Unlike fluid flow, whose spatial (Eulerian) continuum description by the continuity and Navier-Stokes equations is widely accepted, the motion and the causes of motion of granular material within a fluid flow is not amenable to a universal description. The common idealisation is that of a discrete granular phase moving together with or as an effect of the action of a continuum phase. Research on kinematics and dynamics of granular media has led to outstanding achievements, mostly at “engineering” scales. However, these advances have seldom resulted from the integration of processes occurring at a grain-scale. In fact, the vast amount of research work carried out at the grain-scale has yet to be harmonized to match results obtained at macroscale into a consistent theoretical body.

Currently, the description of the motion and causes of motion of the discrete phase still encounters difficulties at fundamental levels. These include the identification of the scales at which an Eulerian-continuum approach is justified, the articulation between the probabilistic definition of sediment discharge, the stochastic nature of the (turbulent) near-bed flow and the development of bed forms or the relation between Lagrangian and Eulerian quantities describing sediment fluxes.

Addressing these open research questions, this session will accommodate contributions that involve Lagrangian and Eulerian grain-scale approaches and its upscaling to larger working scales, thus promoting the harmonization of the vocabulary, premises and mathematical techniques to tackle key theoretical issues. Specific topics of interest include (but are not restricted to): grain-scale mechanics of particle entrainment and disentrainment, upscaling and averaging techniques for stochastic processes related to granular processes, interaction among grain sizes in a poorly sorted mixture, including grain segregation, interaction between fluid flow and grain motion, momentum transfer among grains and between fluid and grains, derivation and solution of conservation equations.
The session is co-organised within the two EU projects "HYTECH" (ITN-Marie Curie FP7-PEOPLE-2013-ITN-316546) and "SEDITRANS (ITN-Marie Curie FP7-PEOPLE-2013-ITN- 607394) by a working group formed to promote fundamental research on mechanics of sediment transport which includes Jochen Aberle, Christophe Ancey, Francesco Ballio, Robert Bialik, Rui M.L. Ferreira, Philippe Frey, Athanasios Papanicolaou, Vladimir Nikora, Andrea Marion, Alessio Radice, Mario Franca and Simon Tait.