  We investigate the very onset of the inertial regime in pore-scale fluid flow
in a three-dimensional, disordered porous media. We analyze how the flow
structure changes with increasing Reynolds number. In particular, we compute
tortuosity, spatial kinetic energy localization, and the pore-space volume
fraction containing negative streamwise velocity. Our analysis shows that the
onset of inertia observed in a standard way by computing the friction factor
appears at a Reynolds number two orders of magnitude higher than indicated by
analyzing tortuosity and spatial distribution of kinetic energy.
