Thanks for your reply. I have around 13 circles that are my fibers. I want all of them as one subdomain. If I have to define the equation of each circle, I think that it is not a very efficient way of declaring the subdomains when we have a very complex geometries.
Is there something else to define subdomains in very complex geometries?
I can do that in abaqus using sets for instance, Is there any way of identifying these sets as subdomains in fenics?
Thank you very much