  In this letter, we investigate the transient rheological behavior of immersed
granular flows using both experiments of submerged granular column collapses
and corresponding numerical simulations. The simulations are performed with the
lattice-Boltzmann method (LBM) coupled with the discrete element method (DEM)
and provide a significant amount of data of the stress and deformation
conditions at different positions and times during the granular collapse. We
derive a new dimensionless number $\mathcal{G}$ that can unify the rheology of
transient granular flows in different regimes for all the simulation data
points. $\mathcal{G}$ smoothly transforms from an inertial number into a
viscous number, unifying both extremes of the rheology law. We also show the
need to introduce the kinetic stresses to achieve a universal relation. The
findings establish a transient constitutive framework for visco-inertial
granular flows and are important for a better understanding of granular-fluid
mixtures in both natural and engineering situations.
