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 hamed.majeed October 11, 2012 03:10

symmetry and wall (free slip) BC

Hi,
What is the difference between symmetry and free slip wall boundary conditions in ANSYS CFX.

 ghorrocks October 11, 2012 05:56

This is described in the documentation.

Symmetry enforces zero normal gradient to all parameters, and a slip wall does not necessarily do this. But for most normal simulations they are equivalent.

And a symmetry plane is required to be planar, but a slip wall can be any shape.

 Far August 20, 2015 13:31

@ghorrocks : Some body showed a case of cylindrical shape domain and he has applied symmetry condition for the 60 deg sector. In my understanding symmetry condition reflect the solution around the mirror plane which in my understating can only be applied to two planes in 3-d case. I suggested him the periodic bc instead.

What do you think?

 flotus1 August 20, 2015 13:47

I did not double-check if this is true in CFX. Another difference between the two boundary types should show up when using a turbulence model.
Near a free-slip wall, the turbulence kinetic energy is zero while at a symmetry boundary condition its normal gradient is zero.

 Far August 20, 2015 13:54

Quote:
 Originally Posted by flotus1 (Post 560401) I did not double-check if this is true in CFX. Another difference between the two boundary types should show up when using a turbulence model. Near a free-slip wall, the turbulence kinetic energy is zero while at a symmetry boundary condition its normal gradient is zero.
If any boundary (wall) is not generating turbulence, will it affect ?? in that case both will behave in similar manner?

This is taken from CFX help :

Quote:
 For turbulent flows, the specified shear is applied directly; no considerations about turbulent wall functions are taken into account.

 Far August 20, 2015 14:02

2 Attachment(s)
and this is from the fluent help (same meaning as shown in CFX help ) and which is very much clear...

http://www.cfd-online.com/Forums/att...1&d=1440093732

Fluent help on symmetry BC:

http://www.cfd-online.com/Forums/att...1&d=1440094135

 ghorrocks August 20, 2015 18:16

Quote:
 In my understanding symmetry condition reflect the solution around the mirror plane which in my understating can only be applied to two planes in 3-d case. I suggested him the periodic bc instead.
A symmetry boundary has zero normal flow. So that means no swirl. A periodic boundary allows normal flow so can allow swirl.

That is the key difference to keep in mind for symmetry versus periodic.

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