Current controversies on climate change highlight the importance of sound scientific foundations, shared by the entire scientific community. The study by similarity laboratory participates in this process, in parallel with advances in theory, numerical modeling and observation.
The Coriolis platform allows to study the combined effects of rotation and density stratification, approaching dynamical similarity with the natural environment. The effects of viscosity and surface tension are small thanks to the large size (13 m diameter) of the set-up.
Although the basic process at stake are well known a priori , modeling turbulent process is notoriously difficult , particularly in the presence of density stratification. The existence of solutions of the Navier-Stokes and regularity has been considered as one of the seven major open problems of mathematics (’ millennium problems ’) by the mathematical foundation ’ Clay ’ . We also know that the predictive ability of any model is limited in time , regardless of its accuracy. In contrast, the complexity of turbulent systems can lead to statistically robust plans , and possibly multiple equilibria . This is what seems to happen to the atmospheric or oceanic flows , leading to climatic fluctuations spontaneous decadal . The Coriolis platform participates in such studies by ANR STATOCEAN (2010-2012) , in collaboration with the Ecole Normale Supérieure de Lyon, Laboratoire de Physique des Oceans Brest, and Nonlinear Physics Institute Nice .
The platform brings together Coriolis and fluid mechanics , applied mathematicians , physicists and researchers in earth sciences and the environment. This is often the case for projects funded by the European (Integrated Infrastructure Initiative) program HYDRALAB recently renewed for 2011-2015. This is also the case of the project ANR OLA ’ (Ocean Layering ) to elucidate the mechanisms of energy dissipation of ocean currents and vertical mixing partner. This project, led by the Laboratoire de Physique des Oceans Brest, addresses this issue by very intensive numerical simulations , the ’ Earth Simulator’ Japanese, by experiments on the Coriolis platform , as well as experiments on a smaller scale direct link with theoretical approaches ( to be carried out LADHYX and IRPHE ) .
These different approaches are complementary. The spatial resolution of digital simulations , even the most extreme , remains below that which can be achieved on the Coriolis platform , where a wide range of from 1 to 10000 mm is commonly accessible. Moreover, there has been a renewed interest in physical models in the laboratory , either at the level of large hydraulic facilities for modeling port or river sites , for more fundamental studies. And rotating small size tanks have recently been equipped with ENSTA and FAST - Orsay. These facilities do not, however, sufficient to cover the same range of parameters as the Coriolis platform .
More technically, a hydrodynamic system is characterized by a horizontal scale L, a vertical scale H , velocity U, leading to the following numbers without dimension:
The Reynolds number UL / ν representing the importance of the effects of viscosity;
The Rossby number U / ( LΩ ) representing the influence of ambient rotation speed angulaire Ω;
The number of internal Froude U / ( LN ) showing the influence of the density stratification , characterized by N = ( gδρ / ρH ) ½ , vertical oscillation frequency of the fluid particles;
The aspect ratio L / H ratio.
Natural environments ( intermediate scales 10-100 km) are characterized by high Reynolds number , numbers Rossby and Froude or small the order of 1 , and large aspect ratios. For a given fluid , a high Reynolds number may be achieved by increasing the velocity U than the dimension L. It is then possible to maintain a small number Rossby also increasing the speed of rotation Ω . But it is not the same Froude number , the stratification N remains limited by physics. The same limitation applies to the use of helium at low temperature, thereby increasing the Reynolds number by reducing the viscosity .
http://coriolis.legi.grenoblep.f ... gravity current on a slope , modeling the descent of dense water to the ocean floor. A large coherent vortex coexists with small turbulent structures (width of 4 m view )
The Coriolis platform is thus the only equipment in the world to combine strong stratification (small Froude number ) and high rotation ( small Rossby number ) , offering more major aspects L / H ratios . This allows for example to reproduce a turbulent boundary layer of air or ocean -type , as in the ANR transtek (2010-2014) , managed by the Dynamic Meteorology Laboratory in Paris . The absence of lateral boundaries around also allows to study wave propagation phenomena , including within the framework of ANR TOPOGI 3D (2005-2009) and Piwo (2009-2011) , respectively controlled by the LEGI and ENS Lyon.
It is planned to further address new phenomena, such as sediment transport and morphodynamics also very accessible to small scale. Finally, the use of large dimensions , although it increases the cost of the initial equipment is sometimes a practical advantage , allowing researchers to enter themselves on the platform rotation and quickly install equipment there various (lasers and probes ) , regardless of weight constraints and congestion and external connections .
Although the primary motivation for this installation is the physical modeling of geophysical and environmental processes , techniques and methods approaches in the tradition of engineering supported by Insis , LEGI main supervision of the CNRS . Researchers working on the installation are also a member of the section 10 of the CNRS. Studies of turbulence in rotation were carried out in a hydraulic loop entirely placed on the rotating platform , and further studies of this type is considered in connection with the modeling work at LEGI in the field of turbomachinery and of turbines . Moreover devices study stratified flows stable or convective apply to problems building ventilation .