Impact of Collapsing Gas-particle Jets: The Initial Conditions for Pyroclastic Flows and Surges

Pyroclastic flows and surges (here called pyroclastic density currents, or PDCs) are a fundamental phenomenon of explosive volcanic eruptions. PDCs are poorly understood because they involve flow of complex mixtures of gasses and particles (such as volcanic ash and rock fragments) at a wide range of speeds and temperatures. The flows are the most destructive of volcanic phenomena, but poor understanding has limited scientists? ability to predict their behavior in a way that optimizes mitigation of hazards and consequences. Direct measurements of their dynamics in nature are rare because of their unpredictability and danger. Instead, much of our understanding of PDCs at a given volcano relies on characteristics of deposits from previous eruptions, along with experiments and computational modeling. Many pyroclastic density currents are caused by the falling back, or collapsing, of jets of gas and debris. As the mixtures impact the ground they change from vertically falling flows to lateral flows (the PDCs), and this transition is complicated by the different ways that particles and gas respond to sudden changes in speed and direction. The impact process critically influences the characteristics of the resulting lateral flows (speeds, particle concentrations), yet this is a topic that has not been addressed by previous quantitative research. The project will combine numerical modeling and experiments, complemented by field studies, to address the following hypotheses: (1) Impact dynamics depend upon collapse height, collapsing particle concentration, clast size and density distribution, slope of the ground at the impact site, and duration of collapse; (2) Collapse of vertical, impulsive jets tend to feed dilute PDCs (pyroclastic surges) by expulsion of fines and gas; (3) Collapse of sustained jets can feed concentrated and/or dilute PDCs immediately upon impact, depending upon the clast size and density population of the collapsing mixture; (4) Impact dynamics define initial conditions for the PDCs and should inform simplified flow models for hazards assessments. The work will define the initial conditions for jet/fountain-collapse fed flows so that simplified PDC models can be better used to guide decision makers coping with volcanic hazards. The experimental setup developed by the project will be available for follow-on used by anyone in the research community. It is anticipated that the fluid dynamic aspects of the research will have applications to industrial processes that involve gas-particle flows. Aspects of the work will comprise a key part of a Ph.D. dissertation. Additionally, a video-based education module will be prepared for university students, which will demonstrate the integrated approach and aid in developing an intuitive understanding of multiphase flow dynamics as a key aspect of geological fluid dynamics.