Static aerodynamic characteristics of symmetrical wind turbine airfoils considering the influence of near-ocean underlying surface

This paper presents a numerical investigation into the effects of the ocean underlying surface (OUS) on the static aerodynamic characteristics of a symmetric NACA0012‌ airfoil ‌near the sea surface, utilizing the ‌volume of fluid (VOF)‌ model. The interference of OUS on the static aerodynamic characteristics of ‌NACA0012‌ is thoroughly studied by examining the time-frequency characteristics of lift coefficients and flow field characteristics under different ‌wind-wave conditions. Results show that the interference exerted by OUS originates from an additional ‌angle of attack. ‌Root mean square (RMS)‌ values of the airfoil lift coefficients and the additional ‌angle of attack exhibit a similar variation trend with wave height and wind speed. The airfoil's static stall ‌angle of attack in the ‌wind-wave field‌ is smaller than that in the wind field, with the decrement depending on the mean value of the additional ‌angle of attack. The interference of OUS persists throughout all flow stages of the airfoil, but it significantly suppresses the development of airfoil vortices only at high ‌angles of attack, leading to different development mechanisms for leading-edge drift vortices and trailing-edge shedding vortices. The former is a long-period (‌wave period‌ of 4.5 s), high-intensity drift disturbed by OUS, while the latter is a short-period, low-intensity shedding induced by the flow separation around the high angles of attack of airfoil itself. Notably, the shedding frequency of trailing edge vortices in the wind wave field is 0.537 Hz lower than that in the wind field. When the airfoil is positioned 30 meters above the ‌still water surface‌, an independent trailing-edge vortex shedding frequency of 0.430 Hz appears, accompanied by a more ‌complex‌ vortex shape. The findings of this study shed light on improving the safety performance of large floating wind turbines.