A Condition On Spherical Surfaces To Non-Existence Of Incompressible   Velocity Fields
  In an incompressible velocity field, the surface area of a volume varies with time, but volume remains unchanged. If incidentally the surface becomes spherical along time, the area reaches a local minimum, since sphere has the least area that surrounds a volume. So the area is a function of time that is locally convex at this point. When applied to an incompressible Navier--Stokes fluid, this property is used to work out an inequality that suggest a criterion to non-existence of initial configurations of velocity fields, revealing its impossibility to evolve with time. Three velocity fields are proposed as examples. One of them agrees the inequality, the other two violate it. 
Manuel Garc\'ia-Casado
physics.flu-dyn