convert dt to fem.Constant

festim/hydrogen_transport_problem.py

@@ -115,6 +115,7 @@ def initialise(self):
         self.assign_functions_to_species()
 
         self.t = fem.Constant(self.mesh.mesh, 0.0)
+        self.dt = self.settings.stepsize.get_dt(self.mesh.mesh)
 
         self.define_boundary_conditions()
         self.create_formulation()
@@ -218,10 +219,6 @@ def create_formulation(self):
         if len(self.species) > 1:
             raise NotImplementedError("Multiple species not implemented yet")
 
-        self.dt = F.as_fenics_constant(
-            self.settings.stepsize.initial_value, self.mesh.mesh
-        )
-
         self.formulation = 0
 
         for spe in self.species:

festim/stepsize.py

@@ -1,3 +1,6 @@
+import festim as F
+
+
 class Stepsize:
     """
     A class for evaluating the stepsize of transient simulations.
@@ -14,3 +17,12 @@ def __init__(
         initial_value,
     ) -> None:
         self.initial_value = initial_value
+
+    def get_dt(self, mesh):
+        """Defines the dt value
+        Args:
+            mesh (dolfinx.mesh.Mesh): the domain mesh
+        Returns:
+            fem.Constant: the dt value
+        """
+        return F.as_fenics_constant(self.initial_value, mesh)