How to define a variable depending on another variable at the other position

Dear community,

I'm trying to define a variable, y, which is determined by the other variable, x. The problem is that y is determined by the values of x at two different positions in the domain. Furthermore, the two different positions are exactly at the surface on two sides of one inertial boundary.

I really hope someone can help me. Thanks a lot in advance! I'm looking forward to your kind reply.


Best,
Meixia