POLYFLOW - Extrusion case: despite swelling, why does the cross section decrease?
An extrusion case is defined, and seems to run properly. However, although swelling is observed, the extrudate cross section decreases in the downstream direction. Why?
Presumably it can be connected to the selected boundary conditions.
1) Extrudate cross-section may decrease if a take-up velocity or force is imposed at the exit; this is part f the physics.
2) For viscoelastic flows, the extrudate cross section may decrease if "outflow" conditions are selected at the exit of the dmain, instead of vanishing forces.
In general, it is recommended to impose vanishing forces at the exit cross section of the extrudate, unless otherwise required by the physics. In particular, selecting outflow boundary doncitions for a viscoelastic flow will result in imposed velocities at the exit, whose value depends on the cross section as initially known in the mesh file. This may deviate from the actual swelled solution.