Experimental Study of Surface Curvature Effects on Aerodynamic Performance of a Low Reynolds Number Airfoil for Use in Small Wind Turbines
Journal of Renewable and Sustainable Energy
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This paper presents wind tunnel experimental results to investigate the effects of surface gradient-of-curvature on aerodynamic performance of a low Reynolds number airfoil Eppler 387 for use in small-scale wind tur- bines. The prescribed surface curvature distribution blade design method is applied to the airfoil E387 to remove gradient-of-curvature discontinuities and the redesigned airfoil is denoted as A7. Both airfoils are manufactured with high precision to reflect the design. Low-speed wind tunnel experiments are conducted to both airfoils at chord based Reynolds numbers 100,000, 200,000 and 300,000. Surface pressure measurements are used to calculate lift and pitching-moment data, and wake survey method is applied to obtain drag data. The experimental results of E387 are compared with NASA LTPT results for validation. The gradient-of-curvature discontinuities of E387 result in a larger laminar separation bubble which causes higher drag at lower angles of attack. As the angle of attack increases the separation bubble of the airfoil E387 moves faster towards the leading edge than that of A7, resulting in a premature bubble bursting and earlier stall on E387. The impact of the gradient-of-curvature distribution on airfoil performance is more profound at higher angles of attack and lower Reynolds number. The aerodynamic improvements are integrated over the 3D geometry of a 3 kW small wind turbine, resulting in up to 10% increase in instantaneous power and 1.6% increase in annual energy production. It is concluded experimentally that an improved curvature distribution results in better airfoil performance, leading to higher energy output efficiency.