Matteo Cerminara | Istituto Nazionale di Geofisica e Vulcanologia (original) (raw)
Papers by Matteo Cerminara
Bulletin of Volcanology
Numerical results of a two-layer depth-averaged model of pyroclastic density currents (PDCs) were... more Numerical results of a two-layer depth-averaged model of pyroclastic density currents (PDCs) were compared with an experimental PDC generated at the international eruption simulator facility (the Pyroclastic flow Eruption Large-scale Experiment (PELE)) to establish a minimal dynamical model of PDCs with stratification of particle concentrations. In the present two-layer model, the stratification in PDCs is modeled as a voluminous suspended-load layer with low particle volume fractions ($$\lesssim {10}^{-3})$$ ≲ 10 - 3 ) and a thin basal bed-load layer with higher particle volume fractions ($$\sim {10}^{-2}$$ ∼ 10 - 2 ) on the basis of the source condition in the experiment. Numerical results for the suspended load quantitatively reproduce the time evolutions of the front position and flow thickness in the experimental PDC. The numerical results of the bed-load and deposit thicknesses depend on an assumed value of settling speed at the bottom of the bed load ($${W}_{\mathrm{sH}}$$ W ...
<p>In the last years, the interest in three-dimensional&... more <p>In the last years, the interest in three-dimensional  physico-mathematical models for volcanic plumes has grown, motivated by the need of predicting accurately the dispersal patterns of volcanic ash in the atmosphere (to mitigate the risks for civil aviation and for the nearby inhabited regions) and pushed by improved remote sensing techniques and measurements. However, limitations due to the mesh resolution and numerical accuracy as well as the complexity entailed model formulations, have so far prevented a detailed study of turbulence in volcanic plumes at high resolution. Eruptive columns are indeed multiphase gas-particle turbulent flows, in which the largest (integral) scale is in the order of tens or hundreds of kilometers and the smallest scale is of the order of microns. Performing accurate numerical simulations of such phenomena remains therefore a challenging task.</p><p>Modern HPC resources and recent model developments enable the study of multiphase turbulent structures of volcanic plumes with an unprecedented level of detail. However, a number of issues of the present model implementation need to be addressed in order to efficiently use the computational resources of modern supercomputing machines. Here we present an overview of an optimization strategy that allows us to perform large parallel simulations of volcanic plumes using ASHEE, a numerical solver based on OpenFOAM and one of the target flagship codes of the project ChEESE (Centre of Excellence for Exascale in Solid Earth). Such optimizations include: mixed precision floating point operations to increase computational speed and reduce memory usage, optimal domain decomposition for better communication load balancing and asynchronous I/O to hide I/O costs. Scaling analysis and volcanic plume simulations are presented to demonstrate the improvement in both computational performances and computing capability.</p>
Remote Sensing
Thermal-infrared remote sensing is used to monitor and study hazardous volcanic phenomena. Therma... more Thermal-infrared remote sensing is used to monitor and study hazardous volcanic phenomena. Thermal cameras are often used by monitoring centers and laboratories. A physical comprehension of their behavior is needed to perform quantitative measurements, which are strongly dependent on camera features and settings. This makes it possible to control the radiance measurements related to volcanic processes and, thus, to detect thermal anomalies, validate models, and extract source parameters. We review the theoretical background related to the camera behavior beside the main features affecting thermal measurements: Atmospheric transmission, object emissivity and reflectivity, camera characteristics, and external optics. We develop a Python package, PythTirCam-1.0, containing pyTirTran, a radiative transfer model based on the HITRAN database and the camera spectral response. This model is compared with the empirical algorithm implemented into a commercial camera. These two procedures are ...
<p&amp... more <p>While computational capabilities in volcano science are developing to progressively higher sophistication levels involving HPC, parallel programming, and extensive use of super-computers, there is an increasing demand for accessibility to low to intermediate-level models and codes that can support multi-disciplinary research carried out by experts other than physical modelers and code developers. Responding to such a need by the international community is the justification and objective of Virtual Access (VA) activities developed under the EUROVOLC project. The Volcano Dynamics Computational Centre (VDCC) at INGV Pisa is renown as one international leader in physical-mathematical modelling and numerical simulation of volcanic thermo-fluid dynamics processes occurring from the deep regions of magma rise and accumulation within the crust, to within the atmosphere during volcanic eruptions. VDCC has been developing a large set of computational tools during last 30 years, that are offered under EUROVOLC for Transnational Access (for the most sophisticated, computational demanding models and codes) as well as for VA for low to intermediate-level models and codes. The latter include from non-ideal, compositional-dependent, multi-component volatile-melt thermodynamics to steady-state magma ascent to fast-performing kinematic modelling of pyroclastic density currents. Here we illustrate the model capabilities, the procedures to both download the codes and perform web-based computation, and a few relevant examples of calculations available through VA, and show relevant statistics of access and download by the volcano community to-date.</p>
Annals of Geophysics, 2017
Processes occurring in volcanic conduits, the pathways through which magma travels from its stora... more Processes occurring in volcanic conduits, the pathways through which magma travels from its storage region to the surface, have a fundamental control on the nature of eruptions and associated phenomena. It has been well established that magma flows, crystallizes, degasses, and fragments in conduits, that fluids migrate in and out of conduits, and that seismic and acoustic waves are generated and travel within conduits. A better understanding of volcanic conduits and related processes is of paramount importance for improving eruption forecasting, volcanic hazard assessment and risk mitigation. However, despite escalating advances in the characterization of individual conduit processes, our understanding of their mutual interactions and the consequent control on volcanic activity is still limited. With the purpose of addressing this topic, a multidisciplinary workshop led by a group of international scientists was hosted from 25 to 27 October 2014 by the Pisa branch of the Istituto Na...
Journal of Volcanology and Geothermal Research, 2016
Abstract In the framework of the IAVCEI (International Association of Volcanology and Chemistry o... more Abstract In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) initiative on volcanic plume models intercomparison, we discuss three-dimensional numerical simulations performed with the multiphase flow model PDAC (Pyroclastic Dispersal Analysis Code). The model describes the dynamics of volcanic and atmospheric gases (in absence of wind) and two pyroclastic phases by adopting a non-equilibrium Eulerian–Eulerian formulation. Accordingly, gas and particulate phases are treated as interpenetrating fluids, interacting with each other through momentum (drag) and heat exchange. Numerical results describe the time-wise and spatial evolution of weak (mass eruption rate: 1.5 × 106 kg/s) and strong (mass eruption rate: 1.5 × 109 kg/s) plumes. The two tested cases display a remarkably different phenomenology, associated with the different roles of atmospheric stratification, compressibility and mechanism of buoyancy reversal, reflecting in a different structure of the plume, of the turbulent eddies and of the atmospheric circulation. This also brings about different rates of turbulent mixing and atmospheric air entrainment. The adopted multiphase flow model allows to quantify temperature and velocity differences between the gas and particles, including settling, preferential concentration by turbulence and thermal non-equilibrium, as a function of their Stokes number, i.e., the ratio between their kinetic equilibrium time and the characteristic large-eddy turnover time of the turbulent plume. As a result, the spatial and temporal distribution of coarse ash in the atmosphere significantly differs from that of the fine ash, leading to a modification of the plume shape. Finally, three-dimensional numerical results have been averaged in time and across horizontal slices in order to obtain a one-dimensional picture of the plume in a stationary regime. For the weak plume, the results are consistent with one-dimensional models, at least in the buoyant plume region, and allow to reckon a variable, effective entrainment coefficient with a mean value around 0.1 (consistently with laboratory experiments). For the strong plume, analysis of the results reveals that the two most critical assumptions of one-dimensional integral models are the self-similarity and the pressure equilibrium. In such a case, the plume appears to be controlled by the dynamics in the jet stage (below the buoyancy reversal) and by mesoscale vorticity associated with the development of the umbrella.
Sensors (Basel, Switzerland), Jan 28, 2017
This study analyzes the capabilities of a LiNbO₃ whispering gallery mode microdisc resonator as a... more This study analyzes the capabilities of a LiNbO₃ whispering gallery mode microdisc resonator as a potential bolometer detector in the THz range. The resonator is theoretically characterized in the stationary regime by its thermo-optic and thermal coefficients. Considering a Q-factor of 10⁷, a minimum detectable power of 20 μW was evaluated, three orders of magnitude above its noise equivalent power. This value opens up the feasibility of exploiting LiNbO₃ disc resonators as sensitive room-temperature detectors in the THz range.
We performed an inter-comparison study of three-dimensional models of volcanic plumes. A set of c... more We performed an inter-comparison study of three-dimensional models of volcanic plumes. A set of common volca-nological input parameters and meteorological conditions were provided for two kinds of eruptions, representing a weak and a strong eruption column. From the different models, we compared the maximum plume height, neutral buoyancy level (where plume density equals that of the atmosphere), and level of maximum radial spreading of the umbrella cloud. We also compared the vertical profiles of eruption column properties, integrated across cross-sections of the plume (integral variables). Although the models use different numerical procedures and treatments of subgrid turbulence and particle dynamics, the inter-comparison shows qualitatively consistent results. In the weak plume case (mass eruption rate 1.5 × 10 6 kg s −1), the vertical profiles of plume properties (e.g., vertical velocity , temperature) are similar among models, especially in the buoyant plume region. Variability...
Bollettino Dell Unione Matematica Italiana Sezione B Articoli Di Ricerca Matematica, 2007
In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Ear... more In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) in-tercomparison study on volcanic plume models, we present three-dimensional (3D) numerical simulations carried out with the ASHEE (ASH Equilibrium Eulerian) model. The ASHEE model solves the compressible balance equations of mass, momentum, and enthalpy of a gas-particle mixture and is able to describe the kinematic decoupling for particles characterized by Stokes number (i.e., the ratio between the particle equilibrium time and the flow characteristic time) lower than 0.2 (or particles smaller than about 1 mm). The computational fluid dynamic model is designed to accurately simulate a turbulent flow field using a Large Eddy Simulation approach , and is thus suited to analyze the role of particle non-equilibrium in the dynamics of turbulent volcanic plumes. The two reference scenarios analyzed correspond to a weak (mass eruption rate = 1.5* 10 6 kg/s) and a strong volcanic plume (mass eruption rate = 1.5*10 9 kg/s) in absence of wind. For each scenario, we compare the 3D results, averaged in space and time, with theoretical results obtained from integral plume models. Such an approach enables quantitative evaluation of the effects of grid resolution and the subgrid-scale turbulence model, and the influence of gas-particle non-equilibrium processes on the large-scale plume dynamics. We thus demonstrate that the uncertainty on the numerical solution associated with such effects can be significant (of the order of 20%), but still lower than that typically associated with input data and integral model approximations. In the Weak Plume case, 3D results are consistent with the predictions of integral models in the jet and plume regions, with an entrainment coefficient around 0.10 in the plume region. In the Strong Plume case, the self-similarity assumption is less appropriate and the entrainment coefficient in the plume region is more unstable, with an average value of 0.24. For both cases, integral model predictions diverge from the 3D plume behavior in the umbrella region. The presented analysis of 3D numerical simulations thus enables identification of the critical hypotheses that underlie integral models used in operational studies. In addition, high-resolution 3D runs allow reproduction of observable quantities (such as infrasound signals) which can be useful for constraining eruption dynamics during real events.
This study compares and evaluates one-dimensional (1D) and three-dimensional (3D) numerical model... more This study compares and evaluates one-dimensional (1D) and three-dimensional (3D) numerical models of volcanic eruption columns in a set of different inter-comparison exercises. The exercises were designed as a blind test in which a set of common input parameters was given for two reference eruptions, representing a strong and a weak eruption column under different meteorological conditions. Comparing the results of the different models allows us to evaluate their capabilities and target areas for future improvement. Despite their different formulations, the 1D and 3D models provide reasonably consistent predictions of some of the key global descriptors of the volcanic plumes. Variability in plume height, estimated from the standard deviation of model predictions, is within ~20% for the weak plume and ~10% for the strong plume. Predictions of neutral buoyancy level are also in reasonably good agreement among the different models, with a standard deviation ranging from 9 to 19% (the latter for the weak plume in a windy atmosphere). Overall, these discrepancies are in the range of observational uncertainty of column height. However, there are important differences amongst models in terms of local properties along the plume axis, particularly for the strong plume. Our analysis suggests that the simplified treatment of entrainment in 1D models is adequate to resolve the general behaviour of the weak plume. However, it is inadequate to capture complex features of the strong plume, such as large vortices, partial column collapse, or gravitational fountaining that strongly enhance entrainment in the lower atmosphere. We conclude that there is a need to more accurately quantify entrainment rates, improve the representation of plume radius, and incorporate the effects of column instability in future versions of 1D volcanic plume models.
A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of po... more A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of polydis-perse 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 com-pressible 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 high-Reynolds 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.
We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forwar... more We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forward approach, the model is able to simulate the plume dynamics from prescribed input flow conditions and generate the corresponding synthetic thermal infrared (TIR) image, allowing a comparison with field-based observations. An inversion procedure is then developed to retrieve vent conditions from TIR images, and to independently estimate the mass eruption rate. The adopted fluid-dynamic model is based on a one-dimensional, stationary description of a self-similar turbulent plume, for which an asymptotic analytical solution is obtained. The electromagnetic emission/absorption model is based on Schwarzschild's equation and on Mie's theory for disperse particles, and we assume that particles are coarser than the radiation wavelength (about 10 μm) and that scattering is negligible. In the inversion procedure, model parameter space is sampled to find the optimal set of input conditions which minimizes the difference between the experimental and the synthetic image. Application of the inversion procedure to an ash plume at Santiaguito (Santa Maria volcano, Guatemala) has allowed us to retrieve the main plume input parameters, namely mass flow rate, initial radius, velocity, temperature, gas mass ratio, entrainment coefficient and their related uncertainty. Moreover, by coupling with the electromagnetic model we have been able to obtain a reliable estimate of the equivalent Sauter diameter of the total particle size distribution. The presented method is general and, in principle, can be applied to the spatial distribution of particle concentration and temperature obtained by any fluid-dynamic model, either integral or multidimensional, stationary or time-dependent, single or multiphase. The method discussed here is fast and robust, thus indicating potential for applications to real-time estimation of ash mass flux and particle size distribution, which is crucial for model-based forecasts of the volcanic ash dispersal process.
We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forwar... more We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forward
approach, the model is able to simulate the plume dynamics from prescribed input flow conditions and
generate the corresponding synthetic thermal infrared (TIR) image, allowing a comparison with
field-based observations. An inversion procedure is then developed to retrieve ash plume properties
from TIR images.
The adopted fluid-dynamic model is based on a one-dimensional, stationary description of a self-similar (top-hat) turbulent plume, for which an asymptotic analytical solution is obtained.
%assuming that the gas and particulate phases are in kinetic and thermal equilibrium.
%by neglecting the effect of atmospheric stratification and in the limit of small density contrasts.
The electromagnetic emission/absorption model is based on the Schwarzschild's equation and on Mie's theory for disperse particles, assuming that particles are coarser than the radiation wavelength and neglecting scattering.
%To apply the procedure, experimental images are preliminary manipulated to obtain a time-averaged image having with a vertically oriented axis.
In the inversion procedure, model parameters space is sampled to find the optimal set of input conditions which minimizes the difference between the experimental and the synthetic image.
Two complementary methods are discussed: the first is based on a fully two-dimensional fit of the TIR image, while the second only inverts axial data.
Due to the top-hat assumption (which overestimates density and temperature at the plume margins), the one-dimensional fit results to be more accurate. However, it cannot be used to estimate the average plume opening angle. Therefore, the entrainment coefficient can only be derived from the two-dimensional fit.
Application of the inversion procedure to an ash plume at Santiaguito volcano (Guatemala) has
allowed us to retrieve the main plume input parameters, namely the initial radius b0b_0b0, velocity U0U_0U0, temperature T0T_0T0, gas mass ratio n0n_0n0, entrainment coefficient kkk and their related
uncertainty. Moreover, coupling with the electromagnetic model, we have been able to
obtain a reliable estimate of the equivalent Sauter diameter dsd_sds of the total particle size
distribution.
The presented method is general and, in principle, can be applied to the spatial distribution of
particle concentration and temperature obtained by any fluid-dynamic model, either integral or
multidimensional, stationary or time-dependent, single or multiphase.
The method discussed here is fast and robust, thus indicating potential for applications to
real-time estimation of ash mass flux and particle size distribution, which is crucial for
model-based forecasts of the volcanic ash dispersal process.
Bulletin of Volcanology
Numerical results of a two-layer depth-averaged model of pyroclastic density currents (PDCs) were... more Numerical results of a two-layer depth-averaged model of pyroclastic density currents (PDCs) were compared with an experimental PDC generated at the international eruption simulator facility (the Pyroclastic flow Eruption Large-scale Experiment (PELE)) to establish a minimal dynamical model of PDCs with stratification of particle concentrations. In the present two-layer model, the stratification in PDCs is modeled as a voluminous suspended-load layer with low particle volume fractions ($$\lesssim {10}^{-3})$$ ≲ 10 - 3 ) and a thin basal bed-load layer with higher particle volume fractions ($$\sim {10}^{-2}$$ ∼ 10 - 2 ) on the basis of the source condition in the experiment. Numerical results for the suspended load quantitatively reproduce the time evolutions of the front position and flow thickness in the experimental PDC. The numerical results of the bed-load and deposit thicknesses depend on an assumed value of settling speed at the bottom of the bed load ($${W}_{\mathrm{sH}}$$ W ...
<p>In the last years, the interest in three-dimensional&... more <p>In the last years, the interest in three-dimensional  physico-mathematical models for volcanic plumes has grown, motivated by the need of predicting accurately the dispersal patterns of volcanic ash in the atmosphere (to mitigate the risks for civil aviation and for the nearby inhabited regions) and pushed by improved remote sensing techniques and measurements. However, limitations due to the mesh resolution and numerical accuracy as well as the complexity entailed model formulations, have so far prevented a detailed study of turbulence in volcanic plumes at high resolution. Eruptive columns are indeed multiphase gas-particle turbulent flows, in which the largest (integral) scale is in the order of tens or hundreds of kilometers and the smallest scale is of the order of microns. Performing accurate numerical simulations of such phenomena remains therefore a challenging task.</p><p>Modern HPC resources and recent model developments enable the study of multiphase turbulent structures of volcanic plumes with an unprecedented level of detail. However, a number of issues of the present model implementation need to be addressed in order to efficiently use the computational resources of modern supercomputing machines. Here we present an overview of an optimization strategy that allows us to perform large parallel simulations of volcanic plumes using ASHEE, a numerical solver based on OpenFOAM and one of the target flagship codes of the project ChEESE (Centre of Excellence for Exascale in Solid Earth). Such optimizations include: mixed precision floating point operations to increase computational speed and reduce memory usage, optimal domain decomposition for better communication load balancing and asynchronous I/O to hide I/O costs. Scaling analysis and volcanic plume simulations are presented to demonstrate the improvement in both computational performances and computing capability.</p>
Remote Sensing
Thermal-infrared remote sensing is used to monitor and study hazardous volcanic phenomena. Therma... more Thermal-infrared remote sensing is used to monitor and study hazardous volcanic phenomena. Thermal cameras are often used by monitoring centers and laboratories. A physical comprehension of their behavior is needed to perform quantitative measurements, which are strongly dependent on camera features and settings. This makes it possible to control the radiance measurements related to volcanic processes and, thus, to detect thermal anomalies, validate models, and extract source parameters. We review the theoretical background related to the camera behavior beside the main features affecting thermal measurements: Atmospheric transmission, object emissivity and reflectivity, camera characteristics, and external optics. We develop a Python package, PythTirCam-1.0, containing pyTirTran, a radiative transfer model based on the HITRAN database and the camera spectral response. This model is compared with the empirical algorithm implemented into a commercial camera. These two procedures are ...
<p&amp... more <p>While computational capabilities in volcano science are developing to progressively higher sophistication levels involving HPC, parallel programming, and extensive use of super-computers, there is an increasing demand for accessibility to low to intermediate-level models and codes that can support multi-disciplinary research carried out by experts other than physical modelers and code developers. Responding to such a need by the international community is the justification and objective of Virtual Access (VA) activities developed under the EUROVOLC project. The Volcano Dynamics Computational Centre (VDCC) at INGV Pisa is renown as one international leader in physical-mathematical modelling and numerical simulation of volcanic thermo-fluid dynamics processes occurring from the deep regions of magma rise and accumulation within the crust, to within the atmosphere during volcanic eruptions. VDCC has been developing a large set of computational tools during last 30 years, that are offered under EUROVOLC for Transnational Access (for the most sophisticated, computational demanding models and codes) as well as for VA for low to intermediate-level models and codes. The latter include from non-ideal, compositional-dependent, multi-component volatile-melt thermodynamics to steady-state magma ascent to fast-performing kinematic modelling of pyroclastic density currents. Here we illustrate the model capabilities, the procedures to both download the codes and perform web-based computation, and a few relevant examples of calculations available through VA, and show relevant statistics of access and download by the volcano community to-date.</p>
Annals of Geophysics, 2017
Processes occurring in volcanic conduits, the pathways through which magma travels from its stora... more Processes occurring in volcanic conduits, the pathways through which magma travels from its storage region to the surface, have a fundamental control on the nature of eruptions and associated phenomena. It has been well established that magma flows, crystallizes, degasses, and fragments in conduits, that fluids migrate in and out of conduits, and that seismic and acoustic waves are generated and travel within conduits. A better understanding of volcanic conduits and related processes is of paramount importance for improving eruption forecasting, volcanic hazard assessment and risk mitigation. However, despite escalating advances in the characterization of individual conduit processes, our understanding of their mutual interactions and the consequent control on volcanic activity is still limited. With the purpose of addressing this topic, a multidisciplinary workshop led by a group of international scientists was hosted from 25 to 27 October 2014 by the Pisa branch of the Istituto Na...
Journal of Volcanology and Geothermal Research, 2016
Abstract In the framework of the IAVCEI (International Association of Volcanology and Chemistry o... more Abstract In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) initiative on volcanic plume models intercomparison, we discuss three-dimensional numerical simulations performed with the multiphase flow model PDAC (Pyroclastic Dispersal Analysis Code). The model describes the dynamics of volcanic and atmospheric gases (in absence of wind) and two pyroclastic phases by adopting a non-equilibrium Eulerian–Eulerian formulation. Accordingly, gas and particulate phases are treated as interpenetrating fluids, interacting with each other through momentum (drag) and heat exchange. Numerical results describe the time-wise and spatial evolution of weak (mass eruption rate: 1.5 × 106 kg/s) and strong (mass eruption rate: 1.5 × 109 kg/s) plumes. The two tested cases display a remarkably different phenomenology, associated with the different roles of atmospheric stratification, compressibility and mechanism of buoyancy reversal, reflecting in a different structure of the plume, of the turbulent eddies and of the atmospheric circulation. This also brings about different rates of turbulent mixing and atmospheric air entrainment. The adopted multiphase flow model allows to quantify temperature and velocity differences between the gas and particles, including settling, preferential concentration by turbulence and thermal non-equilibrium, as a function of their Stokes number, i.e., the ratio between their kinetic equilibrium time and the characteristic large-eddy turnover time of the turbulent plume. As a result, the spatial and temporal distribution of coarse ash in the atmosphere significantly differs from that of the fine ash, leading to a modification of the plume shape. Finally, three-dimensional numerical results have been averaged in time and across horizontal slices in order to obtain a one-dimensional picture of the plume in a stationary regime. For the weak plume, the results are consistent with one-dimensional models, at least in the buoyant plume region, and allow to reckon a variable, effective entrainment coefficient with a mean value around 0.1 (consistently with laboratory experiments). For the strong plume, analysis of the results reveals that the two most critical assumptions of one-dimensional integral models are the self-similarity and the pressure equilibrium. In such a case, the plume appears to be controlled by the dynamics in the jet stage (below the buoyancy reversal) and by mesoscale vorticity associated with the development of the umbrella.
Sensors (Basel, Switzerland), Jan 28, 2017
This study analyzes the capabilities of a LiNbO₃ whispering gallery mode microdisc resonator as a... more This study analyzes the capabilities of a LiNbO₃ whispering gallery mode microdisc resonator as a potential bolometer detector in the THz range. The resonator is theoretically characterized in the stationary regime by its thermo-optic and thermal coefficients. Considering a Q-factor of 10⁷, a minimum detectable power of 20 μW was evaluated, three orders of magnitude above its noise equivalent power. This value opens up the feasibility of exploiting LiNbO₃ disc resonators as sensitive room-temperature detectors in the THz range.
We performed an inter-comparison study of three-dimensional models of volcanic plumes. A set of c... more We performed an inter-comparison study of three-dimensional models of volcanic plumes. A set of common volca-nological input parameters and meteorological conditions were provided for two kinds of eruptions, representing a weak and a strong eruption column. From the different models, we compared the maximum plume height, neutral buoyancy level (where plume density equals that of the atmosphere), and level of maximum radial spreading of the umbrella cloud. We also compared the vertical profiles of eruption column properties, integrated across cross-sections of the plume (integral variables). Although the models use different numerical procedures and treatments of subgrid turbulence and particle dynamics, the inter-comparison shows qualitatively consistent results. In the weak plume case (mass eruption rate 1.5 × 10 6 kg s −1), the vertical profiles of plume properties (e.g., vertical velocity , temperature) are similar among models, especially in the buoyant plume region. Variability...
Bollettino Dell Unione Matematica Italiana Sezione B Articoli Di Ricerca Matematica, 2007
In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Ear... more In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) in-tercomparison study on volcanic plume models, we present three-dimensional (3D) numerical simulations carried out with the ASHEE (ASH Equilibrium Eulerian) model. The ASHEE model solves the compressible balance equations of mass, momentum, and enthalpy of a gas-particle mixture and is able to describe the kinematic decoupling for particles characterized by Stokes number (i.e., the ratio between the particle equilibrium time and the flow characteristic time) lower than 0.2 (or particles smaller than about 1 mm). The computational fluid dynamic model is designed to accurately simulate a turbulent flow field using a Large Eddy Simulation approach , and is thus suited to analyze the role of particle non-equilibrium in the dynamics of turbulent volcanic plumes. The two reference scenarios analyzed correspond to a weak (mass eruption rate = 1.5* 10 6 kg/s) and a strong volcanic plume (mass eruption rate = 1.5*10 9 kg/s) in absence of wind. For each scenario, we compare the 3D results, averaged in space and time, with theoretical results obtained from integral plume models. Such an approach enables quantitative evaluation of the effects of grid resolution and the subgrid-scale turbulence model, and the influence of gas-particle non-equilibrium processes on the large-scale plume dynamics. We thus demonstrate that the uncertainty on the numerical solution associated with such effects can be significant (of the order of 20%), but still lower than that typically associated with input data and integral model approximations. In the Weak Plume case, 3D results are consistent with the predictions of integral models in the jet and plume regions, with an entrainment coefficient around 0.10 in the plume region. In the Strong Plume case, the self-similarity assumption is less appropriate and the entrainment coefficient in the plume region is more unstable, with an average value of 0.24. For both cases, integral model predictions diverge from the 3D plume behavior in the umbrella region. The presented analysis of 3D numerical simulations thus enables identification of the critical hypotheses that underlie integral models used in operational studies. In addition, high-resolution 3D runs allow reproduction of observable quantities (such as infrasound signals) which can be useful for constraining eruption dynamics during real events.
This study compares and evaluates one-dimensional (1D) and three-dimensional (3D) numerical model... more This study compares and evaluates one-dimensional (1D) and three-dimensional (3D) numerical models of volcanic eruption columns in a set of different inter-comparison exercises. The exercises were designed as a blind test in which a set of common input parameters was given for two reference eruptions, representing a strong and a weak eruption column under different meteorological conditions. Comparing the results of the different models allows us to evaluate their capabilities and target areas for future improvement. Despite their different formulations, the 1D and 3D models provide reasonably consistent predictions of some of the key global descriptors of the volcanic plumes. Variability in plume height, estimated from the standard deviation of model predictions, is within ~20% for the weak plume and ~10% for the strong plume. Predictions of neutral buoyancy level are also in reasonably good agreement among the different models, with a standard deviation ranging from 9 to 19% (the latter for the weak plume in a windy atmosphere). Overall, these discrepancies are in the range of observational uncertainty of column height. However, there are important differences amongst models in terms of local properties along the plume axis, particularly for the strong plume. Our analysis suggests that the simplified treatment of entrainment in 1D models is adequate to resolve the general behaviour of the weak plume. However, it is inadequate to capture complex features of the strong plume, such as large vortices, partial column collapse, or gravitational fountaining that strongly enhance entrainment in the lower atmosphere. We conclude that there is a need to more accurately quantify entrainment rates, improve the representation of plume radius, and incorporate the effects of column instability in future versions of 1D volcanic plume models.
A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of po... more A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of polydis-perse 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 com-pressible 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 high-Reynolds 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.
We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forwar... more We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forward approach, the model is able to simulate the plume dynamics from prescribed input flow conditions and generate the corresponding synthetic thermal infrared (TIR) image, allowing a comparison with field-based observations. An inversion procedure is then developed to retrieve vent conditions from TIR images, and to independently estimate the mass eruption rate. The adopted fluid-dynamic model is based on a one-dimensional, stationary description of a self-similar turbulent plume, for which an asymptotic analytical solution is obtained. The electromagnetic emission/absorption model is based on Schwarzschild's equation and on Mie's theory for disperse particles, and we assume that particles are coarser than the radiation wavelength (about 10 μm) and that scattering is negligible. In the inversion procedure, model parameter space is sampled to find the optimal set of input conditions which minimizes the difference between the experimental and the synthetic image. Application of the inversion procedure to an ash plume at Santiaguito (Santa Maria volcano, Guatemala) has allowed us to retrieve the main plume input parameters, namely mass flow rate, initial radius, velocity, temperature, gas mass ratio, entrainment coefficient and their related uncertainty. Moreover, by coupling with the electromagnetic model we have been able to obtain a reliable estimate of the equivalent Sauter diameter of the total particle size distribution. The presented method is general and, in principle, can be applied to the spatial distribution of particle concentration and temperature obtained by any fluid-dynamic model, either integral or multidimensional, stationary or time-dependent, single or multiphase. The method discussed here is fast and robust, thus indicating potential for applications to real-time estimation of ash mass flux and particle size distribution, which is crucial for model-based forecasts of the volcanic ash dispersal process.
We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forwar... more We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forward
approach, the model is able to simulate the plume dynamics from prescribed input flow conditions and
generate the corresponding synthetic thermal infrared (TIR) image, allowing a comparison with
field-based observations. An inversion procedure is then developed to retrieve ash plume properties
from TIR images.
The adopted fluid-dynamic model is based on a one-dimensional, stationary description of a self-similar (top-hat) turbulent plume, for which an asymptotic analytical solution is obtained.
%assuming that the gas and particulate phases are in kinetic and thermal equilibrium.
%by neglecting the effect of atmospheric stratification and in the limit of small density contrasts.
The electromagnetic emission/absorption model is based on the Schwarzschild's equation and on Mie's theory for disperse particles, assuming that particles are coarser than the radiation wavelength and neglecting scattering.
%To apply the procedure, experimental images are preliminary manipulated to obtain a time-averaged image having with a vertically oriented axis.
In the inversion procedure, model parameters space is sampled to find the optimal set of input conditions which minimizes the difference between the experimental and the synthetic image.
Two complementary methods are discussed: the first is based on a fully two-dimensional fit of the TIR image, while the second only inverts axial data.
Due to the top-hat assumption (which overestimates density and temperature at the plume margins), the one-dimensional fit results to be more accurate. However, it cannot be used to estimate the average plume opening angle. Therefore, the entrainment coefficient can only be derived from the two-dimensional fit.
Application of the inversion procedure to an ash plume at Santiaguito volcano (Guatemala) has
allowed us to retrieve the main plume input parameters, namely the initial radius b0b_0b0, velocity U0U_0U0, temperature T0T_0T0, gas mass ratio n0n_0n0, entrainment coefficient kkk and their related
uncertainty. Moreover, coupling with the electromagnetic model, we have been able to
obtain a reliable estimate of the equivalent Sauter diameter dsd_sds of the total particle size
distribution.
The presented method is general and, in principle, can be applied to the spatial distribution of
particle concentration and temperature obtained by any fluid-dynamic model, either integral or
multidimensional, stationary or time-dependent, single or multiphase.
The method discussed here is fast and robust, thus indicating potential for applications to
real-time estimation of ash mass flux and particle size distribution, which is crucial for
model-based forecasts of the volcanic ash dispersal process.
This PhD thesis focuses on numerical and analytical methods for simulating the dynamics of volcan... more This PhD thesis focuses on numerical and analytical methods for simulating the dynamics of volcanic ash plumes.
The study starts from the fundamental balance laws for a multiphase gas– particle mixture, reviewing the existing models and developing a new set of Partial Differential Equations (PDEs), well suited for modeling multiphase dispersed turbulence. In particular, a new model generalizing the equilibrium–Eulerian model to two-way coupled compressible flows is developed.
The PDEs associated to the four-way Eulerian-Eulerian model is studied, in- vestigating the existence of weak solutions fulfilling the energy inequalities of the PDEs. In particular, the convergence of sequences of smooth solutions to such a set of weak solutions is showed.
Having explored the well-posedness of multiphase systems, the three-dimensional compressible equilibrium–Eulerian model is discretized and numerically solved by using the OpenFOAM® numerical infrastructure. The new solver is called ASHEE, and it is verified and validated against a number of well understood benchmarks and experiments. It demonstrates to be capable to capture the key phenomena involved in the dynamics of volcanic ash plumes. Those are: turbulence, mixing, heat transfer, compressibility, preferential concentration of particles, plume entrainment.
The numerical solver is tested by taking advantage of the newest High Perfor- mance Computing infrastructure currently available.
Thus, ASHEE is used to simulate two volcanic plumes in realistic volcanological conditions. The influence of model configuration on the numerical solution is analyzed. In particular, a parametric analysis is performed, based on: 1) the kinematic decoupling model; 2) the subgrid scale model for turbulence; 3) the discretization resolution.
In a one-dimensional and steady-state approximation, the multiphase flow model is used to derive a model for volcanic plumes in a calm, stratified atmosphere. The corresponding Ordinary Differential Equations (ODEs) are written in a compact, dimensionless formulation. The six non-dimensional parameters characterizing a multiphase plume are then written. The ODEs is studied both numerically and analytically. Different regimes are analyzed, extracting the first integral of motion and asymptotic solutions. An asymptotic analytical solution approximating the model in the general regime is derived and compared with numerical results. Such a solution is coupled with an electromagnetic model providing the infrared intensity emitted by a volcanic ash plume. Key vent parameters are then retrieved by means of inversion techniques applied to infrared images measured during a real volcanic eruption.
We have developed a fast Eulerian model for the Large Eddy Simulation (LES) of a poly-disperse mi... more We have developed a fast Eulerian model for the Large Eddy Simulation (LES) of a poly-disperse mixture of gases and ash. The model is based on the equilibrium-Eulerian approximation (Ferry and Balachandar, Int. J. Multiph. Flow 2001; Cantero et al., J. Geophys. Res., 2008) for a polydisperse flow. This approach gives particular attention to the particle grain size distribution and its interaction with the turbulence. We are indeed particularly interested in the disequilibrium between the ash and the gaseous part of the volcanic mixture, our aim being to properly simulate phenomena like preferential concentration and turbulent entrainment.
Our numerical code has been used for simulating the dynamics of a particular Pyroclastic Density Current (PDC) benchmark at volcanic scale. We have letted different parameters vary in order to study the influence of them on the dynamics of the PDC:
numerical grid resolution
subgrid turbulence model
two-dimensional versus three dimensional geometry
initial aspect ratio of the PCD
initial temperature of the PDC
presence of an obstacle
grain size distribution of the ash particles
A quantitative analysis of the influence of these parameters on the dynamics of the PDC has been carried out by comparing — through time — the run-out of the PDC, its mixing with the atmosphere, its stratification, its momentum distribution and its sedimentation properties.
Better understanding the role of this parameters on the development of a PDC could be of great help for the setting up of models for volcanic hazard in critical areas like Campi Flegrei.
We present a new method to coherently compare integral one-dimensional (1D) plume model (cf. Mort... more We present a new method to coherently compare integral one-dimensional (1D) plume model (cf. Morton et al 1956) with three-dimensional (3D) computational fluid mechanical models. The proposed approach applies to the 3D numerical solution the same averaging techniques used to develop 1D models. In this way, results can be coherently compared allowing calibration for the faster 1D models through the slower but more accurate 3D simulations.
Two plume benchmarks have been studied by using a new 3D code based on a fast Eulerian model for the Large Eddy Simulation (LES) of turbulent volcanic plumes. The model is based on the equilibrium-Eulerian approximation (Ferry and Balachandar, Int. J. Multiph. Flow 2001; Cantero et al., J. Geophys. Res., 2008) for a polydisperse flow. This approach gives particular attention to the particle grain size distribution and its interaction with the turbulence. We are indeed particularly interested in the disequilibrium between the ash and the gaseous part of the volcanic mixture, our aim being to properly simulate phenomena like preferential concentration and plume entrainment.
Results from numerical simulations have been averaged over time and space in order to furnish the same output usually given by 1D models, namely the mass, momentum and buoyancy fluxes. Interestingly, we have found that the entrainment coefficient become negative when the plume begins to close its umbrella.
Moreover, we have studied the influence of other parameters on the dynamics of the plume, namely:
disequilibrium between gas and particles
grid resolution
subgrid turbulence model
grain-size distribution of the ash particles
A quantitative analysis of the influence of these parameters on the plume development has been carried out by comparing not only the averaged fields, but also the fluctuations and the infrasonic field generated by the plume itself.
Recently, we have developed a methodology for the retrieving of some key vent parameter and their... more Recently, we have developed a methodology for the retrieving of some key vent parameter and their related uncertainly by analyzing Thermal Infrared (TIR) videos of volcanic plumes (cf. Cerminara et al. JVGR 2015). We have applied this inversion procedure to an ash plume at Santiaguito (Santa Maria volcano, Guatemala). In particular, our methodology allows for estimation of the plume turbulent entrainment and of the following quantities at the vent:
mass flow rate
plume velocity
plume temperature
gas/ash mass ratio
Sauter diameter of the total particle grain size distribution
The methodology is based on a coupled fluid-dynamic (FD) and electromagnetic (EM) model for volcanic ash plumes. In a forward approach, the model is able to simulate the plume dynamics from prescribed input flow conditions and generate the corresponding synthetic TIR image. In the inversion procedure, model parameter space is sampled to find the optimal set of input conditions which minimizes the difference between the experimental and the synthetic image.
The adopted FD model is based on a one-dimensional, stationary description of a self-similar turbulent plume. The EM emission/absorption model is based on Schwarzschild's equation and on Mie's theory for disperse particles.
The presented method is general and, in principle, can be applied to the spatial distribution of particle concentration and temperature obtained by any fluid-dynamic model, either integral or multidimensional, stationary or time-dependent, single or multiphase. The method discussed here is fast and robust (less than 1 min on a standard laptop), thus indicating potential for applications to real-time estimation of parameters which are crucial for model-based forecasts of the volcanic ash dispersal process.