Math Of Turbulence

"What Makes the Hardest Equations in Physics So Difficult?":

Mathematicians classify partial differential equations like Navier-Stokes based on the extent to which they can go haywire at infinitesimally small scales. Navier-Stokes is on the extreme end of the spectrum. The difficulty of the mathematics of the equation is, in some sense, an exact reflection of the complexity of the turbulent flows they’re supposed to be able to describe.