[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"?

[singer1812]

Use a plane, point and normal definition. Point at centerline of your tube, normal outward.

Use a rectangular bound to keep the plane on interior section only.

[rikio]

Three ways to get what you want.

1. If there are two domains for the inner and outer, it will be very easy to create a plane by specifing the inner domain at any location.
2. If the tube is simply straight, transform surface is also a good choice.
3. You can create the Iso Clip by setting the variable range. First create a plane containing inner and outer flows, then setting the variable as velocity. As u said, the flow in inner tube is upward, and downward in the outer. The sign of velocity should be opposite, then you can partition the inner tube section out by setting the variable range, i.e. add new visibility parameters to define the iso clip. If there is back flow in the target region, you need more rules.

[Tomimeister]

Thank you very much for your help!
I have only one domain, because of another geometry speciality but the rectangular bounds were useful.