Air Entrainment and Turbulent Dynamics of the March 22, 1944 Vesuvius Eruption Plume

Abstract:

Quantitative understanding of turbulent entrainment during explosive eruptions is critical for volcanic hazard forecasting and mitigation. Two endmember scenarios can describe many explosive eruptions: i) creation of a stable buoyant plume and ii) collapse of the eruption column and formation of devastating pyroclastic flows. The transition between these two regimes is modulated by the efficiency of air entrainment. Hot, dense material exits the vent with upward momentum. Turbulent eddies, induced by shear along the column margins, entrain, heat, and expand air, reducing the mixture density and driving buoyant ascent. If initial upward momentum cannot sustain the plume until entrainment reduces the bulk density to less than that of the surrounding atmosphere, the column will collapse. Most previous studies of entrainment consist of numerical models and analog laboratory experiments, with fewer quantitative studies of real eruptions. Existing observations of entrainment are generally time- and space-averaged measurements, indicating bulk entrainment coefficients of ~0.1, and do not provide information regarding entrainment mechanisms. We investigate spatial and temporal variations in entrainment of the Plinian phase of the 1944 eruption of Mt. Vesuvius using a feature tracking velocimetry (FTV) algorithm applied to film collected by the US Navy and digitized by the US National Archives. We registered the images using stationary reference points to eliminate camera motion. FTV analysis of the image sequences describes the turbulent velocity field with spatial and temporal resolutions of ~40 m and 24 frames per second, respectively. We use a novel technique to estimate the 3D plume morphology from normalized brightness; projection of the 2D velocity fields onto those 3D surfaces provides 3D velocity fields. The divergence of the velocity fields quantifies local expansion and entrainment, and shows that although kilometer scale eddies are present in the plume, entrainment and expansion occur with length scales on the order of hundreds of meters. Integrating the inward directed velocities over the entraining regions quantifies local air entrainment rates. We find that 30-45% of the plume margin entrains ~6.8×10⁷ m³s^-1 air, with entrainment velocities that average ~3.7 m/s and locally exceed 20 m/s.