The structure concept in the description of mixing turbulence: the 2SFK model

To meet our modelling needs on turbulent flows produced by gravitational instabilities (of Rayleigh-Taylor or Richtmyer-Meshkov type), we have developed an original approach, designated as 2SFK for '2-structure, 2-fluid, 2-turbulent'. We provide the physical elements, theoretical, experimental, and numerical, which support this choice. A full description being out of question here, we give the principles of the model derivation, which hinges around an averaging conditioned by presence functions of the large structures in the flow, and discuss its distinctive properties compared to usual 'single-fluid' models. Numerical 1-dimension results on elementary flows illustrate the satisfactory behaviour of the model. All along this article, emphasis is given on the peculiar characteristics of turbulence in the Rayleigh-Taylor flow (possibly under variable acceleration): energy balance, characteristic size of large eddies, directed transport, enhanced diffusion, etc. (authors)