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 42.85714% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.68%. Comparing base (7c42535) to head (0b1d1df).

Files with missing lines	Patch %	Lines
src/evolve_pressure.cxx	45.00%	11 Missing ⚠️
src/neutral_full_velocity.cxx	0.00%	10 Missing ⚠️
src/evolve_momentum.cxx	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #359   +/-   ##
=======================================
  Coverage   38.68%   38.68%           
=======================================
  Files          94       94           
  Lines        9285     9279    -6     
  Branches     1333     1330    -3     
=======================================
- Hits         3592     3590    -2     
+ Misses       5310     5306    -4     
  Partials      383      383           

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