In an incompressible
flow, when the normal velocity is imposed on the complete boundary,
the pressure level is undetermined from boundary conditions.
In this case to guarantee
uniqueness of the solution, the user specifies the coordinates of
a point at which the pressure is assigned to a selected value. The condition
will be applied to the closiest nodal point to the specified coordinates.
By default, the pressure
equals the selected value at the nodal point closiest to the origin of
the coordinates system.