Numerical Simulation Of Aerodynamics And Modeling Research At High Angles Of Attack

Current and future fighter airplanes are designed to transiently fly at very high angles of attack and require highly maneuverability. So it is very important to simulate the corresponding flowfield structure and predict the aerodynamics of aircrafts accurately in the designation of modern fighter aircrafts and their control systems. In this paper, three-dimensional Navier-Strokes numerical simulations were carried out to predict the characteristics of the vortex-dominated flowfield over delta wings and a wing-body-canard. Besides, according to state-space model that was proposed by Goman, some research work is developed to simulate the unsteady aerodynamics at high angles of attack. And the corresponding data from Navier-Strokes numerical simulations is used for the identification of the unsteady aerodynamic description. The main work developed in this paper is as follows:1. Numerical investigation of the structure of the vortical flowfield over delta wings at high angles of attack in longitudinal and with small sideslip angle is presented here. The methods that analyze the flowfield structure qualitatively and quantitatively are given by using flowfield data from the computational results. In the region before the vortex breakdown, the vortex axes are approximated as being straight line. As the angle of attack increases, the vortex axes are closer to the root chord, and far away from the wing surface. Along the vortex axes, as the adverse pressure gradients occur, the axial velocity decreases, the vortex becomes unstable, and it is possible to breakdown. The occurrence of the breakdown results in the instability of lateral motion for a delta wing, and the lateral moment diverges after a small disturbance occurs.2. Numerical investigation of the structure of the vortical flowfield over a delta wing undergoing maneuverable motions is presented. Three cases are considered to investigate the frequency effect on the flowfield structure and aerodynamic loads forthe delta wing. The lateral moments are more evident as the frequency decreases at high angles of attack. Pitching motion with high frequency delays the vortex breakdown obviously. Besides, the influences of a transient lateral disturbance on the lateral aerodynamics are researched in the paper. For lower frequency case, the lateral moment diverges even if the disturbance occurs at an angle of attack at which the vortex do not breakdown over the wing. As the frequency increases, the lateral moment has no evident change after the transient lateral disturbance occurs, and the lateral-directional stability is more pronounced.3. The static flowfield over a wing-body-canard is calculated and the vortex flow over the fore-body, strake, canard and wings is analyzed. The influence of the vortex flows on each other is also discussed.4. The dynamic flowfield over the wing-body-canard is calculated and the influence of the reduced frequency on the flowfield and the aerodynamics is analyzed.5. According to the aerodynamic characteristics of aircrafts, the state-space model of unsteady aerodynamics at high angles of attack is established. And the unknown parameters in the aerodynamic model are identified by system identification method. The maximum likelihood method is used as identification principle, and the identified arithmetic is Newton-Raphson method. The results show that the model can describe the unsteady aerodynamics at high angles of attack very well.6. The research about the aerodynamic model establishment is one of the basic research for flight dynamics, the designation of modern fighter aircrafts and their control systems. In this paper, the aerodynamic model method integrates with the numerical simulation to avoid great quantity computations of unsteady aerodynamics. The integration of these two methods is a rather effective, economical and practical method.