outlet as inlet (interpolation function)

Good morning,

I'm trying simulare a laminar flow and the transport of a diluted specie (c) in a 3D model. I would like create a loop in which the concentration at the outlet at the previous step became the inlet of a following simulation and so on using an interpolation function.

The geometry is simple but there are some holes.

I've noticed that the value set as extrapolation during the definition of the interpolation function changes the values of c that I try to pass as new inlet.
Why?

Is there a way to assign concentration values node by node in my mesh without the use of an interpolation function?

thanks

G