Design Optimization of Airfoils at Ultra-Low Reynolds Numbers

Spoon shaped airfoil has high lift-to-drag ratio at ultra-low Reynolds numbers (ranges from 10,000 to 100,000), but instable separated flow on lower surface near trailing edge result to obvious unsteady effect, which affects the controllability and stability of the aircraft. This paper aims to design airfoils not only to improve the time-averaged lift-to-drag ratio, but also to weaken the unsteady effect. An efficient optimizer based on Kriging surrogate model, named “SurroOpt” is used for airfoil design. An in-house URANS (unsteady Reynolds-averaged Navier-Stokes) solver with SST transition model for flow transition is utilized to simulate the flows around airfoils. Both single-point and two-point design are investigated. The results demonstrate that the time-averaged lift-to-drag ratio of the optimized airfoil is higher than that of baseline in a wide range of angles of attack. Meanwhile, the unsteady amplitudes of aerodynamic force are reduced. It is concluded that the developed unsteady aerodynamic design method achieves the design goal of improving lift-to-drag ratio and reducing unsteady effect.