Unsteady Effects on Stationary Airfoils due to Unsteady Flow Separations at Low Reynolds Numbers

摘要

This paper presents the unsteady flow analysis of the stationary airfoils at low Reynolds numbers, aiming to study the unsteady effects on the aerodynamic coefficients generated by the unsteadiness of the flow separations occurring at relatively small angles of attack. It was found in this study that the aerodynamic coefficients of lift and drag have periodic variations in time at incidences larger than 6 or 8 degrees, depending on the airfoil shape. Solutions are presented for the unsteady lift and drag coefficients of several symmetric and cambered airfoils, which incorporate the effect of the unsteady flow separations. These unsteady solutions are obtained with an efficient time-accurate numerical method developed by the authors for the solution of the Navier-Stokes equations at low Reynolds numbers, which is second-order-accurate in time and space. The numerical method is validated by comparison with experimental results. The paper presents also a study of the influence of various geometric and flow parameters, such as the airfoil relative thickness and camber and the Reynolds number on the unsteady aerodynamic coefficients. The flow separation is also studied with the aid of flow visuzlizations illustrating the changes in the flow pattern at various moments in time.