On the two classes of global primary modal instability in laminar separation bubbles

Abstract

The self-excited global instability mechanisms existing in flat-plate laminar separation bubbles are studied here, in order to shed light on the causes of unsteadiness and threedimensionality of unforced, nominally two-dimensional separated flows. The presence of two known linear global mechanisms, namely an oscillator behavior driven by local regions of absolute inflectional instability and a centrifugal instability giving rise to a steady threedimensionalization of the bubble, is studied in a series of model separation bubbles. Present results indicate that absolute instability, and consequently a global oscillator behavior, does not exist for two-dimensional bubbles with a peak reversed-flow velocity below 12% of the free-stream velocity. However, the three-dimensional instability becomes active for recirculation levels as low as urev ≈ 7%. These findings suggest a route to the three-dimensionality and unsteadiness observed in experiments and simulations substantially different from that usually found in the literature, in which two-dimensional vortex shedding is followed by three-dimensionalization.

Self-excited primary and secondary instability of laminar separation bubbles

“…Figures 7 and 8 show ω 0 (X) and α 0 (X) for the base flowsq 2D andq 3D , respectively. In line with the analyses of the baseline LSB flows q 0 (Rodríguez, Gennaro & Juniper 2013a), the spanwise-average bifurcated flowsq 2D are found to be only convectively unstable. In contrast, the three-dimensional flowsq 3D present regions of absolute instability localized in the downstream part of the recirculation region.…”