On the role of flow transition in laminar separation flutter

Abstract

This work explores self-sustained pitching oscillations of a NACA0012 airfoil operating at low-to-moderate Reynolds numbers in which the aerodynamic flow is in a transitional regime. One-degree of freedom (DOF) pitching oscillations were explored over a range of Reynolds numbers (7.7×104<Rec≤2.0×105) using high-order implicit large-eddy simulation coupled with structural dynamics. Limit-cycle oscillation is observed at all Reynolds numbers tested but requires a disturbance to initiate at the highest flow speeds identifying a bifurcation in possible solution states. In all cases, aerodynamic loading is dominated by primarily two features. Negative aerodynamic damping is largely provided by suction beneath a separation bubble located behind the elastic axis. This feature induces a moment that reinforces the pitch-rate at small angles of incidence and is directly influenced by flow transition at different Reynolds numbers. Open trailing edge separation on the opposite surface transitions and reattaches immediately preceding the largest angles of incidence. This process imparts a spike in the pitching moment that opposes the pitch-rate and briefly damps oscillations. Transition of the detached shear layer occurs at smaller angles of incidence as Reynolds number is increased, attenuating oscillation amplitude as Reynolds number is increased.