Coriolis-induced instabilities in centrifuge modeling of granular flow

Abstract

Granular flows are typically studied in laboratory flumes based on common similarity scaling, which create stress fields that only roughly approximate field conditions. The geotechnical centrifuge produces stress conditions that are closer to those observed in the field, but steady conditions can be hardly achieved. Moreover, secondary effects induced by the apparent Coriolis acceleration, which can either dilate or compress the flow, often obscure scaling. This work aims at studying a set of numerical experiments where the effects of the Coriolis acceleration are measured and analyzed. Three flow states are observed: dense, dilute, and unstable. It is found that flows generated under the influence of dilative Coriolis accelerations are likely to become unstable. Nevertheless, a steady dense flow can still be obtained if a large centrifuge is used. A parametric group is proposed to predict the insurgence of instabilities; this parameter can guide experimental designs and could help to avoid damage to the experimental apparatus and model instrumentation. Graphic abstract

DEM-LBM insights into unsteady grain-fluid collapses on a geotechnical centrifuge

“…As was the case for the two previous studies [15,23], we set h 0 = 50 mm and r 0 = 54 mm for each phase, resulting in a column aspect ratio of a = h 0 /r 0 = 0.93. The granular phase comprises monodispersed particles with a mean diameter D = 8 × 10 −3 m (±10% to prevent crystallisation within the initial configuration [21]) and a particle density ρ p = 2650 kg m −3 . This is the largest size used in the two previous studies and was selected as it minimises surface tension effects when in contact with a thin film of fluid [15] (see § 5.2 for further discussion).…”

“…Secondly, surface tension effects may contribute to the larger separation between downstream and upstream flow fronts observed in the physical experiments during the retardation phase (Figure 8). The extent of surface tension's influence on decelerating the flow would be greater for the ω + forcing case, where a cf already slows down the flow, and more particles are likely to be in contact due to the higher flow density [21] (see § 6.1). In conclusion, the numerical model reasonably demonstrates the idealised behaviour of a fluidsaturated granular column collapse within a centrifuge, particularly during the acceleration phase.…”

“…The numerical tool chosen for the current work was used previously by Cabrera et al [19] and Leonardi et al [21] to investigate the behaviour and modelling effects of a granular flow down a rough incline within a geotechnical centrifuge. The code is an extension of the original work by Leonardi et al [30] which has been modified and extensively used for the study of laboratoryand large-scale fluid-granular flow mechanisms [e.g.…”

“…As such, the fluid mesh size ∆ f must be smaller than the characteristic DEM particle diameter D. It should be noted that this stipulation on the lattice spacing also implies that the fluid pressure is effectively resolved at the pore scale [34], down to the precision offered by the lattice spacing itself. The work of Leonardi et al [21] has been extended so that both the fluid and granular phases can be submitted to an elevated gravitational acceleration field as would be imposed to a model while being spun on a geotechnical centrifuge. This is achieved by imposing both a cf and a co on each phase as external forces.…”

“…Furthermore, such simulations can serve as effective tools for scaling up our insights from laboratory-scale experiments to these larger geophysical phenomena. Recent investigations have utilised the Discrete Element Method (DEM) to explore the effects of centrifugal and Coriolis accelerations on both steady [19,21] and unsteady [12,22] dry granular flow configurations. Numerical modelling not only facilitates the study of a cf when varying R c but also allows for the independent examination of the effects of a cf and a co .…”