title: Granular-front formation in free-surface flow of concentrated suspensions.
abstract: A granular front emerges whenever the free-surface flow of a concentrated suspension spontaneously alters its internal structure, exhibiting a higher concentration of particles close to its front. This is a common and yet unexplained phenomenon, which is 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 viscoplastic 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, such as fresh concrete or debris flow. In these materials, granular and fluid phases have the natural tendency to separate 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 particle concentration 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 nondimensional number is defined, and is found to be effective in predicting the formation of a granular front.
authors.affiliation: Institute for Building Materials, ETH Zurich, Computational Physics for Engineering Materials, Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland.|Itasca Consulting GmbH, Leithestrasse 111, 45886 Gelsenkirchen, Germany.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Leonardi
authors.affiliation: Institute of Geotechnical Engineering, University of Natural Resources and Life Sciences (BOKU), Feistmantelstrasse 4, 1180 Vienna, Austria.
authors.forename: Miguel
authors.initials: M
authors.lastname: Cabrera
authors.affiliation: Institute for Building Materials, ETH Zurich, Computational Physics for Engineering Materials, Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland.
authors.forename: Falk K
authors.initials: FK
authors.lastname: Wittel
authors.affiliation: Institute of Mountain Risk Engineering, University of Natural Resources and Life Science (BOKU), Peter-Jordan-Strasse 82, 1190 Vienna, Austria.
authors.forename: Roland
authors.initials: R
authors.lastname: Kaitna
authors.affiliation: Institute for Building Materials, ETH Zurich, Computational Physics for Engineering Materials, Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland.
authors.forename: Miller
authors.initials: M
authors.lastname: Mendoza
authors.affiliation: Institute of Geotechnical Engineering, University of Natural Resources and Life Sciences (BOKU), Feistmantelstrasse 4, 1180 Vienna, Austria.
authors.forename: Wei
authors.initials: W
authors.lastname: Wu
authors.affiliation: Institute for Building Materials, ETH Zurich, Computational Physics for Engineering Materials, Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland.
authors.forename: Hans J
authors.initials: HJ
authors.lastname: Herrmann