Titre/Title : Particle-Bed Turbulence Interactions in an Oscillatory Flow: Fully Resolved Simulations and LES-DEM Modeling

Contact : Julien Chauchat (équipe MEIGE)

Résumé/Abstract : Turbulent flows over a layer of densely-packed particles have been subject to several investigation owing to increasing interests in geophysical applications; for example, incipient motion of sediment particles. In order to improve the meager understanding of the mechanisms that lead to the onset of sediment erosion under coastal conditions, detailed knowledge of sediment-bed-turbulence interactions is necessary. This work therefore focusses on fundamental understanding of the particle-turbulence interactions in symmetric, sinusoidally driven transitional and turbulent flows over a range of Reynolds numbers using particle-resolved direct numerical simulations as well as Euler-Lagrange, large-eddy simulations with subgrid scale closures for particle motion. The resulting flowfields are characterized in terms of coherent vortex structures, turbulent kinetic-energy budgets, cross-correlations between forces upon particles and flow variables, along with statistical distributions of near-bed velocities and pressure fluctuations. It is shown that the sediment particles modulate the near-bed turbulence, distort and break the streamwise horse-shoe structures, and reduce the large-scale anisotropy. The lift force is well correlated with the streamwise velocity fluctuations up to distances of the same order as the particle diameter, whereas pressure fluctuations are correlated and anti-correlated with the lift force in the front and aft regions of the particle. A fourth order Gram-Charlier distribution model incorporating higher order turbulence statistics is found to provide a good fit to the near-bed velocity and pressure correlations. This may have important consequences on the Gaussian descriptions of particle pick-up functions typically used in large scale modeling of particle transport.