ExB shear and momentum transport implementation details

[theo-brown]

Starting a thread to discuss the rotation and momentum transport implementation. Following discussions on Thursday, I think @jcitrin @hamelphi wanted to cross-check how some of the terms were calculated in JETTO?

@fcasson will probably be more useful in this than me. We can also arrange a call if that would be more useful?

[fcasson]

I would try to separate two things here

1. Implementation of v_ExB flow shear into QLKNN and TGLFNN input, computed from radial force balance. Three terms contribute to this

• a) Toroidal flow (taken as an input, or computed from the toroidal momentum transport equation). This usually dominates in the core
• b) Poloidal flow (usually insignificant, and requires a neoclassical model)
• c) Pressure gradient (can be significant in transport barriers)

2. Implementation of a momentum transport equation

The latter is likely to be significantly more challenging, and the TGLFNN at present does not have any momentum transport anyway. For me the priority should be 1a) and then 1c). Strictly speaking, only 1a) is consistent with typical GK ordering and is the only one consistent with the TGLF documentation, more here. Nevertheless a lot of the community are doing 1a,b,c) anyway.