Discontinuity at periodic/symmetry boundary

bongbang · April 29, 2015, 13:18

My model is a sixth of a rotationally symmetric (i.e. periodic) cylinder.

The wall shear stress and its derivative, Oscillatory Shear Index (a measure of how frequently the shear vector changes direction) exhibit a sharp discontinuity in places along the boundary. This doesn't happen consistently. At its worst, it looks distinctly off, while it's barely noticeable at its best. OSI is worse than WSS. The discontinuity exists (albeit in different patterns) whether the "rotational periodic" or "symmetry" condition was applied. A match control has been applied to match the mesh on either side.

This shouldn't happen. Is there a way out of this? Thanks.

OSI

WSS average (very much acceptable, though division still visible)

WSS average, cut moved to a different place.

ghorrocks · April 29, 2015, 17:10

This is a post-processing artefact only. The post-processor does not make the contours continuous across periodic boundaries.

It looks OK for symmetry boundaries as the symmetry boundary condition enforces zero normal gradient and the post processor gets the contours about right in this case.

bongbang · April 29, 2015, 20:01

I don't understand how this is an "artifact," since the contours reflect the physical reality according to the solver. And that reality should be continuous, no?

Actually, all of the above images are results of periodic condition (i.e. none of symmetry), just with different variables and geometry. In fact, discontinuity with the symmetry condition (not shown) is even worse.

ghorrocks · April 30, 2015, 02:35

Yes, the contours are just of the variable field. But the contours need to interpolate between control volumes. At periodic boundaries the contour plotting gizmo is not smart enough to realise it has to use control volumes on the other side of the periodic boundary to do the interpolation. So at periodic boundaries it is only using control volumes on one side of the boundary and so when you match up both sides of the boundary in post processing they do not quite match up.

Symmetry is even worse? I will have to think about that.

April 29, 2015, 13:18	Discontinuity at periodic/symmetry boundary	#1
bongbang Member NC Join Date: Jan 2015 Posts: 49 Rep Power: 11	My model is a sixth of a rotationally symmetric (i.e. periodic) cylinder. The wall shear stress and its derivative, Oscillatory Shear Index (a measure of how frequently the shear vector changes direction) exhibit a sharp discontinuity in places along the boundary. This doesn't happen consistently. At its worst, it looks distinctly off, while it's barely noticeable at its best. OSI is worse than WSS. The discontinuity exists (albeit in different patterns) whether the "rotational periodic" or "symmetry" condition was applied. A match control has been applied to match the mesh on either side. This shouldn't happen. Is there a way out of this? Thanks. OSI WSS average (very much acceptable, though division still visible) WSS average, cut moved to a different place.

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April 29, 2015, 17:10		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	This is a post-processing artefact only. The post-processor does not make the contours continuous across periodic boundaries. It looks OK for symmetry boundaries as the symmetry boundary condition enforces zero normal gradient and the post processor gets the contours about right in this case.

April 29, 2015, 20:01		#3
bongbang Member NC Join Date: Jan 2015 Posts: 49 Rep Power: 11	I don't understand how this is an "artifact," since the contours reflect the physical reality according to the solver. And that reality should be continuous, no? Actually, all of the above images are results of periodic condition (i.e. none of symmetry), just with different variables and geometry. In fact, discontinuity with the symmetry condition (not shown) is even worse.

April 30, 2015, 02:35		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Yes, the contours are just of the variable field. But the contours need to interpolate between control volumes. At periodic boundaries the contour plotting gizmo is not smart enough to realise it has to use control volumes on the other side of the periodic boundary to do the interpolation. So at periodic boundaries it is only using control volumes on one side of the boundary and so when you match up both sides of the boundary in post processing they do not quite match up. Symmetry is even worse? I will have to think about that.