A circular tube is a well axis-symetric geometry,have you ever mesh a circular tube by taking the axis line as axis boudary type? in gambit,can you make it? or just make a 2D mesh and specify one side as axis..?

specify the axis boundary in a 2d geometry

but how to form the 3D geometry?(my physical model is 3D,a circular tube)?

Hello, I can't catch your exact meaning. I think a circular tube can be modeled by using a 2D mesh if you are sure that the flow in tube is axisymmetric. You can choose a 2D geometry as your model and just set one of the boundary as axis. And in fluent,go into define----->solver,choose axisymmetric under space.

If you want to create a 2D mesh in gambit firstly and then form the 3D geometry by revolving it, It seems unrealized.

As a whole, I choose a 2D geometry to model an axisymmetric one and a 3D geometry to un-axisymmetric.