A new viewpoint on the mechanism of transonic single-degree-of-freedom flutter

Transonic single-degree-of-freedom (SDOF) flutter can be easily understood by the energy transferred from fluid to structure, but it can not explain why transonic SDOF flutter frequently takes place in special flow states and at the structure's natural frequencies. In this work, we develop a reduced-order model (ROM) for the pitching or plunging NACA 0012 airfoil in transonic pre-buffet flow. The instability of the SDOF system is also caused by the coupling of modes - one structural mode and one fluid mode, different from the classic flutter in potential flow caused by the coupling between two or more structural modes. By eigenvalue analysis, two prerequisites must be satisfied for the occurrence of transonic SDOF flutter: (a) the transonic flow is near the critical unstable state, where the fluid mode damping is small; (b) the ratio of the structure's natural frequency to the fluid characteristic frequency must be within a certain range. For a smaller mass ratio, flutter can be divided into two patterns. One is dominated by the instability of structural mode, and the flutter frequency locks onto the structure's natural frequency. The other is dominated by the instability of the fluid mode, and the flutter frequency follows the fluid characteristic frequency instead of the structure's natural frequency.