Initiation of underwater granular avalanches: Influence of the initial volume fraction

Pailha, Mickaël; Nicolas, Maxime; Pouliquen, Olivier

doi:10.1063/1.3013896

Cited by 93 publications

(129 citation statements)

References 20 publications

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“…The presence of a fluid (rather than air) not only modifies the interstitial pore pressures, but also couples the stress carried by the particles to that carried by the fluid flowing through gaps in the grain matrix (Iverson & LaHusen 1989;Iverson 1997Iverson , 2005. This coupling is particularly significant in unsteady flows, since local changes in the particle volume fraction allow large excess pore pressures to develop, which in turn feedback on the granular motion (du Pont et al 2003;Muite, Hunt & Joseph 2004;Pailha, Nicolas & Pouliquen 2008;Pailha & Pouliquen 2009). However, for steady, dense granular flows such as those sketched in figure 2, the large number of particle-particle contacts mean that frictional interactions are still dominant in …”

Section: Recirculating Particle Motionmentioning

confidence: 99%

Asymmetric breaking size-segregation waves in dense granular free-surface flows

et al. 2016

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Debris and pyroclastic flows often have bouldery flow fronts, which act as a natural dam resisting further advance. Counter intuitively, these resistive fronts can lead to enhanced run-out, because they can be shouldered aside to form static levees that self-channelise the flow. At the heart of this behaviour is the inherent process of size segregation, with different sized particles readily separating into distinct vertical layers through a combination of kinetic sieving and squeeze expulsion. The result is an upward coarsening of the size distribution with the largest grains collecting at the top of the flow, where the flow velocity is greatest, allowing them to be preferentially transported to the front. Here, the large grains may be overrun, resegregated towards the surface and recirculated before being shouldered aside into lateral levees. A key element of this recirculation mechanism is the formation of a breaking size-segregation wave, which allows large particles that have been overrun to rise up into the faster moving parts of the flow as small particles are sheared over the top. Observations from experiments and discrete particle simulations in a moving-bed flume indicate that, whilst most large particles recirculate quickly at the front, a few recirculate very slowly through regions of many small particles at the rear. This behaviour is modelled in this paper using asymmetric segregation flux functions. Exact non-diffuse solutions are derived for the steady wave structure using the method of characteristics with a cubic segregation flux. Three different structures emerge, dependent on the degree of asymmetry and the non-convexity of the segregation flux function. In particular, a novel 'lens-tail' solution is found for segregation fluxes that have a large amount of non-convexity, with an additional expansion fan and compression wave forming a 'tail' upstream of the 'lens' region. Analysis of exact solutions for the particle motion shows that the large particle motion through the 'lens-tail' is fundamentally different to the classical 'lens' solutions. A few large particles starting near the bottom of the breaking wave pass through the 'tail', where they travel in a region of many small particles with a very small vertical velocity, and take significantly longer to recirculate. †

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Section: Recirculating Particle Motionmentioning

confidence: 99%

Asymmetric breaking size-segregation waves in dense granular free-surface flows

et al. 2016

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“…Note that even if the solid volume fraction is used as a control parameter it reflects only partially the complexity of the granular microstructure from which originate the local mechanisms responsible for the micro-failures described in [5][6][7][8][9]. Other quantities should be more relevant than the solid volume fraction as for instance the dilatancy angle [12,23] or the mean coordination number. Moreover, to analyze the role of boundary conditions, we will vary the geometry of the cell by changing independently of each other the width, height and depth of the sample.…”

Section: Conclusion and Prospectsmentioning

confidence: 99%

“…By now, only very few studies have investigated the transient regime [10][11][12] while many works have dealt with the steady regime of dry surface avalanche flows leading in particular to the proposal of a local granular rheology for dense flows (see [13] and references therein). According to the definition of three regimes of avalanches in fluids [14], the local rheology has been subsequently extended to the case of submarine granular flows down inclined planes [15].…”

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Sensitivity to solid volume fraction of gravitational instability in a granular medium

Bonnet¹,

Richard²,

Philippe³

2010

Granular Matter

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We report experimental results on incipient dynamics of gravitational destabilization in an immersed granular medium. Two protocols have been used to reach instability: a progressive loading and a dam-break situation. The instability triggering, analyzed by high-speed imaging and CIV post-processing, reveals distinct dynamical behaviours depending on the initial packing fraction of the medium. Two destabilisation modes have been observed: a surface avalanche and a deeper phenomenon that we called circular collapse.

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“…They observed that the fluid has a twofold effect on the granular motion. On one hand, it may reduce the granular kinetic energy by developing negative pore pressure and fluid viscous drag force (Iverson et al 2000;Pailha et al 1994;Topin et al 2011). On the other hand, it can also enhance the granular flow by lubricating the granular mixture.…”

Section: Introductionmentioning

confidence: 99%

Rockslide and Impulse Wave Modelling in the Vajont Reservoir by DEM-CFD Analyses

2015

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This paper investigates the generation of hydrodynamic water waves due to rockslides plunging into a water reservoir. Quasi-3D DEM analyses in plane strain by a coupled DEM-CFD code are adopted to simulate the rockslide from its onset to the impact with the still water and the subsequent generation of the wave. The employed numerical tools and upscaling of hydraulic properties allow predicting a physical response in broad agreement with the observations notwithstanding the assumptions and characteristics of the adopted methods. The results obtained by the DEM-CFD coupled approach are compared to those published in the literature and those presented by Crosta et al. (Landslide spreading, impulse waves and modelling of the Vajont rockslide. Rock mechanics, 2014) in a companion paper obtained through an ALE-FEM method. Analyses performed along two cross sections are representative of the limit conditions of the eastern and western slope sectors. The max rockslide average velocity and the water wave velocity reach ca. 22 and 20 m/s, respectively. The maximum computed run up amounts to ca. 120 and 170 m for the eastern and western lobe cross sections, respectively. These values are reasonably similar to those recorded during the event (i.e. ca. 130 and 190 m, respectively). Therefore, the overall study lays out a possible DEM-CFD framework for the modelling of the generation of the hydrodynamic wave due to the impact of a rapid moving rockslide or rock-debris avalanche.

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Initiation of underwater granular avalanches: Influence of the initial volume fraction

Cited by 93 publications

References 20 publications

Asymmetric breaking size-segregation waves in dense granular free-surface flows

Asymmetric breaking size-segregation waves in dense granular free-surface flows

Sensitivity to solid volume fraction of gravitational instability in a granular medium

Rockslide and Impulse Wave Modelling in the Vajont Reservoir by DEM-CFD Analyses

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