【作者】 史志伟；

【导师】 明晓；

【作者基本信息】 南京航空航天大学 ， 流体力学， 2006， 博士

【摘要】 随着对战斗机性能要求的不断提高,过失速机动能力已经成为下一代战斗机的重要性能指标之一。虽然关于过失速机动飞行中非定常气动力的研究已进行多年,但是过去的研究,主要集中在飞行姿态快速变化产生的非定常气动力上。然而,战斗机在完成过失速机动飞行时,飞行速度也将产生较大变化,因此在研究战斗机过失速机动飞行中的动态气动特性时,考虑飞行速度变化产生的影响是十分必要的。本文在南航非定常风洞内,以NACA0012翼型和三角翼模型为研究对象,借助动态气动力测量技术、动态压力测量技术、流场显示和PIV测量技术,对非定常自由来流中的二维翼型动态气动特性,非定常自由来流中的静态三角翼气动特性,以及非定常自由来流与三角翼俯仰运动耦合作用下的动态气动特性等,进行了详细和系统的实验研究。分析了来流速度脉动对气动特性产生的影响,分别讨论了对气动特性产生影响的物理机制。应用拓扑分析方法,分析了非定常自由来流下的三角翼空间流场瞬时流线拓扑结构。最后,对实验获得的非定常气动力数据进行了建模,分析了考虑非定常自由来流影响的非定常气动力数据对过失速机动飞行特性产生的影响。非定常自由来流对静态二维翼型气动特性的影响研究表明,来流风速以短周期脉动时,升力系数随来流速度的减小而增加,来流风速以长周期脉动时,升力系数则减小。这与翼型前缘形成的分离涡随来流速度变化在翼面上驻留的时间有关。翼型俯仰运动与非定常自由来流耦合运动下的动态气动特性研究结果表明,耦合运动下的升力系数的迟滞回线比单独俯仰运动时扩大,最大升力系数的值比单独俯仰运动时增加。来流风速的影响作用,在模型静态失速攻角前后表现的最为明显,这是因为来流风速的减速使得模型动态运动过程中产生的前缘涡从翼型上的脱落过程进一步减缓。非定常自由来流对静态三角翼模型气动特性的影响研究表明,来流减速和加速过程对三角翼背风面空间流场结构和强度产生的影响,是导致三角翼模型气动特性产生变化的主要原因。对三角翼上流动结构影响最显著的,是当前缘涡在翼面上发生破碎以后。而对于小攻角下的附着流动以及很大攻角下的完全分离流动,三角翼上的流动结构变化则表现的不明显。通过对非定常自由来流变化过程中,翼面横截面内的瞬时流线拓扑结构的分析表明,随着来流风速的减小,三角翼上的涡的破碎位置向下游移动,而来流风速的加速过程又使涡的破碎位置提前。横截面内的流动拓扑结构的变化可以大致判断前缘集中涡的破碎位置变化。三角翼模型单独俯仰运动的实验研究表明,并不是在所有情况下模型的俯仰运动都会产生明显的动态迟滞特性。产生较大动态迟滞特性的运动,是模型上的流动经历了多种流动形态变化的运动,特别是在模型静态失速攻角前后范围内的运动,此时减缩频率的影响也最明显。来流风速脉动与三角翼模型俯仰运动耦合作用的实验研究表明,在小攻角附着流动和很大攻角完全分离流动的情况下,模型俯仰与来流脉动的耦合作用对模型气动特性的影响并不是很明显。而当模型动态运动经历较大流态变化时,特别是在模型静态失速攻角前后,来流风速脉动使气动特性的迟滞回环进一步扩大,并增大了模型的最大升力系数,推迟了动态失速攻角的出现。流动显示和PIV测量结果说明,模型上仰运动时,来流风速的减速过程对前缘涡的破碎起到进一步的推迟作用,而来流风速的加速过程则促进了前缘涡的破碎。非定常气动力数学模型的研究表明,不论是模糊逻辑模型还是基于模糊聚类分析的模糊神经网络模型,对于多变量输入输出系统来说都是比较好的建模方法,有较强的预测能力。而模糊聚类分析方法则很好地解决了模型的结构辨识问题,提高了计算速度。过失速机动中70迎角定直飞行的仿真计算表明,考虑来流非定常变化的非定常气动力数据,对战斗机的飞行特性和飞行控制律都产生了明显的影响,因此在新一代战斗机的设计、研制中,必须加以考虑。更多还原

【Abstract】 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.更多还原