Projects:

On this page, I am highlighting several current and previous research projects that have intrigued me. These include:

• Segregation of an avalanching granular material: measuring segregation characteristics in a multi-disciplinary program
• Rheology of flowing granular material: probing constitutive laws of laboratory-scale sand avalanches
• Dynamics of large-scale snow avalanches: analyzing the motion of snow avalanches with an advanced Doppler radar system
• Structure and migration of sand dunes: using geophysical methods to probe the internal structure of large desert dunes
• Booming sand dunes: investigating sound generation and propagation in large desert dunes

Segregation of an avalanching granular material

Summary:

In 2011 I was awarded a 3-year Natural Environmental Research Council NERC Postdoctoral Research Fellowship based on my proposal "Segregation in geophysical mass flows". Segregation is the separation of grains with different shape, density or size due to a variety of physical processes. Segregation does not only occur within large-scale flows in industry and nature, but can be observed in our own kitchen as well. Shaking a box of cereal at home is a good illustration of the so-called Brazil nut effect -- the larger particles end up at the surface while the smaller particles sink to the bottom. Intuitively one may think that this (size) sieving, or segregation, is simply due to the smaller particles falling into the holes between the larger particles. This explanation is qualitatively correct, but it has proven extremely difficult to construct a mathematical theory that can accurately predict the speed and degree of segregation. Segregation also occurs for same size but different density particles, as the denser particles move to the bottom of the mixture.

The executive summary of the research project is: The aim of this project is to investigate the segregation within geophysical mass flows such as avalanches, rock slides and debris flows. These geophysical flows are so powerful and destructive because of the continuous interaction between the fluid, solid and gas phases. This NERC postdoctoral research proposal aims to experimentall and numerically investigate segregation in avalanching geometries and validate phenomenological models using the unique facilities within DAMTP. In addition, field experiments on the segregation characteristics of snow avalanche debris complement the research and provide a large scale validation.

Collaboration:

I am collaborating with Dr. Mick Mantle and Dr. Andy Sederman at the Magnetic Resonance Research Centre (MRRC) at the University of Cambridge to conduct MRI experiments. Furthermore, I am collaborating with Dr. Jim McElwaine (from May 2013 onwards based at Durham University, UK) on numerical simulations with a discrete element code. The granular collapse research is conducted by my Ph.D. student Josh Caplan and Katherine Daniels is employed as a research assistent on field and laboratory experiments on segregation in 3D flows.

Progress:

• Segregated granular collapse in a spherical geometry
• Fixed volume release of a bidisperse mixture on an open chute
• Segregation of a bidisperse mixture in a closed-end chute
• Field research on segregation in snow avalanche deposits using a Ground Penetrating Radar survey and GPS

For more information and an extensive description of the different research strands, I would like to refer to the special "segregation" research webpage that is currently in development and whose link will be posted here shortly.

Dissemination:

• Setting up a separate segregation webpage, with a "general public" and a "research section".
• Presentations at the 2nd IMA Conference on Dense Granular Flows in Cambridge (July 1st - July 4th, 2013), including "Granular segregation in a closed-end chute - MRI experiments and numerical simulations" and "Segregation effects in granular collapses".

Rheology of flowing granular material

Summary:

Granular avalanches on an open slope form channels, where a flowing region is bounded by quasistatic levees. Current theories are inaccurate for these flows because they contain static and flowing regions at depths close to hstop. The dimensions of the unconfined channels and the relations between the flowing and static regions provide insight on the rheology of the flow. Previous work highlighted laboratory experiments of unconfined shallow granular flows featuring a curved free surface in the flowing regions, bounded by static margins exerting lateral stresses on the flow. As the velocity profile and the height of the margins are self-similar, the rheological parameters cannot be uniquely determined, since the lateral stresses and flow depth both effect the velocity.

This study investigate the characteristics of a flowing granular material down a low-angle V-shaped inclined plane under the action of gravity. The long-wave (shallow water) approximation relates the geometry of the flow directly to the slight V-shaped angle and therefore the depth dependence of the velocity can be decoupled from the lateral stresses. A combination of laboratory experiments and numerical simulations show that the surface of the flow, steady both in time and down the slope, has significant curvature caused by second normal-stress differences, similar to those observed in non-Newtonian fluids such as granular suspensions.

Experiments and numerical simulations validate the new theoretical model relating the height and velocity of the flow directly to the V-shape angle.

Collaboration:

This research has been conducted in collaboration with Dr. Jim McElwaine (from May 2013 onwards based at Durham University, UK).

Progress and dissemination:

Dynamics of large-scale snow avalanches

Summary:

In early 2010 I was recruited as a postdoctoral research associate at the University of Cambridge to work on the analysis of dynamical data collected with a state-of-the-art phased-array radar system that had been installed in a reinforced bunker on a large-scale snow avalanche test site in Vallée de la Sionne in Wallis, Switzerland and managed by the WSL-Institut für Schnee- und Lawinenforschung SLF.

I participated in two field trips, one in the summer of 2011 to upgrade and calibrate the system and one in the December 2011 when we hoped to conduct a measurement of an artificial release of a large snow avalanche -- unfortunately even after several explosives the snow did not release into an avalanche. Unfortunately, no data was recorded in in the first winter (2009 - 2010) of deployment due to environmental adverse conditions, but we were successfull to capture data with an automated trigger system in the subsequent winter (2010 - 2011). My analysis results show the dynamics of the dense core of snow avalanches from start to finish featuring initial fronts and internal roll waves. The radar data reveal a wealth of structure in the avalanche and allow the tracking of individual fronts and surges down the slope for the first time.

Collaboration:

This is a large research project with several parties involved. The radar system was developed by Dr. Matt Ash, while doing a Ph.D. in Electrical Engineering at the University College London. Field operations in Switzerland have been managed by the WSL-Institut für Schnee- und Lawinenforschung SLF, our project partner. The project has been funded by NERC and included the following PIs: Dr. Chris Keylock (University of Sheffield), Prof. Paul Brennan (University College London) and Dr. Jim McElwaine (from May 2013 onwards: Durham University). The GEODAR project research page can be found here.

Progress and dissemination:

• Discovery Channel TV documentary "x-ray Yellowstone", featuring a segment interviewing me on my avalanche research and its implications.
• Royal Society Summer Science Exhibitor 2012, where I organized together with Chris Keylock and Paul Brennan the display "Setting a Speed Trap for an Avalanche" for 11,000 visitors.
• Scientific publication in Geophysical Research Letters (2013): "High-resolution radar measurements of snow avalanches".

Structure and migration of sand dunes

Summary:

As part of my geophysical interest during my Ph.D., I conducted extensive geophysical field measurements on large (50 - 200 m) desert sand dunes, using Ground Penetrating Radar (100 MHz and 200 MHz). Although these large dunes seem to be stationary at the same position, due to wind action they do move over time. Furthermore, they are not only massive lumps of sand but actually feature a rich internal structure that evolves over the seasons.

Collaboration:

The geophysical equipment, including Ground Penetrating Radar equipment, a 48-channel seismic array and a laserscanner, is owned by the GPS-division at Caltech and was kindly lent to me for the purpose of this research. As a Ph.D.-student, I was supervised by Professor Melany Hunt and Professor Rob Clayton.

Results and dissemination:

Booming sand dunes

Summary:

My main project during my Ph.D. focussed on the investigation of wave propagation on booming sand dunes. A booming sand dune manifests itself by initiating an avalanche from the leeward face of a large dune. The resulting low-frequency booming noise or music is loud and resembles a low-flying propeller airplane. The sound is surprisingly monotone with one dominant audible frequency. The sound is sustained and may continue for to a minute after initiation, even after all visible motion has ceased. Moving a hand through the dry sand of a booming dune shears the upper layer and generates another acoustic phenomenon, the burping emission - pulse-like, short bursts of sound.

Collaboration:

My thesis advisors, Professor Melany Hunt, Professor Rob Clayton and Professor Chris Brennen, joined me on several field trips to the booming dunes of the Eureka expanse in Death Valley NP and Dumont Dunes in the Mojave Desert. Furthermore, several granular flow group members took some time off their own research to join on various field trips, together with many other people from different options and universities joining us occasionally. On average, we would have at least six volunteers in total on each trip, with maximum numbers exceeding 25 when I would invite an undergraduate class to join. Without their input and enthusiasm, the field research could have never been conducted.

Results and dissemination:

• National Geographic TV documentary "Death Valley", featuring a segment where they joined our team on a field trip to Eureka dunes and interviewed me on my booming sand research.
• Scientific publication in Annual Review of Earth and Planetary Sciences (2010): "Booming Sand Dunes".
• Scientific publication in Geophysical Research Letters (2007): "Solving the Mystery of Booming Sand Dunes", including a large amount of popular science media coverage, including Nature, Science and NewScientist.