Volcanic ballistic projectiles in supersonic jets: shock tube experiments with glass beads

Abstract To investigate volcanic ballistic projectile (VBP) interactions with supersonic volcanic jets, shock tube experiments using spherical glass beads as VBP surrogates were conducted. Trajectory data and Schlieren/shadowgraph images of the jets were obtained. Comparisons of experimental and volcanic data indicate that the experiments best represent Strombolian eruptions, with some overlap with Vulcanian and Plinian eruptions. Analysis of the jet images and trajectories revealed the following key findings: (1) the VBP exit velocity at the vent increases with the reservoir-to-ambient pressure ratio and initial particle depth but decreases with the particle size and (2) the VBPs are accelerated by the supersonic jet over a distance exceeding the Mach disk distance before a constant maximum velocity is reached. No trend between the acceleration distance and the pressure ratio was observed. The maximum VBP velocity increases with increasing pressure ratio and decreases with increasing particle size. While the literature provides data from field observations and numerical modeling for VBP acceleration above the vent, experimental studies have thus far predicted that maximum VBP velocities occur at the vent. In contrast, the results of this experimental study reveal that the VBP velocity can increase after the VBPs exit the vent because of acceleration by the flow field of supersonic volcanic jets. The maximum velocity that the VBPs can reach affects their travel distance, making it an essential parameter for VBP hazard assessment. Graphical Abstract