Add detailed documentation to engine nacelles

[joergbrech]

The engine nacelles have been around for a while now, but the documentation only contains an image of a nacelle without a description how to create one with CPACS. We should add more details here. We have the schema, the implementation in TiGL and some CPACS files to start.

[MarAlder]

I added the documentation and an example file:

• Link to online documentation

Documentation

• [x] better descriptions
• [x] engineNacelleType
• [x] rotationCurveType
• [x] nacelleSectionType
• [x] nacelleGuideCurveType
• [x] nacelleCenterCowlType

Example file

• nacelles.xml

Open questions

• [ ] In case of three-dimensional fan blades: is the coordinate center the most forward (in negative streamwise direction) postion or its geometric center?

Derived issues

This does not belong to the present issue, but I want to raise some potential issues which could be discussed in future if necessary:

• The center cowl has no x-scaling. Maybe it would make sense to add it so that the profile can be predefined between 0..1
• There is no overall transformation node. Could it be useful?
• Maybe it makes sense to set the rotationCurve optional?
• guide curves description element should be optional

Review

• [ ] @codingpoets: I think you were involved in developing the nacelles, right? Could you please verify that the documentation correctly reflects the intended approach?
• [x] @joergbrech: Can you check the consistency between the descriptions and the TiGL implementation?
• [ ] @cliersch: Can you check with Patrick whether the descriptions answer the questions you had a few weeks ago?
• [ ] @swoehler: You have already gained some experience with the nacelles. Could you check if the docs are consistent with your understanding/your openAD implementation?

[joergbrech]

Nice! Thanks for the great work @MarAlder!

Regarding your mentioned issues I have no strong opinions.

• We discussed wether the rotation curve should be optional or not, and I think the consensus back than was that it is impossible to build an engine nacelle without a rotationally symmetric interior.
• I also think to remember that we discussed wether we should add x-scaling to the center cowl, I cannot remember why we descided not to do it. It is possible that we just postponed the decision: We wanted to have a working prototype first.

Finally, the nacelle guide curve profiles need some further documentation, or at least I couldn't find it in your links. The guide curve points are interpreted as (r and x) offsets from a cubic polynomial:

This is a bit ugly, but the intuition behind it is that we needed a baseline for the guide curves that works between sections that are positioned at different radial positions. So the baseline is a smooth transition from the radius of the start section to the end section, which is given by the above cubic polynomial.

https://github.com/DLR-SC/tigl/blob/8e1b13dd136cc90184ede4d538f9f0ad039e8fb3/src/engine_nacelle/CTiglNacelleGuideCurveBuilder.cpp#L150-L184

[MarAlder]

Thanks @joergbrech, I added the guide curve description.