Featured research (3)
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.
Numerical models of pyroclastic currents are widely used for fundamental research and for hazard and risk modeling that supports decision-making and crisis management. Because of their potential high impact, the credibility and adequacy of models and simulations needs to be assessed by means of an established, consensual validation process. To define a general validation framework for pyroclastic current models, we propose to follow a similar terminology and the same methodology that was put forward by Oberkampf and Trucano (Prog Aerosp Sci, 38, 2002) for the validation of computational fluid dynamics (CFD) codes designed to simulate complex engineering systems. In this framework, the term validation is distinguished from verification (i.e., the assessment of numerical solution quality), and it is used to indicate a continuous process, in which the credibility of a model with respect to its intended use(s) is progressively improved by comparisons with a suite of ad hoc experiments. The methodology is based on a hierarchical process of comparing computational solutions with experimental datasets at different levels of complexity, from unit problems (well-known, simple CFD problems), through benchmark cases (complex setups having well constrained initial and boundary conditions) and subsystems (decoupled processes at the full scale), up to the fully coupled natural system. Among validation tests, we also further distinguish between confirmation (comparison of model results with a single, well-constrained dataset) and benchmarking (inter-comparison among different models of complex experimental cases). The latter is of particular interest in volcanology, where different modeling approaches and approximations can be adopted to deal with the large epistemic uncertainty of the natural system.
Dilute pyroclastic density currents (dilute PDCs) are frequent and highly dangerous flows of hot gas and particles occurring at explosive volcanoes. The study and interpretation of the sedimentary characteristics of their associated deposits is one of the most important approaches to better understand these violent phenomena and to characterise their dynamics, frequency and magnitude in the geological record of volcanoes. Current strategies are based on sediment transport principles developed for fluvial and aeolian systems. How well these analogies capture the sediment transport behaviour of dilute PDCs remains poorly understood, as the hot and hostile conditions of these fast-moving volcanic flows hamper direct measurements. Here we report the results of large-scale experiments that aim to synthesise the behaviour of dilute PDCs in order to investigate the transport and sedimentation processes inside the hot flows. These flows reproduce the spatiotemporal deposit facies variations seen in natural deposits of dilute PDCs. Through measurements of the evolving flow structure (velocity, particle concentration and flow grain size distribution), we show that the lower region of the density stratified current can be subdivided from the aggrading deposit upwards into a dynamic bedload region with particle concentrations of c. 0.5 to several vol%, a transient region with particle concentrations of c. 0.1–1.5 vol%, a dilute, fully turbulent region with formation of mesoscale clusters, and an upper dilute turbulent region absent of mesoscale clusters. We show that the particle feeding mechanisms of the transient region is related to the occurrence of mesoscale clusters. This process has a key role in modifying the sediment transport modes inside the bedload region. These modifications cause a variation of the dynamics of the lower flow boundary, including erosion/deposition events, and variations of the deposition rate over at least three orders of magnitude. We present images and video footage of these processes that include the formation of shifting sandwaves, rolling and saltation of particles inside the experimental dilute PDC, and analyse how these relate to the development of massive, stratified and laminated deposit structures. Through mapping the isochrones of deposition across the flow length and measuring the propagating flow above, we capture a spatiotemporal view of the aggrading deposit, which can guide the interpretation of natural deposits.