It's very easy. Mind you that 'simulation' means you're trying to solve a system of partial differential equations at tens of thousands to billions of positions

It's very easy. Mind you that 'simulation' means you're trying to solve a system of partial differential equations at tens of thousands to billions of positions simultaneously. The simple truth is that computational fluid dynamics codes are nowhere near a calculator-like fire-and-forget reliability, even though the UI of commercial solvers implies as much when you just have to click a button to get 'an answer'. It's still much more of a black art and you really have to know (1) the physics, and (2) the numerics to reliably generate physically meaningful results.