Fix pneumatic node zero-flow reversal

app/simulation/components/amesim/junctions/nodes.py

@@ -9,6 +9,24 @@ from app.simulation.core.medium import IdealGasMedium
 from app.simulation.core.ports import PortDefinition, PortState
 
 
+_REFERENCE_OUTFLOW_REGULARIZATION_RATIO = 0.05
+
+
+def _regularized_inverse_outflow(flow: float, transition_flow: float) -> float:
+    """Return a C1 inverse that tends to zero as a negative flow vanishes."""
+
+    if flow >= 0.0:
+        return 0.0
+    transition_flow = max(float(transition_flow), 1.0e-12)
+    if -flow >= transition_flow:
+        return 1.0 / flow
+    return (
+        flow
+        * (2.0 * transition_flow * transition_flow - flow * flow)
+        / transition_flow**4
+    )
+
+
 class _AmesimPneumaticNode(AlgebraicComponent):
     """Shared implementation for AMESim pneumatic junction submodels.
 
@@ -102,7 +120,7 @@ class _AmesimPneumaticNode(AlgebraicComponent):
                 else self.temperature_reference_h
             )
 
-        if reference_port.m_flow < -1e-12:
+        if reference_port.m_flow < 0.0:
             energy_without_reference = sum(
                 port.m_flow
                 * (
@@ -113,8 +131,29 @@ class _AmesimPneumaticNode(AlgebraicComponent):
                 for name, port in self.ports.items()
                 if name != self.REFERENCE_PORT
             )
+            non_reference_flow_scale = sum(
+                abs(port.m_flow)
+                for name, port in self.ports.items()
+                if name != self.REFERENCE_PORT
+            )
+            transition_flow = (
+                _REFERENCE_OUTFLOW_REGULARIZATION_RATIO
+                * non_reference_flow_scale
+            )
+            # Port 2 carries AMESim's residual-energy causality.  Exact
+            # division is singular when its outflow reverses through zero, so
+            # use a C1 band that matches the exact balance at its boundary and
+            # tends to the mixed enthalpy at zero flow.
+            inverse_flow = _regularized_inverse_outflow(
+                reference_port.m_flow,
+                transition_flow,
+            )
+            energy_residual_at_mixed_h = (
+                energy_without_reference
+                + reference_port.m_flow * mixed_h
+            )
             reference_port.h_outflow = (
-                -energy_without_reference / reference_port.m_flow
+                mixed_h - energy_residual_at_mixed_h * inverse_flow
             )