  Granular fronts are a common yet unexplained phenomenon emerging during the
gravity driven free-surface flow of concentrated suspensions. They are usually
believed to be the result of fluid convection in combination with particle size
segregation. However, suspensions composed of uniformly sized particles also
develop a granular front. Within a large rotating drum, a stationary
recirculating avalanche is generated. The flowing material is a mixture of a
visco-plastic fluid obtained from a kaolin-water dispersion, with spherical
ceramic particles denser than the fluid. The goal is to mimic the composition
of many common granular-fluid materials, like fresh concrete or debris flow. In
these materials, granular and fluid phases have the natural tendency to
segregate due to particle settling. However, through the shearing caused by the
rotation of the drum, a reorganization of the phases is induced, leading to the
formation of a granular front. By tuning the material properties and the drum
velocity, it is possible to control this phenomenon. The setting is reproduced
in a numerical environment, where the fluid is solved by a Lattice-Boltzmann
Method, and the particles are explicitly represented using the Discrete Element
Method. The simulations confirm the findings of the experiments, and provide
insight into the internal mechanisms. Comparing the time-scale of particle
settling with the one of particle recirculation, a non-dimensional number is
defined, and is found to be effective in predicting the formation of a granular
front.
