Gholamhossein Bagheri

MeMoVolc report on classification and dynamics of volcanic explosive eruptions

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.

TError: towards a better quantification of the uncertainty propagated during the characterization of tephra deposits

We present TError, a Matlab package designed to quantify systematically the uncertainty associated with the characterization of tephra deposits, in which the most commonly used methods to quantify eruption source parameters are implemented. Inputs of the code are a range of field-based, model-based and empirical parameters (i.e., clast diameter, crosswind and downwind ranges, thickness measurement, area of isopach contours, bulk deposit density, empirical constants and wind speed), for which the user defines an uncertainty and an associated distribution. The TError package contains two main functions. The first function deterministically varies one input parameter at a time and quantifies the sensitivity of each Eruption Source Parameter (ESP; i.e., plume height, erupted volume, mass eruption rate) to the variability of input parameters. The second function propagates input parameters as stochastic distributions of noise through all ESPs. The resulting distributions can then be used to express the uncertainty of physical parameters of explosive eruptions in a systematic way. For both functions, comprehensive reports and sets of figures assist the user in the interpretation of the results. As an example, the TError package was applied to Layer 5 of Cotopaxi volcano. Using the median, the 2 percentile and the 98 percentile as central value, lower bound and upper bound respectively, a new quantification of the ESP suggests a plume height of 30± 1 km a.s.l, a mass eruption rate of 1.8+0.3 −0.2 × 108 kg s−1 and a tephra volume between 0.23 +0.13 −0.04 and 0.43 +0.08 −0.06 km3, depending on the empirical model used.

Dedicated vertical wind tunnel for the study of sedimentation of non-spherical particles

A dedicated 4-m-high vertical wind tunnel has been designed and constructed at the University of Geneva in collaboration with the Groupe de compétence en mécanique des fluides et procédés énergétiques. With its diverging test section, the tunnel is designed to study the aero-dynamical behavior of non-spherical particles with terminal velocities between 5 and 27 ms(-1). A particle tracking velocimetry (PTV) code is developed to calculate drag coefficient of particles in standard conditions based on the real projected area of the particles. Results of our wind tunnel and PTV code are validated by comparing drag coefficient of smooth spherical particles and cylindrical particles to existing literature. Experiments are repeatable with average relative standard deviation of 1.7%. Our preliminary experiments on the effect of particle to fluid density ratio on drag coefficient of cylindrical particles show that the drag coefficient of freely suspended particles in air is lower than those measured in water or in horizontal wind tunnels. It is found that increasing aspect ratio of cylindrical particles reduces their secondary motions and they tend to be suspended with their maximum area normal to the airflow. The use of the vertical wind tunnel in combination with the PTV code provides a reliable and precise instrument for measuring drag coefficient of freely moving particles of various shapes. Our ultimate goal is the study of sedimentation and aggregation of volcanic particles (density between 500 and 2700 kgm(-3)) but the wind tunnel can be used in a wide range of applications.

Simulation of Solid Particles Behavior in a Heated Cavity at High Rayleigh Numbers

Transport and deposition of solid particles in a differentially heated cavity at high Rayleigh numbers up to 108 was studied using an Eulerian–Lagrangian computational method. Two-dimensional Navier-Stokes and energy equations were solved and the motions of particles with diameters in the range of 10 nm to 10 μm were simulated. Effects of drag, lift, thermophoresis and Brownian forces on the particle trajectories were investigated. It was observed that the variation of Rayleigh number can significantly change the flow field and the corresponding particle deposition patterns. In particular, recirculation regions were formed near the corners as the Rayleigh number increased. Furthermore, the effects of changes in the Rayleigh numbers on transport and deposition of particles of different sizes were quite different. Increasing Rayleigh number from 107 to 108 caused a decrease in particles deposition except for 10 μm particles. Smaller particles had a higher probability to deposit on the cold wall as the thermophoresis effect becomes important. Increasing the Rayleigh number decreased the influencing zone of the thermophoresis in the vicinity of the walls. Copyright 2012 American Association for Aerosol Research

Aerodynamics of Volcanic Particles: Characterization of Size, Shape, and Settling Velocity

Abstract Volcanic clasts are known to have highly nonspherical and irregular shapes, with physical, chemical, and optical characteristics significantly different from those of spherical particles. Numerous studies show the importance of particle shape on various particle properties, such as their scattering and aerodynamical behavior. Nevertheless, volcanic particles have often been approximated as spheres in numerical descriptions and observation strategies, in part because accurate shape characterization is challenging, time-consuming, and needs special equipment, and in part because scattering analysis is complicated for nonspherical shapes. The technological development of the last few decades years has provided various strategies for quantifying shape of volcanic particles rapidly (eg, image analysis) and accurately (eg, 3-D laser scanning). However, studies of particle shape are dispersed across various scientific fields, which, in some cases, lead to research redundancy or improper method implementation. The main goal of this chapter is to summarize state-of-the-art methods of characterizing particle size and shape, which are crucial for models of particle dispersal and sedimentation.