翼型低雷诺数层流分离诱导转捩对再附效应的作用机理

Laminar separation bubble is the characteristic feature of airfoils at low Reynolds number, which contains the complex phenomenon, such as laminar separation, transition and reattachment. The formation and subsequent turbulent breakdown of laminar separation bubble have long been known to be detrimental to the performance of airfoils, directly affecting endurance and degrading handling and stability of aircrafts. As has been shown in lots of study, the classic laminar separation bubble and attachment effects are the results of time average. The structures and behavior of laminar separation phenomenon have the property of severe nonlinearity and intense unsteadiness, and the transition process is quite different from the natural and bypass transition. So investigating the mechanism of the laminar separation-induced transition and its effect to the time-averaged reattachment is of prominent value in academic as well as great significance in engineering by understanding the reasons for the deterioration of aerodynamic characteristics and inhibition of it. The present project intends to utilise large eddy simulation, , combined with the theory analysis and the low turbulence wind tunnel tests to investigate laminar separation phenomenon on Low-Reynolds-Number airfoils, in order to understand the effects of attack angle and Reynolds number on the flow structures and aerodynamic characteristics. Further insight into the mechanism of separation induced transition and time-averaged attachment effect will be gained, as well as the generation and evolution mechanism of the long and trailing edge laminar separation bubbles, providing theoretical support for the design and flow control of low Reynolds number airfoils.

层流分离泡是翼型低雷诺数流动的典型特征，包括层流分离、转捩、再附等复杂流动现象。层流分离泡的形成、演化及破碎会严重损害翼型气动特性，直接影响飞行器的气动性能和操稳特性。大量研究表明，经典的层流分离泡及再附效应是非定常流动结构的时均化结果，层流分离现象具有强烈的非定常和非线性效应，其转捩过程与“自然转捩”和“旁路转捩”具有明显差异。因此，研究低雷诺数层流分离诱导的转捩机理及其对再附效应的作用机制具有显著的学术意义，对于认识低雷诺数气动特性恶化的原因并加以抑制具有重要的工程价值。本项目拟采用大涡模拟、理论分析和低湍流度风洞试验相结合的方法对翼型低雷诺数层流分离现象进行细致研究，认识迎角及雷诺数对流场结构和气动特性的影响规律，揭示层流分离诱导的转捩过程及其对时均化再附效应的作用机理，认清“长层流分离泡”与“后缘层流分离泡”的转捩特征及演化触发机制，为低雷诺数翼型设计以及流动控制提供理论支撑。

低雷诺数层流分离流动是高空长航时无人机、临近空间太阳能飞行器和微小型飞行器等飞行器中广泛存在的流动现象。层流分离流动包含层流分离、转捩、再附等复杂流动现象，会严重损害翼型气动特性，并直接影响飞行器的气动性能和操纵性和稳定性。项目结合数值模拟与风洞试验方法，对翼型低雷诺数层流分离现象开展了机理性研究，首先发展了适用翼型低雷诺数分离流动的大涡模拟方法，为复杂流动精细化数值模拟及流动机理性研究建立了有效手段。然后，重点对比分析了迎角及雷诺数对层流分离现象及转捩过程的影响，掌握了迎角及雷诺数对翼型气动特性的影响规律及其作用机理，探索了翼型层流分离流动诱导转捩及对再附效应的作用机理，认识了不同形态时均“分离泡”的转捩特征及演化触发机制。最后，开展了低湍流度风洞试验测试研究，获得平均流动速度型、速度脉动幅值及其频谱，并对数值模拟结果进行了验证。项目研究工作获得了翼型低雷诺数分离流动的深入认识，厘清了层流分离、转捩再附流动结构与时均化分离泡形态及气动特性的关系，为低雷诺数飞行器气动特性预测及气动设计提供了理论支撑。

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