Hi,

i have a problem with the periodic boundary conditions. I want to implement the boundary condition for example

which i found it in the internet. The first equotion is u_facebuttom - u_facetop=0

So i know that we can do this with creating Constraint and selecting Equotion.

At first i define Sets FaceButtom and FaceTop which includes all nodes on bottom and top. Here is a picture for the Set Face Top:

Now i want to implement it with creating a   Constraint and selecting  Equotion. My Equotion for the first boundary condition is  u_facebuttom - u_facetop=0

It looks like this:

But now if i click the OK button to finish it for the first boundary condition, i get the Error: All terms  except the first one must use sets consisting of a single point; therefore Set " face Top" cannot be used in row 2. 

Can somebody explain what i have to do ?

How can i make periodic boundary conditions for my Representative Volume Element ?