Role of perturbations generated by micro-cavity in passive control of flow separation

Flow over a compressible open cavity has been extensively studied to identify dominant instability modes responsible for self-sustained oscillations and noise generation. These oscillations often result in intense pressure fluctuations and acoustic emissions. Traditional flow control efforts have targeted instabilities originating near the cavity's leading edge or within the cavity core to suppress or disrupt the feedback loop that sustains the oscillations. However, recent work by Visbal and Garmann (TCFD, 2023) demonstrated that cavities themselves can be repurposed as passive control elements: micro-cavities placed on an airfoil surface generate perturbations that travel downstream, suppress laminar separation bubble (LSB) bursting, and delay dynamic stall. Inspired by this finding, we investigate the downstream amplification of cavity-induced perturbations and their interaction with a developing separation bubble. We conduct bi-global classical and harmonic resolvent analyses using spanwise-averaged base flows from three-dimensional direct numerical simulations at a depth-based Reynolds number of 1500. Our analysis reveals dominant instability mechanisms and characterizes how these perturbations evolve beyond the cavity's trailing edge. Using harmonic forcing modes, we demonstrate how cavity-generated disturbances can passively influence the dynamics of an LSB over a flat plate at Mach 0.25, providing insight into passive flow control via upstream cavity forcing.