Tomographic investigation of turbulence and sediment dynamics

Understanding sediment transport has come a long way since the pioneering works of Shields and Bagnold, nearly a century ago. Yet, the complex mechanisms that drive, at larger scales, the morphological evolution of coastal and estuarine environments remain poorly understood at the ‘noisy’ end of the spectrum. Fundamentally, this is a problem of fluid sediment interaction, with turbulence – the most notorious problem in classical physics – lying at its very heart. Mathematical descriptions of the problem, are riddled with difficulties in theoretical formulation and computational requirements, and hence, tend to empiricism. Even experimental and field measurements are often limited in terms of what parameters or processes they measure, and at what spatial and temporal scales.

A team from the Coastal Research theme within Geology and Geophysics, comprising PhD candidate Hachem Kassem, and Prof Carl Amos, has recently conducted an state-of-the-art experiment looking into the small scale turbulent and sediment transport processes in continuous 3D space. Co-located and time-synchronized measurements by a medical CT Scanner (computed tomography) and Particle Image Velocimetry were carried to study turbulent sediment transport in unidirectional currents.  The Laboratoire multidisciplinaire de tomodensitometrie pour les resources naturelle et le genie civil (or Lab CT SCAN, for short) at the Centre Eau Terre Environment of the Institut national de la recherche scientifique (INRS), Québec, Canada has the unique necessary facilities to allow such measurements to be collected simultaneously. A sandy bed within a 7m long flume passing through the sliding gantry of a Siemens SOMATOM Definition AS+128 high performance X-Ray CT scanner is imaged in both longitudinal and perfusion modes under increasing current velocities. The flume is equipped with a stereo PIV system with 2 cameras and a sheet-laser source, allowing the user to obtain 3D velocity vectors over a 2D plane at the centre of the imaged section. A profiling ADV is also used to measure high frequency turbulence profile in the near bed layer of the flow. Collectively, these measurements will shed light on a number of fundamental physical processes from robust estimates of physical and hydrodynamic roughnesses, the relative importance of skin friction and form drag, to quantifying, for the first time, sediment content of energetic turbulent vortices and their role in drag reduction, and exchanges between interstitial and boundary layer waters due to reattachment of lee eddies due to bedforms.

This collaboration came along building on historic links between Prof Amos, and the visionary of the CT Scan lab, Prof Bernard Long. The Canadian team included the brilliant technicians Louis-Frédéric Daigle and Mathieu Des Roches, who operated the CT scanner and PIV systems, and Ms Corrine Brunelle, PhD Candidate an INRS, and facilitated by Prof Pierre Francus, the current director of the lab.

 

Understanding sediment transport has come a long way since the pioneering works of Shields and Bagnold, nearly a century ago. Yet, the complex mechanisms that drive, at larger scales, the morphological evolution of coastal and estuarine environments remain poorly understood at the ‘noisy’ end of the spectrum. Fundamentally, this is a problem of fluid sediment interaction, with turbulence – the most notorious problem in classical physics – lying at its very heart. Mathematical descriptions of the problem, are riddled with difficulties in theoretical formulation and computational requirements, and hence, tend to empiricism. Even experimental and field measurements are often limited in terms of what parameters or processes they measure, and at what spatial and temporal scales.

A team from the Coastal Research theme within Geology and Geophysics, comprising PhD candidate Hachem Kassem, and Prof Carl Amos, has recently conducted an state-of-the-art experiment looking into the small scale turbulent and sediment transport processes in continuous 3D space. Co-located and time-synchronized measurements by a medical CT Scanner (computed tomography) and Particle Image Velocimetry were carried to study turbulent sediment transport in unidirectional currents.  The Laboratoire multidisciplinaire de tomodensitometrie pour les resources naturelle et le genie civil (or Lab CT SCAN, for short) at the Centre Eau Terre Environment of the Institut national de la recherche scientifique (INRS), Québec, Canada has the unique necessary facilities to allow such measurements to be collected simultaneously. A sandy bed within a 7m long flume passing through the sliding gantry of a Siemens SOMATOM Definition AS+128 high performance X-Ray CT scanner is imaged in both longitudinal and perfusion modes under increasing current velocities. The flume is equipped with a stereo PIV system with 2 cameras and a sheet-laser source, allowing the user to obtain 3D velocity vectors over a 2D plane at the centre of the imaged section. A profiling ADV is also used to measure high frequency turbulence profile in the near bed layer of the flow. Collectively, these measurements will shed light on a number of fundamental physical processes from robust estimates of physical and hydrodynamic roughnesses, the relative importance of skin friction and form drag, to quantifying, for the first time, sediment content of energetic turbulent vortices and their role in drag reduction, and exchanges between interstitial and boundary layer waters due to reattachment of lee eddies due to bedforms.

This collaboration came along building on historic links between Prof Amos, and the visionary of the CT Scan lab, Prof Bernard Long. The Canadian team included the brilliant technicians Louis-Frédéric Daigle and Mathieu Des Roches, who operated the CT scanner and PIV systems, and Ms Corrine Brunelle, PhD Candidate an INRS, and facilitated by Prof Pierre Francus, the current director of the lab.

 

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One thought on “Tomographic investigation of turbulence and sediment dynamics

  1. […] Hachem then went to describe the two classical theories in the field, from energy cascades to the quest for order among chaos. This was a nice introduction into his specific area of research which focuses on ‘coherent turbulence structures’, which he described as bundles of energetic eddies that play an important role in momentum exchanges near boundaries, such as the sea bed. Hachem is interested in the role played by such motions in mobilising sediment on the sea bed under waves, tidal currents and/or combinations of both. He briefly introduced the concept of sediment transport then gave an example of how beaches change their profiles between summer and winter due to the different wave conditions they experience. He then introduced his PhD as ‘a study of these swirly energetic things that move sediment under waves!”. An overview of three major experiments that he conducted followed, starting with the large European Hydralab ‘Barrier Dynamics Experiment 2’ or BARDEX II. During that experiment, a team of international researchers built a life-scale beach, over a 100 m long, and subjected it to different types of waves, monitoring everything from turbulence to sediment transport, and of course changes in the beach profile. Hachem briefly mentioned the different mathematical tools he employs to analyse the role of these motion, and showed how different waves result in different beach layouts and patterns in the transport of sand. His second experiment was the very first to be conducted in the brand new HR Wallingford Fast Flow Facility, a unique scientific facility where he studies how currents interact with waves, and result in different patterns of bedforms on the seabed. Hachem concluded with some videos from his latest endeavour, another experiment at the unique CT Scan lab in Canada, where they actually used a medical CT Scanner to image how the sand moves under currents. […]

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