Modeling volcanic granular flows:
Particle dynamics and emergent structure of pyroclastic density currents
Supervisors: Dr Mark Naylor1, Dr Eliza S. Calder1 & Dr Jin Sun2
1School of Geosciences, University of Edinburgh
2School of Engineering, University of Edinburgh
Contact: [email protected]
Project Summary: This project will investigate particle dynamics in pyroclastic flows including density and
size segregation as well as particle-gas phase coupling to understand the evolving rheology and bulk
mobility of flows.
Background: Pyroclastic density currents (PDCs) are complex, highly unsteady, multiphase flows generated
when poorly-sorted mixtures of decimetre to metre-sized volcanic debris set within an ash and gas matrix
descend the flanks of a volcano. Their high temperatures, inherent mobility and unpredictable nature
render them one of the most hazardous phenomena associated with volcanic activity. Small-volume (< 0.5
km3) end-member PDCs are relatively common volcanic phenomenon, yet despite being frequently
observed their emplacement mechanisms remain poorly understood and their behavior is difficult to
model.
As material within PDCs undergoes progressive breakdown and internal segregation, there is
feedback onto the dynamics of the flow; this history dependence makes granular flows difficult to model.
For example, in PDCs generated from lava dome collapses (Block-and-ash flows), collapsing material
progressively evolves from solid fractured lava, through coarse collapsing mega-blocks, to highly
fragmented, ash-dominated material. In other cases, such as in the formation of small volume pumice
flows, material progressively sediments from a collapsing but dilute mixture, eventually condensing to form
ground-hugging concentrated granular currents, which often show marked vertical and lateral segregation
in terms of their high and low density components. Variations in their uniformity and material make-up are
in turn reflected in the bulk mobilities of different classes of PDC (Calder et al., 1999, Vallance et al., 2010).
Furthermore, the complexities associated with interpretation of field deposits, anticipation of inundation
extents for given flow scenarios over specific terrain and our ability to effectively model these currents, are
all essentially rooted in this unsteady, non-uniform behavior. A key limitation in the application of our
knowledge to real-case scenario hazard mitigation, is that none of the available suite of numerical models,
practical for hazard assessments, can account for non-uniformity, and as such, their success in simulating
past events and in forward-modeling applications to help delineate potential anticipated hazards is limited.
Aims and Key research questions: This research will focus on understanding the dynamics and
emplacement of PDC’s by investigating at the particle dynamic scale using Discrete Element Modelling
(DEM) approaches for particle interactions in granular flows. Both size and density segregation of particles
as well as fragmentation and comminution occur progressively as the flows propagate (e.g,
http://ow.ly/3yktEd). These processes are likely key in affecting the changing bulk material properties, and
flow dynamics as the flows propagate, as well contributing to current unsteadiness and variations in mass
transfer into overriding ash cloud surges. We will focus on the role that size and density segregation plays
in vertically and longitudinal flow variations as well as the resultant gas-phase coupling, to determine how
the emergent structure and rheological behavior during flow emplacement varies over space and time. A
more thorough understanding of material property variations and the stresses resulting from those particle
interactions will result in improved numerical models of large-scale flows.
Key research questions will be:
 Can particle size and density segregation induce larger-scale flow unsteadiness?
 How is gas-phase coupling affected by segregation, and what implications does this have for elutriation
and ash cloud surge generation?
 Can size and density segregation and the resultant changes in gas-phase coupling explain syn-flow
rheology variations?
Methodology: An approach that couples discrete element method with computational fluid dynamics
(DEM-CFD) will be employed to capture the particle dynamics within modelled PDCs. DEMs are particularly
suited to modelling complex granular flows as they encapsulate the physics at the particle scale. Hence
phenomena, such as segregation of particles during the flow, become emergent properties of the
simulation. Using computer simulations we can forensically track the behaviour of individual particles to
their final resting place to enable field observations to be better interpreted in terms of flow dynamics.
Individual particles’ translational and rotational motion is tracked by solving Newton’s equations according
to particle contact forces and torques in DEM simulations. The contact force between the particles will be
modelled by the spring-dashpot model, which calculates the force using the overlap and the relative
velocity. Friction between particles is accounted for through the Coulomb friction law. Different particle
shapes can be represented by composites of primary spheres. The particle dynamics is coupled to fluid
dynamics through volume fraction coupling and momentum transfer. Such resolution at the particle scale
renders the method a powerful tool to study how particle size and density affect the flow dynamics. This
technique has been developed for engineering applications, but its application to volcanology promises a
powerful tool to make substantial advances
in understanding how large particle dynamics
are coupled with that of the fluidized ash
matrix of pyroclastic density currents. Finally
a coarse-graining method will be applied to
obtained the bulk properties from the
particle dynamics, which will allow
investigation of how the segregation at the
gain scale, scales up to affect the larger-scale
flow rheology. The designed DEM-CFD
simulations will be carried out using in-house
software developed by the Engineering
group, which has already been validated
against detailed results from fluidised bed experiments. In this project, validation of PDC simulations will
be performed against existing datasets from small-scale laboratory experiments on segregation of granular
materials in avalanches. There may also be the opportunity for additional experiments to be undertaken
as part of this project.
Research Training: A comprehensive training programme will be provided comprising both specialist
scientific training and generic transferable and professional skills. The student will undertake training in
diverse multidisciplinary areas including volcanic processes, numerical methods, granular dynamics,
multiphase flow, and high-performance scientific computing. Project specific training on the DEM-CFD
method will be provided.
Requirements: Applications are invited from students with geoscience, maths, or engineering background
who have strong quantitative and coding skills. This is a cross-disciplinary project, preference will be given
to students with some experience in numerical modelling, discrete element method, multiphase flow, or
have experience in laboratory experiments on granular flows.
Further reading:
Calder, E.S., et al (1999). Geophysical Research Letters, 26(5), p537-540
Chialvo,et al. 2012. Physical Review E 85 (2): 021305.
Gray (2010). AIP Conf. Proc. 1227, 343-362.
Kokelaar, et al (2014) Earth Planet. Sci. Lett. 385, 172-180.;
Naylor, et al. 2005, Journal of Geophysical Research, (110), B12403;
Sun, et al. 2007. Journal of Fluid Engineering 129 (11): 1394–1403.;
Sun, et al. 2009. International Journal of Computational Fluid Dynamics 23 (2): 81–92.