Experimental and Numerical Study of Surface Curvature Effects on the Performance of the Aerofoils Used in Small Wind Turbines
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The effects of surface curvature and slope-of-curvature on the performance of aerofoils used in small wind turbines are studied experimentally and numerically. A symmetric aerofoil NACA0012 and an asymmetric aerofoil E387 are judiciously selected as an example of an aerofoil with a surface curvature discontinuity and an example of an aerofoil with slope-of-curvature discontinuities respectively. The prescribed surface curvature distribution blade design (CIRCLE) method is applied to both aerofoils to remove the curvature and slope-ofcurvature discontinuities. The newly designed aerofoils have continuous curvature and slope-of-curvature distributions and have nearly identical geometry compared to the original aerofoils, denoted as QM13F and A7. Low-speed wind tunnel experiments, together with two numerical methods, are conducted to aerofoil E387 and A7 to investigate the effects of slope-of-curvature. The slope-of-curvature discontinuities of E387 result in a larger LSB, which causes higher drag at low angles of attack, and result in premature LSB bursting process at higher angles of attack, causing earlier stall. The impact of the slope-ofcurvature distribution on aerodynamic performance is more profound at higher angles of attack and lower Reynolds number. The aerodynamic improvements are estimated over a 3 kW small HAWT, resulting in up to 10% increase in instantaneous power and 1.6% increase in annual energy production. In terms of the effects of surface curvature, the curvature discontinuity at the leading edge affects aerofoil lift and drag performance near the stalling angle in the steady flow, and it is estimated in a 5 kW small VAWT that the power coefficient can be increased by 9.7% by removing the curvature discontinuity. Acoustic experimental measurements were performed on aerofoil E387 and A7 in an anechoic wind tunnel to investigate effects of slope-of-curvature on aerofoil acoustic performance. The in-house CFD code Cgles was modified to perform large eddy simulation (LES) the 3D aerofoil sections to further investigate the experimental phenomenon. The tonal noise of E387 at different angles of attack is reduced by removing slope-of-curvature discontinuities. It is experimentally and numerically concluded that continuous curvature and slope-of-curvature distributions can result in better aerodynamic performance of the aerofoil used in small wind turbines, leading to lower aerofoil self-noise and higher energy output efficiency.
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