Experimental Research Of Oscillating Free-stream Effects On Dynamic Characteristics Of Wind Tunnel Models

Post-stall maneuverability has been one of the important characteristics of the future fighters. The unsteady aerodynamics about the post-stall maneuvering aircrafts has been studied for many years, but the most investigations mainly focus on the unsteady aerodynamics caused by the attitude angle changes of the aircrafts. However, the changes of the fighter velocity are also very large, when the aircrafts perform the post-stall maneuver. It is necessary to research the effects of the variable free-stream in the study of dynamic aerodynamics of the maneuvering aircrafts.In this thesis, the unsteady wind tunnel at NUAA provides a unique experimental apparatus to study the effects of oscillating free stream. The unsteady aerodynamic responses of the stationary or dynamic NACA0012 airfoil and the stationary or dynamic delta wing in the oscillating free stream have been investigated through the measurements of the aerodynamic forces and the dynamic pressure distributions, the flow visualization and Particle Image Velocimetry (PIV) measurements. The effects of frequencies and amplitudes of velocity perturbation are discussed. The physical mechanisms behind those changes are analyzed. Topological features of the instantaneous cross-flow streamlines patterns of delta wing are studied. Finally, the unsteady aerodynamics models are established using the wind tunnel test data. The effects of oscillating free-stream on the performances of maneuvering aircraft are also presented.The test results of the stationary airfoil in oscillating free stream show that, in the short period of oscillating free-stream, the lift coefficients increase with decelerating. For the long period, the lift coefficients decrease with decelerating contrarily. These facts are related with the time scale of the vortex convection from the leading edge to the trailing edge and the externally time scale of velocity oscillating. The test data of the dynamic airfoil in oscillating free stream show that, the hysteresis loops of lift in coupled motion are enlarged. The maximum lift coefficients are increased compared with the model pitching alone. The effects of oscillating free-stream appear significantly at the static stall angle of attack around, because the process of the leading-edge vortex convected from the leading edge becomes slowly when the free-stream velocity decelerates.The test results of the stationary delta wing in oscillating free-stream indicate that, the flow field structures of delta wing are altered by the oscillating free-stream and the vortex strength is increased. These changes will result in the variety of aerodynamics correspondingly. The change of vortex structure is mostly obvious when the vortex burst at the leeward of the delta wing. But when the flow attach or separate completely at the upper surface, the effects of oscillating free-stream on the structure is not obvious. PIV measurements of the instantaneous flowfield indicate that the crossflow streamline topologies can be used to identify the positions of vortex breakdown. The instantaneous streamline topologies also show that, during flow deceleration, the vortex breakdown positions move downstream, while move upstream during flow acceleration.For delta wing pitching alone, it is found that dynamic hysteresis characteristics are not always clear. When the pitching motion of the model goes through the different flow condition with different time scales, especially when the model goes through the static stall angle of attack, the aerodynamic hysteresis loops arise obviously. For this case, the reduced frequency will act an important role for aerodynamic hysteresis phenomena.The experimental results of the pitching delta wing in oscillating free stream show that, the effects of oscillating free-stream are not important at the small angle of attack and at the angle of attack that the upper surface flow has already separated completely. But when the pitching motion of delta wing has the different flow patterns with different time scales, the effects of oscillating velocity appear very strong. The hysteresis loops enlarge further than those in the steady free-stream. The maximum lift coefficients are increased and the dynamic stall angles of attack are delayed. The flow visualizations and PIV measurements indicate that the vortex breakdown are postponed during the decelerating cycle and advanced during the accelerating.Finally, a Fuzzy Logic Model (FLM) and a Fuzzy Neural Network (FNN) model based on fuzzy clustering are developed. The simulating results show that these two models are both better methods to identify the unsteady aerodynamics with multi variables. Meanwhile, the fuzzy clustering method is a best method to design fuzzy neural network structures. The calculating process using FNN can be speeded up. The performances of flying at 70 degree angle of attack are calculated using unsteady aerodynamics of oscillating free-stream. The results indicate that unsteady aerodynamics is very important for the flight performance and controller design. This means that we must consider the effects of oscillating free-stream in the design of future supermaneuverability fighters.