Effect of Airfoil Dimple on Low-Reynolds-Number Differing Laminar Separation Behavior via Multi-Objective Optimization

Micro air vehicles and vertical-takeoff-and-landing drones usually operate at relatively low values of the Reynolds number (Re=10(4)-10(5)). In the present study, three airfoils from different categories (AG14, BE50, and SD7032) are studied at Re=40,000-80,000. How the behavior of the laminar separation bubble changes with the Reynolds number is investigated and is found to give rise to differing aerodynamic forces that can severely degrade the aerodynamic performance of low-Reynolds-number airfoils. To obtain airfoils with satisfactory aerodynamic performance over a wide range of low Reynolds number, a multi-objective optimization approach is proposed and implemented with different design considerations. The airfoils optimized at these Reynolds numbers have a geometric dimple, as confirmed by unsteady Reynolds-averaged Navier-Stokes simulations with a transition model, and by particle image velocimetry wind-tunnel testing. The dimple is effective in restricting the location and size of the laminar separation bubble, leading to robust and significantly improved aerodynamic performance. Also studied is how the number of nonuniform rational B-spline control points affects the optimum airfoil results, and the dimple always appears in the optimum geometry for Re=40,000-80,000.