You're welcome 😀

Navier Stokes is the most common model for most phenomena when the fluid can be assumed to be a continuum. But when

• it involves particles / multiphase flows / phase change / chemical reactions, extra equations are needed
• in microsccopic regime (small devices like a DNA sequencing machine) / rarefied regime (hypersonic fluids) the fluid molecules are too far apart, so it is no longer a continuum. Other models like Molecular dynamics or Monte Carlo methods or Lattice Boltzmann methods work best here.
• in highly turbulent flows, you can't simulate the full Navier Stokes due to limitations in computational resources. Then some turbulence modelling is needed to mimic diffusion at small scales.