Experimental study of jet gas-particle interaction generated during explosive volcanic eruptions

Abstract:

During violent volcanic eruptions, a shock wave may be generated that is immediately followed by the formation of a supersonic jet. The overpressurized vapor-solid-liquid mixture being ejected begins to expand and accelerate. Oblique shock waves and rarefaction waves are generated at the edge of the crater. The oblique shock waves, inclined relatively to the flow axis, intersect forming a structure called a “Mach disk” or “Mach diamond”. This pattern repeats until the jet decelerates into subsonic flow. In an explosive volcanic eruption, unlike other applications involving jets, a mixture of hot gas and solid particles is present. The mixture typically contains a relatively high percentage of solid particles of different sizes. The relationship between jet and particle is one the major parameters affecting the formation of ash plume dynamics and the pyroclastic flows. Therefore, a more comprehensive study is needed in order to understand the mixing occurring within the volcanic eruption jet, specifically, the effect of particle size and concentration.

In this work, a series of analog explosive volcanic experiments using an atmospheric shock tube are performed to generate supersonic jets. High-speed video imaging of the expanding jet as well as the pressure evolution at different points in space are recorded for different values of initial energy and particle sizes and concentrations. Particles of different sizes and in various concentrations are placed inside the jet stream in which all the environmental conditions are monitored. Understanding of the coupling between the particles and the jet dynamics interaction is the first step toward a more thorough understanding of ash plume dynamics and the pyroclastic flows formation.