Difference between FAST v7 and FAST v8


I simulated 10MW DTU RWT to see if FAST v7 and FAST v8 produce the same results under the same conditions.

Input is the same to both. (Tower properties, Blade properties, etc.)

The only difference is the AeroDyn version. (FAST v7 use AeroDyn 13, FAST v8 use AeroDyn 14, But same option)

When comparing the results, there were several differences, and the results as shown in the figure below.

I used the ROSCO controller using default input for 10MW DTU.

Why is there a difference? If you know it, please could you explain it?

Best regards,

Dear Seonghwan.Kim

I don’t really know off the top of my head what would cause these differences. Your plots identify OpenFAST in the legend, but I don’t see any red in the plot; are they identical to the FAST v8 result?

The module-to-module coupling approach (numerics) is quite different between FAST v7 and FAST v8 / OpenFAST; have you confirmed that that the FAST v8 result is converged, e.g., does dropping the time step in half change the solution? There could also be bugs that were fixed between FAST v7 and FAST v8.

If you really want to dig down to identify the specific differences, you’ll likely need to run various iterations of the model with various features enabled/disabled.

Best regards,

Dear Jason,

Thanks for your quick reply!

After seeing that the results of FAST7 and FAST8 were different, I ran the OpenFAST to compare with other results. The results of FAST8 and OpenFAST are the same.

And I checked if FAST8 was converged, yes It’s converged. (dropping the time step in half gave the same results)

I simulated another case about uniform inflow wind.

Please could you recommend how to identify the specific differences like changing some conditions or simulate only rotor?

Best regard,

Dear Seonghwan,

I would assume that the differences are mostly aerodynamic related considering the switch from AeroDyn v13 to AeroDyn v14), but you could first check that the response is the same in the absence of aerodynamic loads (CompAero = 0). And to assess differences in the aerodynamic loads, I would recommend disabling all structural DOFs in ElastoDyn and to start off with uniform inflow (as you’ve now done).

Best regards,

Dear Jason,

Thanks for your kind explanation.

As you recommend, I check whether the response is the same in the absence of aerodynamic loads (CompAero = 0).

At the same time, I turn off the structural DOFs in ElastoDyn like this. (FASTv7 example)

---------------------- FEATURE FLAGS -------------------------------------------
False FlapDOF1 - First flapwise blade mode DOF (flag)
False FlapDOF2 - Second flapwise blade mode DOF (flag)
False EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
True GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)
False TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
False TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
False TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
False TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
False CompAero - Compute aerodynamic forces (flag)
False CompNoise - Compute aerodynamic noise (flag)

Blade moments are almost the same, But some results are weird.

I checked the input file one more time, there are no differences between FASTv7 and FASTv8 input.

I was confused why “RotSpeed” is different. Is it a reasonable result?

Best regards,

Dear Seonghwan,

It looks like the rotor speeds are slightly different. Was RotSpeed initialized to the same value in both FAST v7 and FAST v8? Do you have any control enabled? What happens over the initial 70 s?

Best regards,

Dear Jason,

Yes, RotSpeed initialized to the same value.

I also simulated NREL 5MW RWT.

The simulation results are almost the same except RotSpeed. (like DTU 10MW simulation result)

FAST7 “RotSpeed” = 0.8599
FAST8 “RotSpeed” = 0.8642

To confirm If the ROSCO controller make this difference, I used Discon.dll what you provided(not used ROSCO controller)

The results are the exactly same. (“RotSpeed” also)

I think that some differences due to the ROSCO controller.

To be honest, I don’t know what makes a difference…

Please could you give me an opinion about these results?

Thank you very much.

Best regards,

Dear Seonghwan,

The ROSCO controller and the original controller for the NREL 5-MW baseline turbine are different, so, I would not expect the same response when switching controllers. If you are simply trying to compare differences between FAST versions, I would use the same controller in both versions.

That said, with aerodynamics disabled, I would disable the controller, which would not function properly without aerodynamic loads.

Best regards,

Dear Jason,

I’m sorry I seem to have confused you a little.

I used the original controller for both versions.

By the way, I solved the problem!

When the initial value of the rotor speed was set slightly lower than the rated speed, the same result came out.

Thank you for your advice.

Best regards,