Fix the internal energy used by the under-relaxation factor

[rois1995]

Proposed Changes

The implicit compressible solver limits the update of each point with an under-relaxation factor. To decide how much to limit, it compares the internal energy before and after the update. This internal energy was computed wrongly: the kinetic energy was not divided by the density.

SU2/SU2_CFD/src/solvers/CEulerSolver.cpp

Lines 2555 to 2556 in a6b7496

	e_old -= 0.5 * pow(nodes->GetSolution(iPoint, jVar), 2);
	e_new -= 0.5 * pow(nodes->GetSolution(iPoint, jVar) + LinSysSol(iPoint, jVar), 2);

In cases run with dimensional values the wrong internal energy can become negative. When it does, the point gets no update at all and stays frozen. In an air nozzle about a quarter of the points were frozen, so the case could not converge.

The fix is 5 lines: the kinetic energy is divided by the density, using the old and the new density.

Almost all compressible implicit regression cases converge a little differently, so most regression values change. They are updated from the CI.

This is the first of four PRs that replace #2941, to make the review easier. The other three build on this one:

• [WIP] Fix the HLLC scheme on moving grids and its Jacobians #2943: HLLC on moving grids;
• Fix L2Roe, LMRoe and the general-gas Roe scheme in 3D #2944: Roe-type schemes in 3D;
• Fix the incompressible moving-grid Jacobian and several smaller mean-flow bugs #2945: smaller fixes.

Related Work

• Detect non-physical solutions in a statistic way instead of just P or T <= 0 #2830 added the under-relaxation factor.
• Fix mean-flow solver bugs: under-relaxation, HLLC on moving grids, moving-grid Jacobians and others #2941 (replaced by this PR and the three that follow).

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LEL91DW5WPbPwgFtCvHga6

[rois1995]

Test cases

Both cases: axisymmetric air nozzle, dimensional, 2 MPI ranks, develop against this PR. Files and scripts: pr1 (see the README).

Restart from a converged solution (SST). The start solution was converged with this PR. Develop does not stay converged: the density residual rises from -12 to -10.2 in 300 iterations, because the points with a "negative internal energy" are frozen. With this PR the residual stays at -12 and the run stops after 10 iterations.

Start from rest (SA, CFL adaptation). Develop stays at a density residual of -2 for 20 000 iterations, with a wrong outlet pressure (5.1 bar). With this PR it reaches -11 in 11 647 iterations (outlet pressure 1.64 bar).

[bigfooted]

@rois1995 if this PR is finished can you update the regression tests? looks like it's a small PR that can be merged soon.