From a058736d3ec7cb01ab804a69897248ce7d01d272 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Thu, 24 Jul 2025 17:53:19 +0200
Subject: [PATCH 1/9] FEAT: Add natural convection BC

---
 tidy3d/__init__.py                      |   3 +
 tidy3d/components/tcad/boundary/heat.py | 101 +++++++++++++++++++++++-
 tidy3d/components/tcad/types.py         |   2 +-
 3 files changed, 104 insertions(+), 2 deletions(-)

diff --git a/tidy3d/__init__.py b/tidy3d/__init__.py
index a3993e3863..f7bf678566 100644
--- a/tidy3d/__init__.py
+++ b/tidy3d/__init__.py
@@ -68,6 +68,7 @@
     CaugheyThomasMobility,
     ConstantMobilityModel,
     ConvectionBC,
+    NaturalConvectionVerticalSpec,
     CurrentBC,
     HeatFluxBC,
     HeatFromElectricSource,
@@ -405,6 +406,7 @@ def set_logging_level(level: str) -> None:
 Transformed.update_forward_refs()
 ClipOperation.update_forward_refs()
 GeometryGroup.update_forward_refs()
+ConvectionBC.update_forward_refs()
 
 __all__ = [
     "C_0",
@@ -713,4 +715,5 @@ def set_logging_level(level: str) -> None:
     "set_logging_console",
     "set_logging_file",
     "wavelengths",
+    "NaturalConvectionVerticalSpec",
 ]
diff --git a/tidy3d/components/tcad/boundary/heat.py b/tidy3d/components/tcad/boundary/heat.py
index 9671dee6a9..e9fc097b59 100644
--- a/tidy3d/components/tcad/boundary/heat.py
+++ b/tidy3d/components/tcad/boundary/heat.py
@@ -4,6 +4,7 @@
 
 import pydantic.v1 as pd
 
+from typing import Union
 from tidy3d.components.tcad.boundary.abstract import HeatChargeBC
 from tidy3d.constants import HEAT_FLUX, HEAT_TRANSFER_COEFF, KELVIN
 
@@ -39,6 +40,100 @@ class HeatFluxBC(HeatChargeBC):
         units=HEAT_FLUX,
     )
 
+#TODO Should I adapt to Tidy3D units (e.g. micrometers), or keep the original ones?
+THERMAL_CONDUCTIVITY_UNITS = "W/(m*K)"
+DYNAMIC_VISCOSITY_UNITS = "Pa*s"
+SPECIFIC_HEAT_UNITS = "J/(kg*K)"
+DENSITY_UNITS = "kg/m**3"
+THERMAL_EXPANSIVITY_UNITS = "1/K"
+LENGTH_UNITS = "m"
+ACCELERATION_UNITS = "m/s**2"
+
+class NaturalConvectionVerticalSpec(HeatChargeBC):
+    """
+    Specification for natural convection from a vertical plate.
+
+    This class calculates the heat transfer coefficient (h) based on fluid
+    properties and an expected temperature difference, then provides these
+    values as 'base' and 'exponent' for a generalized heat flux equation
+    q = base * (T_surf - T_fluid)^exponent.
+    """
+    # --- Input Parameters ---
+    fluid_k: pd.NonNegativeFloat = pd.Field(
+        title="Fluid Thermal Conductivity",
+        description="Thermal conductivity (k) of the fluid.",
+        units=THERMAL_CONDUCTIVITY_UNITS,
+    )
+    fluid_mu: pd.NonNegativeFloat = pd.Field(
+        title="Fluid Dynamic Viscosity",
+        description="Dynamic viscosity (μ) of the fluid.",
+        units=DYNAMIC_VISCOSITY_UNITS,
+    )
+    fluid_Cp: pd.NonNegativeFloat = pd.Field(
+        title="Fluid Specific Heat",
+        description="Specific heat capacity (Cp) of the fluid at constant pressure.",
+        units=SPECIFIC_HEAT_UNITS,
+    )
+    fluid_rho: pd.NonNegativeFloat = pd.Field(
+        title="Fluid Density",
+        description="Density (ρ) of the fluid.",
+        units=DENSITY_UNITS,
+    )
+    fluid_beta: pd.NonNegativeFloat = pd.Field(
+        title="Fluid Thermal Expansivity",
+        description="Thermal expansion coefficient (β) of the fluid.",
+        units=THERMAL_EXPANSIVITY_UNITS,
+    )
+    plate_L: pd.NonNegativeFloat = pd.Field(
+        title="Plate Characteristic Length",
+        description="Characteristic length (L), defined as the height of the vertical plate.",
+        units=LENGTH_UNITS,
+    )
+
+    gravity: pd.NonNegativeFloat = pd.Field(
+        default=9.80665,
+        title="Gravitational Acceleration",
+        description="Gravitational acceleration (g).",
+        units=ACCELERATION_UNITS,
+    )
+
+    def _compute_parameters(self):
+
+        # Calculate the Rayleigh Number (Ra_L)
+        rayleigh_numerator_notemp = (
+                self.gravity
+                * self.fluid_beta
+                * self.fluid_rho ** 2
+                * self.fluid_Cp
+                * self.plate_L ** 3
+        )
+        rayleigh_denominator = self.fluid_mu * self.fluid_k
+        Ra_L = rayleigh_numerator_notemp / rayleigh_denominator
+
+        # Calculate the denominator term from the Nusselt number correlation
+        # This term is related to the Prandtl Number (Pr = mu * Cp / k)
+        pr_term_inner_num = 0.492 * self.fluid_k
+        pr_term_inner_den = self.fluid_mu * self.fluid_Cp
+
+        pr_term_inner = pr_term_inner_num / pr_term_inner_den
+        pr_denominator = (1 + pr_term_inner ** (9 / 16)) ** (4 / 9)
+
+        # Select formula based on flow regime (determined by Ra_L)
+        if Ra_L <= 1e9:
+            # Laminar Flow
+            h_factor_linear = 0.68
+            h_factor_non_linear =  (0.670 * Ra_L ** (1 / 6)) / pr_denominator
+        elif 1e9 < Ra_L <= 1e13:
+            # Turbulent Flow
+            h_factor_linear = 0.825
+            h_factor_non_linear = (0.387 * Ra_L ** (1 / 6)) / pr_denominator
+        else:
+            raise ValueError(f"Ra_l={Ra_L} should be smaller than 1e13 for NaturalConvectionVerticalSpec")
+
+        h = (self.fluid_k / self.plate_L) * h_factor_linear
+        h_nonlinear = (self.fluid_k / self.plate_L) * h_factor_non_linear
+        exponent = 1 + 1/6
+        return h, h_nonlinear, exponent
 
 class ConvectionBC(HeatChargeBC):
     """Convective thermal boundary conditions.
@@ -55,8 +150,12 @@ class ConvectionBC(HeatChargeBC):
         units=KELVIN,
     )
 
-    transfer_coeff: pd.NonNegativeFloat = pd.Field(
+    transfer_coeff: Union[pd.NonNegativeFloat, NaturalConvectionVerticalSpec] = pd.Field(
         title="Heat Transfer Coefficient",
         description=f"Heat flux value in units of {HEAT_TRANSFER_COEFF}.",
         units=HEAT_TRANSFER_COEFF,
     )
+
+#TODO Maybe I should find a better place for these lines.
+NaturalConvectionVerticalSpec.update_forward_refs()
+ConvectionBC.update_forward_refs()
diff --git a/tidy3d/components/tcad/types.py b/tidy3d/components/tcad/types.py
index dec3f44f62..79fd754f55 100644
--- a/tidy3d/components/tcad/types.py
+++ b/tidy3d/components/tcad/types.py
@@ -4,7 +4,7 @@
 
 from tidy3d.components.tcad.bandgap import SlotboomBandGapNarrowing
 from tidy3d.components.tcad.boundary.charge import CurrentBC, InsulatingBC, VoltageBC
-from tidy3d.components.tcad.boundary.heat import ConvectionBC, HeatFluxBC, TemperatureBC
+from tidy3d.components.tcad.boundary.heat import ConvectionBC, HeatFluxBC, TemperatureBC, NaturalConvectionVerticalSpec
 from tidy3d.components.tcad.generation_recombination import (
     AugerRecombination,
     RadiativeRecombination,

From bbe0d31e5189236d2f0e6a7629199a3d9c482858 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Thu, 24 Jul 2025 17:59:57 +0200
Subject: [PATCH 2/9] Fixing formatting and minor error

---
 tidy3d/components/tcad/boundary/heat.py | 24 +++++++++++++-----------
 tidy3d/components/tcad/types.py         |  7 ++++++-
 2 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/tidy3d/components/tcad/boundary/heat.py b/tidy3d/components/tcad/boundary/heat.py
index e9fc097b59..22e3567691 100644
--- a/tidy3d/components/tcad/boundary/heat.py
+++ b/tidy3d/components/tcad/boundary/heat.py
@@ -40,7 +40,8 @@ class HeatFluxBC(HeatChargeBC):
         units=HEAT_FLUX,
     )
 
-#TODO Should I adapt to Tidy3D units (e.g. micrometers), or keep the original ones?
+
+# TODO Should I adapt to Tidy3D units (e.g. micrometers), or keep the original ones?
 THERMAL_CONDUCTIVITY_UNITS = "W/(m*K)"
 DYNAMIC_VISCOSITY_UNITS = "Pa*s"
 SPECIFIC_HEAT_UNITS = "J/(kg*K)"
@@ -49,6 +50,7 @@ class HeatFluxBC(HeatChargeBC):
 LENGTH_UNITS = "m"
 ACCELERATION_UNITS = "m/s**2"
 
+
 class NaturalConvectionVerticalSpec(HeatChargeBC):
     """
     Specification for natural convection from a vertical plate.
@@ -58,6 +60,7 @@ class NaturalConvectionVerticalSpec(HeatChargeBC):
     values as 'base' and 'exponent' for a generalized heat flux equation
     q = base * (T_surf - T_fluid)^exponent.
     """
+
     # --- Input Parameters ---
     fluid_k: pd.NonNegativeFloat = pd.Field(
         title="Fluid Thermal Conductivity",
@@ -98,14 +101,9 @@ class NaturalConvectionVerticalSpec(HeatChargeBC):
     )
 
     def _compute_parameters(self):
-
         # Calculate the Rayleigh Number (Ra_L)
         rayleigh_numerator_notemp = (
-                self.gravity
-                * self.fluid_beta
-                * self.fluid_rho ** 2
-                * self.fluid_Cp
-                * self.plate_L ** 3
+            self.gravity * self.fluid_beta * self.fluid_rho**2 * self.fluid_Cp * self.plate_L**3
         )
         rayleigh_denominator = self.fluid_mu * self.fluid_k
         Ra_L = rayleigh_numerator_notemp / rayleigh_denominator
@@ -122,19 +120,22 @@ def _compute_parameters(self):
         if Ra_L <= 1e9:
             # Laminar Flow
             h_factor_linear = 0.68
-            h_factor_non_linear =  (0.670 * Ra_L ** (1 / 6)) / pr_denominator
+            h_factor_non_linear = (0.670 * Ra_L ** (1 / 6)) / pr_denominator
         elif 1e9 < Ra_L <= 1e13:
             # Turbulent Flow
             h_factor_linear = 0.825
             h_factor_non_linear = (0.387 * Ra_L ** (1 / 6)) / pr_denominator
         else:
-            raise ValueError(f"Ra_l={Ra_L} should be smaller than 1e13 for NaturalConvectionVerticalSpec")
+            raise ValueError(
+                f"Ra_L={Ra_L} should be smaller than 1e13 for NaturalConvectionVerticalSpec"
+            )
 
         h = (self.fluid_k / self.plate_L) * h_factor_linear
         h_nonlinear = (self.fluid_k / self.plate_L) * h_factor_non_linear
-        exponent = 1 + 1/6
+        exponent = 1 + 1 / 6
         return h, h_nonlinear, exponent
 
+
 class ConvectionBC(HeatChargeBC):
     """Convective thermal boundary conditions.
 
@@ -156,6 +157,7 @@ class ConvectionBC(HeatChargeBC):
         units=HEAT_TRANSFER_COEFF,
     )
 
-#TODO Maybe I should find a better place for these lines.
+
+# TODO Maybe I should find a better place for these lines.
 NaturalConvectionVerticalSpec.update_forward_refs()
 ConvectionBC.update_forward_refs()
diff --git a/tidy3d/components/tcad/types.py b/tidy3d/components/tcad/types.py
index 79fd754f55..38cb64c74b 100644
--- a/tidy3d/components/tcad/types.py
+++ b/tidy3d/components/tcad/types.py
@@ -4,7 +4,12 @@
 
 from tidy3d.components.tcad.bandgap import SlotboomBandGapNarrowing
 from tidy3d.components.tcad.boundary.charge import CurrentBC, InsulatingBC, VoltageBC
-from tidy3d.components.tcad.boundary.heat import ConvectionBC, HeatFluxBC, TemperatureBC, NaturalConvectionVerticalSpec
+from tidy3d.components.tcad.boundary.heat import (
+    ConvectionBC,
+    HeatFluxBC,
+    TemperatureBC,
+    NaturalConvectionVerticalSpec,
+)
 from tidy3d.components.tcad.generation_recombination import (
     AugerRecombination,
     RadiativeRecombination,

From 4e3013135e3ab74ff8fcdaafe872cfe9bd1d4c6c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Mon, 28 Jul 2025 19:18:43 +0200
Subject: [PATCH 3/9] Further improvements

---
 tidy3d/components/tcad/boundary/heat.py | 90 +++++++++++++++++++------
 tidy3d/constants.py                     | 20 ++++++
 2 files changed, 89 insertions(+), 21 deletions(-)

diff --git a/tidy3d/components/tcad/boundary/heat.py b/tidy3d/components/tcad/boundary/heat.py
index 22e3567691..921f898031 100644
--- a/tidy3d/components/tcad/boundary/heat.py
+++ b/tidy3d/components/tcad/boundary/heat.py
@@ -2,11 +2,23 @@
 
 from __future__ import annotations
 
+from typing import Union
+
 import pydantic.v1 as pd
 
-from typing import Union
 from tidy3d.components.tcad.boundary.abstract import HeatChargeBC
-from tidy3d.constants import HEAT_FLUX, HEAT_TRANSFER_COEFF, KELVIN
+from tidy3d.constants import (
+    ACCELERATION,
+    DENSITY,
+    DYNAMIC_VISCOSITY,
+    HEAT_FLUX,
+    HEAT_TRANSFER_COEFF,
+    KELVIN,
+    MICROMETER,
+    SPECIFIC_HEAT,
+    THERMAL_CONDUCTIVITY,
+    THERMAL_EXPANSIVITY,
+)
 
 
 class TemperatureBC(HeatChargeBC):
@@ -41,16 +53,6 @@ class HeatFluxBC(HeatChargeBC):
     )
 
 
-# TODO Should I adapt to Tidy3D units (e.g. micrometers), or keep the original ones?
-THERMAL_CONDUCTIVITY_UNITS = "W/(m*K)"
-DYNAMIC_VISCOSITY_UNITS = "Pa*s"
-SPECIFIC_HEAT_UNITS = "J/(kg*K)"
-DENSITY_UNITS = "kg/m**3"
-THERMAL_EXPANSIVITY_UNITS = "1/K"
-LENGTH_UNITS = "m"
-ACCELERATION_UNITS = "m/s**2"
-
-
 class NaturalConvectionVerticalSpec(HeatChargeBC):
     """
     Specification for natural convection from a vertical plate.
@@ -65,41 +67,87 @@ class NaturalConvectionVerticalSpec(HeatChargeBC):
     fluid_k: pd.NonNegativeFloat = pd.Field(
         title="Fluid Thermal Conductivity",
         description="Thermal conductivity (k) of the fluid.",
-        units=THERMAL_CONDUCTIVITY_UNITS,
+        units=THERMAL_CONDUCTIVITY,
     )
     fluid_mu: pd.NonNegativeFloat = pd.Field(
         title="Fluid Dynamic Viscosity",
         description="Dynamic viscosity (μ) of the fluid.",
-        units=DYNAMIC_VISCOSITY_UNITS,
+        units=DYNAMIC_VISCOSITY,
     )
     fluid_Cp: pd.NonNegativeFloat = pd.Field(
         title="Fluid Specific Heat",
-        description="Specific heat capacity (Cp) of the fluid at constant pressure.",
-        units=SPECIFIC_HEAT_UNITS,
+        description="Specific heat of the fluid at constant pressure.",
+        units=SPECIFIC_HEAT,
     )
     fluid_rho: pd.NonNegativeFloat = pd.Field(
         title="Fluid Density",
         description="Density (ρ) of the fluid.",
-        units=DENSITY_UNITS,
+        units=DENSITY,
     )
     fluid_beta: pd.NonNegativeFloat = pd.Field(
         title="Fluid Thermal Expansivity",
         description="Thermal expansion coefficient (β) of the fluid.",
-        units=THERMAL_EXPANSIVITY_UNITS,
+        units=THERMAL_EXPANSIVITY,
     )
     plate_L: pd.NonNegativeFloat = pd.Field(
         title="Plate Characteristic Length",
         description="Characteristic length (L), defined as the height of the vertical plate.",
-        units=LENGTH_UNITS,
+        units=MICROMETER,
     )
 
     gravity: pd.NonNegativeFloat = pd.Field(
-        default=9.80665,
+        default=9.80665 * 1e6,
         title="Gravitational Acceleration",
         description="Gravitational acceleration (g).",
-        units=ACCELERATION_UNITS,
+        units=ACCELERATION,
     )
 
+    def from_si_units(
+        fluid_k: pd.NonNegativeFloat,
+        fluid_mu: pd.NonNegativeFloat,
+        fluid_Cp: pd.NonNegativeFloat,
+        fluid_rho: pd.NonNegativeFloat,
+        fluid_beta: pd.NonNegativeFloat,
+        plate_L: pd.NonNegativeFloat,
+        gravity: float = 9.80665,
+    ):
+        """
+        Create an instance from standard SI units.
+
+        Args:
+            fluid_k: Thermal conductivity in [W/(m*K)].
+            fluid_mu: Dynamic viscosity in [Pa*s].
+            fluid_Cp: Specific heat in [J/(kg*K)].
+            fluid_rho: Density in [kg/m**3].
+            fluid_beta: Thermal expansivity in [1/K].
+            plate_L: Plate characteristic length in [m].
+            gravity: Gravitational acceleration in [m/s**2].
+
+        Returns:
+            An instance of NaturalConvectionVerticalSpec with all values
+            converted to Tidy3D's internal unit system.
+        """
+
+        # --- Apply conversion factors ---
+        # value_tidy = value_si * factor
+        k_tidy = fluid_k / 1e6  # W/(m*K) -> W/(um*K)
+        mu_tidy = fluid_mu / 1e6  # Pa*s -> kg/(um*s)
+        cp_tidy = fluid_Cp * 1e12  # J/(kg*K) -> um**2/(s**2*K)
+        rho_tidy = fluid_rho / 1e18  # kg/m**3 -> kg/um**3
+        beta_tidy = fluid_beta  # 1/K -> 1/K (no change)
+        L_tidy = plate_L * 1e6  # m -> um
+        g_tidy = gravity * 1e6  # m/s**2 -> um/s**2
+
+        return NaturalConvectionVerticalSpec(
+            fluid_k=k_tidy,
+            fluid_mu=mu_tidy,
+            fluid_Cp=cp_tidy,
+            fluid_rho=rho_tidy,
+            fluid_beta=beta_tidy,
+            plate_L=L_tidy,
+            gravity=g_tidy,
+        )
+
     def _compute_parameters(self):
         # Calculate the Rayleigh Number (Ra_L)
         rayleigh_numerator_notemp = (
diff --git a/tidy3d/constants.py b/tidy3d/constants.py
index 7edcf22278..5d78c372d5 100644
--- a/tidy3d/constants.py
+++ b/tidy3d/constants.py
@@ -221,6 +221,26 @@
 Amperes per square micrometer
 """
 
+DYNAMIC_VISCOSITY = "kg/(um*s)"
+"""
+Amperes per square micrometer
+"""
+
+SPECIFIC_HEAT = "um^2/(s^2*K)"
+"""
+Square micrometers per (square second Kelvin).
+"""
+
+THERMAL_EXPANSIVITY = "1/K"
+"""
+Inverse Kelvin.
+"""
+
+ACCELERATION = "um/s^2"
+"""
+Acceleration unit.
+"""
+
 LARGE_NUMBER = 1e10
 """
 Large number used for comparing infinity.

From fa13026003341764dea0f163a0f1ad95d946fdc5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Tue, 29 Jul 2025 16:42:35 +0200
Subject: [PATCH 4/9] Removing unnecessary update_forward_refs()

---
 tidy3d/components/tcad/boundary/heat.py | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/tidy3d/components/tcad/boundary/heat.py b/tidy3d/components/tcad/boundary/heat.py
index 921f898031..04f58fd24d 100644
--- a/tidy3d/components/tcad/boundary/heat.py
+++ b/tidy3d/components/tcad/boundary/heat.py
@@ -204,8 +204,3 @@ class ConvectionBC(HeatChargeBC):
         description=f"Heat flux value in units of {HEAT_TRANSFER_COEFF}.",
         units=HEAT_TRANSFER_COEFF,
     )
-
-
-# TODO Maybe I should find a better place for these lines.
-NaturalConvectionVerticalSpec.update_forward_refs()
-ConvectionBC.update_forward_refs()

From 65416202e81b66f5658375e4989aa437968b52a4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Tue, 29 Jul 2025 17:43:38 +0200
Subject: [PATCH 5/9] Fix style with ruff format

---
 tidy3d/__init__.py              | 4 ++--
 tidy3d/components/tcad/types.py | 1 -
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/tidy3d/__init__.py b/tidy3d/__init__.py
index f7bf678566..70b98986c5 100644
--- a/tidy3d/__init__.py
+++ b/tidy3d/__init__.py
@@ -24,6 +24,7 @@
 from tidy3d.components.spice.sources.dc import DCCurrentSource, DCVoltageSource
 from tidy3d.components.spice.sources.types import VoltageSourceType
 from tidy3d.components.tcad.analysis.heat_simulation_type import UnsteadyHeatAnalysis, UnsteadySpec
+from tidy3d.components.tcad.boundary.heat import NaturalConvectionVerticalSpec
 from tidy3d.components.tcad.boundary.specification import (
     HeatBoundarySpec,
     HeatChargeBoundarySpec,
@@ -68,7 +69,6 @@
     CaugheyThomasMobility,
     ConstantMobilityModel,
     ConvectionBC,
-    NaturalConvectionVerticalSpec,
     CurrentBC,
     HeatFluxBC,
     HeatFromElectricSource,
@@ -606,6 +606,7 @@ def set_logging_level(level: str) -> None:
     "ModulationSpec",
     "Monitor",
     "MultiPhysicsMedium",
+    "NaturalConvectionVerticalSpec",
     "NedeljkovicSorefMashanovich",
     "NonlinearModel",
     "NonlinearSpec",
@@ -715,5 +716,4 @@ def set_logging_level(level: str) -> None:
     "set_logging_console",
     "set_logging_file",
     "wavelengths",
-    "NaturalConvectionVerticalSpec",
 ]
diff --git a/tidy3d/components/tcad/types.py b/tidy3d/components/tcad/types.py
index 38cb64c74b..5926eba088 100644
--- a/tidy3d/components/tcad/types.py
+++ b/tidy3d/components/tcad/types.py
@@ -8,7 +8,6 @@
     ConvectionBC,
     HeatFluxBC,
     TemperatureBC,
-    NaturalConvectionVerticalSpec,
 )
 from tidy3d.components.tcad.generation_recombination import (
     AugerRecombination,

From e7036eb427164970fd8761725984734ca9f1925b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Tue, 29 Jul 2025 17:46:51 +0200
Subject: [PATCH 6/9] Minor typo

---
 tidy3d/constants.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tidy3d/constants.py b/tidy3d/constants.py
index 5d78c372d5..f637e451eb 100644
--- a/tidy3d/constants.py
+++ b/tidy3d/constants.py
@@ -223,7 +223,7 @@
 
 DYNAMIC_VISCOSITY = "kg/(um*s)"
 """
-Amperes per square micrometer
+Kilograms per (micrometer second)
 """
 
 SPECIFIC_HEAT = "um^2/(s^2*K)"

From c3ea537173dcfd973e54874ec8bf54d94b686266 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Sat, 2 Aug 2025 12:31:45 +0200
Subject: [PATCH 7/9] Update naming and minor fixes

---
 tidy3d/__init__.py                      |  5 ++---
 tidy3d/components/tcad/boundary/heat.py | 13 +++++++------
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/tidy3d/__init__.py b/tidy3d/__init__.py
index 70b98986c5..a7ec9f24db 100644
--- a/tidy3d/__init__.py
+++ b/tidy3d/__init__.py
@@ -24,7 +24,7 @@
 from tidy3d.components.spice.sources.dc import DCCurrentSource, DCVoltageSource
 from tidy3d.components.spice.sources.types import VoltageSourceType
 from tidy3d.components.tcad.analysis.heat_simulation_type import UnsteadyHeatAnalysis, UnsteadySpec
-from tidy3d.components.tcad.boundary.heat import NaturalConvectionVerticalSpec
+from tidy3d.components.tcad.boundary.heat import VerticalNaturalConvectionCoeffModel
 from tidy3d.components.tcad.boundary.specification import (
     HeatBoundarySpec,
     HeatChargeBoundarySpec,
@@ -406,7 +406,6 @@ def set_logging_level(level: str) -> None:
 Transformed.update_forward_refs()
 ClipOperation.update_forward_refs()
 GeometryGroup.update_forward_refs()
-ConvectionBC.update_forward_refs()
 
 __all__ = [
     "C_0",
@@ -606,7 +605,6 @@ def set_logging_level(level: str) -> None:
     "ModulationSpec",
     "Monitor",
     "MultiPhysicsMedium",
-    "NaturalConvectionVerticalSpec",
     "NedeljkovicSorefMashanovich",
     "NonlinearModel",
     "NonlinearSpec",
@@ -696,6 +694,7 @@ def set_logging_level(level: str) -> None:
     "UnsteadyHeatAnalysis",
     "UnsteadySpec",
     "Updater",
+    "VerticalNaturalConvectionCoeffModel",
     "VisualizationSpec",
     "VoltageBC",
     "VoltageSourceType",
diff --git a/tidy3d/components/tcad/boundary/heat.py b/tidy3d/components/tcad/boundary/heat.py
index 04f58fd24d..84276bb27b 100644
--- a/tidy3d/components/tcad/boundary/heat.py
+++ b/tidy3d/components/tcad/boundary/heat.py
@@ -6,6 +6,7 @@
 
 import pydantic.v1 as pd
 
+from tidy3d.components.base import Tidy3dBaseModel
 from tidy3d.components.tcad.boundary.abstract import HeatChargeBC
 from tidy3d.constants import (
     ACCELERATION,
@@ -53,14 +54,14 @@ class HeatFluxBC(HeatChargeBC):
     )
 
 
-class NaturalConvectionVerticalSpec(HeatChargeBC):
+class VerticalNaturalConvectionCoeffModel(Tidy3dBaseModel):
     """
     Specification for natural convection from a vertical plate.
 
     This class calculates the heat transfer coefficient (h) based on fluid
     properties and an expected temperature difference, then provides these
     values as 'base' and 'exponent' for a generalized heat flux equation
-    q = base * (T_surf - T_fluid)^exponent.
+    q = base * (T_surf - T_fluid)^exponent + base * (T_surf - T_fluid).
     """
 
     # --- Input Parameters ---
@@ -124,7 +125,7 @@ def from_si_units(
             gravity: Gravitational acceleration in [m/s**2].
 
         Returns:
-            An instance of NaturalConvectionVerticalSpec with all values
+            An instance of VerticalNaturalConvectionCoeffModel with all values
             converted to Tidy3D's internal unit system.
         """
 
@@ -138,7 +139,7 @@ def from_si_units(
         L_tidy = plate_L * 1e6  # m -> um
         g_tidy = gravity * 1e6  # m/s**2 -> um/s**2
 
-        return NaturalConvectionVerticalSpec(
+        return VerticalNaturalConvectionCoeffModel(
             fluid_k=k_tidy,
             fluid_mu=mu_tidy,
             fluid_Cp=cp_tidy,
@@ -175,7 +176,7 @@ def _compute_parameters(self):
             h_factor_non_linear = (0.387 * Ra_L ** (1 / 6)) / pr_denominator
         else:
             raise ValueError(
-                f"Ra_L={Ra_L} should be smaller than 1e13 for NaturalConvectionVerticalSpec"
+                f"Ra_L={Ra_L} should be smaller than 1e13 for VerticalNaturalConvectionCoeffModel"
             )
 
         h = (self.fluid_k / self.plate_L) * h_factor_linear
@@ -199,7 +200,7 @@ class ConvectionBC(HeatChargeBC):
         units=KELVIN,
     )
 
-    transfer_coeff: Union[pd.NonNegativeFloat, NaturalConvectionVerticalSpec] = pd.Field(
+    transfer_coeff: Union[pd.NonNegativeFloat, VerticalNaturalConvectionCoeffModel] = pd.Field(
         title="Heat Transfer Coefficient",
         description=f"Heat flux value in units of {HEAT_TRANSFER_COEFF}.",
         units=HEAT_TRANSFER_COEFF,

From 144650fb37c507eb711d47948c2959515778ee08 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Tue, 5 Aug 2025 17:06:22 +0200
Subject: [PATCH 8/9] Addressing last comments

---
 tidy3d/components/material/tcad/heat.py       |  98 +++++++++++++++-
 tidy3d/components/tcad/boundary/heat.py       | 108 +++---------------
 .../components/tcad/simulation/heat_charge.py |  89 +++++++++++++++
 tidy3d/constants.py                           |   5 +
 4 files changed, 205 insertions(+), 95 deletions(-)

diff --git a/tidy3d/components/material/tcad/heat.py b/tidy3d/components/material/tcad/heat.py
index a33925f210..42f5fbf3a6 100644
--- a/tidy3d/components/material/tcad/heat.py
+++ b/tidy3d/components/material/tcad/heat.py
@@ -10,8 +10,11 @@
 from tidy3d.components.base import Tidy3dBaseModel
 from tidy3d.constants import (
     DENSITY,
+    DYNAMIC_VISCOSITY,
+    SPECIFIC_HEAT,
     SPECIFIC_HEAT_CAPACITY,
     THERMAL_CONDUCTIVITY,
+    THERMAL_EXPANSIVITY,
 )
 
 
@@ -46,11 +49,100 @@ class FluidMedium(AbstractHeatMedium):
     """Fluid medium. Heat simulations will not solve for temperature
     in a structure that has a medium with this 'heat_spec'.
 
-    Example
-    -------
-    >>> solid = FluidMedium()
+    The full set of parameters is primarily intended for calculations involving natural
+    convection, where they are used to determine the heat transfer coefficient.
+    In the current version, these specific properties may not be utilized for
+    other boundary condition types.
+
+    Attributes
+    ----------
+    thermal_conductivity : float, optional
+        Thermal conductivity ($k$) of the fluid in $W/(\\mu m \\cdot K)$.
+    viscosity : float, optional
+        Dynamic viscosity ($\\mu$) of the fluid in $kg/(\\mu m \\cdot s)$.
+    specific_heat : float, optional
+        Specific heat ($c_p$) of the fluid in $\\mu m^2/(s^2 \\cdot K)$.
+    density : float, optional
+        Density ($\rho$) of the fluid in $kg/\\mu m^3$.
+    expansivity : float, optional
+        Thermal expansion coefficient ($\beta$) of the fluid in $1/K$.
+
+    Examples
+    --------
+    >>> # It is most convenient to define the fluid from standard SI units
+    >>> # using the `from_si_units` classmethod.
+    >>> # The following defines air at approximately 20°C.
+    >>> air_from_si = FluidMedium.from_si_units(
+    ...     thermal_conductivity=0.0257,  # Unit: W/(m*K)
+    ...     viscosity=1.81e-5,          # Unit: Pa*s
+    ...     specific_heat=1005,         # Unit: J/(kg*K)
+    ...     density=1.204,              # Unit: kg/m^3
+    ...     expansivity=1/293.15        # Unit: 1/K
+    ... )
+
+    >>> # One can also define the medium directly in Tidy3D units.
+    >>> # The following is equivalent to the example above.
+    >>> air_direct = FluidMedium(
+    ...     thermal_conductivity=2.57e-8,
+    ...     viscosity=1.81e-11,
+    ...     specific_heat=1.005e+15,
+    ...     density=1.204e-18,
+    ...     expansivity=0.00341
+    ... )
     """
 
+    thermal_conductivity: pd.NonNegativeFloat = pd.Field(
+        default=None,
+        title="Fluid Thermal Conductivity",
+        description="Thermal conductivity (k) of the fluid.",
+        units=THERMAL_CONDUCTIVITY,
+    )
+    viscosity: pd.NonNegativeFloat = pd.Field(
+        default=None,
+        title="Fluid Dynamic Viscosity",
+        description="Dynamic viscosity (μ) of the fluid.",
+        units=DYNAMIC_VISCOSITY,
+    )
+    specific_heat: pd.NonNegativeFloat = pd.Field(
+        default=None,
+        title="Fluid Specific Heat",
+        description="Specific heat of the fluid at constant pressure.",
+        units=SPECIFIC_HEAT,
+    )
+    density: pd.NonNegativeFloat = pd.Field(
+        default=None,
+        title="Fluid Density",
+        description="Density (ρ) of the fluid.",
+        units=DENSITY,
+    )
+    expansivity: pd.NonNegativeFloat = pd.Field(
+        default=None,
+        title="Fluid Thermal Expansivity",
+        description="Thermal expansion coefficient (β) of the fluid.",
+        units=THERMAL_EXPANSIVITY,
+    )
+
+    def from_si_units(
+        thermal_conductivity: pd.NonNegativeFloat,
+        viscosity: pd.NonNegativeFloat,
+        specific_heat: pd.NonNegativeFloat,
+        density: pd.NonNegativeFloat,
+        expansivity: pd.NonNegativeFloat,
+    ):
+        thermal_conductivity_tidy = thermal_conductivity / 1e6  # W/(m*K) -> W/(um*K)
+        viscosity_tidy = viscosity / 1e6  # Pa*s -> kg/(um*s)
+        specific_heat_tidy = specific_heat * 1e12  # J/(kg*K) -> um**2/(s**2*K)
+        density_tidy = density / 1e18  # kg/m**3 -> kg/um**3
+        expansivity_tidy = expansivity  # 1/K -> 1/K (no change)
+
+        return FluidMedium(
+            thermal_conductivity=thermal_conductivity_tidy,
+            viscosity=viscosity_tidy,
+            specific_heat=specific_heat_tidy,
+            density=density_tidy,
+            expansivity=expansivity_tidy,
+        )
+
 
 class FluidSpec(FluidMedium):
     """Fluid medium class for backwards compatibility"""
diff --git a/tidy3d/components/tcad/boundary/heat.py b/tidy3d/components/tcad/boundary/heat.py
index 84276bb27b..3bc26c5165 100644
--- a/tidy3d/components/tcad/boundary/heat.py
+++ b/tidy3d/components/tcad/boundary/heat.py
@@ -7,18 +7,15 @@
 import pydantic.v1 as pd
 
 from tidy3d.components.base import Tidy3dBaseModel
+from tidy3d.components.material.tcad.heat import FluidMedium
 from tidy3d.components.tcad.boundary.abstract import HeatChargeBC
 from tidy3d.constants import (
     ACCELERATION,
-    DENSITY,
-    DYNAMIC_VISCOSITY,
+    GRAV_ACC,
     HEAT_FLUX,
     HEAT_TRANSFER_COEFF,
     KELVIN,
     MICROMETER,
-    SPECIFIC_HEAT,
-    THERMAL_CONDUCTIVITY,
-    THERMAL_EXPANSIVITY,
 )
 
 
@@ -64,64 +61,35 @@ class VerticalNaturalConvectionCoeffModel(Tidy3dBaseModel):
     q = base * (T_surf - T_fluid)^exponent + base * (T_surf - T_fluid).
     """
 
-    # --- Input Parameters ---
-    fluid_k: pd.NonNegativeFloat = pd.Field(
-        title="Fluid Thermal Conductivity",
-        description="Thermal conductivity (k) of the fluid.",
-        units=THERMAL_CONDUCTIVITY,
+    medium: FluidMedium = pd.Field(
+        default=None,
+        title="Interface medium",
+        description="Medium to use for heat transfer coefficient.",
     )
-    fluid_mu: pd.NonNegativeFloat = pd.Field(
-        title="Fluid Dynamic Viscosity",
-        description="Dynamic viscosity (μ) of the fluid.",
-        units=DYNAMIC_VISCOSITY,
-    )
-    fluid_Cp: pd.NonNegativeFloat = pd.Field(
-        title="Fluid Specific Heat",
-        description="Specific heat of the fluid at constant pressure.",
-        units=SPECIFIC_HEAT,
-    )
-    fluid_rho: pd.NonNegativeFloat = pd.Field(
-        title="Fluid Density",
-        description="Density (ρ) of the fluid.",
-        units=DENSITY,
-    )
-    fluid_beta: pd.NonNegativeFloat = pd.Field(
-        title="Fluid Thermal Expansivity",
-        description="Thermal expansion coefficient (β) of the fluid.",
-        units=THERMAL_EXPANSIVITY,
-    )
-    plate_L: pd.NonNegativeFloat = pd.Field(
+
+    plate_length: pd.NonNegativeFloat = pd.Field(
         title="Plate Characteristic Length",
         description="Characteristic length (L), defined as the height of the vertical plate.",
         units=MICROMETER,
     )
 
     gravity: pd.NonNegativeFloat = pd.Field(
-        default=9.80665 * 1e6,
+        default=GRAV_ACC,
         title="Gravitational Acceleration",
         description="Gravitational acceleration (g).",
         units=ACCELERATION,
     )
 
     def from_si_units(
-        fluid_k: pd.NonNegativeFloat,
-        fluid_mu: pd.NonNegativeFloat,
-        fluid_Cp: pd.NonNegativeFloat,
-        fluid_rho: pd.NonNegativeFloat,
-        fluid_beta: pd.NonNegativeFloat,
-        plate_L: pd.NonNegativeFloat,
-        gravity: float = 9.80665,
+        medium: FluidMedium,
+        plate_length: pd.NonNegativeFloat,
+        gravity: pd.NonNegativeFloat = GRAV_ACC * 1e-6,
     ):
         """
         Create an instance from standard SI units.
 
         Args:
-            fluid_k: Thermal conductivity in [W/(m*K)].
-            fluid_mu: Dynamic viscosity in [Pa*s].
-            fluid_Cp: Specific heat in [J/(kg*K)].
-            fluid_rho: Density in [kg/m**3].
-            fluid_beta: Thermal expansivity in [1/K].
-            plate_L: Plate characteristic length in [m].
+            plate_length: Plate characteristic length in [m].
             gravity: Gravitational acceleration in [m/s**2].
 
         Returns:
@@ -131,59 +99,15 @@ def from_si_units(
 
         # --- Apply conversion factors ---
         # value_tidy = value_si * factor
-        k_tidy = fluid_k / 1e6  # W/(m*K) -> W/(um*K)
-        mu_tidy = fluid_mu / 1e6  # Pa*s -> kg/(um*s)
-        cp_tidy = fluid_Cp * 1e12  # J/(kg*K) -> um**2/(s**2*K)
-        rho_tidy = fluid_rho / 1e18  # kg/m**3 -> kg/um**3
-        beta_tidy = fluid_beta  # 1/K -> 1/K (no change)
-        L_tidy = plate_L * 1e6  # m -> um
+        plate_length_tidy = plate_length * 1e6  # m -> um
         g_tidy = gravity * 1e6  # m/s**2 -> um/s**2
 
         return VerticalNaturalConvectionCoeffModel(
-            fluid_k=k_tidy,
-            fluid_mu=mu_tidy,
-            fluid_Cp=cp_tidy,
-            fluid_rho=rho_tidy,
-            fluid_beta=beta_tidy,
-            plate_L=L_tidy,
+            medium=medium,
+            plate_length=plate_length_tidy,
             gravity=g_tidy,
         )
 
-    def _compute_parameters(self):
-        # Calculate the Rayleigh Number (Ra_L)
-        rayleigh_numerator_notemp = (
-            self.gravity * self.fluid_beta * self.fluid_rho**2 * self.fluid_Cp * self.plate_L**3
-        )
-        rayleigh_denominator = self.fluid_mu * self.fluid_k
-        Ra_L = rayleigh_numerator_notemp / rayleigh_denominator
-
-        # Calculate the denominator term from the Nusselt number correlation
-        # This term is related to the Prandtl Number (Pr = mu * Cp / k)
-        pr_term_inner_num = 0.492 * self.fluid_k
-        pr_term_inner_den = self.fluid_mu * self.fluid_Cp
-
-        pr_term_inner = pr_term_inner_num / pr_term_inner_den
-        pr_denominator = (1 + pr_term_inner ** (9 / 16)) ** (4 / 9)
-
-        # Select formula based on flow regime (determined by Ra_L)
-        if Ra_L <= 1e9:
-            # Laminar Flow
-            h_factor_linear = 0.68
-            h_factor_non_linear = (0.670 * Ra_L ** (1 / 6)) / pr_denominator
-        elif 1e9 < Ra_L <= 1e13:
-            # Turbulent Flow
-            h_factor_linear = 0.825
-            h_factor_non_linear = (0.387 * Ra_L ** (1 / 6)) / pr_denominator
-        else:
-            raise ValueError(
-                f"Ra_L={Ra_L} should be smaller than 1e13 for VerticalNaturalConvectionCoeffModel"
-            )
-
-        h = (self.fluid_k / self.plate_L) * h_factor_linear
-        h_nonlinear = (self.fluid_k / self.plate_L) * h_factor_non_linear
-        exponent = 1 + 1 / 6
-        return h, h_nonlinear, exponent
-
 
 class ConvectionBC(HeatChargeBC):
     """Convective thermal boundary conditions.
diff --git a/tidy3d/components/tcad/simulation/heat_charge.py b/tidy3d/components/tcad/simulation/heat_charge.py
index 7b4b693c74..3631a8f241 100644
--- a/tidy3d/components/tcad/simulation/heat_charge.py
+++ b/tidy3d/components/tcad/simulation/heat_charge.py
@@ -9,6 +9,8 @@
 import numpy as np
 import pydantic.v1 as pd
 
+from tidy3d import FluidMedium, VerticalNaturalConvectionCoeffModel
+
 try:
     from matplotlib import colormaps
 except ImportError:
@@ -455,6 +457,93 @@ def check_voltage_array_if_capacitance(cls, values):
             )
         return values
 
+    @pd.root_validator(skip_on_failure=True)
+    def check_natural_convection_bc(cls, values):
+        """Make sure that natural convection BCs are defined correctly."""
+        boundary_spec = values.get("boundary_spec")
+        if not boundary_spec:
+            return values
+
+        structures = values["structures"]
+        boundary_spec = values["boundary_spec"]
+        bg_medium = values["medium"]
+
+        # Create mappings for easy lookup of media and structures by name.
+        media = {s.medium.name: s.medium for s in structures if s.medium.name}
+        if bg_medium and bg_medium.name:
+            media[bg_medium.name] = bg_medium
+        structures_map = {s.name: s for s in structures if s.name}
+
+        def check_fluid_medium_attr(fluid_medium):
+            if (
+                (fluid_medium.thermal_conductivity is None)
+                or (fluid_medium.viscosity is None)
+                or (fluid_medium.specific_heat is None)
+                or (fluid_medium.density is None)
+                or (fluid_medium.expansivity is None)
+            ):
+                raise SetupError(
+                    f"Boundary spec at index {i}: The fluid medium at the natural convection interface "
+                    f"must have 'thermal_conductivity', 'viscosity', 'specific_heat', 'density' and 'expansivity' defined."
+                )
+
+        for i, bc in enumerate(boundary_spec):
+            if not (
+                isinstance(bc.condition, ConvectionBC)
+                and isinstance(bc.condition.transfer_coeff, VerticalNaturalConvectionCoeffModel)
+            ):
+                continue
+
+            natural_conv_model = bc.condition.transfer_coeff
+            placement = bc.placement
+
+            # Case 1: The fluid medium is inferred from the placement interface.
+            # We use direct dictionary access, assuming 'names_exist_bcs' validator has already run.
+            if natural_conv_model.medium is None:
+                if isinstance(placement, MediumMediumInterface):
+                    if len(placement.mediums) != 2:
+                        continue
+                    med1 = media[placement.mediums[0]]
+                    med2 = media[placement.mediums[1]]
+                elif isinstance(placement, StructureStructureInterface):
+                    if len(placement.structures) != 2:
+                        continue
+                    med1 = structures_map[placement.structures[0]].medium
+                    med2 = structures_map[placement.structures[1]].medium
+                else:
+                    raise SetupError(
+                        f"Boundary spec at index {i}: 'VerticalNaturalConvectionCoeffModel' with no medium specified requires "
+                        f"the 'placement' to be of type 'MediumMediumInterface' or 'StructureStructureInterface', "
+                        f"but got '{type(placement).__name__}'."
+                    )
+                specs = [
+                    med1.heat if isinstance(med1, MultiPhysicsMedium) else med1,
+                    med2.heat if isinstance(med2, MultiPhysicsMedium) else med2,
+                ]
+
+                # Check for a single fluid in the interface.
+                is_fluid = [isinstance(s, FluidMedium) for s in specs]
+                if is_fluid.count(True) != 1:
+                    raise SetupError(
+                        f"Boundary spec at index {i}: A natural convection boundary at an interface "
+                        f"must be between exactly one solid and one fluid medium. "
+                        f"Found types '{type(specs[0]).__name__}' and '{type(specs[1]).__name__}'."
+                    )
+                fluid_medium = specs[is_fluid.index(True)]
+                check_fluid_medium_attr(fluid_medium)
+
+            # Case 2: The fluid medium IS specified directly in the convection model.
+            else:
+                fluid_medium = natural_conv_model.medium
+                if not isinstance(fluid_medium, FluidMedium):
+                    raise SetupError(
+                        f"Boundary spec at index {i}: The medium '{fluid_medium.name}' specified in "
+                        f"'VerticalNaturalConvectionCoeffModel' must be a fluid, but it has a heat "
+                        f"spec of type '{type(fluid_medium).__name__}'."
+                    )
+                check_fluid_medium_attr(fluid_medium)
+        return values
+
     @pd.validator("size", always=True)
     def check_zero_dim_domain(cls, val, values):
         """Error if heat domain have zero dimensions."""
diff --git a/tidy3d/constants.py b/tidy3d/constants.py
index f637e451eb..a9a0e3808d 100644
--- a/tidy3d/constants.py
+++ b/tidy3d/constants.py
@@ -53,6 +53,11 @@
 Boltzmann constant [eV/K]
 """
 
+GRAV_ACC = 9.80665 * 1e6
+"""
+Gravitational acceleration (g) [um/s^2].",
+"""
+
 # floating point precisions
 dp_eps = np.finfo(np.float64).eps
 """

From 3494f6d6aef930253893c3d88a123f69c17a1695 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <damiano@flexcompute.com>
Date: Tue, 5 Aug 2025 20:50:21 +0200
Subject: [PATCH 9/9] Small fix

---
 tidy3d/components/tcad/boundary/heat.py          | 2 +-
 tidy3d/components/tcad/simulation/heat_charge.py | 4 ----
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/tidy3d/components/tcad/boundary/heat.py b/tidy3d/components/tcad/boundary/heat.py
index 3bc26c5165..59337a9998 100644
--- a/tidy3d/components/tcad/boundary/heat.py
+++ b/tidy3d/components/tcad/boundary/heat.py
@@ -81,8 +81,8 @@ class VerticalNaturalConvectionCoeffModel(Tidy3dBaseModel):
     )
 
     def from_si_units(
-        medium: FluidMedium,
         plate_length: pd.NonNegativeFloat,
+        medium: FluidMedium = None,
         gravity: pd.NonNegativeFloat = GRAV_ACC * 1e-6,
     ):
         """
diff --git a/tidy3d/components/tcad/simulation/heat_charge.py b/tidy3d/components/tcad/simulation/heat_charge.py
index 3631a8f241..f68975de10 100644
--- a/tidy3d/components/tcad/simulation/heat_charge.py
+++ b/tidy3d/components/tcad/simulation/heat_charge.py
@@ -501,13 +501,9 @@ def check_fluid_medium_attr(fluid_medium):
             # We use direct dictionary access, assuming 'names_exist_bcs' validator has already run.
             if natural_conv_model.medium is None:
                 if isinstance(placement, MediumMediumInterface):
-                    if len(placement.mediums) != 2:
-                        continue
                     med1 = media[placement.mediums[0]]
                     med2 = media[placement.mediums[1]]
                 elif isinstance(placement, StructureStructureInterface):
-                    if len(placement.structures) != 2:
-                        continue
                     med1 = structures_map[placement.structures[0]].medium
                     med2 = structures_map[placement.structures[1]].medium
                 else: