`compute_flux_q` passes total pressure to hybrid momentum flux in radiation mode

Summary

In the HYBRID_MOMENTUM branch of compute_flux_q, the state passed to compute_hybrid_flux uses qint.p (total pressure) for GDPRES, while the surrounding radiation flux logic uses gas pressure (qint.p_g) for hydro momentum/energy fluxes. This creates an inconsistent pressure source in hybrid momentum flux construction.

Location

• Source/hydro/riemann_solvers.H:125
• Source/hydro/riemann_solvers.H:136
• Source/hydro/riemann_solvers.H:206

Problem Details

In radiation mode, the base flux is built as:

F(..., im1) += qint.p_g;
F(..., UEDEN) = qint.un * (rhoetot + qint.p_g);

But the hybrid override uses:

qgdnv_zone[GDPRES] = qint.p;
compute_hybrid_flux(qgdnv_zone, ...);

compute_hybrid_flux uses state[GDPRES] directly in pressure-force terms, so this substitutes total pressure into hybrid momentum fluxes while non-hybrid
hydro terms are gas-pressure based. The same routine later stores reduced Godunov state with GDPRES = qint.p_g under RADIATION, which further indicates the
intended field is gas pressure.

Impact

• In RADIATION + HYBRID_MOMENTUM builds, hybrid momentum components can include a pressure contribution inconsistent with the rest of the hydro flux update.
• Can introduce direction-dependent momentum/energy mismatch and non-physical force balance when radiation pressure is significant.

Suggested Patch

diff --git a/Source/hydro/riemann_solvers.H b/Source/hydro/riemann_solvers.H
--- a/Source/hydro/riemann_solvers.H
+++ b/Source/hydro/riemann_solvers.H
@@
-    qgdnv_zone[GDPRES] = qint.p;
+#ifdef RADIATION
+    qgdnv_zone[GDPRES] = qint.p_g;
+#else
+    qgdnv_zone[GDPRES] = qint.p;
+#endif

Prepared by Codex

Read Source/hydro/riemann_solvers.H at the compute_flux_q and compute_hybrid_flux locations listed in the issue. Compare the GDPRES source used by the HYBRID_MOMENTUM branch with the surrounding radiation-mode flux construction. Done means radiation-mode hybrid momentum fluxes use the pressure source intended by the surrounding hydro flux logic without changing non-radiation behavior.