Study On Aeroelastic Analysis In Separated Flows

Separated flow is a common phenomenon in many fields,such as aeronautics and astronautics engineering,bridge enginering,ship industry,and biochemical industry.The flow field of separated flow is quite complex,and the corresponding aerodynamic force is strongly unsteady and nonlinear.So it has a great impact on the strength of the structure,and makes the system unstable.Many complicated aeroelastic problems emerge in separated flows.While,the primary physical mechanism for these aeroelastic problems is still not clear,and the existing research on aeroelastic analyasis of separated flows is lack of focus on the relaevant mechanism study.Therfore,it is still necessary to carry out the aeroelastic analysis in separated flows and its mechanism study thoroughly.In this thesis,the separated flows in aeronautics and astronautics engineering are mainly concerned,such as the stall flow at high angle of attack,the vortex shedding flow,transonic buffet flow and the separated vortex flow around a delta wing.A series of studies on the aeroelastic stability analysis and the relative induced mechanism are performed in separated flows,by applying the CFD/CSD coupled technology or using the analysis method based on the Reduced-Order-Model technology.The main contents of this thesis are as follows:(1)Numerical simulations of a NACA0012 airfoil with forced vibration are performed at high angle of attack,by using the unsteady Reynolds-averaged Navier-Stokes equations.The influences of the airfoil motion on the characteristics of the vortex shedding flow are investigated by a series of computations for various amplitudes,frequencies and phase angles.The frequency lock-in phenomenon is found,and the performance of lock-in is also carried out.When lock-in occurs,the flow dominant frequency no longer appears as the vortex shedding frequency,but keeps the same value as the forced vibration frequency.The boundary of the lock-in regime is affected by the vibration frequency,amplitude and phase angle.It is just like a V-shaped funnel,and shows evident asymmetry.Due to the strong nonlinearity of the vortex shedding flow,the vibration phase angle has an important effect on the flow properities to a certain extent.Especially for the flow in transition state,it is easily affected by the vibration phase angle and the lock-in phenomenon presents.Then,the boundary of the lock-in regime is changed widely.(2)An aeroelastic analysis method based on the Reduced-Order-Model(ROM)technique is developed by using the system identification method to construct the reduced order unsteady aerodynamic model,and coupling with the structural motion equations together.It is applied to study the aeroelastic analysis of a pitching airfoil at high angle of attack,in which the vortex shedding has not yet occurred.The stall flutter and its induced mechanism are mainly concerned in this paper.It is found that,there are potential unstable fluid modes in the stall flow field at high angle of attack.As the angle of attack is closer to the critical vortex shedding angle,the potential instability of the flow becomes stronger,and the stall flutter is more likely to happen.The mode veering phenomenon is observed in the stall flutter at high angle of attack.The interaction between the fluid mode and structural mode is the main cause to make the system exhibit different instability characteristics.When the structural natural frequency or mass ratio is higher,the system appears as the structural mode instability.Whereas,when the structural natural frequency or mass ratio is lower,the system appears as the fluid mode instability.So the stall flutter at high angle of attack is in essence a single degree of freedom flutter,which is caused by the coupling of the unstable fluid mode and the structural mode.(3)The aeroelastic responses of an elastically suspended airfoil in transonic buffet flow are analyzed deeply by adopting the CFD/CSD coupling method.The frequency lock-in phenomenon is mainly focused in current work.Great attentions are paid to studying the physical mechanism of lock-in and its characteristics.The common point of lock-in resulting from resonance has been questioned,and a new viewpoint of lock-in due to coupled-mode flutter is proposed.It is found that,the frequency range of lock-in in current work is far away from the typical resonance region,so it does not satisfy the frequency nearness condition anymore.And the amplitude of the airfoil in pitch displacement does not yet reach the maximum value at the resonant frequency.Moreover,the effects of the mass ratio and structural damping on the lock-in region are also beyond the interpretation of the traditional resonance theory.So it can no longer use the traditional resonance theory to explain the lock-in phenomenon in this paper.In current results of this paper,when the system is far away from the typical resonance region,the force coefficient response shows obvious competition between the fluid mode and the structural mode,and undergoes a transition from forced vibration to self-excited flutter.Ultimately,the lock-in occurs with the structural mode instability.It can be seen that the frequency lock-in phenomenon in transonic buffet flow is not caused by resonance,but induced by the coupling of unstable fluid mode and structural mode.It is just as a single-degree-of-freedom flutter.(4)By using the CFD/CSD coupling method,the complex aeroelastic analyses of a flexible delta wing before and after vortex breakdown are performed in this paper.The separated flow fields are simulated by applying the Euler equation and the DES method,respectively.It is shown that before the vortex breakdown,the flow exhibits strong inviscid characteristic,and its aeroelastic instability presents as just a simple flutter,induced by the coupling of the struatural first mode and second mode together.However,after the vortex breakdown,the flow becomes highly unsteady and nonlinear.The unsteady fluctuation loads have a significant impact on the aeroelastic characteristic of the wing,so the corresponding aeroelastic problem is very complicated.Instead of a simple flutter or buffet,there are both flutter and buffet concomitant and competing together.