Fluid equation of state with density floor

[bendudson]

A relatively simple fix that seems to greatly help reduce unphysical behavior near density floors.

Defines the internal energy E as E = Cv * Nlim * T rather than the usual Cv * N * T. This effectively puts a floor on the heat capacity of the fluid. Then solve the pressure equation consistent with this equation of state.

Changes made to pressure equations in evolve_pressure, neutral_mixed and neutral_full_velocity. Unfortunately I ran clang-format on the files so the changes seem numerous. The actual change is quite small: In the pressure advection replace (5/3)P with Nlim * T + (2/3) * P.

This is an example from the midplane of a DIII-D simulation using neutral_full_velocity. The density floor is 1e-5, and previously produced quite unphysical behavior.

[mikekryjak]

@bendudson this is awesome. Looking at evolve_pressure only for now: am I understanding correctly that this in effect makes the density floor in use everywhere consistently, whereas before it was only used where we divided by zero? And P_solver is the pressure without floors that the solver sees, while P is the pressure calculated using floored density?

What about flooring pressure itself? I see that now it's only flooring the density.

I raised an issue to put clang-format in the CI: #360

[bendudson]

Hey @mikekryjak . Thanks! The solver now evolves a modified internal energy P_solver = N_lim T, which is divided by N_lim to get temperature, and then the pressure is P = N T. This is why neutral pressure can continue to fall into the core, even when temperature is increasing and the density has hit the floor: the internal energy P_solver is going up into the core, while pressure P is going down.

[codecov]

Codecov Report

❌ Patch coverage is 6.66667% with 126 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.20%. Comparing base (e027256) to head (66da9ad).

Files with missing lines	Patch %	Lines
src/neutral_mixed.cxx	0.00%	101 Missing ⚠️
src/evolve_pressure.cxx	42.85%	12 Missing ⚠️
src/neutral_full_velocity.cxx	0.00%	10 Missing ⚠️
src/evolve_momentum.cxx	0.00%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (e027256) and HEAD (66da9ad). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (e027256)	HEAD (66da9ad)
integration	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #359       +/-   ##
===========================================
- Coverage   40.45%   19.20%   -21.25%     
===========================================
  Files          89       87        -2     
  Lines        8236     8069      -167     
  Branches     1170     1143       -27     
===========================================
- Hits         3332     1550     -1782     
- Misses       4506     6366     +1860     
+ Partials      398      153      -245     

Flag	Coverage Δ
integration	?
unit	19.20% <6.66%> (-0.06%)	⬇️

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