Author
L. Girolami
Other affiliations: LMV, University of Toulouse, Blaise Pascal University ...read more
Bio: L. Girolami is an academic researcher from François Rabelais University. The author has contributed to research in topics: Settling & Discrete element method. The author has an hindex of 7, co-authored 18 publications receiving 349 citations. Previous affiliations of L. Girolami include LMV & University of Toulouse.
Papers
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TL;DR: In this paper, a novel variant of Discrete Element Method (DEM) is proposed to simulate the flow dynamics of granular material made of non-spherical particles, which is limited to particles of convex shape but permits to consider any combination of shape and size.
146 citations
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TL;DR: In this article, a 3D discrete-element model, Grains3D, was used to simulate the behavior of granular columns that propagate down a rough horizontal surface from different initial conditions (varying the initial aspect ratio).
Abstract: In this paper, we used a 3-D discrete-element model, Grains3D, which allows the simulation of unsteady granular flows of monodisperse soft spherical particles in a common situation (i.e., down a rectangular channel). A series of numerical dam-break experiments was performed to predict the behavior of granular columns that propagate down a rough horizontal surface from different initial conditions (varying the initial aspect ratio). Numerical results were compared to those obtained experimentally by Lajeunesse et al. (Phys Fluids 17:103302, 2005) from a similar configuration. Runout distance, temporal flow evolution, deposit morphology and internal flow structures of similar laboratory experiments were quantitatively reproduced as well as prediction of empirical and theoretical scaling laws. This paper highlights that such fully 3-D simulations of soft-spheres can remarkably capture dam-break collapses performed in a rectangular channel. Moreover, Grains3D can provide a complete physical description of such complex unsteady systems which will be the topic of future on-going studies.
72 citations
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TL;DR: In this article, the authors conducted laboratory experiments on dam-break flows of sub-250-µm volcanic ash, generated by the release of gas-fluidized and variably nonexpanded to expanded (up to 35%) beds, in order to gain insights into the internal kinematics of pyroclastic flows.
Abstract: We conducted laboratory experiments on dam-break flows of sub-250-µm volcanic ash, generated by the release of gas-fluidized and variably non-expanded to expanded (up to 35%) beds, in order to gain insights into the internal kinematics of pyroclastic flows. The flows were typically several cm thick and had frontal speeds of up to ∼2 m s−1. High-speed videos taken through the transparent sidewall of the 3-m-long channel were analyzed with a particle-tracking algorithm, providing a spatial and temporal description of transport and sedimentation. The flows deposited progressively as they traveled down the flume, being consumed by sedimentation until they ran out of volume. Deposition commenced 5–20 cm rearward of the flow front and (for a given expansion) proceeded at a rate independent of distance from the lock gate. Deposit aggradation velocities were equal to those inferred beneath quasi-static bed collapse tests of the same ash at the same initial expansions, implying that shear rates of up to ∼300 s−1 have no measurable effect on aggradation rate. The initially non-expanded (and just fluidized) flow deposited progressively at a rate indicative of an expansion of a few percent, perhaps due to shear-induced Reynolds dilation during initial slumping. The fronts of the flows slid across the flume floor on very thin basal shear layers, but once deposition commenced a no-slip condition was established at the depositional interface. Within the flows, the trajectory of the constituent particles was linear and sub-horizontal. The velocities of the particles increased with height above the depositional interface, reached a maximum, then declined slightly towards the flow surface, perhaps due to air drag. At a given location, the velocity profiles were translated upwards as the deposit aggraded. The results show that even cm-thin, poorly expanded flows of ash deposit progressively, as inferred for many pyroclastic flows. The change from (frontal) slip to (rearward) no-slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits.
52 citations
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TL;DR: The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects as discussed by the authors, which can be attributed to the high gaspore pressures associated with high fluidization effect.
Abstract: The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects We carried out experiments on flows of fluidized volcanic ash at 170°C, which is hot enough to reduce cohesive effects of moisture The flows were generated in a 3‐m‐long, horizontal lock‐exchange flume The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released Each flow defluidized progressively down the flume until motion ceased Initial expansion E and initial height h0 were varied independently of one another The flows traveled in a laminar manner Flow fronts exhibited three main phases of transport: (1) a brief initial phase of gravitational slumping, (2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase Phase 2 frontal velocities scaled with equation image, like other types of dam‐break flow Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E Despite rates of shear up to 80 s−1, aggradation rates were identical to those determined independently, at the same value of E, in quasi‐static collapse tests Sedimentation caused the flows to thin progressively during transit until they ran out of volume The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h0/x0 in the lock reservoir and (2) the ratio tsett/tgrav of two timescales: a particle settling time tsett and a gravitational acceleration time tgrav
40 citations
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01 May 1993
TL;DR: Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems.
Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.
29,323 citations
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TL;DR: In this paper, a review of recent developments in the discrete element method (DEM) to model particles of non-spherical shape is presented, including shape representation, algorithms for the efficient detection of contacts and the determination of contact parameters.
417 citations
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TL;DR: A review of the recent efforts in developing discrete element method (DEM) approaches to model non-spherical particulate systems (NSPS) and strategies of coupling such a nonspherical DEM model with computational fluid dynamics (CFD) for particle-fluid flows is presented in this paper.
414 citations
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TL;DR: In this paper, the authors describe laboratory experiments of granular material flowing over an inclined plane covered by an erodible bed, designed to mimic erosion processes of natural flows travelling over deposits built up by earlier events.
Abstract: [1] We describe laboratory experiments of granular material flowing over an inclined plane covered by an erodible bed, designed to mimic erosion processes of natural flows travelling over deposits built up by earlier events. Two controlling parameters are the inclination of the plane and the thickness of the erodible layer. We show that erosion processes can increase the flow mobility (i.e., runout) over slopes with inclination close to the repose angle of the grains θr by up to 40%, even for very thin erodible beds. Erosion efficiency is shown to strongly depend on the slope of the topography. Entrainment begins to affect the flow at inclination angles exceeding a critical angle θc ≃ θr/2. Runout distance increases almost linearly as a function of the thickness of the erodible bed, suggesting that erosion is mainly supply-dependent. Two regimes are observed during granular collapse: a first spreading phase with high velocity followed by a slow thin flow, provided either the slope or the thickness of the erodible bed is high enough. Surprisingly, erosion affects the flow mostly during the deceleration phase and the slow regime. The avalanche excavates the erodible layer immediately at the flow front. Waves are observed behind the front that help to remove grains from the erodible bed. Steep frontal surges are seen at high inclination angles over both rigid or erodible bed. Finally, simple scaling laws are proposed making it possible to obtain a first estimate of the deposit and emplacement time of a granular collapse over a rigid or erodible inclined bed.
243 citations
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TL;DR: The physical processes that modulate magma accumulation in the upper crust, transport magma to the surface, and control eruptive activity have been investigated in this paper, focusing on the physical processes of volcanic activity.
Abstract: Over the past 25 years, our understanding of the physical processes that drive volcanic eruptions has increased enormously thanks to major advances in computational and analytical facilities, instrumentation, and collection of comprehensive observational, geophysical, geochemical, and petrological data sets associated with recent volcanic activity. Much of this work has been motivated by the recognition that human exposure to volcanic hazard is increasing with both expanding populations and increasing reliance on infrastructure (as illustrated by the disruption to air traffic caused by the 2010 eruption of Eyjafjallajokull volcano in Iceland). Reducing vulnerability to volcanic eruptions requires a thorough understanding of the processes that govern eruptive activity. Here, we provide an overview of our current understanding of how volcanoes work. We focus particularly on the physical processes that modulate magma accumulation in the upper crust, transport magma to the surface, and control eruptive activity.
225 citations