Identification of flow phenomena over NACA 4412 wind turbine airfoil at low Reynolds numbers and role of laminar separation bubble on flow evolution

Objective of this study is to elucidate relationship with aerodynamic properties and vortex shedding from suction surface and wake of wind turbine blade at low Reynolds numbers. Force measurement of NACA 4412 airfoil was conducted at various angles of attack and Reynolds numbers. Furthermore, smoke-wire experiment was performed to clearly visualize flow patterns such as flow separation or laminar separation bubble (LSB) over the airfoil. Also, velocity measurements at near wake region were done to highlight coactions among LSB and trailing edge vortices. Hot-film experiment was performed to obtain more information with regards to progress and formation of LSB. Experiments indicated that location and formation of LSB were affected by variety of both Reynolds number and angle of attack. Besides, mean and rms velocities showed that flow characteristics near wake were influenced by LSB and trailing-edge separation. At lower angles of attack, short bubble occurred, and frequency of vortex shedding due to short bubble were high whereas shedding frequency of long bubble at moderate angles of attack were low. The bubbles can cause vibration and noise at the wind turbine blade, hence they should be better known and eliminated, which aerodynamic performance and energy efficiency of turbine can be increased.