Groupe de Recherche : Turbulence & Instabilités
tél : 04.72.18.62.00
email : firstname.lastname@example.org
Current position :
I am a post-doctoral researcher at the Laboratoire de Mecanique des Fluides et d’Acoustique, at Ecole Centrale de Lyon, working under the ANR project StratiMix.
Current research :
My current research is related to stable stratified fluids. I am working in the study of turbulence mixing of stratified fluids through high resolution DNS. In addition, I keep collaborating in experimental work related to my PhD thesis at ENS Lyon, such as internal waves near-critical reflection, and transport of particles in suspension due to internal waves. Find my thesis manuscript "Transport properties of internal gravity waves" here.
Turbulence mixing in high resolution DNS
Figure : Snapshot of a 3D buoyancy field of a turbulent decaying flow with initial linear stratification.
Stratified fluids are common to many geophysical and industrial environments. The dynamics of these systems are driven by the complex balance between turbulent decay, buoyancy restoring force and irreversible mixing ; where the local mixing can produce an effect in the global energy budget of the system.
In particular, turbulent mixing in the ocean interior plays a crucial role in its global energy budget. This mixing partially drives large scale dynamics, as evidenced in the meridional overturning circulation (Wunsch and Ferrari (2004)). In addition, vertical transport in the ocean is substantial for sequestering large quantities of dissolved greenhouse gases from the atmosphere to the deep ocean. The proportion of energy transferred from turbulent structures to effective mixing is very difficult to estimate through observations (Ivey et al. (2008)), and the details of this energy transfer is yet not fully understood.
In order to resolve irreversible mixing produced by turbulence in a stable stratification, we introduce boundaries at the top and bottom of our domain which allows the mean stratification to evolve in time. This differs with the classical approach of homogeneous stratified turbulence where the background stratification is fixed. The main interest of our approach is that the irreversible mixing is directly computed from the full density field. A porous penalization region is introduced to take into account non-flux conditions at the bottom and at the top of the box (see Kadoch et al. (2012)).
Figure : (a) Vertical cut of the instantaneous reduced density field . (b)Vertical profile of the horizontal mean reduced density field (red line) and horizontal mean of the sorted reduced density field (green line). The initial reduced density profile is also indicated (dashed line). The penalization region is indicated by two arrows and the letter P between both figures.
Internal gravity waves
Watch a video of an internal plane wave propagating in experimental conditions here.
When difference of density exists within a fluid, it will tend to redistribute driven by the force of gravity so that the lighter fluid remains above the heavier forming a stable stratification profile. This particular configuration will be stable in time and if not perturbed, static. When the fluid is slightly vertically displaced, it will feel a buoyancy restoring force acting in a direction opposite to the displacement. The force will act as a spring, and therefore the fluid will oscillate around an equilibrium position. These oscillations are know as internal gravity waves, which differ from the well known surface waves, as they occur inside the fluid where the density of the fluid changes continuously.
The atmosphere is stratified in temperature, and the ocean is stratified in both salinity and temperature. The main motivation for understanding the dynamics of internal gravity waves is that they occur naturally in these systems. These waves have an effect over the dynamics of stratify systems, and may be taken into account to be able to better predict large scale effects such as transport of energy and matter.
Thanks to several experimental techniques internal waves can be generated and observed in laboratory conditions.
Internal waves near-critical reflection
Watch a video of an internal wave near-critical reflection in experimental conditions here
Figure : Velocity field of an internal wave near-critical reflection. The incident wave is coming from left to right, and the generator is located 30 cm from the center of the image.
The peculiar dispersion relation and the nonintuitive relation between group velocity and the wavenumber of internal gravity waves lead to some very unusual physical consequences. In particular, when internal waves are reflected on a sloped boundary the frequency is conserved, and therefore, its angle of propagation. In consequence, nonintuitive effects including reflection, focalization and wave attractors will emerge when internal waves interact with boundaries.
The detailed study of this process is principally motivated by the peculiar characteristics of internal waves reflection that can enhance the shear stress developed near boundaries and therefore generate erosion of particles settle at a boundary.
Many observational studies have been done that indicate that internal gravity waves are a cause of sediment resuspension, as for example Bogucki et al 1997., Quaresma et al. 2007 and Hosegood & van Haren 2004. In every case the capacity of generating sediment transport through the interaction of internal waves over the seafloor is limited by the shear stress generated at the boundary and the physical characteristics of the particles
Transport of particles by internal gravity waves
Find a video here
Figure : Internal waves passing through a column of particles in suspension. The boundaries of the column are perturbed by the pass of the wave.
Find a video here
Figure : Displaced column of particles in suspension when internal waves pass through. The column is displaced towards the wave generator.
I performed my PhD titled "Transport properties of internal gravity waves" (find my thesis manuscript here) at the Laboratoire de Physique of the ENS Lyon under the supervision of Sylvain Joubaud and Philippe Odier.