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Trajectories of large particles in supersonic jets: Implications for volcanic ballistic projectiles

Nils Steinau 1, Kae Tsunematsu1, Kiyonobu Ohtani2, Kazuya Seo3, Akuto Kaneko3, Toshihiro Ogawa2

  • Affiliations:  1Faculty of Science, Yamagata University, Yamagata, Japan; 2Institute of Fluid Science, Tohoku University, Sendai, Japan; 3Department of Mechanical Engineering, Kogakuin University, Tokyo, Japan 

  • Presentation type: Talk

  • Presentation time: Monday 11:15 - 11:30, Room R290

  • Programme No: 3.3.4

  • Theme 3 > Session 3

Abstract

Explosive volcanic eruptions are known to produce supersonic jets, which play an important role in quantifying volcanic processes and their hazards. One such hazard is the explosive ejection of rocks that describe ballistic trajectories, so-called volcanic ballistic projectiles (VBPs). Their initial acceleration by the jet is an active area of research, however, further insights are still needed to fully understand the process. To study the effect of the supersonic jet, open-ended shock tube experiments with spherical glass beads as VBP surrogates were conducted, and bead trajectory data and schlieren/shadowgraph images of the jet obtained. Numerical simulations using OpenFOAM were conducted to obtain quantifiable flow field data by reproducing the steady-state jet structure from the images. Trajectory calculations with the flow field data obtained from OpenFOAM simulations successfully reproduced the measured vertical velocity profiles of the beads. This trajectory calculation method was applied to ballistic data from the 1997 Vulcanian eruption of Soufrière Hills and the observed maximum impact distances were successfully reproduced. The analysis of the calculated trajectories at Soufrière Hills revealed that VBPs are accelerated by the supersonic jet until they cross the Mach disk. VBPs cross the Mach disk after the jet reaches its steady-state and before the steady-state ends, with the Mach disk and shock structure collapsing towards the vent. This implies that VBPs are predominantly accelerated by the steady-state flow field of the supersonic jet.