# Aerodynamic Derivatives

Dear All,

we need the derivatives of aerodynamic power and thrust force with respect to the tip speed ratio. Is it possible to compute it by using the linearization capability of FAST? If not, do you have any suggestion how to do it?
We are using FAST 8 with AeroDyn 14. Is it possible to set FrozenWake = True here or we have to change to AeroDyn 15?

Thank you very much in advance for any help.

Best regards

Yes, the linearization functionality of FAST v8 or OpenFAST can be used to calculate these derivatives. That said, FAST v8 and OpenFAST permit full-system linearization using AeroDyn v15, but not AeroDyn v14; so, I suggest upgrading your AeroDyn v14 model to v15.

The process we’ve used to calculate various aerodynamic derivatives is described in section 3.2 of our TORQUE 2016 paper: nrel.gov/docs/fy17osti/67015.pdf. To calculate derivatives of the aerodynamic power and thrust, you should use AeroDyn outputs RtAeroPwr and RtAeroFxh, respectively. The derivatives of these outputs will show up in the full-system linear state-space matrices C and D. While you can’t calculate the derivatives with respect to tip-speed ratio (TSR) directly, you can calculate the derivatives with respect to rotor speed in the C matrix (assuming that the generator or drivetrain-torsion degree of freedom are enabled in ElastoDyn) and with respect to the uniform wind speed in the D matrix (assuming you’ve set LinInputs=1 in the FAST/OpenFAST primary input file), from which you can convert to TSR through appropriate algebra.

Best regards,

Dear Jason,

thank you very much for your advices. We are now moving to AeroDyn v15. However, we have discussing if the effort should be to move only to AeroDyn v15 or if it is meaningful to move complete to OpenFAST 2.1. Are there remarkable advantages of OpenFAST 2.1 compared with FAST 8.16?

Best regards