FAST Library: add access to hub position and velocity

[rafmudaf]

Feature or improvement description
Adds a subroutine to access the hub position, velocity, and orientation DCM through the FAST Library for use in AMR Wind.
cc @psakievich @ndevelder @andrew-platt

Related issue, if one exists
None

Impacted areas of the software
Added a subroutine to FAST Library

[psakievich]

This looks great. It should work just fine for the AMR-Wind API. I'll defer to @ndevelder to confirm if the variables are sufficient for their calculations.

[rafmudaf]

@ndevelder just checking in again before I merge this - everything good here for your use in AMR Wind?

[ndevelder]

Hi @rafmudaf! We're finally getting around to implementing this. One thing we found was a spelling inconsistency in "HubPosition" in the .h file. I'll try to keep updating here as we test.

[rafmudaf]

Thanks for pointing this out @ndevelder, I've fixed it

[rafmudaf]

I've added a unit test for this in the last commit as well as a method to get the hub position through the Python interface. However, the test is currently running in the automated system because it litters the repository with extra files. For GH Actions, this is not a problem, but during day to day development and running tests it will get cumbersome.

It is located in openfast/modules/openfast-library/tests/test_openfast_library.py and you can run it with this command:

python test_openfast_library.py ../../../reg_tests/r-test/glue-codes/openfast/5MW_OC4Jckt_ExtPtfm/5MW_OC4Jckt_ExtPtfm.fst

It simply initializes the NREL 5MW turbine model and compares the position, rotational velocity and orientation from the initial conditions.

def test_hub_position(library_path, input_file):
    openfastlib = openfast_library.FastLibAPI(library_path, input_file)
    openfastlib.fast_init()
    absolute_position, rotational_velocity, orientation_dcm = openfastlib.get_hub_position()

    # Initial hub position is at -5, 0, 90.55
    np.testing.assert_allclose(
        absolute_position,
        np.array([-5.0, 0.0, 90.55]),
        rtol=1e-5,
        atol=1e-8,
        verbose=True
    )

    # This case is initially still
    # Velocities should be 0 and the DCM should be identity
    np.testing.assert_array_equal( rotational_velocity, np.zeros(3) )
    np.testing.assert_array_equal( np.reshape( orientation_dcm[:], (3,3) ), np.eye(3) )

[rafmudaf]

And lastly, I've added the same to the C++-based FastLibAPI. This is not the one used to integrate with external CFD codes. It is a close copy of the Python interface, and it is mostly for testing the FAST_Library header file.