Total surface area to volume (#5165)

* calculate total surface are to volume

* composite surface form test

* changelog

* skip newman tobias

Author: rtimms

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Features
 
+- Added support for algebraic and differential surface form in composite models. ([#5165](https://github.com/pybamm-team/PyBaMM/pull/5165))
 - Adds a composite electrode electrode soh model ([#5160](https://github.com/pybamm-team/PyBaMM/pull/5129))
 - Generalises `set_initial_soc` to `set_initial_state` and adds Thevenin initial state setting. ([#5129](https://github.com/pybamm-team/PyBaMM/pull/5129))
 

src/pybamm/models/submodels/active_material/total_active_material.py

@@ -49,4 +49,25 @@ def get_coupled_variables(self, variables):
             )
             variables.update({f"{Domain} electrode capacity [A.h]": C})
 
+        if self.options["particle shape"] == "spherical":
+            # Ignore factor of 4pi that will cancel
+            total_surface_area = sum(
+                variables[f"{Domain} electrode {phase} active material volume fraction"]
+                * variables[f"{Domain} {phase} particle radius [m]"] ** 2
+                for phase in phases
+            )
+            total_volume = 3 * sum(
+                variables[f"{Domain} {phase} particle radius [m]"] ** 3
+                for phase in phases
+            )
+            total_surface_area_to_volume_ratio = total_surface_area / total_volume
+            variables.update(
+                {
+                    f"{Domain} electrode surface area to volume ratio [m-1]": total_surface_area_to_volume_ratio,
+                    f"X-averaged {domain} electrode surface area to volume ratio [m-1]": pybamm.x_average(
+                        total_surface_area_to_volume_ratio
+                    ),
+                }
+            )
+
         return variables

tests/unit/test_models/test_full_battery_models/test_lithium_ion/base_lithium_ion_tests.py

@@ -655,3 +655,17 @@ def test_well_posed_composite_LAM(self):
             "loss of active material": "stress-driven",
         }
         self.check_well_posedness(options)
+
+    def test_well_posed_composite_differential_surface_form(self):
+        options = {
+            "particle phases": ("2", "2"),
+            "surface form": "differential",
+        }
+        self.check_well_posedness(options)
+
+    def test_well_posed_composite_algebraic_surface_form(self):
+        options = {
+            "particle phases": ("2", "2"),
+            "surface form": "algebraic",
+        }
+        self.check_well_posedness(options)

tests/unit/test_models/test_full_battery_models/test_lithium_ion/test_newman_tobias.py

@@ -51,3 +51,11 @@ def test_well_posed_sei_VonKolzenberg2020_model(self):
     @pytest.mark.skip(reason="Test currently not implemented")
     def test_well_posed_sei_tunnelling_limited(self):
         pass  # skip this test
+
+    @pytest.mark.skip(reason="Test currently not implemented")
+    def test_well_posed_composite_differential_surface_form(self):
+        pass  # skip this test
+
+    @pytest.mark.skip(reason="Test currently not implemented")
+    def test_well_posed_composite_algebraic_surface_form(self):
+        pass  # skip this test