【作者】 李娟；

【导师】 吴子牛；

【作者基本信息】 清华大学 ， 力学， 2018， 博士

【摘要】 大迎角突然启动问题在扑翼飞行、飞行器机动时控制面的快速偏转运动和突遇阵风等问题中有着广泛的应用。这类问题的主要特点是:流场高度非定常,不断有自由涡从前缘和尾缘溢出,气动力随时间演化。在过去,人们只针对小迎角突然启动问题或大迎角突然启动问题的短时间特性有理论研究,而针对大迎角、长时间演化的突然启动问题,只有数值模拟和实验研究。由于缺乏基于理论分析的研究,对于大迎角突然启动问题长时间过程涉及到的物理规律认识不足,尤其对旋涡与气动力的关联认识不足。因此,需要对大迎角长时间演化的突然启动问题进行理论研究与分析,揭示此类问题的物理规律。本文针对大迎角突然启动问题构造了长时间尺度的理论分析方法,辅以数值计算进行验证。首先,将小迎角Wagner突然启动理论推广到了中等迎角的情况,这样得到的修正理论考虑了前缘与尾缘集中旋涡对气动力的影响。分析表明,前缘涡抬高Wagner升力曲线。接着,针对任意大迎角,获得了确定涡作用力的方法,将涡作用力表示为涡作用力因子的函数。涡作用力因子只与坐标有关。基于涡作用力因子,我们提出了涡作用力地图(也称为涡力线地图)的概念。在涡作用力地图上,可以依据旋涡的强度、速度和位置,识别其对气动力的贡献。以此,我们发现了前缘涡和尾缘涡增强气动力的临界区域。除了人们所熟知的前缘吸力区,我们还发现了尾缘吸力区。依据大迎角突然启动问题的理论与数值计算结果,我们发现法向力随时间的演化,可以分解为四个阶段:力的释放阶段、力的增强阶段、失速阶段和力的恢复阶段。结合涡力线地图,我们对各阶段产生的物理机制给出了合理的解释。此外,结合集中涡分析方法、压力的定常与非定常分解等手段,对涡作用力及非定常效应进行了深入的分析。对粘性效应的计算和分析发现,流场中主旋涡和大尺度气动力的演化规律对雷诺数不敏感,粘性会导致复合前缘涡的产生,引起小尺度力的振荡。本文的研究结果可对大迎角突然启动问题涉及的力的增强、失速等给出更深入的认识,对利用和控制旋涡气动力有重要意义。更多还原

【Abstract】 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.更多还原