Could symmetry BC cause relatively large residual?

Anna Tian · January 30, 2015, 23:34

Hi,

I currently met a quite strange problem. When I run a simulation with a symmetry BC, I got relatively quite large residual (above 5e-5) near that symmetry BC. So I change the symmetry BC into free slip wall BC. Then the large residual disappears and the simulation converges to 1e-7.

I used CFX for more than two years. That never happened before. Could anyone explain why this happens? Sorry that I can’t attach the CFX file here because it’s too large. Has anyone had the same experience or bring any hint?

ghorrocks · January 31, 2015, 04:36

A free slip wall and a symmetry boundary condition are mathematically similar, but not identical.

* They both have zero flow velocity across the boundary.
* They both have zero shear stress for the tangential velocity component at the boundary

But the symmetry boundary enforces zero normal gradient on all the scalars. The Slip wall does not do this. In a fully converged solution this should converge to the same result, but it does mean the symmetry boundary is more constraining and therefore can affect convergence. I would have thought a symmetry boundary would converge quicker than a slip wall - but you say you experienced the opposite. But I would think the number of cases where the difference between symmetry and free slip is important will be quite small.

January 30, 2015, 23:34	Could symmetry BC cause relatively large residual?	#1
Anna Tian Senior Member Meimei Wang Join Date: Jul 2012 Posts: 494 Rep Power: 15	Hi, I currently met a quite strange problem. When I run a simulation with a symmetry BC, I got relatively quite large residual (above 5e-5) near that symmetry BC. So I change the symmetry BC into free slip wall BC. Then the large residual disappears and the simulation converges to 1e-7. I used CFX for more than two years. That never happened before. Could anyone explain why this happens? Sorry that I can’t attach the CFX file here because it’s too large. Has anyone had the same experience or bring any hint? __________________ Best regards, Meimei

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January 31, 2015, 04:36		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	A free slip wall and a symmetry boundary condition are mathematically similar, but not identical. * They both have zero flow velocity across the boundary. * They both have zero shear stress for the tangential velocity component at the boundary But the symmetry boundary enforces zero normal gradient on all the scalars. The Slip wall does not do this. In a fully converged solution this should converge to the same result, but it does mean the symmetry boundary is more constraining and therefore can affect convergence. I would have thought a symmetry boundary would converge quicker than a slip wall - but you say you experienced the opposite. But I would think the number of cases where the difference between symmetry and free slip is important will be quite small.