fix: align PNL0003 Reynolds diagnostic with AMESim

app/simulation/components/amesim/media/mediums.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from collections.abc import Callable, Sequence
 from dataclasses import dataclass
-from math import isfinite
+from math import exp, isfinite, log
 from typing import ClassVar
 
 from app.simulation.core.errors import RecoverableTrialStateError
@@ -55,6 +55,12 @@ class AmesimHeliumPengRobinsonMedium(IdealGasMedium):
     fluid: ClassVar[PengRobinsonFluid] = HELIUM_PR
     nasa_cp_over_R: ClassVar[float] = 2.5
     nasa_enthalpy_constant_K: ClassVar[float] = -745.375
+    nasa_viscosity_coefficients: ClassVar[tuple[float, float, float, float]] = (
+        0.7501594,
+        35.76324,
+        -2212.129,
+        0.9212635,
+    )
 
     name: str = "AMESimHeliumPengRobinson"
     R_gas: float = HELIUM_PR.specific_gas_constant
@@ -73,6 +79,19 @@ class AmesimHeliumPengRobinsonMedium(IdealGasMedium):
         del T
         return self.cv
 
+    def diagnostic_dynamic_viscosity(self, T: float) -> float:
+        """Return the AMESim NASA-table viscosity used by pipe diagnostics.
+
+        pn2pipefr reports Reynolds number with sagum viscosity. Keep this
+        separate from dynamic_viscosity so matching that diagnostic cannot
+        alter the already-validated pipe flow or friction dynamics.
+        """
+
+        if T <= 0.0:
+            raise ValueError("Temperature must be positive.")
+        a, b, c, d = self.nasa_viscosity_coefficients
+        return 1.0e-7 * exp(a * log(T) + b / T + c / (T * T) + d)
+
     @profile_property("density")
     @cache_property_calculation("density")
     def density(self, p: float, T: float) -> float: