DOF enabled in FAST? The rotor torque output by FAST not only includes contributions from the aerodynamic loads, but it also includes contributions from rotor inertia if the rotor is accelerating or decelerating. This is exexplained in detail in the June 10 post of the forum topic found here: http://forums.nrel.gov/t/fast-aerodyn-beddoes-stall-mode/258/1.

Best regards,

Jason,

I’ve turned the generator DOF per your other post and run the model to steady state, so I have a constant rotor speed.

The hand calculated torque from the data in the AeroDyn output file is -2.5285e+05 and the one I get in the FAST output file is -2.592E+05. We are trying to get our RCAS results to match FAST. The rotor torque calculated by RCAS matches the hand calculated value within 90 N-m.