Pyroclastic density currents (PDCs) are dangerous multiphase flows originating from volcanic eruptions. PDCs cause more than a third of volcanic fatalities globally and, therefore, development of robust PDC hazard models is a priority in volcanology and natural hazard science. However, the complexity of gas–particle interactions inside PDCs, as well as their hostile nature, makes quantitative measurements of internal flow properties, and the validation of hazard models, challenging. Within the last decade, major advances from large-scale experiments, field observations and computational and theoretical models have provided new insights into the enigmatic internal structure of PDCs and identified key processes behind their fluid-like motion. Recent developments have also revealed important links between newly recognized processes of mesoscale turbulence and PDC behaviour. In this Review, we consider how recent advances in PDC research close the gaps towards more robust hazard modelling, outline the need to measure the internal properties of natural flows using geophysical methods and identify critical future research challenges. Greater understanding of PDCs will also provide insights into the dynamics of other natural gravity currents and high-energy turbulent multiphase flows, such as debris avalanches and turbidity currents.
We are not yet learning quickly enough about pyroclastic density currents (PDCs) to save lives.
Recent advances in experimental and computational studies delineate the concentration boundaries that separate dilute, intermediate and concentrated regimes of PDC transport.
Mass and momentum transfer between dilute and concentrated flow regions, and, thus, the evolving transport behaviour, is controlled by the recently identified intermediate regime.
Identification of pore-pressure feedbacks in experimental PDCs, combined with multiphase modelling, provide insights into the origin of the high mobility and extremely low effective friction of PDCs.
New geophysical methods to probe the internal flow structure are becoming available and will provide data to test existing PDC flow models and drive future research.
The advanced understanding of PDCs gained from combining experimental, computational and field approaches must be used to benchmark, validate and advance PDC hazard models.
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This work was supported by the Royal Society of New Zealand Marsden Fund (contract nos. MAU1506 and MAU1902), the National Science Foundation (contract no. EAR 1650382), the New Zealand Ministry of Business, Innovation and Employment’s Endeavour Fund (contract no. RTVU1704) and Resilience to Nature’s Challenges Science Challenge Fund (GNS-RNC047). We thank Michael Manga for the inspiration and help to create the additional figure, and Ermanno Brosch for assisting in the preparation of the figures.
The authors declare no competing interests.
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- Gravity currents
When a fluid of one density propagates into a different fluid of contrasting density.
- Hazard footprints
The area impacted by one or more hazard(s).
- Dynamic pressure
One-half of the product of flow density and the squared flow velocity. A measure of the destructiveness of pyroclastic density currents.
- Compressible turbulent flow
Turbulent flow where the density and temperature change with gas pressure.
- Pore-pressure-modified granular flow
Granular flow where excess pore pressure is present and can alter the friction force inside the flow.
- Dry granular flow
A dense flow of dry granular material whose dynamics are envisaged to be dominated by the stresses associated with particle–particle interactions, rather than hydrodynamic stresses.
- Pyroclastic flows
Pyroclastic density currents dominated by concentrated transport.
- Pyroclastic surges
Pyroclastic density currents dominated by dilute transport.
The characteristics of a mixture of particles that contains a range of particle sizes.
- Isotropic turbulence
An idealistic state of turbulence, where turbulent fluctuations are assumed to decay statistically uniformly in every direction.
- Pore pressure
The pressure of the fluid contained in the interstices of a granular medium.
- Subgrid models
The representation of physical processes occurring at scales that are not resolved on a computational mesh.
A mixture of particles with equivalent size.
- Mesoscale particle clusters
Gatherings of particles into coherent band-like structures.
- Kolmogorov scale
The length, time and velocity scales in turbulent flows below which the effects of molecular viscosity are non-negligible.
- Kelvin–Helmholtz instabilities
A hydrodynamic instability in which immiscible, incompressible and inviscid fluids are in relative and irrotational motion.
- Lift-off distances
The distance at which the flow density becomes lower than that of ambient air, resulting in the buoyant rise of part of the pyroclastic density current.
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Lube, G., Breard, E.C.P., Esposti-Ongaro, T. et al. Multiphase flow behaviour and hazard prediction of pyroclastic density currents. Nat Rev Earth Environ (2020). https://doi.org/10.1038/s43017-020-0064-8