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Abstract:
Dense pyroclastic density currents are granular mass flows consisting of a mixture of solid particles and gas, denser than the surrounding atmosphere, which flow down the slopes of a volcano edifice under the action of gravity.They are one of the most hazardous phenomena related to volcanic eruptions and their destructive power is related to their high velocity, high temperature and large dynamic pressure. Due to their very complex nature (multiphase, polydisperse mixtures), our knowledge on the fluid dynamics of these flows is still limited, especially on their rheology and interaction with the ground. Still, this complexity must be addressed in order to improve the existing numerical models used for quantifying their hazard.Here we present results of the rheological characterization of volcanic granular material carried out with the FT4 Powder Rheometer (Freeman Technology). Shear test, compressibility tests and wall friction tests have been performed to characterise the flowability of the powders and their interaction with a variable-roughness substrate, while the rheological behaviour has been described by linking the shear stress τ and shear rate γ̇ in a steady state flow curve and using a coaxial cylinder flow configuration. The same experiments have been performed on aerated samples in order to assess the influence of the fluidisation on the rheological behaviour, and also the role played by the fines to enhance the mobility.
