复杂跨声速气动弹性现象及其机理分析

The complex transonic aeroelastic phenomena and it's mechanisms

虽然跨声速颤振、抖振和嗡鸣等问题不是新的研究课题,但这些跨声速气动弹性问题时常困扰着航空航天飞行器的研制,这与其复杂性密切相关.跨声速气动弹性的复杂性源于流动的非线性和非定常特性,但其中的物理机制仍有待深入研究.本文以跨声速单自由度颤振、抖振锁频以及机翼弹性特征降低抖振边界等现象为例,阐述了这些复杂流固耦合现象的诱发机理,并对其相关性进行了系统性研究.研究指出,跨声速状态下的结构大幅振动大多与气动弹性稳定性相关,线化模型也能够很好地预报一些复杂动力学现象的发生边界.跨声速气动弹性的复杂性主要源于跨声速状态下流动的稳定性降低,与经典气动弹性问题相比,多出了一个流动模态,流动模态和结构模态之间的耦合常导致跨声速状态下发生特殊的气动弹性现象.

Modern high-speed aircraft is easily encountered into various transonic aeroelastic problems,such as transonic flutter,buffeting and buzz.Although they are not fresh topics,the physical mechanism underlying these phenomena are still not clear,which directly leads to frequent occurrences of these problems,causing a negative impact on the design process and service life.The complexity of transonic aeroelastic dynamics is derived from the nonlinearity and unsteadiness of the flow.In this paper,based on the refined numerical simulation and modeling of complex transonic flows,we first constructed a unified analytical method for the aeroelastic stability and response problems.Then,we performed a series studies on the mechanism of three complex fluid-structure interaction phenomena and the relationships between the different phenomena.（1）Transonic buzz is in essence a single degree of freedom（SDOF）flutter caused by the coupling of most unstable aerodynamics flow mode and structural mode.For this kind of SDOF flutter to arise,the fluid must exhibit sufficiently low damping i.e.the free stream flow condition is near the buffet boundary or at the low supersonic zone.Besides,the unstable frequency boundary is determined by those of zero and pole of the open loop system.（2）Frequency lock-in phenomenon in transonic buffeting flow is not caused by the resonance,but it is in essence the SDOF flutter in unstable separated flow.In this process,the response undergoes a transition from the forced vibration to the self-excited flutter,which is the root cause why the lock-in region being far away from the resonance point.While the traditional uncoupled method will misestimate the risk range and underestimate the amplitude of the vibration.（3）Transonic buffet will be induced in a lower angle of attack when the pitching degree of freedom is released.This indicates the drawbacks of the traditional idea to guide the aircraft design based on the predicted buffet onset by the rigid model.The elastic characteristic,in contrast,should be a crucial factor to predict the buffet onset in the engineering.From the above study,we find the large amplitude structural vibration in transonic flow is mostly related to aeroelastic stability.The dynamic linear model can predict the boundary of some complex dynamic phenomena well.Complexity of transonic aeroelasticity mainly arises from the stability reduction of the flow.Compared with the classical aeroelastic problems,a new flow mode derived from the reduction of fluid stability must be considered.The coupling between the flow mode and the structural mode often leads to the complex phenomena in transonic flows.