Still with my bird migration, I would like to represent the return of the birds in Europe after spending time in Africa.
To achieve that, I need to be able to "reverse" the diffusion of the birds. I tried to put a negative coefficient for diffusion, but it doesn't work of course. So I wonder if it is possible to achieve this with FiPy.
That won't work. Diffusion is not a reversible operator. Try to define the problem with maths and then we can help to implement the equations in FiPy.