Effects of particle mixtures and nozzle geometry on entrainment into volcanic jets

Efficient turbulent entrainment causes otherwise dense volcanic jets to rise high into the atmosphere as buoyant plumes. Classical models suggest that the inflow of air is 10–15% of the axial velocity, giving predictions for the height of the plume and, in turn, the composition and structure of the resulting umbrella clouds. Crucially, entrainment is assumed independent of source geometry and mechanically unaffected by the pyroclastic mixture properties. We show that particle inertia and vent geometry act to modify the shape of the largest eddies defining the jet’s edge and thus entrainment of the ambient. Whereas particle-free flows are essentially unaffected by vent shape, entrainment into particle-laden flows is enhanced for flared vents and reduced for cylindrical vents. Our results predict that vent erosion during an explosive eruption reduces the height of volcanic jets, alters the structure and sedimentation regime of the umbrella cloud, and the resulting deposit.