4.7 Article

Impact of collisional versus viscous dissipation on flow instabilities in gas-solid systems

Journal

JOURNAL OF FLUID MECHANICS
Volume 727, Issue -, Pages -

Publisher

CAMBRIDGE UNIV PRESS
DOI: 10.1017/jfm.2013.268

Keywords

absolute/convective instability; granular media; multiphase and particle-laden flows

Funding

  1. National Science Foundation [CBET 1236157, 1236490]
  2. Department of Energy [DE-FE0007450]
  3. Directorate For Engineering
  4. Div Of Chem, Bioeng, Env, & Transp Sys [1236157, 1236490] Funding Source: National Science Foundation

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Flow instabilities encountered in the homogeneous cooling of a gas solid system are considered via lattice-Boltzmann simulations. Unlike previous efforts, the relative contribution of the two mechanisms leading to instabilities is explored: viscous dissipation (fluid-phase effects) and collisional dissipation (particle-phase effects). The results indicate that the instabilities encountered in the gas solid system mimic those previously observed in their granular (no fluid) counterparts, namely a velocity vortex instability that precedes in time a clustering instability. We further observe that the onset of the instabilities is quicker in more dissipative systems, regardless of the source of the dissipation. Somewhat surprisingly however, a cross-over of the kinetic energy levels is observed during the evolution of the instability. Specifically, the kinetic energy of the gas solid system is seen to become greater than that of its granular counterpart (i.e. same restitution coefficient) after the vortex instability sets in. This cross-over of kinetic energy levels between a more dissipative system (gas solid) and a less dissipative system (granular) can be explained by the alignment of particle motion found in a vortex. Such alignment leads to a reduction in both collisional and viscous energy dissipation due to the more glancing nature of collisions.

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