[Tomimeister]

I have a pipe-in-pipe arrengement geometry with counterflow (in the inner tube the water flows upward, in the outer ring the water flows downward). The shape (cutplane) of the pipes is rectangular with rounded corner. I want to examine only the inner flow in a cutplane. The "conventional" way to creat a simle plane is incorrect now because it would contain the outer ring too.
How can I define a special surface only in the inner tube? I tried to define an "User Surface" "From File" method but I have never done this before and the help was not so useful to me.
Can somebody help me? Shoud I try with other method for example "Transformed Surface"?