Research On The Mechanism And Control Of Complex Aeroelastic Problems In Transnic Flows

With the increasing requirement/pursuit on the high-speed,high mobility and light-weight,modern aircraft,especially the military aircraft are more and more easily encountered into the cases of transonic separated flows.Various transonic aeroelastic issues associated with separated flows have become bottlenecks for the design and use of these aircrafts.In the past decades,academic and engineer circles have carried out a lot of research on transonic aeroelastic problems,however,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.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 and control for the complex transonic aeroelastic problems.The main research contents of this paper are listed as following:(1)Towards the typical transonic complex flow——transonic buffet flow,we studied its instability characteristics by the numerical simulation and the data driven dynamic modal decomposition method.The two-dimensional buffet flow over airfoils displays a narrow spectrum feature with a single instability mode,in which the instability and its unsteadiness are dominated by the oscillating shock along the chord.While the three-dimensional buffet flow over a wing displays broad spectrum with multiple instability modes.The low frequency response is dominated by the chordwise and spanwise shock oscillations,in which the spanwise instability is associated with the three-dimensional effect caused by the factors of span and sweep.The high frequency response is dominated by the K-H instability mode near the wing tip.(2)Two kinds of reduced-order model(ROM)for the unsteady aerodynamics including the state of the transonic buffet are constructed by ARX and ERA method,respectively.These ROMs both can accurately capture stability features of the flow with the changes of angle of attack and Mach number.The predicted buffet boundaries are in good agreement with the results from CFD simulation and wind tunnel experiment.The ROM-based aeroelastic model is further constructed by coupling the ROM with the structural motion equation in state space.This model can be applied to the unified analysis of aeroelastic stability and response problems.The calculation examples indicate that this model has a high accuracy.(3)The induced mechanisms of some complex transonic aeroelastic phenomena are revealed.(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 freestream 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)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.(3)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.The above researches provide some new viewpoints to understand the complex aeroelastic problems in transonic flow,which are potential guidance on how to prevent these problems in engineering.(4)For transonic buffet flow,several active controls are conducted and a novel passive feedback control strategy is proposed.The actuator of active control is the trailing-edge flap.Under the framework of CFD simulation,the open-loop manner with the prescribed periodic oscillating flap can reduce the buffeting load by more than 70%,while the closed-loop manner based on the optimal delay feedback of the lift coefficient can completely suppress the buffeting unsteadiness through expensive numerical iterations.Based on the aerodynamic ROM,two kinds of closed-loop control with the output feedback of lift and moment are designed by the pole assignment and linear quadratic methods respectively.Results show that these control laws are both able to completely suppress the buffet instability with good robustness,and the best performance is obtained when the control operates close to the anti-resonance point.In passive feedback control,buffet flow can be stabilized by releasing the structural degree of freedom with reasonable parameters design.The essence of this strategy is to efficiently utilize the coupling effect of the aeroelastic system,improving the stability of the fluid mode by properly decreasing the stability margin of the structural mode.This approach has a potential application prospect in the field of flow control,which does not need extra power and complex control law.