The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review

Abstract

The experimental approach has become a major tool increasingly used by volcanologists in recent decades to investigate the physics of eruptive processes in complement to field and theoretical works. Researchers have developed various methodologies to study volcanic phenomena at reduced length scale. The works involve natural or analogue materials and their types range from first-order tests, to identify fundamental processes and make qualitative comparison with field observations, to more sophisticated experiments in which precise data obtained in a controlled environment can be used to validate outputs of theoretical models. Scaling is a central issue to ensure dynamic similarity between the small-scale experiments and the large-scale volcanic phenomena when natural conditions cannot be simulated due to inherent length scale difference and/or technical limitations. In this respect, dimensionless numbers are used to map physical regimes and to define scaling laws, which allow experimental results to be extrapolated to natural scale. This review presents the variety of experimental studies conducted to investigate subterraneous and aerial volcanic phenomena involving in particular fluid-particle mixtures. We focus on the major scaling issues and the physical regimes investigated, and we also highlight in a historical perspective some of the major advances achieved through experimental studies. Finally we conclude on some perspectives for future works.

The Influence of Particle Concentration on the Formation of Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds

“…Finally, laboratory experiments, unlike volcanic plumes, exist in a confined space affected by boundary conditions and the formation of return flows (Roche and Carazzo, 2019). In our experiments, boundary conditions have an impact on convection in the lower layer, which has only a limited effect on measurements of finger characteristic velocity and dimensions, especially in the early part of the experiments, and a negligible influence on finger formation.…”

“…Altogether, this scaling analysis shows that our experiments do a good job reproducing most dimensionless numbers associated with natural clouds and fingers, although the variability of natural phenomena means that they are associated with larger ranges of Re and Gr, potentially extending to greater values than in our experiments. However, it is inevitable that laboratory models of volcanic clouds cannot capture the full range of variability of the natural system (Carazzo and Jellinek, 2012;Kavanagh et al, 2018;Roche and Carazzo, 2019). Despite this, given that that the ranges of Re and Gr in both the experiments and natural clouds are close to or above expected transitional values (Reynolds, 1883;Hoyal et al, 1999b), we can regard our experiments as suitable analogues to study settling-driven gravitational instabilities at the base of volcanic clouds.…”

“…The difference in complexity and scale between the natural phenomenon and small-scale experiments raises the problem of the applicability of the analogue experimental results. To address this, we perform a scaling analysis (Burgisser and Bergantz, 2002;Burgisser et al, 2005;Carazzo and Jellinek, 2012;Kavanagh et al, 2018;Roche and Carazzo, 2019). Dimensionless numbers relevant to our problem are calculated for volcanic ash clouds and compared with particle suspensions (Figure 2D).…”

“…For multiphase flows involving particles, the Stokes St and Sedimentation Σ numbers quantify the momentum transfer between the fluid phase and the particles (Burgisser et al, 2005;Roche and Carazzo, 2019). They are calculated as…”

Modelling Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds Using the Lattice Boltzmann Method

“…This Eulerian description is valid under the assumptions of sufficiently small particles and a large enough number of particles such that the drag and gravitational forces are in equilibrium. The condition on the particle size can be quantified through the Stokes number (Burgisser et al, 2005;Roche and Carazzo, 2019), one possible definition of which is…”