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  • 1
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    Combined effects of grain size, flow volume and channel width on geophysical flow mobility: three-dimensional discrete element modeling of dry and dense flows of angular rock fragments (2017)
    Copernicus
    Details
    Publication Date: 2017-02-16
    Description: We have carried out new three-dimensional numerical simulations by using a discrete element method (DEM) to study the mobility of dry granular flows of angular rock fragments. These simulations are relevant for geophysical flows such as rock avalanches and pyroclastic flows. The model is validated by previous laboratory experiments. We confirm that (1) the finer the grain size, the larger the mobility of the center of mass of granular flows; (2) the smaller the flow volume, the larger the mobility of the center of mass of granular flows and (3) the wider the channel, the larger the mobility of the center of mass of granular flows. The grain size effect is due to the fact that finer grain size flows dissipate intrinsically less energy. This volume effect is the opposite of that experienced by the flow fronts. The original contribution of this paper consists of providing a comparison of the mobility of granular flows in six channels with a different cross section each. This results in a new scaling parameter χ that has the product of grain size and the cubic root of flow volume as the numerator and the product of channel width and flow length as the denominator. The linear correlation between the reciprocal of mobility and parameter χ is statistically highly significant. Parameter χ confirms that the mobility of the center of mass of granular flows is an increasing function of the ratio of the number of fragments per unit of flow mass to the total number of fragments in the flow. These are two characteristic numbers of particles whose effect on mobility is scale invariant.
    Print ISSN: 1869-9510
    Electronic ISSN: 1869-9529
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 2
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    Stresses at the base of dry and dense flows of angular rock fragments in 3-D discrete element modeling: Scaling of basal stress fluctuations versus grain size, flow volume and channel width (2018)
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    Publication Date: 2021-03-08
    Description: We simulate granular flows of angular rock fragments by means of a three-dimensional discrete element modeling to study the basal stresses that these flows exert on the subsurface. These granular flows have different grain sizes and different flow volumes and they model dry rock avalanches and dense pyroclastic flows. These flows travel on four different concave–upward chutes that represent channels on a mountainside or on the flank of a volcano. Each chute has a different width. The stress data demonstrate the validity of a linear relation between two scaling parameters: D and ψ. Parameter D is a scaled basal stress deviation that is equivalent to a scaled particle agitation. Particle agitation is ultimately responsible for the energy dissipation that governs the mobility of dense geophysical flows in nature. Parameter ψ contains grain size, flow volume and channel width. This second parameter is equal to the product of the reciprocal of characteristic numbers of fragments in granular flows. Since these numbers of particles are dimensionless, the linear relation is valid at any scale, either in the laboratory or in nature.
    Description: Published
    Description: 230-239
    Description: 5V. Processi eruttivi e post-eruttivi
    Description: JCR Journal
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 3
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    Stress level effect on mobility of dry granular flows of angular rock fragments (2021)
    Springer
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    Publication Date: 2021-09-06
    Description: Granular flows of angular rock fragments such as rock avalanches and dense pyroclastic flows are simulated numerically by means of the discrete element method. Since large-scale flows generate stresses that are larger than those generated by small-scale flows, the purpose of these simulations is to understand the effect that the stress level has on flow mobility. The results show that granular flows that slide en mass have a flow mobility that is not influenced by the stress level. On the contrary, the stress level governs flow mobility when granular flow dynamics is affected by clast agitation and collisions. This second case occurs on a relatively rougher subsurface where an increase of the stress level causes an increase of flow mobility. The results show also that as the stress level increases, the effect that an increase of flow volume has on flow mobility switches sign from causing a decrease of mobility at low stress level to causing an increase of mobility at high stress level. This latter volume effect corresponds to the famous Heim’s mobility increase with the increase of the volume of large rock avalanches detected so far only in the field and for this reason considered inexplicable without resorting to extraordinary mechanisms. Granular flow dynamics is described in terms of dimensionless scaling parameters in three different granular flow regimes. This paper illustrates for each regime the functional relationship of flow mobility with stress level, flow volume, grain size, channel width, and basal friction.
    Description: Published
    Description: 3085-3099
    Description: 5V. Processi eruttivi e post-eruttivi
    Description: JCR Journal
    Keywords: Pyroclastic Flows ; Rock Avalanches ; Stress Level ; Flow Volume ; Mobility ; mobility of pyroclastic flows and rock avalanches
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 4
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    Slope-break Collisions: Comment on “Insight Into Granular Flow Dynamics Relying on Basal Stress Measurements: From Experimental Flume Tests” by K. Li et al. (2023)
    Wiley-AGU
    Details
    Publication Date: 2023-05-25
    Description: Numerical simulations show that the positive correlation observed in laboratory experiments by Li et al. (2022) between an increase of grain size and particle agitation, on the one hand, and an increase of granular flow mobility, on the other hand, is not a valid cause-and-effect relationship. In other words, their mobility differential is not caused by a different energy dissipation rate that results from a different grain size content. Instead, the flows stop because of a head-on collision with the horizontal flume at the bottom of a steep 40º incline. Essentially, the slope-break jams the granular movement. Indeed, a combination of laboratory experiments and numerical simulations demonstrated that the mobility of unhindered dense granular flows increases as grain size and clast agitation decrease. Consequently, there is no evidence that the high mobility of large natural rock avalanches is due to an increase of particle agitation.
    Description: Published
    Description: e2022JB024799
    Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio
    Description: JCR Journal
    Keywords: Pyroclastic Flows ; Mobility ; 04.08. Volcanology
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 5
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    Flow front mobility of rock avalanches as a function of flow volume, grain size, channel width, basal friction and flow scale (2024)
    Springer
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    Publication Date: 2024-02-01
    Description: The ability to predict the mobility of rock avalanches is necessary when designing strategies to mitigate the risks they pose. A popular mobility indicator of the flow front is the Heim’s apparent friction coefficient muH. In the field, muH shows a decrease in value as flow volume V increases. But this correlation has been a mystery as to whether it is due to a causal relationship between V and mobility since: (1) field data of muH do not collapse onto a single curve because typically widely scattered and (2) laboratory experiments have shown an opposite volume effect on the center of mass mobility of miniature flows. My numerical simulations confirm for the first time the existence of a functional relationship of scaling parameters where muH decreases as V increases in unsteady and nonuniform 3D flows. Data scatter is caused by muH that is affected by numerous other variables besides V. The interplay of these variables produces different granular regimes with opposite volume effects. In particular, muH decreases as V increases in the regime characterized by a relatively rough subsurface. The relationship holds for large-scale flows that, like rock avalanches, consist of a very large number of fine clasts traveling in wide channels. In these dense flows, flow front mobility increases as flow volume increases, as channel width increases, as grain size decreases, as basal friction decreases and as flow scale increases. Larger-scale flows are more mobile because they have larger Froude number values.
    Description: In press
    Description: OSV1: Verso la previsione dei fenomeni vulcanici pericolosi
    Description: JCR Journal
    Keywords: Pyroclastic Flows ; Rock Avalanches ; Flow Front ; Mobility
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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