Sensitivity of the dynamics of volcanic eruption columns to their shape

Ishimine, Yasuhiro

doi:10.1007/s00445-005-0027-4

Cited by 15 publications

(23 citation statements)

References 32 publications

(94 reference statements)

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“…The entrainment coefficient also determines the plume shape (Ishimine, 2006) and can be empirically assessed by means of direct field observations or ad hoc laboratory measurements.…”

Section: Dusty-gas Modeling Of Volcanic Plumesmentioning

confidence: 99%

ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

2016

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Abstract.A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of polydisperse gas-particle mixtures forming volcanic plumes. Starting from the three-dimensional N-phase Eulerian transport equations for a mixture of gases and solid dispersed particles, we adopt an asymptotic expansion strategy to derive a compressible version of the first-order non-equilibrium model, valid for low-concentration regimes (particle volume fraction less than 10 −3 ) and particle Stokes number (St -i.e., the ratio between relaxation time and flow characteristic time) not exceeding about 0.2. The new model, which is called ASHEE (ASH Equilibrium Eulerian), is significantly faster than the N-phase Eulerian model while retaining the capability to describe gas-particle non-equilibrium effects. Direct Numerical Simulation accurately reproduces the dynamics of isotropic, compressible turbulence in subsonic regimes. For gas-particle mixtures, it describes the main features of density fluctuations and the preferential concentration and clustering of particles by turbulence, thus verifying the model reliability and suitability for the numerical simulation of highReynolds number and high-temperature regimes in the presence of a dispersed phase. On the other hand, Large-Eddy Numerical Simulations of forced plumes are able to reproduce the averaged and instantaneous flow properties. In particular, the self-similar Gaussian radial profile and the development of large-scale coherent structures are reproduced, including the rate of turbulent mixing and entrainment of atmospheric air. Application to the Large-Eddy Simulation of the injection of the eruptive mixture in a stratified atmosphere describes some of the important features of turbulent volcanic plumes, including air entrainment, buoyancy reversal and maximum plume height. For very fine particles (St → 0, when non-equilibrium effects are negligible) the model reduces to the so-called dusty-gas model. However, coarse particles partially decouple from the gas phase within eddies (thus modifying the turbulent structure) and preferentially concentrate at the eddy periphery, eventually being lost from the plume margins due to the concurrent effect of gravity. By these mechanisms, gas-particle non-equilibrium processes are able to influence the large-scale behavior of volcanic plumes.

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“…The entrainment coefficient also determines the plume shape (Ishimine, 2006) and can be empirically assessed by means of direct field observations or ad hoc laboratory measurements.…”

Section: Dusty-gas Modeling Of Volcanic Plumesmentioning

confidence: 99%

ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

2016

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“…The calculation was performed with a vent temperature of 1000 K, vent velocity of 300 m/s, vent gas mass fraction of 0.03, and vent radii of 20, 100, and 200 m. A computer program developed by Ishimine [2006] was used. The upward velocity in the jet region, which is determined from the conditions in section 2.3, is plotted with dotted lines to focus the discussion on features as a buoyancygenerating plume.…”

Section: Discussionmentioning

confidence: 99%

“…Only the region below 15 km is shown because the increase rate rapidly decreased with height in the stratosphere owing to strong density stratification. See Ishimine [2006] for the influence of the decrease in the atmospheric pressure in the stratosphere. The discontinuity at the basal part of each eruption column is caused by the change in the entrainment coefficient prescribed in the Woods [1988] model.…”

Section: Discussionmentioning

confidence: 99%

“…We discuss a turbulent plume that is basically the same as that in the work of Morton et al [1956] except that the fluid significantly changes its volume upon mixing. See Morton et al [1956] and Ishimine [2006] for the fundamental assumptions, which are used in this paper to develop an alternative model.…”

Section: Fundamental Variablesmentioning

confidence: 99%

“…For simplicity, we apply equation (17) here, similarly to the situation in a uniform environment. See Ishimine [2006] for a discussion regarding the adequacy of this treatment. We can obtain the equation set of a dimensionless form by combining equations (16), (17), and (29): There are flow transitions from a jet through DP to LP.…”

Section: Stratified Environmentmentioning

confidence: 99%

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A simple integral model of buoyancy‐generating plumes and its application to volcanic eruption columns

Ishimine

2007

J. Geophys. Res.

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[1] This paper discusses the importance of the increase in buoyancy flux due to thermal expansion of entrained air to clarify the fundamental features of volcanic eruption columns. A one-dimensional steady state model, which is simplified as much as possible, is developed to elucidate the essential differences in behavior between a typical incompressible turbulent plume and an eruption column in which the buoyancy flux significantly increases with height. An analytical solution of a simple form is specifically derived for an idealized axisymmetric turbulent plume of a linearly increasing buoyancy flux in a uniform environment. The solution predicts that the upward velocity of the plume is constant along the height, in contrast to the upward velocity of a common incompressible plume, the velocity of which decreases inversely proportional to the third root of the height. More realistic plumes in both uniform and density-stratified environments are also investigated by modifying the one-dimensional model. The model yields several scaling parameters, some of which are used to estimate the terminal height of an eruption column. Numerical investigations using the parameters indicate that the thermal energy in an eruption column is exhausted for heating and expanding entrained air before reaching the terminal height. Numerical investigations also imply that the buoyancy flux in an eruption column may be less than half of that predicted by the conventional theory of an incompressible plume. The discussion based on the present model also sheds new light on the physical background of the superbuoyant behavior of an eruption column.

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Modelling pyroclastic density currents from a subplinian eruption at La Soufrière de Guadeloupe (West Indies, France)

et al. 2020

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We have used a three-dimensional, non-equilibrium multiphase flow numerical model to simulate subplinian eruption scenarios at La Soufrière de Guadeloupe (Lesser Antilles, France). Initial and boundary conditions for computer simulations were set on the basis of independent estimates of eruption source parameters (i.e. mass eruption rate, volatile content, temperature, grain size distribution) from a field reconstruction of the 1530 CE subplinian eruption. This event is here taken as a reference scenario for hazard assessment at La Soufrière de Guadeloupe. A parametric study on eruption source parameters allowed us to quantify their influence on the simulated dynamics and, in particular, the increase of the percentage of column collapse and pyroclastic density current (PDC) intensity, at constant mass eruption rate, with variable vent diameter. Numerical results enabled us to quantify the effects of the proximal morphology on distributing the collapsing mass around the volcano and into deep and long valleys and to estimate the areas invaded by PDCs, their associated temperature and dynamic pressure. Significant impact (temperature > 300 °C and dynamic pressure > 1 kPa) in the inhabited region around the volcano is expected for fully collapsing conditions and mass eruption rates > 2 × 107 kg/s. We thus combine this spatial distribution of temperature and dynamic pressure with an objective consideration of model-related uncertainty to produce preliminary PDC hazard maps for the reference scenario. In such a representation, we identify three areas of varying degree of susceptibility to invasion by PDCs—very likely to be invaded (and highly impacted), susceptible to invasion (and moderately impacted), and unlikely to be invaded (or marginally impacted). The study also raises some key questions about the use of deterministic scenario simulations for hazard assessment, where probability distributions and uncertainties are difficult to estimate. Use of high-performance computing techniques will in part allow us to overcome such difficulties, but the problem remains open in a scientific context where validation of numerical models is still, necessarily, an incomplete and ongoing process. Nevertheless, our findings provide an important contribution to the quantitative assessment of volcanic hazard and risk at La Soufrière de Guadeloupe particularly in the context of the current unrest of the volcano and the need to prepare for a possible future reawakening of the volcano that could culminate in a magmatic explosive eruption.

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Sensitivity of the dynamics of volcanic eruption columns to their shape

Cited by 15 publications

References 32 publications

ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

A simple integral model of buoyancy‐generating plumes and its application to volcanic eruption columns

Modelling pyroclastic density currents from a subplinian eruption at La Soufrière de Guadeloupe (West Indies, France)

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