Supervisors: and Stuart Dalziel (DAMTP)
Importance of the area of research:
Geophysical mass flows are large movements of snow, mud or rocks down steep slopes. Varying topography and three-dimensional effects create a complicated interplay between internal friction and segregation, resulting in a mobile and volatile mass flow obtaining much higher velocities and impact pressures than otherwise expected. The effect of basal friction on flow instabilities, e.g. granular fingering and roll waves, is poorly understood. A constriction or object in the flow can dramatically change the flow path and parameters, leading to unexpected outcomes.
These flows form a significant hazard for communities and directly influence the environment, infrastructure, economy and tourism of a region. A fundamental and thorough understanding of granular flow dynamics governing them, grounded in mathematical modeling and carefully-controlled scaled-down laboratory experiments, is therefore necessary to mitigate damage and loss of life. The goal of this project is to develop better models and increase our confidence in avalanche predictions.
This project aims to obtain a thorough understanding of the influence of the basal surface and objects in the path on the behaviour of granular avalanching flows. Careful experimentation in a unique large (~2.5 m), fast and high-speed (~ 6 m/s), large (~ 20 kg/s) recirculating granular chute is performed with a variety of basal surfaces (smooth, rough) and objects (shape, size). The created granular avalanches down an incline show a variety of outcomes mimicking processes observed in nature, which can be explored with observational field work.
What the student will do:
The student will conduct granular experiments on a unique experimental set-up, based at the GK Batchelor Laboratory in DAMTP. The continuous recirculating avalanche allow the precise measurement, using a high-speed camera, a laserscanner and force sensors, of the velocities, heights and the exerted forces of an avalanche. The slope of the chute can be adjusted at different angles and with different mass flow rates, thus representing different avalanche regimes. Different base materials (continuously rough or smooth, or a transition) can be inserted and, in the latter case, the properties of a hydraulic jump will carry rheological information. For different objects in the flow, the force and torque exerted can be quantified, while downstream granular flow profiles reveal further information on the friction characteristics of an avalanche.
A natural extension of the experimental component of project includes theoretical modelling and/or numerical simulations with either continuum or particle-based methods.
Johnson, C. G., Kokelaar, B. P., Iverson, R. M., Logan, M., LaHusen, R. G. & Gray, J. M. N. T. 2012. Grain-size segregation and levee formation in geophysical mass flows. Journal of Geophysical Research: Earth Surface, vol. 117, pp. F1.
Forterre, Y. & Pouliquen, O. 2003. Long-surface-wave instability in dense granular flows. Journal of Fluid Mechanics, vol. 486, pp 21-50.
Woodhouse, M.J., Thorton, A.R., Johnson, C.G., Kokelaar, B.P. & J.M.N.T. Gray. Segregation-induced fingering instabilities in granular free surface flows. Journal of Fluid Mechanics, vol. 709, pp 543-580.
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