Controlling laminar-to-turbulent transition with superhydrophobic surfaces
Orateur : Francesco Picella
Abstract : Tailoring bio-mimetic rough surfaces researchers are accessing new approaches reducing drag in wall bounded shear flows. Among them Underwater SuperHydrophobic Surfaces (U-SHS) have proven to be capable of dramatically reduce skin friction of an overlying liquid turbulent flow, providing a stable, lubricating layer of gas bubbles trapped within the surface’s nano-sculptures. As long as a specific set of geometrical and thermodynamical conditions are ensured, wetting transition is avoided and the no-slip boundary condition at the wall is relaxed ; this so called ’Lotus effect’ is typically achieved when the length scale of U-SHS roughnesses is several order of magnitudes smaller than the overlying flow, bringing out both experimental and numerical challenges. In this framework we want to study, by means of numerical simulations, the influence of U-SHS in a closed channel, following the complete evolution from laminar, to transitional and fully developed turbulent flow. We report the results of transition over U-SHS taking into account the dynamics of each microscopic liquid-gas free-surface by means of a fully coupled fluid-structure solver and show that U-SHS can triple transition time to turbulence. This work has been supported by the French Research Agency (ANR- 15-CE29-0008).
What controls atmospheric and oceanic motions on planetary scales ?
Orateur : Simon Cabanes
Abstract : Within our solar system, substantial atmospheres and oceans are found on a series of planetary bodies : The Earth, Venus, Mars and the gas giants, i.e. Jupiter, Saturn, Neptune. All these planets show the formation of zonal jets, i.e. bands of east-west directed flows, which are the most energetic features that fundamentally structure atmospheric and oceanic circulations on a planetary scale, see Figure 1. Zonal jets are known to play a critical role in the organization of the climatic system by transporting essential substances, such as heat, humidity, gases and nutrients, around the planet. An understanding of the physical ingredients controlling the formation of these planetary jets is a prerequisite to our understanding of climate variability, for both past and future climate changes. So far, it has been accepted that the energetic strength of the zonal jets is directly related to the transformation of solar energy into sources of flow motion within the atmosphere. This assertion, however, disregards a fundamental conundrum formulated 20 years ago by Ingersoll et al. (2004), why are zonal jets on Jupiter 4 times stronger of that on Earth while Jupiter receives 30 times less of solar energy ? These facts seem to contradict each other. Thanks to the project the JUpiter Modeling Platform (JUMP), we conducted an in-depth analysis of gas giants’ zonal jets emulated from numerical and laboratory simulation as well as from direct imaging of the planets and we offered a partial answer to this conundrum. We showed that the strength of the jets is set by the « Rotational Kinetic Energy » of the planet rather than solar energy input.
Date et lieu : vendredi 25/02 11h00, en ligne sur Zoom.