The Aerodynamic Research Of Incompressible Starting Flow At Large Angle Of Attack

Starting flow problem at large angle of attack(AoA)has important applications in flapping flights,sudden deflection of control plane of an aircraft during maneuvering,gust wind interaction and so forth.The flow of such problems is highly unsteady,with vorticity shedding from both leading and trailing edges,and the time evolution curve of aerodynamic force is highly oscillating.Previously,analytical studies for starting flow were performed either for small AoA(Wagner problem)or for short time.Only numerical and experimental studies were done for large AoA and long time behavior of starting flow.The lack of theoretical research leads to insufficient recognition of the physical mechanism of the starting flow problem at large AoA and for long time,especially of the relevance between aerodynamic force and vortex structure in the flow field.Thus,we need to carry out theoretical analysis of starting flow problem at large AoA and for long time.The purpose of this work is to perform theoretical studies for starting flow at large AoA and for long time evolution.Firstly,the original Wagner theory for starting problem at small AoA is extended to the case of middle AoA.The modified Wagner theory considers the influence of concentrated leading edge vortex(LEV)and trailing edge vortex(TEV)on aerodynamic force.By this study,we find that LEV elevates the Wagner lift curve.Secondly,for arbitrary large AoA,a vortex force method is obtained.The vortex force is represented as the function of vortex force factor,which is only dependent on coordinates.A vortex force line(VFL)map is proposed based on the vortex force factor.On the VFL map,one can easily identify the force enhancing/decreasing role of each given vortex according to its strength,velocity and position.Then,we find the critical region for force enhancement by LEV and TEV.Except for the widely known leading edge suction region,a trailing edge suction region is also found.Based on theoretical and CFD computation,we find that the force evolution can be split into four stages: the force release stage,the force enhancement stage,stall stage and the force recovery stage.The physical mechanism to have these force stages is explained.We have also studied the unsteady effect,concentrated vortex behavior and viscous effect in starting flow.For instance,it is found that viscosity may cause the production of multiple leading edge vortex,giving rise to small scale oscillation of the force curve.The results of this work give a deep insight into the force enhancement or stall phenomenon in starting flow at large AoA.They are also meaningful to the application and control of vortex force.