Aeolian transport of cohesive particles: From sand to snow – EOLE

The transport of solid particles by the shearing of a turbulent air flow over a bed of particles occurs in many geophysical processes, such as sand transport by wind, snow transport or dust emission. Particle transport involves a myriad of coupled physical mechanisms, including fluid-particle, particle-particle (mid-air collision) and bed-particle interactions. A precise quantification of these interactions in the context of the complex particulate beds, including in particular cohesive particles, remains a major challenge. Aeolian sand transport with spherical and non-cohesive particles can be considered as one of the simplest natural two-phase flows, while wet sand or snow transport is at the other end of the complexity scale.

The difficulty in describing the aforementioned natural systems comes essentially from the fact that cohesion can evolve strongly in time and space. The monitoring of these variations at the relevant temporal and spatial scales is beyond current instrumental capacities. Likewise, the potential effects of inter-particle cohesion on the transfer of mass, momentum and energy between moving particles and the static bed remain poorly documented. Thus, the prediction of the mass flows transported for these complex systems is very uncertain.

The objective of this project is to elucidate the role of cohesion in air-particle flows with the ultimate goal of better describing the complexity of natural systems such as wet sand and snow. The originality and strength of our approach is to model the complexity of natural cohesive beds using "model" particles of controlled size, density and shape and whose cohesion can be tuned with appropriate liquid or solid bonds.

Our strategy is based both on the achievement of controlled experiments in a wind tunnel with model granular systems and on the development of numerical simulations based on new hybrid fluid-particle approaches allowing the modeling of the granular interactions at the particle scale. Experiments with model particles will be a first step before tackling natural systems, including wet sand and snow. Access to the cold wind-tunnel of the National research Institute for Earth science and Disaster resilience in Japan (NIED), a unique instrument in the world, will make it possible to deal with the case of snow.

The detailed characterization of these systems will require the development and implementation of innovative instrumental techniques. To obtain an accurate description of the mobile particles in the transport layer, we will develop stereo micro-imaging techniques never used previoulsy. The characterization of the properties of the bed (in terms of cohesion and packing fraction) is a technical barrier that must be overcome by developing specific tools based on new capacitive and acoustic methods. Finally, to document the effect of cohesion on the bed-particle collision process with an unprecedented accuracy, we will develop a unique tool reproducing this process in a controlled manner.

To achieve this project, a team with complementary skills has been formed, gathering physicists from the « Institut de Physique de Rennes » (IPR, Partner 1), environmental research scientists from the «Erosion Torrentielle, Neige et Avalanches » lab in Grenoble (ETNA, Partner 2), and fluid mechanics experts from the « Laboratoire de Thermique et Energie de Nantes » (LTeN, Partner 3). The members of this team include both experimentalists, theoreticians and computer simulation experts, and collaborate with internationally recognized partners.