3-D numerical simulations of eruption column collapse: Effects of vent size on pressure-balanced jet/plumes

Buoyant columns or pyroclastic flows form during explosive volcanic eruptions. In the transition-state, these two eruption styles can develop simultaneously. We investigated the critical condition that separates the two eruption styles (referred to as “the column collapse condition”) by performing a series of three-dimensional numerical simulations. In the simulation results, we identify two types of flow regime: a turbulent jet that efficiently entrains ambient air (jet-type) and a fountain with a high-concentration of the ejected material (fountain-type). Hence, there are two types of column collapse (jet-type and fountain-type). Which type of collapse occurs at the column collapse condition depends on whether the critical mass discharge rate for column collapse (MDRCC) is larger or smaller than that for the generation of a fountain (MDRJF) for a given exit velocity. Temperature controls the relative magnitude of MDRCC relative to MDRJF, and hence the type of collapse. For given magma properties (e.g., temperature and water content), the column collapse condition is expressed by a critical value of the Richardson number (RiCC ≡ g′0L0/w02, where g′0 is the source buoyancy, L0 is the vent radius, and w0 is the exit velocity). When the jet-type collapse occurs at the column collapse condition, RiCC is independent of the exit velocity. When the fountain type collapse occurs at the column collapse condition, on the other hand, RiCC decreases as the exit velocity increases. As the exit velocity exceeds the sound velocity, a robust flow structure with a series of standing shock waves develops in the fountain, which suppresses entrainment of ambient air and enhances column collapse.