POLYFLOW  Conflict of velocity boundary conditions
Some flow situations may require to impose velocity boundary conditions that are not necessarily compatible. A typical example occurs when boundaries A and B are perpendicular to each other, and when a vanishing velocity is imposed along boundary A while a nonzero normal velocity component is imposed on boundary B. The conflict appears at the intersection between these boundaries, where a decision is made by te program. This concerns all versions of POLYFLOW. In order to circumvent this trouble, the user may proceed as follows.  Impose the zero velocity boundary condition on the first wall  Select vanishing forces on the second wall  Impose the required velocity conditions on the second wall by means of a submodel. Another way consists of replacing the vanishing velocity condition by a slipping condition with a high coefficient in the slipping law (this will guarantee that a nearly vanishing velocity is obtained). This solution can also be applied when a nonvanishing tangential velocity is imposed; there the definition of the slipping boundary condition must involve the definition of the required nonvanishing wall velocity. 

