Modelling the ejection velocity of ballistic blocks based on shock tube experiments
Kae Tsunematsu1, Kiyonobu Ohtani2, Nils Steinau1, Kazuya Seo3, Akuto Kaneko3, Toshiro Ogawa2
Affiliations: 1 Faculty of Science, Yamagata University, Yamagata, Japan; 2Institute of Fluid Science, Tohoku University, Sendai, Japan, 3Department of Mechanical Engineering, Kogakuin University, Tokyo Japan
Presentation type: Poster
Presentation time: Thursday 16:30 - 18:30, Room Poster Hall
Poster Board Number: 73
Programme No: 3.14.7
Abstract
Explosive eruptions produce pyroclasts of various sizes. In explosive eruptions such as Vulcanian eruptions, some large blocks are ejected, fly into the air, and finally deposit on the ground near the vent. These blocks are called ballistic projectiles or blocks. The size distribution of ballistic blocks varies with the distance from the vent, sometimes from large to small, from small to large, or randomly. Such block size variation has not been clearly modeled because the dynamics of the ejection velocity in conjunction with the block size remains unclear. Therefore, we conducted shock tube experiments, ejecting small glass beads of 0.6-2.0 mm in size. We visualized the shock wave structure using Schlieren imaging and filmed the gas and bead movement with high-speed cameras. The ejection velocity near the vent mainly depends on the pressure ratio of the pressure reservoir to the ambient air pressure, and the bead size. Therefore, the ejection velocity can be modeled using these two parameters, although the velocity can be varied with thermal conditions or the nozzle length. We present how the ejection velocity depends on the pressure ratio and the bead size. A model of ballistic ejection velocity is suggested based on the experimental results. Furthermore, we show how the trajectories change with the numerical model taking into account the ejection velocity variations.