Numerical simulations of volcanic jets: Importance of vent overpressure

Abstract

Explosive volcanic eruption columns are generally subdivided into a gas‐thrust region and a convection‐dominated plume. Where vents have greater than atmospheric pressure, the gas‐thrust region is overpressured and develops a jet‐like structure of standing shock waves. Using a pseudogas approximation for a mixture of tephra and gas, we numerically simulate the effects of shock waves on the gas‐thrust region. These simulations are of free‐jet decompression of a steady state high‐pressure vent in the absence of gravity or a crater. Our results show that the strength and position of standing shock waves are strongly dependent on the vent pressure and vent radius. These factors control the gas‐thrust region's dimensions and the character of vertical heat flux into the convective plume. With increased overpressure, the gas‐thrust region becomes wider and develops an outer sheath in which the erupted mixture moves at higher speeds than it does near the column center. The radius of this sheath is linearly dependent on the vent radius and the square root of the overpressure. The sheath structure results in an annular vertical heat flux profile at the base of the convective plume, which is in stark contrast to the generally applied Gaussian or top‐hat profile. We show that the magnitude of expansion is larger than that predicted from previous 1D analyses, resulting in much slower average vertical velocities after expansion. These new relationships between vent pressure and plume expansion may be used with observations of plume diameter to constrain the pressure at the vent.