# symmetry and wall (free slip) BC

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 October 11, 2012, 03:10 symmetry and wall (free slip) BC #1 Member   Hamed Abdul Majeed Join Date: May 2012 Posts: 72 Rep Power: 5 Hi, What is the difference between symmetry and free slip wall boundary conditions in ANSYS CFX.

 October 11, 2012, 05:56 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,295 Rep Power: 88 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.

 August 20, 2015, 13:31 #3 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,010 Blog Entries: 6 Rep Power: 40 @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?

 August 20, 2015, 13:47 #4 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,134 Rep Power: 19 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.

August 20, 2015, 13:54
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Sijal
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Quote:
 Originally Posted by flotus1 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.

August 20, 2015, 14:02
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Sijal
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and this is from the fluent help (same meaning as shown in CFX help ) and which is very much clear...

Fluent help on symmetry BC:

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August 20, 2015, 18:16
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Glenn Horrocks
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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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