Abstract: 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.

Key words: low Reynolds number, airfoil, laminar separation, transition, Large Eddy Simulation (LES)