PhD studentship: The Weakest Link: Triggering of giant submarine landslides

This PhD studentship is now open for applications. For more information and to apply follow this link.

Description of the project:

Giant landslides and sediment avalanches on the seafloor are a demonstrated hazard to seafloor infrastructure (e.g. internet cables and oil pipelines) as well as being the key mechanism by which terrestrial sediment is transported thousands of kilometres before ultimate burial in the deep sea. Our understanding of these landslides and avalanches, from how seafloor slopes fail to how the flows evolve is limited because we know little about the material properties (i.e. the rheology) of the sands, silts and clays that make up the seafloor in the deep sea. Understanding these properties will lead to a better understanding of where and why landslides and avalanches occur, how such flows evolve and therefore enable better modelling capabilities. This will ultimately inform, where to locate and how to protect seafloor infrastructure; how such flows interact with seafloor habitats and how sediment is transported in our oceans.

This project will establish the composition and rheology of seafloor sediments from a wide variety of environments from seafloor channels and canyons, open slopes and deep sea fans. This will be achieved by making use of samples from deep sea sediment cores that have been collected from all over the world and are housed in the British Ocean Sediment Core Research Facility (BOSCORF) at the National Oceanography Centre, Southampton. The composition of the sediments will be established using X-ray diffraction and the grain size will be established using a laser particle analyser.  The main method of research will be to generate a highly controlled flow over ‘miniature seafloors’ using a rheometer. The first step will be to determine the rheological properties of the sediment suspension. These rheological properties will then be correlated to the sediment composition and grain size. Following on from this, customised rheometer setups with optical access will be used to perform controlled perturbations of the sediment sample. These experiments will investigate how laminar but potentially unsteady flow conditions can generate stable sediment structures and where their point of failure is using image analysis techniques.

PhD Studentship: Cavity collapses in complex geometries

This studentship is now open for application. To apply, follow this link.

Understanding the behaviour of bubbles in the vicinity of structures is important for many practical applications. Of particular interest is how bubbles collapse as a result of strong changes in pressure, which happens in sonochemistry applications, ultrasonic cleaning and is responsible for the cavitation damage in hydraulic systems. In the absence of any boundaries, an initially spherical bubble will remain spherical and collapse towards a single point. The same collapsing bubble near a plane wall, however, will result in a fast liquid jet shooting towards the wall. But what happens to such a bubble in more complex geometries, as is the case in most applications, is currently unknown. This project aims to answer this fundamental question by experimentally investigating the controlled generation and collapse of bubbles in increasingly complex geometries.

There are two main subjects that will be investigated in this project. The first one is large cylindrical cavities that are created through the impact of circular discs. These experiments serve as a model for the collapse of two-dimensional cavitation bubbles. The large scale of these bubbles allows us to investigate the evolution of the shape and the flow field around the bubble in great detail (see figure below). The second subject is laser-induced cavitation, where three-dimensional vapour bubbles are generated through a short focused laser pulse. Both techniques allow us to accurately control the position and size of the cavitation bubble and investigate the effect of the surrounding geometry on the collapse.

collapse_near_wall

Time series of a collapsing cylindrical cavity, created by a circular disc impacting at 1.0 m/s close to a wall (vertical dashed line), viewed from the top. The disc appears grey with a bright spot indicating its centre. The dashed ellipe in (a) and (b) indicates the contour of the cavity at t=0. After the initial formation (c), the jet penetrates the complete cavity (d).

The experimental methods will rely heavily on high-speed imaging and image processing (e.g. surface detection and tracking). Experience with image processing in Python, MATLAB, or a comparable language is recommended. The project requires the construction of experimental setups, so machine-shop experience is advantageous. The candidate should have a degree in physical sciences or engineering with a strong background in fluid dynamics.

 

Oil entrainment

Peters_PRF_image

As a disc moves through an oil-water interface, it entrains part of the oil into the water phase through a funnel-shaped volume. The shape of this volume is determined by a combination of potential flow and a starting vortex, but surprisingly, its shape remains independent of the disc velocity.

Read more about this in Physical Review Fluids: Volume entrained in the wake of a disc intruding into an oil-water interface, Ivo R. Peters, Matteo Madonia, Detlef Lohse, and Devaraj van der Meer Phys. Rev. Fluids 1, 033901 (2016)

Dynamic Shear Jamming

nature_web-01

A suspension of cornstarch particles in water is liquid-like when perturbed lightly, but turns into a jammed solid when sheared strongly. We explored how this solidification propagates through the material using high-speed imaging after applying a sudden deformation. This jamming transition is related to, but distinct from the shear-thickening that this material also exhibits.
Our findings are published in Nature:
I.R. Peters, S. Majumdar and H.M. Jaeger, Direct observation of dynamic shear jamming in dense suspensions, Nature (2016) DOI: 10.1038/nature17167