11:00 AM - 11:15 AM
[MIS23-02] The in-flight dynamics of volcanic ballistics based on eruption simulation experiments and computational fluid dynamics
Keywords:ballistic blocks, Trashcano, CFD, gas flow
Ballistic blocks in a volcanic setting are centimeter to meter-sized volcanic pyroclasts that follow ballistic trajectories during an explosive eruption. In existing numerical models, gravity and air drag are the main considerations in calculating the trajectory of ballistics given the ejection velocity and ejection angle as input parameters. However, ballistics are released by the gas flow in an actual eruption, so the gas flow may affect the ejection velocity and the flight of the ballistics after the eruption. In this study, we introduce two approaches: computational fluid calculation and eruption simulation experiments in order to investigate the interaction between the motion of ballistics and the gas flow. In the fluid calculation, the open-source computational fluid dynamics (CFD) tool "OpenFOAM" was used to calculate the gas flow around multiple ballistic blocks. When the volume fraction of the ballistic block is large, it is expected that the gas flow will change due to the number of blocks, and the gas flow force acting on the blocks will also change. We measured the gas flow force received by the ballistic blocks varying the volume fraction between 0.01 and 0.05. As a result, the force received by the blocks from the flow did not vary as the volume fraction increased. Therefore, the effect of the gas flow on the ballistic blocks is more dominant than the effect of the ballistic blocks on the gas flow. We conducted the Trashcano experiment. The Trashcano is a field experiment that uses liquid nitrogen to explode a plastic bottle and release ejecta from a trash can. We used three types of balls and two types of model blocks with different grain sizes as ejecta. Their motion was recorded with high-speed cameras, and the three-dimensional trajectories of 50 ejecta were reconstructed by image analysis using stereophotogrammetry. The ejection velocity and acceleration of the ejecta were estimated from the trajectories. As a result of comparing the ejection velocity with the cross-sectional area / mass (A/m) of the ejecta, it was found that the ejection velocity tended to increase as the A/m becomes larger. Assuming that the density of the ballistic blocks is constant, the smaller blocks have a larger A/m. Therefore, the ejection velocity of small blocks may be larger than the large blocks if the experimental results are applied to an actual eruption, which consequently enable the smaller blocks to fly farther. The estimated acceleration of the ejecta ranged from -162.5 to 181.9 m/s2. In particular, it was verified that the ejecta flying inside the gas flow were accelerating. This result suggests that the gas flow also affects the motion of the ballistic blocks after the eruption, and it is important to calculate the trajectory considering the gas flow.