a fluid flow is usually described by a set of partial differential equations known as the navier-stokes equations
let us suppose for a moment that we are studying a steady viscous in compressible fluid flow at a given reynolds number
what makes one such flow different from another
all flows are described by the same set of equations but obviously not all flows are the same
the answer is that the particular flow is determined by the physical boundaries of the problem being considered and the conditions applied to those boundaries