Webb, William
(2024)
Axisymmetric saturated granular column collapses at elevated gravitational accelerations.
PhD thesis, University of Nottingham.
Abstract
Debris flows represent a prevalent natural hazard, posing a significant threat to communities and infrastructure located in mountainous regions susceptible to intense precipitation events worldwide.
Furthermore, the escalating impacts of climate change are poised to intensify both the frequency and severity of these events.
The dynamics of debris flows, in contrast to other geophysical granular flows, exhibit heightened complexity owing to substantial fluid volumes and a broad spectrum of particle sizes present within the flow.
The diverse composition of debris flows gives rise to highly heterogeneous flow states, wherein many contributing mechanisms, notably the interplay between the granular and fluid phases, remain inadequately understood within the wider scientific community.
Consequently, there persists a lack of consensus on the optimal approach to incorporate the influence of grain-fluid interactions into numerical models which are essential for predicting debris flow behaviour and formulating effective mitigation strategies.
This study aims to shed light on grain-fluid interactions within debris flows through a programme of physical scaling analysis complemented by two numerical approaches, inspired by the classical granular column collapse experiment.
Focus was given to the just-saturated case, where granular pores were filled with fluid up to the column's free surface.
Crucially, a geotechnical centrifuge controlled the stress state within the granular-fluid flow, enabling experiments across a wide parameter space, including cases where force balances matched those in geophysical flows.
The study's parameter space considered variables such as gravitational acceleration, inertial particle size, fluid viscosity, and the contribution of fine granular material, with different concentrations of fine kaolin clay particles suspended within the fluid phase.
High-speed imaging and basal fluid pressure measurements were used to quantify characteristic acceleration stage flow outcomes as functions of dimensionless parameters defined from the initial column configuration.
The results from the physical experiments formed a substantial dataset applicable for calibrating or validating numerical models.
Two numerical modelling schemes, a continuum-continuum (shallow water) approach, which was implemented within Matlab, and a discrete-continuum (Discrete Element Method-Lattice Boltzmann Method) approach, were used to replicate the observed behaviour from the experiments and gain further insights into the nature of the grain-fluid interactions.
The latter model was then employed to investigate how the dynamics of the column collapses were influenced by the rotation of the geotechnical centrifuge, focusing on the effects of centrifugal and Coriolis accelerations. Key takeaways include the development of a design criterion based on the log of the ratio of the centrifuge's radius and the height of the mounted model.
It was found that when this ratio exceeds 4, it can be assumed that the centrifuge model is subjected to a constant gravitational field, where the influence of horizontal centrifugal and Coriolis accelerations are negligible.
This study emphasises that although complex experimental setups and numerical models are necessary to replicate the flow conditions observed in natural debris flows, simplicity is crucial for gaining insight into the specific mechanisms and processes that drive their dynamics.
Actions (Archive Staff Only)
 |
Edit View |
Tools
Tools
