[AS_Aero]

Hi All

I have a very basic question about the difference between Axisymmetric and Symmetric Boundary condition in fluent.
When should I use Axisymmetric and when can I use symmetric boundary conditon ?
I am trying to simulate a bubble rise from an orifice at the center of a Cylindrical pipe containing water. So in this case doing 2D simulation, I took a plane and at one corner I inject the bubble with a parabolic velocity profile. so now my doubt is if I should use Symmetric boundary or Axisymmetric boundary.

Some people told me that if I use symmetric boundary at oneside then its like a planer and its for Square channel or rectangular channel. So I must use axisymmetric boundary condition. But as far as I understand, if I have to do some simulation for a circular channel then I will have to do by taking a wedge out of the cylinder and give periodic boundary condition for two sides. But then its not 2D anymore then.
Kindly let me know what is the difference here if I want to do a 2D simulation for a cylindrical geometry.

[Jiricbeng]

Hi,


2D axisymmetric means that circumferential derivatives of flow variables (pressure, velocity etc.) are zero. It means that you assume that the solution should not change in circumferential direction and this assumption allows you to use axisymmetry.


Symmetry would mean your channel is rectangular, that is right. Because you want to analyse cylindrical domain, you really cannot use symmetry.


The closer approximation is using the axisymmetry in your case. Beware that any bubble which will not be in the axis of rotation will actually be treated as toroid (circle revolved around axis) and this is likely not a correct approach. I guess your problem is 3D.


I did not understand what you mean by taking a wedge out of a cylinder.. Basically in Fluent a 2D axisymmetry analysis of cylinder should be done as a rectangle whose one side is coincident with "x" axis - this is axis of rotation. And there is no need to take a wedge out or even set periodicity. Periodicity will be set automatically around "x" axis by switching the solver to 2D axisymmetry.