Séminaire de Joran Rolland (Ecole Centrale, LMFL, Lille)

Joran Rolland (Laboratoire de Mécanique des Fluides de Lille Kampé de Fériet). 

Experimental study and modeling of turbulent wake bistability

Many out of equilibrium turbulent flow organise themselves into a non-trivial large-scale flow, like a recirculation, a jet etc. There are more and more reports of turbulent flows where several distinct large scale flow configurations are possible for given values of the control parameters. These turbulent flows are multistable: observation indicates that they spend a lot of time in one configuration suddenly transit to another.
I will present the properties of such a bistable turbulent flow in the wake of two parallel bars, realised in the LMFL wind tunnel. This is ongoing work started this spring which is already showing promising results. In that case, the jet that comes out of the bars can either point left or right for a long time instead of fluttering around the central plane. The position of the jet can be followed either with a simple weighted average or using the amplitude of the first two POD modes of the flow. From this, the pdf of the position, as well as the pdf of the waiting times before a switch of position can be sampled. They clearly indicate bistability. In order to understand the bistable flow in more details, a stochastic model is constructed from the data, that can be used to regenerate new time evolutions of the large- scale flow. Eventually, I will present new, time revolved data that has been sampled to study in more details the switching events.
This work has been done using the data sampled experimentally by Jiangang Chen, Pierre Braganca et Christophe Cuvier, and has been processed with the help of Antoine Barlet (now starting a PhD in CEA Saclay) and Indra Kanshana (starting a PhD in LMFL). 
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