Turbulent flows, like planetary atmospheres, oceans, or flow around an airfoil or a wind turbine, undergo strong fluctuations. Sometimes, the system may even undergo abrupt transitions between entirely different flow configurations. Such transitions seem to occur at random times, in an unpredictable manner.
CNRS researcher at Laboratoire de physique at ENS in Lyon, Corentin Herbert developed the research project TransTurb (Large deviations and rare transitions in turbulent flows) in collaboration with Freddy Bouchet, CNRS research director. This project was supported from 2017 to 2019 by the Marie actions Skłodowska-Curie.
The richness of turbulence continues to pose many problems for theoretical physics. In particular, the study of extreme events is crucial for weather, climate and many technical applications, but these events remain poorly understood.
The first deadlock is that we are interested in rare events, for which, by definition, we have few observations. Direct numerical simulations of the system do not really alleviate the problem, because models for turbulent flows or the climate system are computationally expensive. To solve this sampling problem, numerical algorithms have been developed over the past few years to simulate rare events efficiently. The first main goal of the project is to show that these algorithms can be adapted to address relevant questions for rare events in turbulent flows. Many of the digital tools developed during the project are now available to the public via Github.
The second main objective of the project was to illustrate the importance of rare fluctuations in natural turbulent flows. In collaboration with Rodrigo Caballero (Stockholm University), Corentin Herbert and Freddy Bouchet investigated whether there are spontaneous transitions between bi-stable states in the atmosphere and whether they resemble the noise-induced transitions studied in statistical physics. The three researchers showed that there is indeed a mechanism for abrupt reversal of tropical winds that could be activated by turbulent fluctuations in the atmosphere, leading to abrupt changes in climate.