Analysis of the Flow past Airfoils with Gurney Flaps at Low Reynolds Numbers

Abstract

This paper presents the unsteady flow analysis of the low-Reynolds-number flows past stationary airfoils equipped with Gurney flaps, which are simple but very efficient liftincreasing devices. Due to its mechanical simplicity, the utilization of Gurney flaps is of particular interest for the small size micro-air-vehicles (MAV) flying at low speed and very low Reynolds number. The aerodynamic analysis is performed in the paper with an efficient time-accurate numerical method developed by the authors for the solution of the NavierStokes equations at low Reynolds numbers, which is second-order-accurate in time and space. The paper presents solutions for the aerodynamic coefficients of lift and drag and for the lift-to-drag ratio of several symmetric and cambered airfoils. It was found that although the airfoil is considered stationary, starting from a relatively small incidence (about 8 degrees) the flow becomes unsteady due to the unsteadiness of the flow separations occurring at low Reynolds numbers, and the aerodynamic coefficients display periodic oscillations in time. A detailed study is presented in the paper on the influence of various geometric and flow parameters, such as the Gurney flap height, Reynolds number, airfoil relative thickness and relative camber, on the aerodynamic coefficients of lift, drag and lift-to-drag ratio. The flow separation is also studied with the aid of flow visualizations illustrating the changes in the flow pattern at various moments in time.