POLYFLOW  Interpretation of streamlines when they do not match the velocity field
A 2D calculation is run, and the result show wrong patterns for the stream function. What is happening? The velocity vector plot should be trusted if the calculation has converged. In 2D flow simulation the stream function is obtained by solving a partial differential equation. The boundary conditions are set mainly by the code (the user can explicitly specify one node where the stream function equals to zero). By default, a constant (and unique) value for the stream function is assumed on a boundary where the normal velocity is set to zero. If a wall boundary includes several discontinuous sections (isolated patches), this is actually no longer not true. This may explain why wrong stream lines are obtained. The most reasonable workaround consists of defining a different boundary set for each disconnected part of the boundary, especially when it is suspected that material flows between two successive disconnected parts. 

