Fabio Dioguardi

Conduit flow experiments help constraining the regime of explosive eruptions

Accepted for publication in (Geophysical Research Letters). Copyright (2009) American Geophysical Union.

Volcanic jets, plumes, and collapsing fountains: Evidence from large-scale experiments, with particular emphasis on the entrainment rate

The source conditions of volcanic plumes and collapsing fountains are investigated by means of large-scale experiments. In the experiments, gas-particle jets issuing from a cylindrical conduit are forced into the atmosphere at different mass flow rates. Dense jets (high particle volumetric concentration, e.g., C0 > 0.01) generate collapsing fountains, whose height scales with the squared exit velocity. This is consistent with Bernoulli’s equation, which is a good approximation if air entrainment is negligible. In this case, kinetic energy is transformed into potential energy without any significant loss by friction with the atmosphere. The dense collapsing fountain, on hitting the ground, generates an intense shear flow similar to a pyroclastic density current. Dilute hot jets (low particle volumetric concentration, e.g., C0 < 0.01) dissipate their initial kinetic energy at much smaller heights than those predicted by Bernoulli’s equation. This is an indication that part of the total mechanical energy is lost by friction with the atmosphere. Significant air entrainment results in this case, leading to the formation of a buoyant column (plume) from which particles settle similarly to pyroclastic fallout. The direct measurement of entrainment coefficient in the experiments suggests that dense collapsing fountains form only when air entrainment is not significant. This is a consequence of the large density difference between the jet and the atmosphere. Cold dilute experiments result in an entrainment coefficient of about 0.06, which is typical of pure jets of fluid dynamics. Hot dilute experiments result in an entrainment coefficient of about 0.11, which is typical of thermally buoyant plumes. The entrainment coefficients obtained by experiments were used as input data in numerical simulations of fountains and plumes. A numerical model was used to solve the classic top-hat system of governing equations, which averages the field variables (e.g., column velocity and density) across the column. The maximum heights calculated with the model agree well with those observed experimentally, showing that our entrainment coefficients are compatible with a top-hat model. Dimensional analysis of the experimental data shows that a value of 3 for the source densimetric Froude number characterizes the transition between dense collapsing fountains and dilute plumes. This value delimits the source conditions (exit velocity, conduit radius, and particle volumetric concentration) for pyroclastic flow (<3) and fallout (>3).

Investigating Source Conditions and Controlling Parameters of Explosive Eruptions: Some Experimental-Observational- Modelling Case Studies

Explosive volcanic eruptions are complex systems that can generate a variety of hazardous phenomena, for example, the injection of volcanic ash into the atmosphere or the generation of pyroclastic density currents. Explosive eruptions occur when a turbulent multiphase mixture, initially predominantly composedf of fragmented magma and gases, is injected from the volcanic vent into the atmosphere. For plume modelling purposes, a specific volcanic eruption scenario based on eruption type, style or magnitude is strictly linked to magmatic and vent conditions, despite the subsequent evolution of the plume being influenced by the interaction of the erupted material with the atmosphere. In this chapter, different methodologies for investigating eruptive source conditions and the subsequent evolution of the eruptive plumes are presented. The methodologies range from observational techniques to large-scale experiments and numerical models. Results confirm the relevance of measuring and observing source conditions, as such studies can improve predictions of the hazards of eruptive columns. The results also demonstrate the need for fundamental future research specifically tailored to answer some of the still open questions: the effect of unsteady flow conditions at the source on the eruptive column dynamics and the interaction between a convective plume and wind.

PYFLOW_2.0: a computer program for calculating flow properties and impact parameters of past dilute pyroclastic density currents based on field data

This paper presents PYFLOW_2.0, a hazard tool for the calculation of the impact parameters of dilute pyroclastic density currents (DPDCs). DPDCs represent the dilute turbulent type of gravity flows that occur during explosive volcanic eruptions; their hazard is the result of their mobility and the capability to laterally impact buildings and infrastructures and to transport variable amounts of volcanic ash along the path. Starting from data coming from the analysis of deposits formed by DPDCs, PYFLOW_2.0 calculates the flow properties (e.g., velocity, bulk density, thickness) and impact parameters (dynamic pressure, deposition time) at the location of the sampled outcrop. Given the inherent uncertainties related to sampling, laboratory analyses, and modeling assumptions, the program provides ranges of variations and probability density functions of the impact parameters rather than single specific values; from these functions, the user can interrogate the program to obtain the value of the computed impact parameter at any specified exceedance probability. In this paper, the sedimentological models implemented in PYFLOW_2.0 are presented, program functionalities are briefly introduced, and two application examples are discussed so as to show the capabilities of the software in quantifying the impact of the analyzed DPDCs in terms of dynamic pressure, volcanic ash concentration, and residence time in the atmosphere. The software and user’s manual are made available as a downloadable electronic supplement.