Inconsistent OpenFAST - HAWC2 rated rotor speed

[jennirinker]

Description

OpenFAST rated rotor speed is 0.78788 rad/s (7.5237 RPM). Here
HAWC2 rated rotor speed is 0.792 rad/s (7.563 RPM). Here

Neither of these values match the design rated rotor speed of 7.56 RPM (0.79168 rad/s), although HAWC2 is just off due to roundoff error.

Steps to reproduce issue

MWE not necessary -- bug is related to model implementations.

Current behavior

Rated rotor speed values do not match each other or documentation.

Expected behavior

Rated rotor speed values will match both each other and documentation.

Code versions

Irrelevant.

@dzalkind @gbarter @ptrbortolotti What would you suggest for aligning our models? Should we both use, e.g., 0.7917 rad/s (7.560 RPM)?

[ptrbortolotti]

Is this PR IEAWindSystems/windIO#175 helping solve the discrepancy?

[dzalkind]

Probably not quite. I think we need to ensure the DISCON has the right rated speed, which will happen on the ROSCO side.

I'm working on that over here and can check that we're in alignment: NatLabRockies/ROSCO#492

[dzalkind]

They key input for rated rotor speed in ROSCO is PC_RefSpd for the pitch controller, which seems okay:

IEA-15-240-RWT/OpenFAST/IEA-15-240-RWT-Monopile/IEA-15-240-RWT-Monopile_DISCON.IN

Line 67 in 31ec0e3

0.791680000000 ! PC_RefSpd - Desired (reference) HSS speed for pitch controller, [rad/s].

The torque reference speed is just an upper limit on the reference speed, which changes with wind speed to track the optimal TSR. It actually helps slightly to have it a bit lower than the pitch reference speed.