翼型低雷诺数层流分离现象随雷诺数的演化特征

层流分离现象是翼型低雷诺数条件下出现的典型流场特征。层流分离流动中包含流动分离、转捩、再附等非定常流动结构，层流分离流动的形成与演化会对翼型气动特性产生恶化作用。采用大涡模拟（LES）方法对低雷诺数范围内不同雷诺数下的翼型层流分离流动开展精细数值模拟，研究了雷诺数对翼型气动特性的影响规律及作用机理。LES方法采用隐式亚格子模型，基于结构化拼接网格，对流项离散和时间推进方法分别采用AUSM⁺格式以及双时间步方法。验证算例计算结果表明数值模拟方法的正确性及可靠性，雷诺数对翼型气动特性具有显著影响。随雷诺数降低，时均分离泡外形增大、位置后移，平均阻力系数增大，特别是在较低雷诺数下，翼型升阻力系数随时间出现振荡现象。进一步研究表明，造成不同时均分离泡形态和气动特性的原因在于翼型上表面分离剪切层的失稳与转捩特征。随雷诺数降低，流动黏性增大，导致分离剪切层速度梯度减小，流动发生转捩及再附位置后移，直至翼型表面不再发生转捩和再附。

Evolution of laminar separation phenomenon on low Reynolds number airfoil at different Reynolds numbers

Laminar separation phenomenon is the characteristic feature of airfoils at low Reynolds number conditions. The laminar separation flow contains the complex flow structures of laminar separation, transition and reattachment. The formation and evolution of laminar separation flow are detrimental to the performance of airfoils. The Large Eddy Simulation (LES) method is utilized to predict the laminar separation flow over airfoils at different Reynolds numbers in the low range of Reynolds numbers. The effects of Reynolds number on aerodynamic characteristics of airfoils and the corresponding mechanism are studied. On the structurally patched mesh, the LES method adopt the implicit sub-grid-scale model, and utilize the AUSM⁺ scheme for spatial discretization and dual-time-step method for time marching. The correctness and reliability of the numerical method are proved by the validation example. The results indicate that the Reynolds number has a significant effect on the aerodynamic characteristics of airfoil. With the decrease of Reynolds numbers, the shape of the bubble increases, and the position moves towards the trailing edge, which leads to the increment of average drag coefficient. Besides, at the lower Reynolds numbers, the lift drag coefficient oscillates with time significantly. Further studies show that the instability and transition characteristics of the separated shear layer over the airfoil surface are responsible for the different time-average bubble configurations and aerodynamic characteristics. With the decrease of Reynolds numbers, the flow viscosity increases. So the velocity gradient of the shear layer decreases, and the positions of transition and reattachment move towards the trailing edge. At the lower Reynolds numbers, transition and reattachment do not occur over the airfoil.