A systematic investigation on the aerodynamics of ash particles

A systematic analysis of the physical parameters that influence the aerodynamics of ash, i.e. the attitude of a particle to be transported and/or settled throughout a fluid, is presented. We investigate juvenile particles from eruptions of Somma-Vesuvius, Campi Flegrei and Vulcano (southern Italy), which encompass a wide range of particle characteristics. The analysed samples were selected from dilute pyroclastic density current (DPDC) and fall deposits, and cover an ample spectrum of magma composition and fragmentation mechanisms.Data show that particles have often highly irregular shapes, as determined by the shape factor Ψ. The more irregular is the shape the higher the drag coefficient, Cd, and the lower the terminal velocity. The Cd of DPDC particles is lower than that of fall particles, as due to rounding by attrition at the base of a density current.As a consequence of the irregular shape, the terminal velocity of ash (0.5 mm) can be less than half of the value that results by hypothesising a spherical shape, as it is frequently done in volcanology.In the fall deposits, for the same size fraction, the settling velocity can be different for samples extracted at different locations along the main dispersal axis, especially if the clast population shows heterogeneity of vesicularity. Particle shape becomes more irregular as grain size decreases down to 0.25 mm, whereas at finer sizes the values are almost constant.This study has important implications for how long and how far volcanic particles can be dispersed aloft; this is crucial for dispersal models quantifying risk, including for international air traffic.

Research highlights
► Settling velocity of ash is highly influenced by particle shape.
► Particle shape of ash is highly irregular.
► Ash shape is highly variable amongst eruptions.
► Our shape factor well describes the irregular ash shape.
► Settling velocity of ash is lower than other sedimentary particles.