2025/12/08 by Chiarini, Alessandro, Gallorini, Emanuele, Rosti, Marco Edoardo
#FOS: Physical sciences #Fluid Dynamics (physics.flu-dyn)
paper · doi:10.48550/arxiv.2512.07258
We use direct numerical simulations to investigate fluid-solid interactions in suspensions of rigid fibres settling under gravity in a quiescent fluid. The solid-to-fluid density ratio is O(100), while the Galileo number (Ga) and fibre concentration (nℓf3) are varied over the ranges Ga ∈ [180, 900] and nℓf3 ∈ [0.36, 23.15]; ℓf denotes the fibre length and n the number density. At high Ga and/or low nℓf3, fibres cluster into gravity-aligned streamers with elevated concentrations and enhanced settling velocities, disrupting the flow homogeneity. As Ga increases and/or nℓf3 decreases, the fluid-phase kinetic energy rises and the energy spectrum broadens, reflecting enhanced small-scale activity. The flow anisotropy is assessed by decomposing the energy spectrum into components aligned with and transverse to gravity. Vertical fluctuations are primarily driven by fluid-solid interactions, while transverse ones are maintained by pressure-strain effects that promote isotropy. With increasing Ga, nonlinear interactions become more prominent, producing a net forward energy cascade toward smaller scales, punctuated by localised backscatter events. Analysis of the local velocity gradient tensor reveals distinct flow topologies: at low Ga, the flow is dominated by axisymmetric compression and two-dimensional straining; at high Ga, regions of high fibre concentration are governed by two-dimensional strain, while voids are associated with axisymmetric extension. The fluid motion is predominantly extensional rather than rotational.