text: The two-dimensional stream function is based on the following assumptions:
  * The space domain is three-dimensional.
  * The flow field can be described as two-dimensional plane flow, with velocity vector
  * The velocity satisfies the continuity equation for incompressible flow:
  Although in principle the stream function doesn't require the use of a particular coordinate system, for convenience the description presented here uses a right-handed Cartesian coordinate system with coordinates .