  A linear stability analysis of the Navier-Stokes (NS) granular hydrodynamic
equations is performed to determine the critical length scale for the onset of
vortices and clusters instabilities in granular dense binary mixtures. In
contrast to previous attempts, our results (which are based on the solution to
the inelastic Enskog equation to NS order) are not restricted to nearly elastic
systems since they take into account the complete nonlinear dependence of the
NS transport coefficients on the coefficients of restitution $\alpha_{ij}$. The
theoretical predictions for the critical length scales are compared to
molecular dynamics (MD) simulations in flows of strong dissipation
($\alpha_{ij}\geq 0.7$) and moderate solid volume fractions ($\phi\leq 0.2$).
We find excellent agreement between MD and kinetic theory for the onset of
velocity vortices, indicating the applicability of NS hydrodynamics to
polydisperse flows even for strong inelasticity, finite density, and particle
dissimilarity.
