Control of vent geometry on the fluid dynamics of volcanic plumes: insights from numerical simulations

AbstractWe present three‐dimensional numerical simulations of eruption clouds from circular to linear fissure vents to investigate the control of vent shape on the height and stability of volcanic plumes during large explosive eruptions. Our results show that clouds ejected from circular or low‐aspect‐ratio (nearly square‐like) fissure vents can be associated with radially suspended flow (RSF) at the top of the jet region, whereas those emitted from narrow‐fissure vents are not. Non‐RSF plumes are more stable than those associated with RSF because the highly concentrated parts of the ejected mixture are easily dissipated and mixed with air near the vent. Plume height in the RSF regime decreases while that in the non‐RSF regime increases with increasing aspect ratio, even for a fixed magma flow rate. These observations suggest that the efficiency of air entrainment is influenced by the vent shape, which in turn controls the dynamics of eruption plumes

The dynamics of volcanic jets : Temporal evolution of particles exit velocity from shock‐tube experiments

Pyroclast ejection during explosive volcanic eruptions occurs under highly dynamic conditions involving great variations in flux, particle sizes, and velocities. This variability must be a direct consequence of complex interactions between physical and chemical parameters inside the volcanic plumbing system. The boundary conditions of such phenomena cannot be fully characterized via field observation and indirect measurements alone. In order to understand better eruptive processes, we conducted scaled and controlled laboratory experiments. By performing shock‐tube experiments at known conditions, we defined the influence of physical boundary conditions on the dynamics of pyroclast ejection. If applied to nature, we are focusing in the near‐vent processes where, independently of fragmentation mechanism, impulsively released gas‐pyroclast mixtures can be observed. These conditions can be met during, e.g., Strombolian or Vulcanian eruptions, parts of Plinian eruptions, or phreatomagmatic explosions. The following parameters were varied: (1) tube length, (2) vent geometry, (3) particle load, (4) temperature, and (5) particle size distribution. Gas and particles in the experiments are not coupled (St >> 1). The initial overpressure, with respect to atmosphere, was always at 15 MPa. We found a positive correlation of pyroclast ejection velocity with (1) particle load, (2) diverging vent walls, and (3) temperature as well as a negative correlation with (1) tube length and (2) particle size. Additionally, we found that particle load strongly affects the temporal evolution of particle ejection velocity. These findings stress the importance of scaled and repeatable laboratory experiments for a better understanding of volcanic phenomena and therefore volcanic hazard assessment