(543e) Impact of Collisional Vs. Viscous Dissipation On Flow Instabilities in Gas-Solid Homogeneous Cooling Systems
Flow instabilities encountered in the homogeneous cooling of a gas-solid system are considered via lattice-Boltzmann simulations. Previous studies using dissipative molecular dynamics considered the effect of normal restitution coefficient and solid fraction on the onset and evolution of the instabilities in homogeneous cooling granular systems1,2. In this study, the relative contribution of the two mechanisms leading to instabilities, the viscous dissipation (gas-phase effects) and the collisional dissipation (particle-phase effects), is assessed. 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 crossover of the kinetic energy levels is observed during the evolution of the instability. Specifically, the kinetic energy of a more dissipative system is seen to become greater than that of a less dissipative system after the vortex instability sets in. This crossover of kinetic energy levels between a more dissipative system and a less dissipative system 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. More disspative system therefore is able to preserve its kinetic energy due to the early onset of vortex instability.
1Mitrano, P. P., Dahl, S. R., Cromer, D. J., Pacella, M. S. & Hrenya, C. M. (2011) Instabilities in the homogeneous cooling of a granular gas: A quantitative assessment of kinetic-theory prediction. Phys. Fluids 23:093303.
2Mitrano, P. P., Garzo, V., Hilger, A. M., Ewasko, C. J. & Hrenya, C. M. (2012) Assessing a hydrodynamic description for instabilities in highly dissipative, freely cooling granular gases. Phys. Rev. E 85:141303.