Séminaire de Mécanique d'Orsay

Le Jeudi 3 février 2022 à 14h00 - Webinaire - le lien sera communiqué ultérieurement

Bifurcations in the cylinder wake: from soap film to transonic wind tunnel.

Marie Couliou
Onera, DAAA

Flow around bluff bodies as the cylinder usually experiences boundary layer separation and very strong flow oscillations in the wake region behind the bodies. The wake of a cylinder is a canonical configuration in fluid mechanics and is relevant to many engineering applications. From low Reynolds number in 2D flow to large Reynolds and transonic Mach number, this seminar will be the occasion to present recent experimental studies on the wake of a cylinder. Fluid-structure interactions between the wake of a cylinder and a clapped flexible splitter plate at its rear are first examined. Using a gravity-driven soap film, we investigate the influence of the splitter plate length. It is found that long splitter plates flap symmetrically around the flow direction, while a time-averaged bending occurs for short splitter plates. We have carefully characterized the deviation for short splitter plates and found that the symmetry-breaking vanishes for very short splitter plates. 2D numerical simulations and a quasi-static model have been used to further explore and explain the physical mechanism responsible for the deviations. The second study is on a cylinder mounted in a cross-flow configuration. The experiment is carried in a ONERA transonic wind tunnel where Mach numbers from 0.3 to 0.85 have been achieved.  Measurements combine temporal recordings of the wall pressure and Schlieren high-speed visualisations of the flow past and downstream the cylinder. Several flow states are observed as the Mach number Ma is varied. The focus is here given to the narrow region Ma=0.80 to Ma=0.85 which yields not less than three different states, namely the vortex shedding state (VS), the parallel shear layer state (PSL), and the crossed shear layer state (CSL). The main finding is that the transitions as a function of the Mach number between these three states show a hysteresis behavior. This indicates that in the transonic region the flow past the cylinder is highly sensitive to its history.

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