Wind tunnel testing airfoil with screens at low reynolds number

Abstract

It is well known that ducted wind turbine (DWT) captures more mass flow through the interior of the duct compared to the same size of bare wind turbine. The duct geometry is usually generated by airfoils shaped annular wings. To this end, a new high-lift airfoil for duct application is developed. This paper describes the wind tunnel testing methodology that has been applied to determine the two-dimensional characteristic of an airfoil for a Reynolds number of 1.0 × 10⁵. A particular aspect of the experiments is the presence of a screen, simulating the pressure drop over a real rotor. The experiments are performed in the 0.4 m × 0.4 m W-wind tunnel of the Delft University of Technology. The two-dimension model is composed of an airfoil and a uniform porous screen. Measurements are conducted with the varying angle of attack and wind speed. To investigate the effect of screen loading on the airfoil pressure distribution, three different screens, with thrust coefficient equals to 0.46, and 1.06, respectively, are tested. Tests on the airfoil with screens show the lift coefficient increases compared to the pure airfoil configuration. For fixed angle of attack, the airfoil pressure difference does not linearly increases with the increasing screen loading. By coupling with different screen loading has different influence on the airfoil lift, combining with the screen has C_T = 0.46 achieves the highest C_l in the vicinity of angle of attack (4° ≤ α ≤ 15°), and coupling with the high screen loading (C_T = 0.65 and 1.06) at high angle of attack (α ≥ 20°) is also beneficial for the tested airfoil in terms of C_l. The results also show the combination with screen delays the stall angle.