Fix Viterna extrapolation bug to avoid repeated data

[abhineet-gupta]

Purpose

Fix a small bug in Viterna extrapolation code where if alpha_low == -alpha_high, then the code repeats the data corresponding to alpha_low in the resulting extrapolation. This causes OpenFAST to error out due to duplicate alpha in the airfoil polar files.

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which
  adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Try the Viterna extension with this code (rename code to .py) and with this file in the same folder.

Checklist

• I have run existing tests which pass locally with my changes
• I have added new tests or examples that prove my fix is effective or that my feature works
• I have added necessary documentation

[ebranlard]

Hi @abhineet-gupta
Thanks for doing that. I'm happy to merge it as it is. If you want to add a test you could add a test here:
https://github.com/OpenFAST/python-toolbox/tree/main/pyFAST/airfoils/tests

For instance adding test_extrapolation.py and adding your airfoil file here:
https://github.com/OpenFAST/python-toolbox/tree/main/pyFAST/airfoils/data

But maybe there is no need to add an additional airfoil file, you could use one already in the repository, or set one up using a sine function (with its argument scaled a bit) or a simple linear function.

Preferably you would add a test to verify that the values of your extrapolated polar are as expected (for instance that the original data is the same and the the extrapolated data at few angles of attack match what you would expect).

Let me know if you have any questions

[ebranlard]

I'll merge this for now. Please feel free to add tests later on.