Aeroelastic mode selection mechanism for flexible membranes interacting with separated flow

Flexible membrane immersed in separated flow can experience self-excited vibrations and induce particular vortex shedding patterns via flow-excited instability. To examine how the specific aeroelastic characteristics are selected in the coupled system, a three-dimensional membrane at moderate to high angles of attack in unsteady flow is simulated by a high-fidelity aeroelastic framework. With the aid of a global Fourier mode decomposition technique, we extract the correlated aeroelastic modes (in frequency ranking) from the coupled system. Using the aeroelastic mode decomposition, we show that the dominant structural mode exhibits a chordwise second and spanwise first mode at different angles of attack. Frequency synchronization between the dominant membrane vibration and the vortex shedding process is reported from the mode frequency spectrum. We propose an approximate analytical formula to estimate the structural natural frequency and show the aeroelastic mode selection process is primarily driven by the frequency lock-in between the structural natural frequency and the dominant vortex shedding frequency. By comparing the flow features among a rigid wing, a rigid cambered wing and the flexible membrane, we find that the non-periodic aeroelastic responses at higher angles of attack are associated with the bluff-body vortex shedding instability.