Our research focuses on multi-phase turbulent systems, involving waves, drops and bubbles in turbulent environment. We develop laboratory and numerical experiments to explore the physics at play and build simple models. Our work is motivated by environmental and industrial applications, as diverse as the statistics of ocean surface waves, non-linear wave dynamics and breaking, wave impact on structures, gas transfer by surface breaking waves in the ocean, spray generation and dynamics and cloud formation in the atmosphere.
Current projects include:
1) bubble dynamic in turbulent flow, break-up processes and statistics
2) spray generation by breaking waves, droplet dynamics and evaporation
3) bursting bubbles, modeling and experiments
4) the role of wave breaking in air-sea interaction, in particular the development of gas transfer parameterization and their inclusion in large scale models
5) non-linear waves, wind-wave coupling and boundary layer
6) complex flow modeling, visco-elastic jets, evaporation, gas exchange
Some past and current studies are described below.
Bubble fragmentation in turbulence
Bubble break up in turbulent flows is important in many environmental and industrial situations. These include oil spill mitigation, air entrained in bow waves in ships and submarines, and ocean-atmosphere interactions associated with breaking waves. The bubble interface may deform and break violently under the action of turbulent flows, and the newly formed interfaces dramatically increase transfer of heat and mass between the gas and liquid phases.
This work will develop an understanding of the multi-scale nature of bubble break-up phenomena under realistic conditions in terms of turbulent flow and the physico-chemical complexity of the interfaces. The break-up of bubbles far from the critical break-up size exhibits dramatic behavior, with the formation of multiple tiny satellite bubbles. The presence of surfactants at the bubble interface, ubiquitous in nature and industry, lowers the interfacial tension and affects the break-up processes. However, this latter aspect remains largely unexplored. Systematic experiments and direct numerical simulations for various turbulence configurations will probe these multi-scale regimes. The researchers will also characterize experimentally the role of the physico-chemical properties of the interface by working with controlled surfactant and salinity conditions. In the absence of surfactant, researchers will determine how the break-up dynamics and child bubble statistics are controlled by the ratio between the bubble size and the turbulent Taylor microscale, which controls the excitation range scale, and the ratio between the bubble size and the critical Hinze scale, which controls the break-up mode and the local or non-local range of child bubble size production. In the presence of surfactants, researchers will characterize the filamentary structures that arise due to the extended stability of interfaces, leading to the generation of much smaller bubble fragments. From these extensive data sets, a mathematical framework that can be implemented in multi-phase simulations of the Navier-Stokes equations will be developed. This will allow the modeling of fragmentation in turbulent flow with complex surface rheology.
Wave breaking and air-sea interactions:
Surface wave breaking plays an important role in the coupling between the atmosphere and the ocean from local weather to global climatic scales. It generates turbulence, entrains air bubbles into the ocean and ejects sea spray into the atmosphere. These processes enhance air-sea exchanges of momentum, gases, heat, moisture and marine aerosols. Quantifying these exchanges is necessary to improve our understanding of the ocean, atmosphere and climate systems.
Bubbles in the ocean are formed after a breaking wave has entrained air below the sea surface. Sea spray drops created by bubble bursting affect the exchange between the ocean and atmosphere by transporting water, heat, dissolved gases, salts, surfactants, and biological materials, and are major players in weather prediction and hurricane intensification. Large uncertainties in sea spray production predictions remain despite long standing efforts; this is attributed to the lack of knowledge in the original distribution of sea spray drop sizes and velocities. The research will span all scales relevant to the problem, from individual bubbles to wave statistics at the ocean surface; it will include detailed studies of the impact of key variables such as the concentration of surfactants, temperature, turbulence within a single experimental and numerical framework. A better understanding and improved parameterizations of sea spray production are necessary to improve the exchange between ocean-atmosphere of heat, moisture and aerosols, key elements of climate and weather models and forecast. Sea spray aerosols serve as cloud condensation nuclei and a better understanding of clouds generation and growth is seen by many as one of the biggest challenges in climate sciences.
The complex droplet formation process by collective bursting depends on multiple variables: the size and density of the bubbles rising in the turbulent upper ocean, the coalescence and foam formation at the surface and the physical-chemical properties of water. This project will fill gaps in our knowledge by conducting a comprehensive study that integrates high quality laboratory and numerical data to build models for the distribution of sizes and velocities of sea spray drops. Single bubble bursting will be studied with state of the art numerical simulations, developing a fundamental understanding at the bubble scale. The role physical and chemical variables, such as salinity, temperature and surfactant on the collective bubble bursting, foam aging and droplet formation will be investigated with laboratory experiments. Finally, an upscaling approach will be employed, going from one bubble plume entrained by a single breaking wave to the ocean scale by integrating the results over the breaking statistics, leading to a better parameterization of sea spray fluxes for numerical models.
Example of direct Numerical Simulation of wave breaking, using the open source solver Gerris. Various aspects of breaking waves can be analyzed: importance of surface tension effects at small scale, parasitic capillary wave generation, dissipation due to breaking and capillaries, air bubble entrainment, spray droplet generation, mass transport. DNS results are systematically compared to laboratory experiments.
Ocean surface taken from R/P FLIP in the pacific ocean just after a strong breaking event. Bubbles of various sizes and at various depth are visible.
aims to describe the statistical and dynamical evolution of a set of non-linear interacting waves, I have been working on various wave turbulence systems
– Stationary and decaying capillary wave turbulence and the influence of dissipation at all scale (though small scale lab experiments). We have shown that the existence of dissipation at all scale induce a “leaking cascade” phenomenology, with steeper spectrum for highly dissipative regimes.
– Gravity wave turbulence, direct and inverse cascade, together with finite size effects and boundary conditions; through experiments in small lab tank (20cm) and the large waves tanks (10 by 15 m and 30 by 50 m) located in Nantes (Collaboration Turbulon).
– Direct numerical simulation of capillary wave turbulence, using the open source flow solver Gerris.
I studied Hydro-elastic waves through small scale experiments by developing a laboratory setup consisting on a floating elastic sheet. This idealized setup presents formal analogies with floating ice sheets in the ocean. I have investigated the propagation of linear and non-linear hydro-elastic waves, the role of the sheet tension, the dissipation of the waves and the three-wave resonant interaction process.