I play with CFD for a while, and suddenly, a transcendantal question raises: :-) Navier Stokes is basically Newton applied on a continuum in Eulerian. For solids, we would consider linear, but also angular momentum. Why don't we have to do that for fluids ?
Conversely, you can take the curl of Navier Stokes and have an equation expressed in vorticity, which looks like our angular momentum world. Does it mean that the equation with velocity somehow embed the both kind of momentum, and they are totally correlated for fluids ?
But how it's not the same for solids ? i.e., where is the intrinsic difference that makes it different degrees of freedom in one case and equivalent in the second ?