Adding unsteady heat and custom source to changelog.

Author: marc-flex

CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `priority` field in `Structure` and `MeshOverrideStructure` for setting the behavior in structure overlapping region. When its value is `None`, the priority is automatically determined based on the material property and simulation's `structure_priority_mode`.
 - Automatically apply `matplotlib` styles when importing `tidy3d` which can be reverted via the `td.restore_matplotlib_rcparams()` function.
 - Added `TriangleMesh.from_height_expression` class method to create a mesh from an analytical height function defined on a 2D grid and `TriangleMesh.from_height_grid` class method to create a mesh from height values sampled on a 2D grid.
+- Added heat sources with custom spatial dependence. It is now possible to add a `SpatialDataArray` as the `rate` in a `HeatSource`.
+- Added Transient Heat simulations. It is now possible to run transient Heat simulations. This can be done by specifying `analysis_spec` of `HeatChargeSimulation` object as `UnsteadyHeatAnalysis`. 
 
 ### Fixed
 - Fixed bug in broadband adjoint source creation when forward simulation had a pulse amplitude greater than 1 or a nonzero pulse phase.

docs/api/heat/analysis.rst

@@ -0,0 +1,11 @@
+.. currentmodule:: tidy3d
+
+Transient Heat Analysis
+-----------------
+
+.. autosummary::
+   :toctree: ../_autosummary/
+   :template: module.rst
+
+   tidy3d.UnsteadySpec
+   tidy3d.UnsteadyHeatAnalysis

tidy3d/components/tcad/simulation/heat_charge.py

@@ -125,7 +125,16 @@ class HeatChargeSimulation(AbstractSimulation):
 
          .. math::
 
-            -k \\cdot \\nabla(T) = q
+            -\\nabla \\cdot (-k \\nabla T) = q
+
+    It is also possible to run transient heat simulations by specifying ``analysis_spec=UnsteadyHeatAnalysis(...)``. This adds
+    the temporal terms to the above equations:
+
+        .. math::
+
+            \\frac{\\partial \\rho c_p T}{\\partial t} -\\nabla \\cdot (k \\nabla(T)) = q
+
+    where :math:`\\rho` is the density and :math:`c_p` is the specific heat capacity of the medium.
 
 
     The steady-state electrical ``Conduction`` equation depends on the electric conductivity (:math:`\\sigma`)  of a