Adding ionization capabilities for viscous simulations (NEMO)

[WallyMaier]

Proposed Changes

This PR adds the capability to simulate the AIR-7 gas model using SU2TCLib for viscous problems. Specific contributions include:

• Fix Gupta-Yos transport models for all flows (including ionized)
• inclusion of Coulomb interaction cross sections for ion-ion collisions

Following the implementation detailed in Scalabrin's thesis.

Related Work

PRs #1497 #1555

Update to NEMO documentation includes description of GY model, see: su2code/su2code.github.io#116

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[jtneedels]

Reading Scalabrin, he says the WBE transport model breaks down at Ttr > 10,000 K, and he uses the GY model for incorporating ionization in LeMANS. So I think the roadmap to for viscous/ionized flows is:

1. Implement GY model according to Scalabrin wth Coulomb potential for ion-ion collisions
2. Add some flag to make sure transport model for viscous-ionized is always GY

[WallyMaier]

The path forward sounds good. Gupta-Yos needs to be reworked/fixed :(

[jtneedels]

The path forward sounds good. Gupta-Yos needs to be reworked/fixed :(

Yeah, I'm working on that now. I think one way to do this would be to code in whether to use the Coulomb potential using the omega00 tables for each collision pair. For example, if omega00(ispecies, jspecies, 0) == 1.0, then use Coulomb. We could use 1.0, or some other non-physical value.

[jtneedels]

An update: I re-worked this so the computation of the delta_ij values (used in each of the GY functions) is itself a function to remove repeated code. It is now also set up to handle ionized flows. Use of the Coulombic potential will signaled by the omega00 and omega11 values. Forward work is ensuring the correct collision integral curve fit values are used, modifying the omega00 and omega11 tables.

[jtneedels]

Okay, all tests should be passing (except some random new codefactor issues). Main issue with the GY models hanging is disambiguating which matrix of collision integral (omega) values to use, Scalabrin's notation is a bit unclear.

Are there other places we need to modify for including electrons? Anything in viscous numerics?