Appel à candidatures, Recrutement | Recherche, Emploi

PhD position on Bistability and turbulent cascade in a wake flow, LMFL, Lille (France)

Du 1 septembre 2021 au 30 août 2024

Début de la thèse : Septembre- Octobre 2021 

Site actualite
LMFL, Lille
Contact :
Joran Rolland : joran.rolland@centralelille.fr
Jean-Philippe Laval : Jean-Philippe.laval@univ-lille.fr

An aerodynamic study for transport (aerial, terrestrial) or energy (wind turbines etc.), aimed at minimising energy costs (of transport) or maximising the produced power, requires that one educes the fluid flow upstream of the object that is to be designed, on said object and downstream of the object. The downstream part of the flow is commonly termed a wake. A precise description of the wake un a body (a vehicle, a turbine etc.) is fundamental in the understanding of the lift and the drag exerted by the flow on said object. Wakes can be investigated numerically or experimentally in rather complex and realistic geometries.

 

page1image31315856page1image20868352

PhD on Bistability and turbulent cascade in a wake flow

Directeur (supervisor) Joran Rolland 
E-mail : joran.rolland@centralelille.fr

Co-Directeur (co-supervisor) Jean-Philippe Laval
E-mail : Jean-Philippe.laval@univ-
lille.fr

Laboratoire (research unit) : LMFL
Web : https://lmfl.cnrs.fr/

Résumé du sujet (abstract):

An aerodynamic study for transport (aerial, terrestrial) or energy (wind turbines etc.), aimed at minimising energy costs (of transport) or maximising the produced power, requires that one educes the fluid flow upstream of the object that is to be designed, on said object and downstream of the object. The downstream part of the flow is commonly termed a wake. A precise description of the wake un a body (a vehicle, a turbine etc.) is fundamental in the understanding of the lift and the drag exerted by the flow on said object. Wakes can be investigated numerically or experimentally in rather complex and realistic geometries.

However, in order to study novel or poorly understood phenomenon, one often need to focus on simpler configurations: this is the case of bistability. Indeed, the wake is often turbulent, but that does not mean that the flow is entirely incoherent. It often displays a large scale circulation like one or several large whirls, a jet etc. The situation is often more complex than that: there can be several possible large-scale flows for one set of control parameters. The flow is then multistable (bistable if there are only two possible large-scale flows). In practice, the wake spends a very long time in one or the other configuration, and can switch from one configuration to the other randomly and very rapidly. This leads to very large fluctuations of lift and drag, which impede the control of these flows.

page1image31155968 page1image31153680

In order to have to be able to describe wakes of industrial and energetical interest, we first wish to work on canonical configurations. For this thesis, we propose to work on a configuration studied by a postdoctoral fellow in LMFL, for which a wealth of data has been sampled [3]. The PhD candidate will work on the wake between two parallel rectangular bars. This configuration has been studied numerically and PIV measurements have been performed in the LMFL wind tunnel (see figure below):

A turbulent jet is formed downstream of the two bars. When the two bars are close enough, the direction of the jet is inclined with respect to the symmetry plane of the set up. Two configurations are then possible for the jet, as illustrated in PIV measurements:

The turbulent flow is then bistable. Up to this point, only the metastable position has been studied in details. The thesis will therefore be focused on the dynamics of change of position of the jet. Two main axes of study can be followed. On the one hand, the project aims at documenting on the control parameters of the bistability, in the same spirit as the studies of simpler turbulent flows [2]. On the other hand, the project will also try to elucidate the manner in which energy is transferred in space and between scales during the changes of configuration. We will especially focus on the consequences of bistability on the properties of turbulence and of the turbulent cascade. We will also compare the results to recently observed laws of out of equilibrium turbulence [1].

page2image31385344 page2image31385552

The PhD candidate should have a strong background in fluid mechanics and have ideally already worked with experiments and/or numerical simulations of fluid flows: both these methods of investigations could be used during the thesis. The candidate will be able to access the experimental facilities of LMFL and will be supported by the experimental team (research engineer, faculty members) for the wind tunnel experiments designed to perform new measurements. He will also be able to use a finite volume numerical simulation code, which is able to efficiently simulate this flow, as well as run it in the IDRIS supercomputers.

[1] F. Alves Portela, G. Papadakis, J. C. Vassilicos, The role of coherent structures dans inhomogeneity in near-field interscale turbulent energy transfers, J. Fluid Mech. 896 (2020).

[2] F. Bouchet, J. Rolland, E. Simonnet, Rare events algorithm links transitions in turbulent flows with activated nucleations, Phys. Rev. Let. 122, 074502 (2019).