DrAdrien Lefauve

Current projects include:

Understanding turbulent processes in the coastal ocean
​Humans are largely a coastal species, with most of Britain’s population and 40% of the world’s population living within 100 km of the coast. Estuaries are particularly important, being vital transition and exchange zones between rivers and the sea and often hosting great cities like London. Rising sea levels, pollution, and population growth threaten these regions and their essential services. Fresh water will likely become the "gold" of the 21st century, central to health, industry and increasingly scarce. In estuaries and coastal waters, turbulence drives the transport of salt, pollutants, sediments and nutrients affecting water quality, erosion, fisheries, and infrastructure like wind turbines and subsea pipelines and communication cables. Turbulence remains one of the last unsolved problems in classical physics, and better understanding how it mixes stratified water layers is key to environmental engineering efforts for climate change mitigation. My research tackles this challenge using the latest advances in field observations, lab experiments, and computer simulations that probe turbulence in unprecedented detail.

Revealing shear instabilities in estuaries with multibeam sonar imagery
In most of the ocean, including estuaries, denser water layers lie below lighter ones. In such stably-stratified systems, turbulence is often initiated by shear instabilities triggered by tidal forces or underwater topography. Instability grows into a beautiful coherent wave structure, as visualised above by sonar from an autonomous underwater vehicle in the Connecticut River estuary. This growing underwater wave then breaks into disorganised turbulence, mixing the layers. We are currently analysing game-changing multi-beam sonar imagery of turbulent mixing collected by US collaborators. These observations reveal that mixing does not always occur by breaking in the ‘cores’ of the wave on a scale of 1 m (seen above), but also by smaller secondary shear instabilities along the ‘braids’ on a cm scale (see yellow regions above, ignoring the bright fish!) These findings provide new insights into the turbulent 'energy cascade' from large to small scales controlling the mixing rates of salt, nutrients, dissolved gasses, etc.

Measuring stratified turbulence in the lab with high-speed laser and cameras
To complement direct observations, laboratory experiments are crucial, like the Stratified Inclined Duct (SID)​. This experiment, which I developed at Cambridge, uniquely sustains geophysically-relevant stratified turbulence under more controlled conditions than in the field. It also allows us to measure the flow with much higher resolution and accuracy using fast lasers and cameras to perform simultaneous, 3D Particle Image Velocimetry and Laser Induced Fluorescence. These data led to breakthroughs on the energetics of sustained stratified mixing and 3D coherent structures (see past projects). We are now extrapolating these findings from the laboratory scale to the geophysical scale using theory and models developed in tandem with observations (see above) and simulations (see below). Imaging stronger and faster turbulence with higher-speed laser and cameras (up to 1 kHz) would be an exciting research frontier to pursue for an ambitious PhD or postdoc.

Probing the finest scales of mixing with exascale simulations on GPU clusters
Computational fluid dynamics is a precious tool to complement field observations and laboratory experiments and build better models of turbulence. To do this, we must solve the Navier-Stokes equations on massive grids because of the extreme range of scales involved. This is particularly challenging in stratified turbulence, because the energy cascades down to very small (micrometre) scales where the mixing of salt, heat or other tracers occurs. The simulations that we are currently analysing faithfully reproduce, for the first time, the same kind of mean-shear-forced turbulence that takes place in the above estuary and lab experiment. These simulations were pioneered and run by collaborators in the USA on the world's first exascale computer, Frontier. They involved tens of thousands of GPUs, performing in parallel several billion billion operations per second (exaFLOPS), and are also quite challenging to analyse.

For more details, see my personal website.