Study On Mechanism Of Transonic Shock Oscillation And Alleviation Of Buffet Load On The Airfoil

Shock wave oscillation on the wing surface has a strong influence on airplane aerodynamic performance at transonic flight.So the buffet onset boundary becomes a limitation of the airplane flight envelope.If the transonic buffet onset boundary is penetrated,the control and safety performance of airplane would be impacted strongly by transonic buffet load.Aiming at the issues of transonic buffet onset,buffet onset delay,transonic buffet mechanism and transonic buffet alleviation,the mechanism of transonic buffet origination,buffet sustainability,transonic buffet onset delay and transonic buffet alleviation with microtab were investigated in this thesis.The main works of this study are as follows:(1)A maximum curvature method for predicting transonic buffet onset based on aerodynamic parameters.It is difficult to predict the transonic buffet onset by kink analysis in steady aerodynamic parameter curves such as the lift coefficient curve,pitching moment coefficient curve and trailing edge pressure coefficient curve at a certain flight conditions.Thus,a maximum curvature method is proposed which is based on the aerodynamic parameter curves,to find the buffet onset point at which the curvature of curve achieves an extreme.This method can be applied to predict the transonic buffet onset for airfoils based on steady aerodynamic parameters and good agreement is achieved with the comparison of wind tunnel test data.Furthermore,it is applicable to different airflow conditions and different aerodynamic parameters.(2)The characteristics of transonic buffet onset and the instability mechanism of transonic flow field around the NACA0012 airfoil is studied.By solving the N-S equations,which are enclosed by SST k-? turbulence model,the steady transonic flow field around the NACA0012 arifoil is simulated.By analyzing the variation characteristics of flow field around the airfoil with the increasing of angle of incidence or Mach number of free stream before buffet onset,the instability mechanism of transonic flow field around the airfoil is presented.The intensity of mutually reinforcing interaction between the shock and the vortices reaches the instability level when the angle of incidence of the airfoil or Mach number of free stream reaches a certain degree.The intensities of shock and vortices incease sharply and continually.And this causes the circulation of airfoil varying periodically.As a result,the flow field around the airfoil is shifted from the steady flow to the unsteady flow,and buffet occurs.(3)The effect of microtab on transonic buffet onset of NACA0012 airfoil is investigated.The CFD simulation using RANS method with SST turbulence is conducted to determine the transonic flow around NACA0012 airfoil with a microtab which is installed perpendicular to the airfoil surface in the vicinity of the trailing edge.The influence of two different installing schemes of microtab(‘Upper Tab' refers to a tap installed on upper airfoil surface,‘Lower Tab' refers to a tap installed on lower airfoil surface)on the controlling of transonic buffet onset are analyzed.Subsequently,research is conducted to determine the effect of the extruding height of the microtab on the transonic buffet onset.The results indicate that the improvement of the buffet onset incidence is achieved by ‘Upper Tab'.Furthermore,the buffet onset incidence tends to be improved as the extending height of ‘Upper Tab' increases.While the improvement of the lift coefficient at buffet onset is attained by ‘Lower Tab'.Similarly,the lift coefficient at buffet onset can be improved with the increasing of ‘Lower Tab' height.By analyzing the effects of different installing schemes and different extruding height of microtab on flow field around the airfoil at indentical free stream conditions and angle of incidence,the mechanism of buffet delay with the microtab is presented.(4)The mechanism of transonic shock oscillation sustainability on a supercritical airfoil is investigated.The unsteady flow field around NASA SC(2)-0714 supercritical airfoil on which the shock is oscillating is simulated by solving N-S equations.By analyzing the characteristics of unsteady flow field,the mechanism of second type(model B)shock oscillation sustainability is presented.The alternation of intentisifying and weakening interactions among the shock,the vortices and the flow above the boundary layer after shock causes the thickness of boundary layer after shock varying periodically.And this makes the airfoil circulation change periodically.As a result,the shock oscillats periodically on the airfoil upper surface.(5)The shock oscillation suppression and buffet load alleviation of NASA SC(2)-0714 airfoil with a microtab are investigated.The unsteady flow fields around the NASA SC(2)-0714 airfoil with a microtab on its upper surface are simulated by solving N-S equations.The effects of the chord wise installing location and extruding height of the microtab on the flow field and the buffet load are investigated.The results show that the chord wise installing location of microtab has a large effect on the characteristics of unsteady flow field.When the microtab is installed at x/c=0.8 chord wise location the range of shock oscillation,and the amplitude of buffet load has the most decrease.And the amplitude of buffet load decreases with the increasing of extruding height of microtab located at x/c=0.8 chord wise position.By analyzing the characteristics of unsteady flow field around the airfoil with a microtab on the airfoil upper surface,the mechanism of buffet load alleviation with a microtab is presented.In summary,the present studies on mechanism of transonic shock oscillation and alleviation of buffet load on the airfoils are believed to provide a reference for the transonic aerodynamic design and optimization of airplane wings.And the results of the control effects on the transonic buffet onset and buffet load by using the microtab scheme can provide a technical approach to the transonic buffet onset delay and buffet load alleviation in engineering applications.