Its all to do with these equations.
Navier
They give engineering students nightmares until you discover that they just zero out the hard stuff.
And there still isn't a computer thats powerful enough to solve the whole lot.
They are pretty good at doing 2D linear models but as soon as its gets turbulent your into wag land. When your into 3D nonlinear and transient dynamic flows it produces some very pretty pictures and gives you an idea of what might happen but thats about it.