LOW-FREQUENCY MODAL AND NON-MODAL STABILITY OF A TURBULENT SEPARATION BUBBLE

Abstract

© TSFP 2024.All rights reserved.The low-frequency receptivity of an incompressible, pressure-gradient-induced turbulent separation bubble (TSB) is investigated with the aim of studying the mechanism responsible for the low-frequency unsteadiness commonly observed in experimental studies. The investigated flow consists of a TSB generated on a flat plate by means of adverse and favorable pressure gradients. The direct numerical simulation of Coleman et al. (2018) is used for the linear analysis and compared to the unsteady flow database of Le Floc’h et al. (2018, 2020). Resolvent analysis (RA) reveals receptivity to external forcing at low frequency and low, non-zero spanwise wavenumber. This low-frequency receptivity can be related to the experimentally observed low-frequency “breathing” of the bubble through the alignment metric, which exceeds 94 %. Finally, the strong similarities between the least stable mode of the global linear stability analysis and the optimal response of RA suggests that the low-frequency breathing motion might be caused by a modal mechanism, driven by this weakly damped global mode.