# Strange questions

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 February 4, 2009, 10:49 Strange questions #1 s Guest   Posts: n/a Let me explain my geometry. I have inner pipe with 0.05 m radius and then outer pipe with 0.1 m diameter and then another pipe with 0.2 meter diameter. All of them have same axis of rotation. Now I am not modeling the flow through smaller pipe. But I want to model flow through the other two outer pipes. Now my questions whether this case is 2D? Well If I take it as 2D. But then in Fluent when I specify the velocity based upon the area and mass flow rate from 3D and then use that velocity as input to 2d model? I do not get the same mass flow rate as 3D one. The reason is Fluent calculates the area using a depth of 1 m (this is available in reference values). Of course one solution is to change this depth so that you get specified mass flow rate. But the change can be done only either of tube. I hope you guys what my question is ? Is there any possibility to model this domain in 2D or not. Regards S

 February 4, 2009, 15:45 Re: Strange questions #2 sa Guest   Posts: n/a are you modeling as axisymetric?

 February 5, 2009, 04:38 Re: Strange questions #3 s Guest   Posts: n/a The questions is this model axis-symmetric or not. I do not think it is axis-symmetric. What do you say?

 February 5, 2009, 06:22 Re: Strange questions #4 hervĂ© Guest   Posts: n/a I cannot see why it would not be axi-symetric. The geometry certainly is. Yet, maybe you do not want axi symetrical boundary conditions for various reasons (non uniform velocity profile, walls at distinctive temperatures, gravity in the perpendicular direction of the cylinders, etc ...). Though, I would start with an axi-symetrical model and make sure that I have set all parameters correctly.

 February 5, 2009, 06:38 Re: Strange questions #5 s Guest   Posts: n/a So how Do I define the axis in Fluent for axis-symmetric model when the inner pipe is not modeled which contains the axis. Thanks for your answers S

 February 5, 2009, 14:23 Re: Strange questions #6 sa Guest   Posts: n/a the x axis will be the axis of your pipes hope you got it regards

 February 6, 2009, 03:37 Re: Strange questions #7 B Guest   Posts: n/a what is the command in Fluent?

 February 6, 2009, 07:08 Re: Strange questions #8 hervĂ© Guest   Posts: n/a This is a good question for, given your description of the geometry, the axis location is outside the computational domain. Well, just select "2D axi-symetrical" in "Model", and make sure the cylinder direction is in X and its center (even if not part of the mesh) at the 0,0 location in Y,Z. I (probably naively) believe that the solver should get it right. Check it carefully by reporting the "areas" of a couple of surfaces. If it is not the case, maybe you should contact your user support, they should know better. Hopefully. At the worse, you may simply have to mesh (coarsly) the smaller-radius cylinder, so as the axis is actually part of the mesh. If you solve for temperature and you do not want the inner cyclinder to play a role, just freeze the heat exchange at the separation-wall (heat flux 0 at the wall facing the cylinder, and whatever you want for its "shadow" facing the fluid). Does this make sense? Good luck...

 February 8, 2009, 14:19 Re: Strange questions #9 sa Guest   Posts: n/a herve is correct, just choose the axi symetric model.

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