Harden boundary_condition_slip_wall for Euler

[jlchan]

The implementation of boundary_condition_slip_wall for Euler takes the power of an expression when computing the intermediate pressure state.

Trixi.jl/src/equations/compressible_euler_1d.jl

Lines 257 to 259 in 12f0483

	p_star = p_local *
	(1 + 0.5f0 * (equations.gamma - 1) * v_normal / sound_speed)^(2 *
	equations.gamma *

However, this can be negative, resulting in an error being thrown. This PR hardens this by setting the intermediate pressure state to zero when this happens. This hardening retains entropy stability based on the analysis in https://arxiv.org/abs/1901.04924

This modification was necessary for running the Mach 3 cylinder with no shock capturing and Liu-Zhang positivity limiting.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.84%. Comparing base (12f0483) to head (0c4d6e3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3137   +/-   ##
=======================================
  Coverage   96.84%   96.84%           
=======================================
  Files         662      662           
  Lines       51197    51206    +9     
=======================================
+ Hits        49579    49588    +9     
  Misses       1618     1618           

Flag	Coverage Δ
unittests	96.84% <100.00%> (+<0.01%)	⬆️

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[DanielDoehring]

Interesting that this did not cause problems before - if this goes below zero, its means that v_normal is quite negative, right? This somewhat smells a bit like that the boundary condition is not imposed "strong enough" (due to a too-coarse mesh)?

[andrewwinters5000]

Thanks for adding this fallback in the limiting vacuum situation.

[jlchan]

Interesting that this did not cause problems before - if this goes below zero, its means that v_normal is quite negative, right? This somewhat smells a bit like that the boundary condition is not imposed "strong enough" (due to a too-coarse mesh)?

Yeah, v_normal would have to be larger relative to local Mach number. I think shock capturing tended to control things in previous cases. It is a bit odd to me that the naive mirror state imposition would just return (0, p * nx, p * ny, 0) independently of v_normal though.

[DanielDoehring]

It is a bit odd to me that the naive mirror state imposition would just return (0, p * nx, p * ny, 0) independently of v_normal though.

Yeah that is also what I thought - maybe somewhere in the textbook some assumptions on v_normal are made