articles by Mattia de' Michieli Vitturi
Since the 1970s, multiple reconstruction techniques have been proposed and are currently used, to... more Since the 1970s, multiple reconstruction techniques have been proposed and are currently used, to extrapolate and quantify eruptive parameters from sampled tephra fall deposit datasets. Atmospheric transport and deposition processes strongly control the spatial distribution of tephra deposit; therefore, a large uncertainty affects mass derived estimations especially for fall layer that are not well exposed. This paper has two main aims: the first is to analyse the sensitivity to the deposit sampling strategy of reconstruction techniques. The second is to assess whether there are differences between the modelled values for emitted mass and grainsize, versus values estimated from the deposits. We find significant differences and propose a new correction strategy. A numerical approach is demonstrated by simulating with a dispersal code a mild explosive event occurring at Mt. Etna on 24 November 2006. Eruptive parameters are reconstructed by an inversion using available tephra information collected after the eruption. A full synthetic deposit is created by integrating the deposited mass computed by the model over the computational domain (i.e., an area of 7.5 × 10^4 km2). A statistical analysis based on 2000 sampling tests of 50 sampling points shows a large variability, up to 50% for all the reconstruction techniques. Moreover, for some test examples Power Law errors are larger than estimated uncertainty. A similar analysis, on simulated grain-size classes, shows how spatial sampling limitations strongly reduce the utility of available information on the total grain size distribution. For example, information on particles coarser than φ(−4) is completely lost when sampling at 1.5 km from the vent for all columns with heights less than 2000 m above the vent. To correct for this effect an optimal sampling strategy and a new reconstruction method are presented. A sensitivity study shows that our method can be extended to a wide range of eruptive scenarios including those in which aggre-gation processes are important. The new correction method allows an estimate of the deficiency for each simulated class in calculated mass deposited, providing reliable estimation of uncertainties in the reconstructed total (whole deposit) grainsize distribution.
Mathematical and Computer Modelling, 2005
A numerical study for the simulation of rock deformation due to nonlinear temperature and pressur... more A numerical study for the simulation of rock deformation due to nonlinear temperature and pressure waves in fluid saturated porous rock is presented. The problem of an homogeneous, thermoelastic, and isotropic fluid-saturated matrix, lying over an aquifer in hyperthermal domain an limited by an upper free surface is considered. The equations of thermo-poro-elasticity, the energy balance and Darcy equation are employed to model the deformation induced by pressure and temperature gradients originating from the source at depth. The numerical approach adopted to solve the nonlinear problem is based on a cubic finite-element method for the spatial discretization and an implicit scheme for the temporal discretization. This work essentially provides a numerical tool for interpreting some geophysical phenomena, in particular for the study of ground uplift in volcanic areas such as Rabaul caldera, Long Valley caldera, and Campi Flegrei. Some experiments and results are presented.
Earth and Planetary Science Letters, 2008
Keywords: conduit dynamics conduit geometry magma ascent effusion rate computational model
Computational Geosciences, 2007
An immersed boundary technique suitable for the solution of multiphase compressible equations of ... more An immersed boundary technique suitable for the solution of multiphase compressible equations of gas–particle flows of volcanic origin over complex 2D and 3D topographies has been developed and applied. This procedure combines and extends different existing methods designed for incompressible flows. Furthermore, the extension to compressible multiphase flows is achieved through a flux correction term in the mass continuity equations of the immersed cells that accounts for density variations in the partial volumes. The technique is computationally accurate and inexpensive, if compared to the use and implementation of the finite-volume technique on unstructured meshes. The first applications that we consider are the simulations of pyroclastic density currents generated by the collapse of a volcanic column in 2D axisymmetric geometry and by a dome explosion in 3D. Results show that the immersed boundary technique can significantly improve the description of the no-slip flow condition on an irregular topography even with relatively coarse meshes. Although the net effect of the present technique on the results is difficult to quantify in general terms, its adoption is recommended any time that cartesian grids are used to describe the large-scale dynamics of pyroclastic density currents over volcano topographies.
Journal of Geophysical Research, 2010
A new 2D/3D Lagrangian particle model (named LPAC) for the dynamics of clasts ejected during expl... more A new 2D/3D Lagrangian particle model (named LPAC) for the dynamics of clasts ejected during explosive eruptions is presented. The novelty of the model lies in the one-way coupling of the carrier flow field, given by a Eulerian multiphase flow code, and the particles. The model is based on a simplification of the Basset-Boussinesq-Oseen equation, expressing the Lagrangian equation of
Geophysical Research Letters, 2007
1] We applied a new simulation model, based on multiphase transport laws, to describe the 4D (3D ... more 1] We applied a new simulation model, based on multiphase transport laws, to describe the 4D (3D spatial coordinates plus time) dynamics of explosive eruptions. Numerical experiments, carried out on a parallel supercomputer, describe the collapse of the volcanic eruption column and the propagation of pyroclastic density currents (PDCs), for selected medium scale (sub-Plinian) eruptive scenarios at Vesuvius, Italy. Simulations provide crucial insights into the effects of the generation mechanism of the flows -partial collapse vs boiling-overon their evolution and hazard potential, the unstable dynamics of the fountain, and the influence of Mount Somma on the propagation of PDCs into the circum-Vesuvian area, one of the world's most hazardous volcanic settings. Results also show that it is possible to characterize the volcanic column behavior in terms of percentage of the mass of pyroclasts collapsed to the ground and how this parameter strongly influences the dynamics and hazard of the associated PDCs. Citation: Neri, A., T.
Earth and Planetary Science Letters, 2008
We develop a steady-state, two-phase flow model of magma ascent through an axisymmetric conduit o... more We develop a steady-state, two-phase flow model of magma ascent through an axisymmetric conduit of variable radius R and length L in order to quantify relationships between conduit geometry and magma ascent dynamics. Holding boundary conditions and chamber magma properties constant, we vary conduit geometry systematically and independently, such that the upper conduit radius increases or decreases by a factor of Rt/ Rb (radius ratio; 0.4 ≤ Rt/ Rb ≤ 2.5), above a change initiation height H (0.1 ≤ H/ L ≤ 0.7), and over length Le ( Le/ L = 0.2), where Rt and Rb are conduit radius above ( t) and below ( b) the radius change and H is the height above the top of the magma chamber. Conduit widening causes a drop in overpressure and corresponding increase in gas volume fraction and magma acceleration over the whole length of the conduit, with all changes increasing in magnitude with increasing radius ratio. Magma ascent rate increases roughly as R2 and volumetric flow rate subsequently increases as R4 when Rt = Rb = R. Both increasing Rt for a fixed Rb (increasing radius ratio) and increasing Rb for a fixed Rt (decreasing radius ratio), increase volume flow and magma ascent rates. Compared to changes in geometry, small changes in chamber pressure (< 5%) have a weak effect on flow rate. Many model runs produce a magma plug at the top of the conduit, largely due to permeable gas loss through conduit walls. In general, large radii and low radius ratios (i.e., nearly cylindrical conduits) favor thin, low-density plugs, which may facilitate sudden destruction of a plug, and thus enhance the likelihood of explosive over extrusive eruptions. These findings suggest that changes in conduit geometry, such as those caused by conduit erosion during explosive eruptions or by accretion of magma along conduit walls, are strongly coupled to magma ascent dynamics and should not be ignored when interpreting changes in eruptive behavior.
Earth and Planetary Science Letters, 2010
The transient dynamics of magma ascent during dome-forming eruptions were investigated and the ef... more The transient dynamics of magma ascent during dome-forming eruptions were investigated and the effects of magma chamber pressure perturbations on eruption rate are illustrated. The numerical model DOMEFLOW, developed by the authors for this work, is applied to the problem. DOMEFLOW is a transient 1.5D isothermal two-phase flow model of magma ascent through an axisymmetric conduit of variable radius, which accounts for gas exsolution, bubble growth, crystallization induced by degassing, permeable gas loss through overlying magma and through conduit walls, as well as viscosity changes due to crystallization and degassing. For runs in which chamber pressure increases, the time required to reach the new steady state (transition time) is a complex function of the pressure perturbation, while for decreasing chamber pressure, transition time is a monotonic function of the magnitude of the pressure perturbation. The transition to the new steady state is mainly controlled by magma compressibility, travel time (time required for one parcel of magma to travel from chamber to surface), and the time over which the pressure perturbation occurs. Results of many runs (> 300) were analyzed using dimensional analysis to reveal a general relationship which predicts the temporal evolution of magma effusion rate for a given sudden increase in chamber pressure; the product of the change in steady-state extrusion rate and the time required to reach the new steady state is linearly proportional to the normalized change in chamber pressure, the volume of the conduit, and the ratio of top and bottom conduit radii, and inversely proportional to the cubic root of volatile fraction. This relationship is used to interpret observed variations in two ongoing dome-building eruptions, the Soufrière Hills volcano, Montserrat, and Merapi volcano, Indonesia.
Environmental Modelling and Software, 2010
... 1). The total depth of the bed and the main characteristics of each layer (thickness and ... ... more ... 1). The total depth of the bed and the main characteristics of each layer (thickness and ... for the outlet also allow defining aeration from the bottom of the bed to simulate ... The vertical water flow through porous media in unsaturated conditions is described using the Richards ...
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articles by Mattia de' Michieli Vitturi