DAta science, TrAnsition, Fluid instabiLity, contrOl, Turbulence (DATAFLOT)

Sergio Chibbaro, Didier Lucor, Lionel Mathelin, Onofrio Semeraro

Flow control remains one of the means of controlling the energy efficiency of systems and designing more efficient energy systems. Our activities in the field of flow control is a particularly strong and visible activity of the laboratory. In particular, it is supported by the Lidex ICODE (Université Paris-Saclay) on “decision support and control of complex dynamic dynamic processes”. Part of these activities is focused on instationnarity control. The other part focuses on non-linear closed-loop control techniques based on model-free methods model-free methods or reinforcement control..

In parallel with these activities, we are developing our know-how in data processing from numerical simulations and experiments in fluid mechanics and transfer mechanics. These developments are, on the one hand, useful for increasing our understanding of physical phenomena (modal decomposition, infinite-dimensional operator hollow sampling) and, on the other hand, necessary for the development of increasingly reliable representations or modeling (inference, assimilation, hollow representation), notably for application to control (Machine Learning, in particular). We are also working on the development of Uncertainty Quantification (UQ) techniques, which are a useful addition to the landscape of techniques for analyzing parametric sensitivity, particularly for dealing with complex model identification and inference problems.
In addition to methodological developments, the dissemination of UQ techniques should be intensified towards more applications (Bio-medical Engineering, Geosciences, Aerodynamics, …).
Our efforts will focus more specifically on :

• data analysis for fluid mechanics estimation and assimilation:
  • dictionary learning, variety learning ;
  • infinite-dimensional operator hollow sampling (Koopman), function train
  • tensor approximation ;
  • random projection for model reduction;
  • data assimilation and optimization;
• Uncertainty quantification (UQ):
  • efficient methodological developments ;
  • transfer to applications ;
  • Flow control :
  • control by reinforcement ;
  • model-free, machine-learning control;
  • setting up experimental demonstrators.

Yohann Duguet, Francois Lusseyran, Laurent Martin-Witkowski, Stéphanie Pellerin

Fluid flows can be classified into several regimes, such as laminar, transitional or turbulent, corresponding to significant differences from an energy point of view. The dynamic processes involved in moving from one regime to another, or in stabilizing one of these regimes, are still poorly understood. The instability of a given laminar flow in the face of arbitrary disturbances, of either infinitesimal or finite amplitude, gives rise to interesting and varied mathematical and numerical developments, depending on the type of flow considered. Transitions, often hysteretic, between different regimes also exist within turbulent flows. An original focus is placed on the analysis of spatial symmetries and their breaking by instability mechanisms. These are described qualitatively and quantified using innovative and efficient numerical algorithms, within the framework of three-dimensional unsteady simulations requiring considerable resources and specific methods for large-scale data. An experimental cell also enables the visualization and quantification of these same flows, in direct complementarity with numerical studies. Finally, a detailed understanding and modeling of the hydrodynamic mechanisms at work naturally leads to experimental and/or numerical control methods, enabling the system to be steered towards the desired regime.

Configurations

• experimental and numerical study of the effects of ambient pollution on the stability of rotating flows with rigid or deformable free surfaces
• numerical simulation, modeling and control of symmetry breaking in turbulent wakes
• understanding the mechanisms of sub-critical transition to turbulence in sheared wall flows, from both a non-linear (description of the corresponding phase space) and spatio-temporal (analysis of intermittency) point of view
• experimental and numerical study and closed-loop control of open shear flows