The Fluid Mechanics of Pyroclastic Density Currents

Abstract

Pyroclastic density currents are generated in explosive volcanic eruptions when gas and particle mixtures remain denser than the surrounding atmosphere. These mobile currents have a diversity of flow regimes, from energetic granular flows to turbulent suspensions. Given their hazardous nature, much of our understanding of the internal dynamics of these currents has been explored through mathematical and computational models. This review discusses the anatomy of these currents and their phenomenology and places these observations in the context of forces driving the currents. All aspects of the current dynamics are influenced by multiphase interactions, and the study of these currents offers insight into a high-energy end-member of multiphase flow. At low concentration, momentum transfer is dominated by particle-gas drag. At higher concentration, particle collisions, friction, and gas pore pressure act to redistribute momentum. This review examines end-member theoretical models for dilute and concentrated flow and then considers insight gained from multiphase simulations of pyroclastic density currents. 459

A review of laboratory and numerical modelling in volcanology

“…Above the basal layer of the flow, the particle concentrations are slightly lower and momentum is distributed by particle momentum (described by the collisional stokes number St c and the Bagnold Ba number, which is the ratio of collisional stresses relative to viscous stress). Within the dilute, turbulent portion of the flow, particle motion is influenced primarily by particle-gas drag as described by the Stokes number St. Other dimensionless parameters used in describing flow propagation are the Peclet and Reynolds numbers, the Froude number Fr (the flow speed relative to wave speed), and the dense-dilute transition number D D (the timescale of collisions relative to particle drag response time; Dufek, 2016).…”

“…While numerical studies of pyroclastic density currents still largely follow these two strands, open-source computational fluid dynamics programs (e.g MFiX and OpenFOAM) are being increasingly used to capture the multiphase behaviour that occurs within these volcanic flows. A comprehensive overview of the numerical models describing pyroclastic density currents is provided in Dufek (2016).…”

PYFLOW_2.0: a computer program for calculating flow properties and impact parameters of past dilute pyroclastic density currents based on field data

“…While the process that generates DPDCs can vary significantly, spanning from the collapse of an ash cloud to a direct lateral blast related to a volcanic dome collapse Sulpizio et al 2014;Dufek 2016), their motion is mainly controlled by the density contrast with the surrounding atmosphere and topography Jenkins et al 2013;. In particular, DPDCs represent the low-particle concentration type of pyroclastic density currents (Valentine 1987;Burgisser and Bergantz 2002;Sulpizio et al 2007;Dellino et al 2008;Sulpizio and Dellino 2008;Sulpizio et al 2014;Breard et al 2015;Dufek 2016). In a DPDC, while particles are transported mainly by the mechanism of turbulent suspension, inter-particle collisions and fluidization phenomena play a negligible role and are mainly restricted to a thin basal layer (Fig.…”

“…1a) (Iverson and Vallance 2001;Dellino et al 2008; Editorial responsibility: H. Dietterich Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00445-017-1191-z) contains supplementary material, which is available to authorized users. Sulpizio and Dellino 2008;Roche 2012;Sulpizio et al 2014;Dufek 2016). Due to their mobility and capability to transport dangerous amounts of hot volcanic ash and gases, DPDCs represent a source of hazard for human life, activities, and infrastructure in the potentially affected areas (Neri et al 2015a).…”

Geophysical Investigations of Pyroclastic Density Current Processes and Deposit Properties at Mount St. Helens, Washington (USA)

“…Erosion and its effects on PDC dynamics remains one of the most poorly understood aspect of PDCs [Dufek, 2016] The objectives of this study are to:…”