Reactive terms that depend on the value of the field variables and other condition

[dokken]

You should collapse the sub space, project into the collapsed sub-space and use a FunctionAssigner as done in:

Usage of FunctionAssigner

As you are working with a 3 dimensional mesh, the vector function space has three components: T_S_1 = interpolate(S_11, T) T_S_2 = interpolate(S_12, T) T_s_0 = Function(T) # Function containing 0 values assigner = FunctionAssigner(V, [T, T, T]) assigner.assign(v, [T_S_1, T_S_2, T_s_0])

and
https://bitbucket.org/fenics-project/dolfin/raw/ec57db53f2b13f1768214829e8b4f80fc32205cf/python/demo/undocumented/sub-function-assignment/demo_sub-function-assignment.py