Explosive volcanic eruptions are some of the most energetic flows on the planet, the largest of which can have global impact. The more common, smaller, events are a proximal hazard and still encompass scales of several kilometers. Despite their large size and long duration, mass and energy transfer in these flows are fundamentally controlled by processes at much smaller spatial and temporal scales, where individual particles interact with each other, with gas, or with the surface over which the flows travel. Our ability to predict large-scale behavior of volcanic flows can ultimately be limited by our understanding of very small-scale, or microphysical, processes. This proposal examines a suite of particle-scale mass and energy transfer mechanisms in the laboratory with the aim of understanding the physics of these processes and to incorporate them into large-scale simulations of explosive volcanic eruptions. One of the long term goals of this effort is to provide a technology for students, scientists and civil officials to better understand hazards during times of volcanic unrest.
Advances in computational power and algorithm design enable detailed studies of the turbulent structures that develop in explosive volcanic eruptions. However, even with increases in computational resources, achieving resolution below meter-scale in large-scale three-dimensional simulations may never be possible. Accounting for subgrid-scale physical processes requires developing constitutive relationships for volcanic materials and conditions. Past work on steam explosions has shown that subgrid models developed from experiments can be readily coupled to multiphase numerical simulations. More importantly, these subgrid relations are critical for predicting the dynamics reflected in volcanic deposits; models that neglect subgrid processes can fail to produce the energy transfer manifest in volcanic deposits by several orders of magnitude. This work will focus on 1) heat transfer between particles and gas, 2) comminution and agglomeration in active flows and the impact of a evolving grain size distribution on the dynamics of flows, and 3) particle-boundary interactions, and in particular the role of resuspended particles from the bed. All of the proposed experiments will be conducted with materials and conditions similar to those in natural flows, minimizing the potential difficulties with scaling to large-scale multiphase flows. In the methodology proposed, the numerical models are integrally connected to the experimental data. The dual approach emphasizes the strength of both techniques: the strength of numerical models is the ability to solve non-linear, complexly coupled equations and determine emergent behavior, and the strength of the experiments is to understand in detail the physical processes operating at small scales.
Explosive volcanic eruptions are among the most energetic flows on the planet and are damaging and deadly natural hazards. The largest eruptions have global impact, and even the more common smaller events encompass scales of several kilometers. Despite the large size of eruptions, mass and energy transfers in these flows are fundamentally controlled by processes that occur at much smaller spatial (as small as microns) and temporal scales (as small as milliseconds), where individual particles interact with each other, with gas, or with the surface over which the flows travel. Our ability to predict large-scale behavior of volcanic flows can ultimately be limited by our understanding of very small-scale, or microphysical, processes. We performed a suite of experiments to quantify particle-scale mass and energy transfer for natural volcanic materials. We measured 1) heat transfer coefficients, 2) momentum and energy transfers that occur when particles collide with each other or with a water surface, 3) the rate of ash generation when particles interact. We developed sub-grid scale models for these processes that can be included in large-scale numerical multiphase simulations. We performed simulations of explosive eruptions to study phenomena that occur within volcanic conduits and within pyroclastic flows. We found that significant amounts of ash continue to be generated even after the magma first fragments – ash is generated by collisions between particles inside the conduit and within pyroclastic flows. We showed that the long-distance transport of pyroclastic flows over water probably occurs after a raft of floating pumice accumulates. To validate our models we compared their predictions with measurements made on deposits from the 1980 Mount St Helens eruption, one of the 1912 Lassen pyroclastic flows, and the prehistoric Kos eruption. We showed that the numerical models must account for particle-scale physics in order to capture fundamental aspects of explosive eruptions. The improved understanding of mass and energy transfer in volcanic flows improves our understanding of the potential eruption dynamics of future events. Models similar to those we have developed and are improving are currently being used to assess volcanic hazard, e.g., at Vesuvius. One of the long-term goals of this effort is to provide a technology for students, scientists and civil officials to better understand hazards during times of volcanic unrest. This project supported the training of 2 graduate students, 1 postdoctoral researcher, and provided research experiences for 11 undergraduate students.
