Where Do You Go When the Volcano Blows? Fluid flow modeling aids Italian university researchers in characterizing the effects of explosive eruptions on nearby buildings.

When a volcano erupts, the geological process that results often releases devastating amounts of energy from beneath the planet’s surface through volcanic flows. such flows are classified into two categories: well-known lava flows when the eruption is effusive, and pyroclastic flows when it is explosive. Explosive eruptions, like the ancient events at Mount Vesuvius in Italy and the modern ones at Mount st. Helens in the United states and Mount Pinatubo in the Philippines, represent the more hazardous of the two classes. The explosions themselves can be caused either by magmatic fragmentation due to high stress on the sub-terranean molten rock (magma) or phreatomagmatic fragmentation, which is caused when magma shatters after interacting with water [1]. In both cases, a multiphase mixture of hot gas and solid (ash and pumice) particles is generated and is ejected out the volcano. If the mixture at the crater consists of a pressure-balanced jet, it expands only upward and generates a volcanic column; if it has a pressure greater than atmospheric pressure, it expands in all directions and generates a volcanic mushroom. When columns and mushrooms reach the maximum height allowed by the issuing flow rate against gravity, they then collapse, impacting the ground and generating pyroclastic flows. Pyroclastic flows are extremely dangerous because they move rapidly down from the volcanic crater to spread over the surrounding terrain [2] and interact with buildings in populated areas through the dynamic pressure action on structures. Generally, a flow-building interaction zone, that is, a village or a city, is located far enough from the crater so that pyroclastic flows arrive diluted, turbulent and fully developed — characterizations supported by recent geophysical studies [3]. The multiphase behavior of pyroclastic flows is also very important both on the turbulence modulation and on the dynamic pressure. The flow currents are strongly stratified by velocity and particle concentration [4] so turbulence and pressure must account for the gas–particle coupling.