Étude expérimentale d’une famille de bulbes de décollement turbulents

Abstract

A comparative analysis of pressure and velocity measurements of three very different turbulent separation bubble geometries, induced by pressure gradient on a flat plate at Reo=5 000, was conducted within the framework of an experimental set-up in a boundary layer wind tunnel. A large, massively separated bubble, a medium-size with shorter recirculation region and a small one with no mean backflow are the focus of this experimental campaign using PIV and pressure measurements. Both horizontal and vertical PIV measurements are performed on the medium geometry with the intent to document the spanwise scales at stake in the flow. The unsteady behavior associated with the separation bubble is two-fold : first, a convective movement of the release of medium-frequency vortices is widely known in the literature in the field of fluid mechanics, in experimental as well as in numerical simulations, and is linked to the phenomenon of Kelvin-Helmholtz type instabilities which can typically be found in the case of a flat plane mixing layer. The second instability is associated with the cycles of bubble expansion and contraction, which causes a significant variation in the zone of active recirculation. This clear oscillation of the detachment and reattachment points, analogous to a breathing motion, is to this day poorly understood, and the physical meaning of the phenomenon is still being debated, seeing the large disparity of experimental and numerical results which need to be reconciled. Some important information on the physics of the flow were obtained, most importantly that the breathing phenomenon is characterized as a spatio-temporal problem, which manifests as an amplification of very large structures (VLSM), inactive according to the Townsend paradigm, and which are already present in the upstream boundary layer at zero pressure gradient (ZPG). The signature of quadrants of large scale mouvements in ZPG boundary layers is the same as that of a breathing motion for a detached bubble : the cycles of sweeping (Q4inactif) and ejection (Q2inactif), which resemble longitudinal structures at high and low velocities, are associated with the movement of bubble contraction and expansion respectively. The breathing motion presents itself as an amplified ZPG bursting cycle, further amplified by the adverse pressure gradient which contributes strongly to the active motion with a vertical velocity component and creates quasi-longitudinal large vortices. The dual affect of the active motion (APG) and inactive motion (breathing) are superimposed in the creation of the first spike of cp. The cycles of regeneration of active vortex clusters (LSM) which generate a larger inactive streak (VLSM), then starts oscillating before breaking and reforming an LSM. This constitutes a probable image of breathing and is part of the attached vortex model by Townsend (1976). Depending on whether the inactive part of the large scale is developed enough or not, we categorize the low-frequency detachment and reattachment into three possible groups : (1) mobile-mobile, (2) fixed-mobile, and (3) fixed-fixed. In order to reproduce the breathing motion, the inactive part must therefore be properly taken into account. Finally, the activity of the coherent structures tied to the Kelvin-Helholtz instability suggests that the breathing motion and the release of spanwise vortices do not have a common origin, but rather suggests a modulation of the vortex shedding caused by the low-frequency which is felt globally in the entire detached region.

Date	8 Dec 2021
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Julien Weiss (Supervisor) & Louis Dufresne (Co-supervisor)