Numerical determination of the efficiency of entrainment in volcanic eruption columns

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Abstract
Entrainment of air by turbulent mixing plays a central role in the dynamics of eruption clouds; the amount of entrained air controls eruption styles and heights of eruption columns. The efficiency of entrainment is quantified by the entrainment coefficient: the ratio between mean inward radial velocity at the edge and the mean vertical velocity. We directly determined the entrainment coefficient of eruption columns as a function of height on the basis of three‐dimensional numerical simulations. The value of entrainment coefficient is similar to that for pure jets (∼0.07) just above the vent, and approaches that for pure plumes (0.10–0.15) far from the vent. Between these two regions, we identify a new transitional zone with a significantly small entrainment coefficient (∼0.05). This spatial variation in the entrainment coefficient is correlated with the change in vortical structure accompanying the addition of off‐source buoyancy in the transitional zone.

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