Numerical Analysis of Aerodynamic Characteristics of Airfoils Equipped with Trailing Edge Flaps for Wind Turbine Blades

Abstract

This study reveals the capability of airfoils equipped with trailing edge flaps(TEFs) to alleviate the fluctuating load applied on large wind turbine blades. Compared to other active control devices, TEFs are relatively simple and effective and various attempts have been made to optimize control techniques of TEFs for wind turbines. As a preliminary to developing an active control algorithm for TEFs, the quantification of primary control variables related to configurations of airfoils is required. For that purpose, it is proper to analyze an aerodynamic characteristics of airfoils using computational fluid dynamics(CFD) which has the lower cost and the higher efficiency than wind tunnel measurements and could be a useful guideline on the determination of the optimal TEF design. In this study, OpenFOAM that is one of the Reynolds-averaged Navier Stokes solver analyzed a two dimensional incompressible steady state flow around airfoils with TEFs. Simulation results of baseline airfoils were compared with measurements to validate numerical analysis methods. Additionally, a comparison with XFOIL which is a two dimensional panel code with a coupled integral boundary layer analysis compute program was presented to give the reliability of OpenFOAM computation. First, the grid independence test on NACA 643-418 airfoil was implemented in order to establish overall requirements of the grid generated on an analysis domain around an airfoil. This test helped to obtain converged solutions and to evaluate the suitability of numerical methods applied to OpenFOAM. To suggest optimized design condition of a TEF configuration applicable to wind turbines, a parametric study was conducted. This study covered flap length to total chord ratio ranging from cf/c=0.05 to 0.15, flap deflection angle ranging from β=-20° to +20° and angle of attack ranging from α=-6° to +20° at Re=3,000,000. OpenFOAM computations of the lift coefficient curve agreed with experiments closely when there was the attached flow on the airfoil. The variation in a lift coefficient increased with an increase in the flap length and flap deflection. Also, the rotating direction of the flap deflection affected the change in the lift coefficient. The slope of lift curves maintained to be constant for all the analysis cases. Within the angle of attack that made a drag coefficient constant partly, the moderate flap deflection angle marginally influenced on the change in the drag coefficient. On the other hand, above the flap deflection angle β=+15°, the drag coefficient extremely increased about twice than the constant value, which could cause undesirable problems with a structure and noise from wind turbine blades. Thus, more progressive studies would be required. The flap effectiveness (∂Cl/∂β)/(∂Cl/∂α) is also good for comparison of the aerodynamic performance of airfoils with TEFs. Below the critical angle of attack, the increase in cf/c helped the flap to improve the effectiveness. Deflecting the flap upward decreased the flap effectiveness, while it was not relatively sensitive to downward deflection of TEFs. Therefore, the optimized range of the flap deflection angle would be limited to actuate TEFs efficiently.As a result, TEFs had a substantial effect on a pressure distribution on airfoils so that the unsteady loads on wind turbine blades could be mitigated using the active control for TEFs. This study showed that the range of both the change in lift and the load control became wider as β and cf/c increased. Considering the drag and the flap effectiveness, the optimized configuration conditions could be drown. This analysis suggests that the appropriate range of the flap deflection should be from -10° to +10°.