Match Control and Symmetry Boundary Condtions in a quasi 2D calculation
 

Hi,
i ve a question regarding "Match Control" in Workbench Meshing. For illustration issues i attached a screenshot of my geometry. It represents a fluid slice of 0.5° in circumferential direction with an inflow at the bottom, a cavity, a throat section and an outflow at the top.
As boundary conditions, i set walls, inlet and opening at the faces in circumferential direction and an interface with fluid fluid 1:1 connection at the faces normal to circumferential direction.
In the Ansys WB meshing tool, l i set "match control" for faces with fluid fluid interface, so that i get a 1:1 matching mesh in circumferential direction. Settings for "match control" are "transformation: Cyclic", "axis of rotation: global coordinate system".
If i run the solver with these settings i get an error msg. If i dont use "match control" for these faces the calculation runs w/o any problem.
I dont get the problem, since the mesh should be better for the solver if i add "match control", so i force it to be a 1:1 sweep in circumferential direction.
If you look closely at the two pictures attached, left one shows boundary conditions for the case w/o "match control", right one shows boundary conditions with "match control".
For the second case the solver seems to ve problems with the interface boundary condition.
So if anyone could explain me that behavior.
Regards

+--------------------------------------------------------------------+
| ERROR #002100048 has occurred in subroutine SU_BNEXT. |
| Message: |
| All vertices for a fluid domain lie on boundaries. This is |
| considered to be a fatal error because control volume gradients |
| cannot be calculated, leading to serious discretization error. |
| |
| A common cause for this error is a mesh which is only one |
| element thick, without symmetry or 1:1 periodicity on the lateral |
| boundaries. If you have this situation, and the domain is |
| two-dimensional, please change the lateral boundary conditions |
| to symmetry or 1:1 periodicity. Alternatively, for |
| three-dimensional simulations, please ensure that your mesh |
| has at least two elements across. |
| |
| Execution is terminating. This error message can be bypassed by |
| setting the expert parameter 'boundary vertex check = f', but |
| be aware that doing so may lead to sigificant solution error.