Adding nacelle drag to AeroDyn

[mayankchetan]

Ready to merge.

Feature or improvement description

This PR adds a simple drag model to account for aerodynamic drag on the nacelle. The implimentation is primarily based on code shared by @bjonkman with the addition of froces and moments in the z-direction. The current implimentation allows the user to define the projected areas of the nacelle in the X, Y, Z diections and assign coefficent of drag (Cd) to each of them. The loads are passed to the nacelle mesh and is added to the nacelle buoyancy loads if activated. Outputs added for nacelle drag as well as total loads on the nacelle.

This replaces PR #1888.

Related issue, if one exists

#162

Impacted areas of the software

AeroDyn module

Additional supporting information

AeroDyn input file updated to include, nacelle projected area, Cd along projected area, and the aerodynamic center where the nacelle drag force is applied.

Test results, if applicable

Below figure shows the calculated nacelle drag in inertial x-axis (blue -> calcualted, orange -> OpenFAST) when inflow velocity HWindSpeed = 1 m/s, AirDens = 1.0, NacArea = [2, 2, 2], and NacCd = [1, 1, 1]. Yaw angle is varied from [180, -180)

few of the new outputs added

Two new OpenFAST test cases are added in PR (OpenFAST/r-test#131) to the r-test repo,

• 5MW_Land_DLL_WTurb_wNacDrag - modified from 5MW_Land_DLL_WTurb to add nacelle drag
• MHK_RM1_Floating_wNacDrag - modified from MHK_RM1_Floating to add nacelle drag

TODO from #1888

• Update api_change.rst for API changes
• Update test files to account for API changes
• Add test for the nacelle drag model
• Link r-test PR
• Document implementation

attn: @hkross, @gdeskos

[andrew-platt]

@hkross

[ebranlard]

This looks great. Would it be possible to add a small section in the documentation of the AeroDyn theory as well?

[hkross]

@hkross

@mayankchetan Looks good! I added a couple of small comments above.

[mayankchetan]

Thank for for the comments @hkross, I have pushed a few changes.