hello i am a student of iit delhi and working on annular diffuser problem (2d)on fluent 5.5. i am facing great difficulty in defining the axis for this annular pipe flow configuration. i have meshed the flow regime in the annulus but how do i define the axis to denote the cylindrical shape of the pipe and not a flow between 2 plates in 2d. since the axis for the annulus flow is outside the flow domain, i wanted to know how to define the same in a 2D case and not 3D. i am using gambit for drawing the geometry.

Can't you model a small portion upstream of the diffuser? This would also better define the flow going into the diffuser.

Thank you for your suggestion but I guess i could not explain it well. Let me take a very simple case. How do I declare axis for an annular flow in two concentric cyclindrcal pipes. Unlike in flow thru a normal pipe where the axis is a part of flow regime, for annular flow, the axis lies outside the flow domain. so how do i define it in Gambit?

You define this inside Fluent (Boundary Conditions, Fluid, Axis of Rotation... or something like that). I think that in some cases Fluent requires that it is the x-axis which is the rotational axis, so the x-axis is a safe choice. In Gambit you just place your two end-walls so that the x-axis becomes the rotational axis.

Hello Thank you for your advise. I will definitely try looking for the option of axiz of rotation in fluent. But I am still not clear about Gambit. I have constructed the annular geometry at some distance parallel to the x axis but can not figure out how to declare the axis ( isnt my problem definition in gambit still incomplete in Gambit) Looking forward to your help Puneet IIT Delhi

I assume that you want to create a 2D mesh, in that case you don't need to define any axis of rotation in Gambit. It is enough to define it in Fluent.

In a 2d case the x-axis is the axis of rotation, in a 3d case the z-axis is the axis of rotation. To start a 2d axisymmetirc calculation in Fluent go to Define-Solver in the menu and check the corresponding box on the left.