Aerodynamic performance of blunt trailing-edge airfoil considering roughness sensitivity position

Abstract: Wind turbine is often exposed to dramatically different operational conditions, from icy environments to deserts with sand storms, and there are contaminants in these environments, like dust, dirt, ice, and even insects. These contaminants change the aerodynamic shape of blade and increase the surface roughness, which results in the lower utilization rate of wind energy. The aerodynamic performance of wind turbine blade can be improved through the airfoil modification, so the blunt trailing-edge structure is adopted during the design of an airfoil. Compared with the original airfoil, the blunt trailing-edge modification with bigger trailing-edge thickness and cross-section area not only has a great improvement in the maximum lift coefficient and the stall angle of attack, but also makes the maximum lift less sensitive to the leading-edge roughness. Therefore, it is of great significance to study the aerodynamic performance of blunt trailing-edge modification of the airfoil with rough surface for the improvement of the power utilization coefficient of wind turbine. The aerodynamic performance of the airfoil with rough surface and the blunt trailing-edge modification have been numerically and experimentally investigated in recent years. However, these 2 problems have been discussed separately, and the effects of the blunt trailing-edge modification on the aerodynamic performance improvement of wind turbine airfoil have been less investigated considering the roughness sensitivity position. In the present study, the aerodynamic performance of wind turbine airfoil and its blunt trailing-edge modification considering the roughness sensitivity position was numerically investigated to reveal the effect of the blunt trailing-edge modification on the lift enhancement of airfoil with rough surface. The dedicated wind turbine airfoil S822 from National Renewable Energy Laboratory (NREL) was used for the simulation. The lift and drag coefficients of S822 airfoil with smooth or rough surfaces were calculated by the k-ω SST turbulence model, and were compared with the aerodynamic data from wind tunnel tests, which offered a good opportunity to examine the capability of CFD (computational fluid dynamics) simulation. The mathematical expression of the blunt trailing-edge airfoil profile was established using the coordinates' rotation combined with the zoom coefficient of coordinates, and the airfoil S822 was modified to be symmetrical blunt trailing-edge airfoil. The lift enhancement of modified airfoils was analyzed to get the best trailing-edge thickness. In order to obtain the roughness sensitivity position of suction and pressure surfaces, the aerodynamic performance of the airfoil with rough surface was studied. The lift and drag coefficients and the lift-drag ratio were calculated for the airfoils with the roughness sensitivity position and their symmetrical modifications with the best trailing-edge thickness. And the roughness sensitivity of sharp and blunt trailing-edge airfoils was also analyzed. The results indicated that the best trailing-edge thickness was 2% of chord length for symmetrical blunt trailing-edge airfoil. The roughness sensitivity positions of suction and pressure surfaces were 1% and 5% of chord length away from the leading-edge, respectively. After the blunt trailing-edge modification, the lift coefficient and the maximum lift-drag ratio of the airfoil with the roughness sensitivity position significantly increased. The lift-drag ratio of the blunt trailing-edge airfoil was higher than that of the original airfoil for the angle of attack less than 11.19° when the suction surface of airfoil is rough, and so does the airfoil with rough suction and pressure surfaces. It is the same change ruler as above for the airfoil with rough pressure surface at different angles of attack ranging from 1° to 13.23°. The blunt trailing-edge modification makes the lift coefficient and the maximum lift-drag ratio significantly increase, which remarkably improves the aerodynamic performance of rough airfoil. The compositive index of the roughness sensitivity was 10.68% and 8.15% for sharp and blunt trailing-edge airfoils, respectively. The modification reduces the airfoil's sensitivity to the roughness position. The research provides a significant guidance for designing and optimizing the wind turbine airfoil under rough blade surface conditions.