Wake-added turbulence in FAST.Farm

[ebranlard]

This pull request is ready to be merged

Feature or improvement description
Implementation of the wake-added turbulence into FAST.Farm

Impacted areas of the software
FAST.Farm

Additional supporting information
The feature and input file changes has been documented, but it requires the calibration of its two main constants.
r-tests and additional documentation will follow after the calibration.

The following additional inputs are added to the input file:

--- WAKE-ADDED TURBULENCE ---
0                  WAT                - Switch between wake-added turbulence box options {0: no wake added turbulence, 1: predefined turbulence box, 2: user defined turbulence box} (switch)
"Inflow/WAT/isoturb_1024x32x16.u" WAT_BoxFile  - Filepath to the file containing the u-component of the turbulence box (either predefined or user-defined) (quoted string)
1024, 32, 16       WAT_NxNyNz         - Number of points in the x, y, and z directions of the WAT_BoxFile [used only if WAT=2, derived value if WAT=1] (-)
5.5, 4.0, 4.0      WAT_DxDyDz         - Distance (in meters) between points in the x, y, and z directions of the WAT_BoxFile [used only if WAT=2, derived value if WAT=1] (m)
default            WAT_ScaleBox       - Flag to scale the input turbulence box to zero mean and unit standard deviation at every node [DEFAULT=False] (flag)
0.5                WAT_k_Def          - Calibrated parameter for the influence of the wake deficit in the wake-added turbulence [>=0.0, no upper bound] or DEFAULT [DEFAULT=NONE]
0.5                WAT_k_Grad         - Calibrated parameter for the influence of the radial velocity gradient of the wake deficit in the wake-added turbulence [>=0.0, no upper bound] or DEFAULT [DEFAULT=NONE]
default            WAT_D_BrkDwn       - Downstream distance in rotor diameter after which WAT scaling has reached 100% capacity [>=0] or DEFAULT [DEFAULT=1] (D)

Test results, if applicable
Test results are unchanged.

• Implement the downstream evolution of kdef and kgrad using eddy viscosity function and 4 input parameters

[andrew-platt]

Temporary links to readthedocs changes for proofing:
https://ap-openfast.readthedocs.io/en/f-ffwat/source/user/fast.farm/InputFiles.html#wake-added-turbulence-wat
https://ap-openfast.readthedocs.io/en/f-ffwat/source/user/fast.farm/FFarmTheory.html#wake-added-turbulence-wat

[jjonkman]

I've reviewed the documentation part of this PR. Please find my edits and comments below.

[jjonkman]

We've discussed potentially replacing input WAT_D_BrkDwn with one of the filter functions involved in the eddy viscosity formulation. I would guess F_vShr(x) would make sense because it represents the development of turbulent stresses generated by the wake shear layer, which seems to be what WAT_D_BrkDwn is trying to achieve. The only issue I see is the DEFAULT values of C_vShr_DMin, C_vShr_DMax, C_vShr_FMin, and C_vShr_Exp used to define F_vShr(x) result in a filter function that does not approach unity until 25D. This would impact the calibration of WAT_k_Def and WAT_k_Grad.

[ebranlard]

I think it might be risky to rely on the same inputs indeed.

But it's true that we could use the function EddyFilter, and call it with C_Dmin=0 and C_Dmax = WAT_D_BrkDwn
As for the FMin and the exponent, we could use the values from the input file, but they might represent something a bit different, maybe it's best to hardcode them to say FMin=0 and Exp=1.

[jjonkman]

I'm wondering if we should hold off on deciding this until the calibration is further along. I recall thinking before that the calibration may require different values at different downstream distances, which may work well if we have a filter function that is downstream-distance-dependent.

[andrew-platt]

For finalizing:

• implement the k_def and k_grad functions from paper (@ebranlard)
• Merge in dev-unstable-pointers to resolve conflicts
• Update all FF test cases to include WAT by default, then regen results
• User documentation (link to paper for now -- maybe pull in pieces from paper)