Small-Scale Flow Topologies, Pseudo-Turbulence, And Impact On Filtered Drag Models In Turbulent Fluidization

The small-scale flow structures in a moderately dense turbulent (gas-particle) fluidization are investigated using highly resolved simulation data obtained from a kinetic-theory-based two-fluid model. As a main feature, the invariants of traceless and phase-weighted velocity gradient tensors (rate-of-strain and rate-of-rotation) are computed on the gas and solid phase. The invariant-space joint probability density functions indicate a dominant small-scale topology of tubelike vortical-prevalent clusters and sheetlike viscous dissipative slots on the solid phase. The gas phase, nevertheless, reveals a kind of tendency to a boundary-layer turbulence with a vortex-sheet topology prevalence and balanced mechanism of stretching and contracting enstrophy. From other crucial perspectives, by considering the turbulent interfacial work and drag production [Capecelatro et al., J. Fluid Mech. 780, 578 (2015)], an a priori analysis of the cluster-induced turbulence (pseudo-turbulence) or the subgrid-scale drift velocity parametrizations is outlined. It has been shown that the parametrizations, which depend linearly on the resolved gradient of the solid volume fraction, are invalid in this moderately dense turbulent fluidization. They reveal a clear misalignment with the actual filtered subgrid (solid-induced) drift velocity because of the boundary-layer-like turbulence on gas phase. Alternatively, the tensorial dispersion approach arises to be a valid choice for modeling the drift velocity. The topological analysis, on the other hand, suggests that the linear turbulent dispersion paradigm and the hypothesis of a constant dispersion Prandtl number [Burns et al., ICMF04, (2004), p. 392] is applicable only in the large-scale strain-dominated areas. These and other scrutinies and arguments, from a topological viewpoint, are afforded in this paper.