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

This paper presents a numerical investigation of the effects of the ocean underlying surface (OUS) on the static aerodynamic characteristics of a symmetric airfoil ‌NACA 0012‌ near the sea surface utilizing the ‌Volume of Fluid (VOF)‌ model. The interference of OUS on the static aerodynamic characteristics of ‌NACA 0012‌ is studied thoroughly by examining the time-frequency characteristics of lift coefficients and flow field characteristics under different ‌wind-wave environments‌. Results show that the interference exerted by OUS originates from the additional ‌angle of attack (AoA)‌. ‌Root mean square (RMS)‌ values of the airfoil lift coefficients and the additional ‌AoA‌ exhibit a similar variation trend with wave height and wind speed. The airfoil’s static stall ‌AoA‌ in the ‌wind-wave field‌ is smaller than in the wind field, with the decrement depending on the average value of the additional ‌AoA‌. The interference of OUS persists throughout all flow stages of the airfoil, but it only significantly suppresses the development of airfoil vortices at high ‌AoA‌, leading to different development mechanisms of leading edge drift vortices and trailing edge shedding vortices. The former is a long-period (‌4.5 s wave period‌), high-intensity drift disturbed by OUS, while the latter is a short-period, low-intensity shedding induced by the flow separation around the high-‌AoA‌ airfoil itself. Notably, the shedding frequency of the trailing edge vortices in the wind wave field is lower than that in the wind field by 0.537 Hz. When the airfoil is positioned 30 meters above the ‌still water surface‌, an independent trailing edge vortex shedding frequency of 0.43 Hz appears, accompanied by a more ‌complex‌ vortex shape. The findings of this study will shed light on improving the safety performance of large floating wind turbines.