feat(FXC-5154) Warn gmsh min cylinder radii

Author: marc-flex

tests/test_components/test_heat_charge.py

@@ -2789,3 +2789,54 @@ def test_heat_charge_simulation_plot():
         "for CHARGE simulations, resulting in at least 2 more visual elements "
         "than charge_sim.scene.plot()"
     )
+
+
+def test_cylinder_small_radius_warning():
+    """Test that warning is issued for very small cylinder radii in HeatChargeSimulation."""
+    solid = td.MultiPhysicsMedium(
+        heat=td.SolidSpec(conductivity=1, capacity=1),
+        name="solid",
+    )
+    background = td.MultiPhysicsMedium(
+        heat=td.FluidSpec(),
+        name="background",
+    )
+
+    # Test non-tapered cylinder with tiny radius
+    tiny_cylinder = td.Structure(
+        geometry=td.Cylinder(center=(0, 0, 0), radius=1e-8, length=1, axis=2),
+        medium=solid,
+        name="tiny",
+    )
+    with AssertLogLevel("WARNING", contains_str="radius"):
+        _ = td.HeatChargeSimulation(
+            center=(0, 0, 0),
+            size=(2, 2, 2),
+            medium=background,
+            structures=[tiny_cylinder],
+            grid_spec=td.UniformUnstructuredGrid(dl=0.1),
+            monitors=[td.TemperatureMonitor(size=(1, 1, 1), name="tmp")],
+        )
+
+    # Test tapered cylinder with steep sidewall causing negative radius_top
+    tapered_cylinder = td.Structure(
+        geometry=td.Cylinder(
+            center=(0, 0, 0),
+            radius=0.1,
+            length=1,
+            axis=2,
+            sidewall_angle=np.pi / 3,  # 60 degrees - causes negative radius_top
+            reference_plane="bottom",
+        ),
+        medium=solid,
+        name="tapered",
+    )
+    with AssertLogLevel("WARNING", contains_str="radius_top"):
+        _ = td.HeatChargeSimulation(
+            center=(0, 0, 0),
+            size=(2, 2, 2),
+            medium=background,
+            structures=[tapered_cylinder],
+            grid_spec=td.UniformUnstructuredGrid(dl=0.1),
+            monitors=[td.TemperatureMonitor(size=(1, 1, 1), name="tmp")],
+        )

tidy3d/components/tcad/simulation/heat_charge.py

@@ -24,6 +24,8 @@
     StructureStructureInterface,
 )
 from tidy3d.components.geometry.base import Box
+from tidy3d.components.geometry.primitives import Cylinder
+from tidy3d.components.geometry.utils import traverse_geometries
 from tidy3d.components.material.tcad.charge import (
     ChargeConductorMedium,
     SemiconductorMedium,
@@ -127,6 +129,9 @@
 # define some limits for transient heat simulations
 TRANSIENT_HEAT_MAX_STEPS = 1000
 
+# Minimum radius for gmsh meshing (OpenCASCADE tolerance)
+MIN_GMSH_RADIUS = 1e-6
+
 
 class TCADAnalysisTypes(str, Enum):
     """Enumeration of the types of simulations currently supported"""
@@ -363,6 +368,42 @@ def check_unsupported_geometries(cls, val):
                 )
         return val
 
+    @pd.validator("structures", always=True)
+    def _warn_small_cylinder_radius(cls, val):
+        """Warn if any Cylinder geometry has radius too small for meshing."""
+        for structure in val:
+            for geometry in traverse_geometries(structure.geometry):
+                if isinstance(geometry, Cylinder):
+                    r_bottom = geometry.radius_bottom
+                    r_top = geometry.radius_top
+                    is_tapered = r_bottom != r_top
+
+                    # Compute minimum allowed radius (matches backend heat_mesh.py logic)
+                    min_radius = max(MIN_GMSH_RADIUS, 0.01 * max(abs(r_bottom), abs(r_top)))
+
+                    # Warn if radii are below minimum
+                    if is_tapered:
+                        if r_bottom < min_radius:
+                            log.warning(
+                                f"Cylinder 'radius_bottom' ({r_bottom:.3e}) is below the minimum "
+                                f"radius for meshing ({min_radius:.3e}). The sidewall angle may be "
+                                f"too steep. During meshing, this will be clamped to {min_radius:.3e}."
+                            )
+                        if r_top < min_radius:
+                            log.warning(
+                                f"Cylinder 'radius_top' ({r_top:.3e}) is below the minimum "
+                                f"radius for meshing ({min_radius:.3e}). The sidewall angle may be "
+                                f"too steep. During meshing, this will be clamped to {min_radius:.3e}."
+                            )
+                    else:
+                        if r_bottom < min_radius:
+                            log.warning(
+                                f"Cylinder 'radius' ({r_bottom:.3e}) is below the minimum "
+                                f"radius for meshing ({min_radius:.3e}). "
+                                f"During meshing, this will be clamped to {min_radius:.3e}."
+                            )
+        return val
+
     @staticmethod
     def _check_cross_solids(objs: tuple[Box, ...], values: dict) -> tuple[int, ...]:
         """Given model dictionary ``values``, check whether objects in list ``objs`` cross