Download Granular flows on steep slopes Granular flows are found in many

Granular flows on steep slopes
Granular flows are found in many industrial processes, especially in the mining,
food-processing and building industries. They are also important in nature, for
example in the gravitational destabilization of cliffs, mountain slopes, and
volcanic edifices and represent severe natural hazards for the populations and
infrastructures. The behaviour of such granular flows is not yet well
understood; there is no unified theory describing all the properties of these
flows. They often exhibit flow regimes where "solid", motionless, phases
coexist with "liquid" ones (dense flows) and "gaseous" ones (diluted flows). The
complexity of the granular flows mainly arises from the nature of the
interactions between grains: according to the contribution of brief, collisional
contacts dominant in the dilute parts compared to enduring contacts
associated with friction, dominant in the dense parts, the macroscopic
behaviour of the flows changes drastically in space and time.
Flow on an inclined plane under gravity is particularly interesting to study
because, in addition to appearing in nature and industry, it can be used as a
rheological test to verify theoretical models. Many experiments and
simulations have been done, but these studies consider mainly gentle slopes
and low mass fluxes that lead to dense, uni-directional flows that are now
relatively well understood. More complex flow regimes appear at higher
inclinations and fluxes. For example, we have shown that, when the flux is
increased, the flow evolves toward the « Sidewall Stabilized Heap » (SSH)
regime where rapid flow occurs in a thin layer on top of a quasi-static heap of
grains. The density varies strongly with depth, passing from a very dilute,
collisional region at the top to very dense, extremely slow moving regions deep
in the flow.
This shows that granular flows can exhibit unexpected behaviors even when
the configuration is very simple. Recent numerical discrete-element simulations
have revealed many different flow regimes characterized by a rich and complex
internal structure. One of these regimes, called « supported flow » is
particularly interesting: a rapidly moving dense core of grains is supported by a
dilute and energetic « gas » of grains. The reduction of the effective friction by
the layer of « gas » that supports the core could be the cause of long-runout
landslides sometimes observed in nature. These numerical results pose several
challenging questions: first of all, the influence of boundary conditions
(presence and spacing of the side walls, the nature of the bottom and side
surfaces) on these regimes needs to be studied. Next, the physical mechanisms
and conditions that lead to the appearance of these regimes must be
identified. Finally, it is important to verify that these regimes can be observed
experimentally, especially the supported regime.
The goal of this thesis is to try to answer the above questions, by numerical and
experimental means. For numerical work, a code based on the discrete
element method already exists. The experimental work will be done on an
existing laboratory set-up. The analysis of the experiments will combine optical
(particle tracking), mechanical (stress sensors), and acoustic (passive and active
probing) methods. Both monodisperse and polydisperse flows can be
considered. We expect that size-segregation will appear in the polydisperse
flows, and that this segregation will modify the flow. The study of polydisperse
flows will be done in collaboration with Philippe Frey of IRSTEA in Grenoble and
Nico Gray of the University de Manchester, within the ANR project « Segsed ».