Study On Some Aeroelastic Problems In The Vicinity Of Transonic Buffet Onset

Transonic buffet and transonic flutter are all classical aeroelastic phenomena in transonic flow. They may lead to structural fatigue or even flight accidents, which often procrastinate the design process of the new aircraft. The idea is prevailed in the academic field that the effect of the fluid–structure interaction is hardly distinct, so many studies on transonic buffet are based on uncoupled method. But in fact, the elastic characteristic may be a vital factor in the study of transonic buffet.With the development of computers and CFD, numerical simulation has become the main approach for the studying of buffet and flutter. The numerical examples show that the predicted buffet onset, buffet frequency and buffet loads are precise based on URANS equations enclosed with S-A turbulence model. The coupled CFD/CSD method has been used in the analysis of flutter characteristics with high accuracy but low efficiency. The expensive computational time cost makes it not an appropriate approach for the parameters analysis and the mechanism investigation. An efficient model for transonic aeroelastic system is constructed based on the reduced order modeling(ROM) for unsteady aerodynamics. The specific contents are as follows:Transonic single-degree-of-freedom(SDOF) flutter in pre-buffet flow is studied based on coupled CFD/CSD method. The elastic system may be induced SDOF self-excited oscillations, which occur only when the angle of attack of the elastic airfoil is slightly lower than that of buffet onset, and the ratio of the structural characteristic frequency to the buffet frequency is in a limited range. There is no mutation for the response characteristics in the vicinity of buffet onset, and there exists “lock-in” phenomenon like in buffet flow. The transonic SDOF flutter may closely relate to transonic buffet.The transonic SDOF flutter is studied based on ROM. The essential mechanism for the invocation of SDOF flutter is the coupling of aerodynamic mode and structural mode. It must meet two conditions:(a) the flow is near the critical instable state, where the aerodynamic mode damping is small;(b) the structural characteristic frequency and aerodynamic mode frequency must be in some certain ranges. For a stronger stiffness airfoil, the coupled frequency follows the structural characteristic frequency, which is the “lock-in” phenomenon. While for a weaker stiffness one, the coupled frequency follows the aerodynamic mode frequency, where is not suitable for forced flap oscillations method.The contesting phenomena of flutter and buffet may occur for elastic airfoil in the transonic flow beyond the flutter onset velocity. When the airfoil engages in a flutter, the instantaneous pitching angle may exceed the buffet onset angle, thus, buffeting will appear in a segment of the vibration cycle. The buffet high-frequency aerodynamic loads destroy the original flutter model, and then the amplitude of the structural motion decays. And then the flutter occurs again. The mechanism is that the buffet loads changes the phase angle difference between the plunging and pitching displacements. This may be a new approach for flutter suppression.