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Home > News > Thesis defense > 2019 thesis defense

Soutenance de thèse ECL

Zecong QIN

Jeudi 19 septembre 2019 14h, ECL bât. W1 amphi 201

Zecong QIN

Transitions and Structures in Axisymmetric Turbulence

Alexandros Alexakis - Rapporteur
Wouter Bos - Co-directeur de Thèse
Jan-Bert Flór - Examinateur
Jean-Philippe Laval - Rapporteur
Emmanuel Lévêque - Examinateur
Aurore Naso - Co-directeur de Thèse

Axisymmetric turbulence is a two-dimensional three-component flow. The investiga-
tion of this type of turbulence is motivated by the fact that it represents an asymptotic
limit of anisotropic flows and that it has been the subject of theoretical investigations in
the past. In the present manuscript such a flow is investigated in wall-bounded cylindrical geometry using spectral and pseudo-spectral numerical simulations.

Previous results on the generation of coherent structures, obtained for freely decaying
flows, are here assessed in the context of statistically steady flows, where the energy is supplied by either a spectrally localized forcing, or by moving top and bottom plates of the cylinder. The mean flow displays coherent structures whose properties are compatible with theoretical predictions.

When an anisotropic forcing protocol is used, a bifurcation is observed from a non-swirling (two-dimensional two-component, 2D2C) to a swirling (two-dimensional three-component 2D3C) turbulent flow. This transition is modelled by a system of two ordinary differential equations (ODE). It is shown that this model is able to reproduce
the essential physics of the transition.

The transition of the axisymmetric (2D3C) to three-dimensional (3D3C) flow is then
investigated using a non-integer dimension, by smoothly introducing azimuthal variations into the system. It is shown that the 2D2C limit is singular and that small azimuthal variations allow a redistribution of energy over the different energy components. The ODE model is adapted for this system by modelling the pressure-strain correlation. It is shown how the swirl level depends on the non-integer dimension. Large-Eddy Simulations are carried out to assess the robustness of these results at higher Reynolds number.


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