The complexity and mechanism of transonic aeroelastic problems

Abstract

In high-speed flight, transonic aeroelasticity commonly exists, leading to various problems such as transonic flutter, buffeting and buzz. To solve these problems, a good understanding of the underlying physical mechanisms is critical. Unfortunately, the nonlinearity and unsteadiness of airflows complicate this process and consequently impede the development of aircraft design. Here using the numerical simulation and modeling of complex transonic flows, we developed a unified analytical method for aeroelastic stability and response problems. We then explored the mechanisms of three complex aeroelastic phenomena and unveiled their relationships. (1) Transonic buzz is in essence a single degree of freedom (SDOF) flutter caused by the coupling between the most unstable fluid mode and structural mode. SDOF flutter of this kind is possible only when the fluid exhibits sufficiently low damping; that is, the freestream flow is near the buffet boundary or in the low supersonic zone. Besides, the unstable frequency boundaries are determined by zero and pole of the open loop system. (2) The frequency lock-in phenomenon in the transonic buffeting flow is caused by the SDOF flutter in the unstably separated flow rather than resonance. In this process, the response undergoes a transition from the forced vibration to the self-excited flutter, which results in a deviation of the lock-in region from the resonance point. By contrast, the traditional uncoupled method misestimates the risk range and underestimates the amplitude of a vibration. (3) When the pitching degree of freedom is released, transonic buffet will be induced at a lower angle of attack, indicative of the drawbacks of predicting buffet onset by a rigid model. Actually, the elastic characteristic plays a key role in predicting buffet onset in aircraft design. Collectively, the large-amplitude structural vibration in the transonic flow is closely related to aeroelastic instability. The dynamic linear model can correctly predict the boundary of some complex dynamic phenomena. The complexity of transonic aeroelasticity mainly arises from the stability reduction of airflows. Different from classical aeroelastic problems, the new fluid mode is derived from the reduction of fluid stability. The coupling between such a fluid mode and structural mode often leads to complex phenomena in transonic flows.