Fix pneumatic node zero-flow reversal

tests/test_mechanical_solver_causalization.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from math import exp
+from math import exp, isfinite
 import unittest
 
 from app.simulation.components.amesim.mechanical.translational import (
@@ -130,7 +130,93 @@ def _single_mass_system(
     return GenericFluidSystem(network), mass
 
 
+def _high_stiffness_contact_impact_system() -> GenericFluidSystem:
+    """Return the reduced mechanical core of the four-branch stop impact.
+
+    The production model has four 50 kg piston groups coupled through LSTP00A
+    contacts to one 170000 kg support group.  When the support reaches its
+    ideal upper stop, its velocity is reset while each contact still carries
+    the finite, extremely fast LSTP damping transient.  One branch is enough
+    to retain that stiffness and state-transition interaction in a fast test.
+    """
+
+    medium = IdealGasMedium()
+    zero_left = AmesimF000("zero_left")
+    moving_mass = AmesimMecmas21(
+        "moving_mass",
+        medium,
+        mass=50.0,
+        useFriction=1.0,
+        stoptype=4.0,
+        x0=0.0090025,
+        v0=1.0,
+    )
+    contact = AmesimLstp00a(
+        "contact",
+        medium,
+        gap0=0.0,
+        kcont=1.0e11,
+        rcont=1.0e11,
+        Pdis=1.0e-7,
+        discContactOption=1.0,
+    )
+    support_mass = AmesimMecmas21(
+        "support_mass",
+        medium,
+        mass=170000.0,
+        useFriction=1.0,
+        stoptype=1.0,
+        xmin=0.0,
+        xmax=0.01,
+        x0=0.009,
+        v0=1.0,
+    )
+    zero_right = AmesimF000("zero_right")
+
+    network = SimulationNetwork("high-stiffness-contact-impact")
+    for component in (
+        zero_left,
+        moving_mass,
+        contact,
+        support_mass,
+        zero_right,
+    ):
+        network.add_component(component)
+    network.connect("zero_left", "port_1", "moving_mass", "port_1")
+    network.connect("moving_mass", "port_2", "contact", "port_1")
+    network.connect("contact", "port_2", "support_mass", "port_1")
+    network.connect("support_mass", "port_2", "zero_right", "port_1")
+    return GenericFluidSystem(network)
+
+
 class MechanicalSolverCausalizationTests(unittest.TestCase):
+    def test_high_stiffness_lstp_survives_ideal_stop_transition(self) -> None:
+        system = _high_stiffness_contact_impact_system()
+
+        result = system.simulate(
+            SolveIVPConfig(
+                t_start=0.0,
+                t_stop=0.002,
+                max_step=0.002,
+                method="BDF",
+                rtol=1.0e-6,
+            ),
+            sample_step=0.00025,
+        )
+
+        self.assertTrue(result.success, result.message)
+        totals = result.diagnostics["integration"]["totals"]
+        self.assertEqual(totals["stateTransitionCount"], 1)
+        self.assertEqual(totals["solverStartCount"], 2)
+        self.assertEqual(totals["recoverableRetryCount"], 0)
+
+        self.assertAlmostEqual(result.series["support_mass.x"][-1], 0.01)
+        self.assertAlmostEqual(result.series["support_mass.v"][-1], 0.0, delta=1.0e-9)
+        self.assertAlmostEqual(result.series["moving_mass.v"][-1], 0.0, delta=1.0e-4)
+        self.assertGreater(max(result.series["contact.force"]), 1.0e10)
+        for values in result.series.values():
+            self.assertTrue(all(isfinite(value) for value in values))
+
     def test_lstp_contact_uses_exponential_damping_ramp_and_negative_force_option(
         self,
     ) -> None: