Inertial effects in just-saturated axisymmetric column collapses

Abstract

This work introduces a scaling analysis of sub-aerial axisymmetric column collapses of glass beads and Newtonian glycerol-water solutions mimicking some of the behaviours of debris flows. The beads were in a size range where their inertia partly decouples their collapse behaviour from the water column. Experiments explored a range of viscous, surface tension and particle inertia effects through systematic variation of particle size and fluid viscosity. Crucially a geotechnical centrifuge was used to access elevated effective gravitational accelerations driving the collapse, allowing field-scale viscous and surface tension effects to be replicated. Temporal pore pressure and run out front position evolution data was extracted using a pressure transducer and high speed imaging, respectively. A least-squares fitted scale analysis demonstrated that all characteristic dimensionless quantities of the acceleration phase could be described as a function of the column-scale Bond number $$\text{ Bo }$$ Bo , the Capillary number $$\text{ Ca }$$ Ca , the system size $$r^*$$ r ∗ , and the grain-fluid density ratio $$\rho ^*$$ ρ ∗ . This analysis demonstrated that collapses as characterised by pore pressure evolution and front positions were controlled by the ratio of $$\text{ Bo}/\text{Ca}$$ Bo / Ca . This indicates that grain-scale surface tension effects are negligible in such inertial column collapses where they may dominate laboratory-scale granular-fluid flow behaviour where geometric similarity between grain and system size is preserved. Graphical abstract

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

“…The evolution of the collapses were recorded using two high speed cameras and image processing was used to extract the location of both phase fronts throughout the duration of the experiment (see Figure 3). A full description of the physical system and image analysis can be found in Webb et al [15]. Given, that the flow spreads axisymmetrically, specifically, sections of the flow travel 'upstream' and 'downstream' of centrifuge motion [15], this experimental configuration provides an ideal test case for comprehending the role of centrifugal and Coriolis accelerations in influencing the behaviour of dynamic flows.…”

“…A full description of the physical system and image analysis can be found in Webb et al [15]. Given, that the flow spreads axisymmetrically, specifically, sections of the flow travel 'upstream' and 'downstream' of centrifuge motion [15], this experimental configuration provides an ideal test case for comprehending the role of centrifugal and Coriolis accelerations in influencing the behaviour of dynamic flows. By using experimental comparison to assess the numerical model, we aim to gain valuable insights into the mechanics of granular-fluid flows in a geotechnical centrifuge setting that would be unachievable from experimental testing alone.…”

“…The initial configuration of the granular-fluid column is characterised by two parameters: the initial column height h 0 and radius r 0 . 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 .…”

“…Stress-dependent mechanisms also significantly affect the dynamics of partially or fully saturated granular flows. For instance, phenomena such as surface tension effects caused by capillary bridges between adjacent grains [14], or at the surface of the flow itself, can have a significant impact on flow behaviour in laboratory experiments, even though they are relatively minor compared to gravitational forces in natural mass movements [15]. Moreover, in addition to the flow's particle size distribution, the magnitude of the confining stress at a specific point within the flow greatly influences the buildup and dissipation of excess pore pressures.…”

“…The study aims to enhance our understanding by employing numerical modelling to examine a just-saturated granular column collapse configuration within a geotechnical centrifuge. Two recent experimental studies [15,23] have provided valuable insights into the system, with a strong focus on grain-fluid interaction and its…”