POLYFLOW - Fluid escapes at contact. Why?

In a blow molding or a thermoforming simulation, it may happen that fluid escapes through a segment in contact. Such a situation is illustrated in the figures below. What is happening, and how can this be circumvented?
<a target=_blank href="http://www.fluentusers.com/support/solutions/1339/Fluid_escape_2cnt_quad_glob.gif">http://www.fluentusers.com/support/solutions/1339/Fluid_escape_2cnt_quad_glob.gif</a>http://www.fluentusers.com/support/solutions/1339/Fluid_escape_2cnt_quad_glob.gif

<a target=_blank href="http://www.fluentusers.com/support/solutions/1339/Fluid_escape_2cnt_quad_loc.gif">http://www.fluentusers.com/support/solutions/1339/Fluid_escape_2cnt_quad_loc.gif</a>http://www.fluentusers.com/support/solutions/1339/Fluid_escape_2cnt_quad_loc.gif

In the case shown, one sees at first that the mould consists of two parts (red and blue); this was motivated e.g. for assigning different contact attributes, such as slipping. One also sees that the incriminated segment is indeed in contact with the mould, but each vertex is actually in contact with a different part of the mould. When this happens, a decision cannot be made for the mid-side node of the element (where a velocity is calculated in 2D). Indeed, the mid-side node is considered in contact when both its neighbouring points are also in contact with the same mould part. Therefore, fluid can escape through this segment, and the velocity field is affected as shown above.
There are possible remedies to the present situation:
- use a linear interpolation for the velocity instead of a quadratic one, so that the mid-side node does no longer exist
- both individual mould parts could overlap over a distance that is larger than the element size, in order to make sure that both vertices of a given segment are always seen in contact at least one mould part.

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