Reconstruction of Large-Scale Coherent Structures in Turbulent Separation Bubbles Using Phase-Consistent DMD

Experimental measurements of velocity fields using 2D-2C particle image velocimetry are performed on a pressure-induced turbulent separation bubble (TSB) at Reθ = 5000. Such flows are largely characterized by two unsteady phenomena occurring within two different frequency regimes. At the higher frequency, a classical vortex shedding develops due to the Kelvin-Helmholtz instability which is associated with the roll-up of the shear layer. At the lower frequency, cycles of contraction and expansion of the entire recirculation region are observed, commonly referred to as “breathing” of the TSB. In view of the size of the TSB, the experimental measurements consist of multiple fields of view (FoVs) to capture the entire recirculation region from incipient detachment to full reattachment of the boundary layer. The data in the different FoVs are therefore not synchronized in time and as of yet only statistical quantities have been reported from this experiment [1, 2]. We explore the use of a novel phase-consistent reduced-order modeling technique recently proposed by Nair et al. [3]. The method is based on dynamic mode decomposition and aligns the phases of the dynamic modes in the different FoVs by taking advantage of spatial overlap in the data. The method permits, for the first time, a phase-consistent modal analysis of the full experimental TSB and therefore the study of time-resolved phenomena upon flow reconstruction. As a result, we illustrate that the low frequency unsteadiness is linked to the passage of large coherent structures throughout the whole TSB.