Defining the outflow boundary condition for PDE interface

Hi,

I am modeling species flow and transport in fracture-matrix system. Fracture is a thin space on left-side with connected matrix on the right-side. Dissolved species in water enter at inlet of the fracture (bottom left of the domains) and are advected through fracture in the upward direction. At the fracture-matrix interface the species diffuse into the matrix.

There are no-flux conditions at all other boundaries except for inlet and outlet of the fracture. At inlet of the fracture I have applied Dirichlet Boundary Condition(specifying the fixed species concentration). I want to implement the outflow boundary condition at top of the fracture with the objective that the species can leave there due to convection and diffusion processes.

Can you help me how to implement the outflow boundary condition at outlet of the fracture.


Best Regards,

Nawaz Ahmad
PhD student ,KTH, Stockholm