Difference between periodicity and symmetry boundary conditions
Hi,
I have an axisymmetric geometry (i.e. a pipe) and I want to know what's the best boundary conditions to apply. Since it's axisymmetric I only consider the top half and include only a 10 degree angle. To me it make sense to apply rotational periodicity between the two 'sidewalls'. However, I've seen people use symmetric boundary conditions. How are these two different? I presume both of them still makes the solution 2D. 
In such a case, I agree with what you are using. Using a periodic BC allows a lot of vortices and other flow phenomena which might exist (say shocks, temperature fluctuations etc) to travel continously instead of being damped or getting 'bounced' off the symmetry BC.

But for an axisymmetric geometry, if you only consider the top half and then apply symmetry BCs at the two side planes (and a segment of say 10 degrees), how does the simulation become axisymmetric? I'm confused.

The appropriate boundary condition depends on what is happening on the other boundaries and in the flow. For example:
If the flow is swirling then you must use periodic and not symmetry. If the flow is swirling significantly then the vortex on the centre line will move around/precess breaking symmetry  you will have to simulate 360 degrees. Other nonlinearities in the flow can also break symmetry particularly if you have diffusing regions that are pushed until they separate. If you have 20 upstream geometries repeated in the circumferential direction then you will need to use a sector size of 20/360 with periodic bcs if the upstream flow swirls or, perhaps, 10/360 with symmetry bcs if the upstream geometries are themselves symmetric. The centre line bc can also be a source of difficulty particularly for an unsteady simulation. 
Thanks for the reply.
I'm confused about how the code will know whether the it's an axisymmetric geometry when you just apply symmetric boundary conditions at the two sidewalls? Surely if you apply symmetry BCs it just means geometric symmetry. When using Centreline BC do you select the axis as the centreline? 
Hi,
The code does not find out that the problem is axisymmetric, what it does is that when you apply symmetric BCs on the two sides, then it says the code that gradients are zero on those planes. You can think of it as mirroring. When you have a symmetric BC, you can mirror the geometry, loads, and also the solution with respect to that plane. Now lets have a little visualization game. Consider your wedge (for example 5 degrees). As you have axisymmetry on sides, you can mirror it with respect to those, so put a mirror side by side to the original one (and the solution, the geometry, and the loads will repeat themselves). Now if we look at our image, we'll see that the added wedge is actually the same wedge rotated by the wedge angle (e.g. 5 degrees). We can continue this process 70 more times (360/5=72) and we will get the whole cylinder. I hope this helps in understanding the symmetric boundry condition, and how it leads to axisymmetry. Reza. 
Quote:
If you have azimuthal velocity components then symmetry boundary conditions will effectively act as (frictionless) walls, most likely giving you a wrong result with a lot of reflections. 
Hi,
You are right, if a problem is axisymmetric we can model it as a 2D problem, but unfortunately some of the softwares demand us to have a 3D mesh (like CFX) so we have to model the axisymmetric flow as a flow in a wedge with symmetric BCs, and if there is swirl, or tangential velocity, to model the BCs as periodic boundary conditions. 
boundary condition (symmetry vs periodic doubt)
Dear friends,
i am doing 3D wing analysis, i have doubt on boundary conditions in fluent ie both symmetry and periodic, anybody have the answer please let me know briefly,for what kind of purpose to use symmetry & periodic boundary conditions, please tell any example with problem. 
periodicity: given a period L, the solution implies u(L +/ h) = u(+/ h)
simmetry: given a plane symmetry z=L the solution implies u(L+h)=u(Lh) 
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