If one considers a flow which is inviscid, this is where the viscous forces are small and can therefore be neglected in the calculations, then one arrives at Euler's equations:
Although in this case we have assumed an inviscid fluid this assumption does not hold for flows where there is a boundary. The presence of a boundary causes some viscosity at the boundary layer which cannot be neglected and one arrives back at the Navier–Stokes equation. Finding the solutions to these governing equations under different circumstances and determining their stability is the fundamental principle in determining the stability of the fluid flow itself.