Effect of partial flexibility over both upper and lower surfaces to flow over wind turbine airfoil

Fluid-structure interaction phenomena on NACA 4412 airfoil having membrane material which was partially mounted on both its suction and pressure surfaces were experimentally investigated in wind tunnel measurements. Different experiments including Digital Image Correlation, smoke-wire, force measurement, and hot-wire systems were conducted at varying angles of attack, α (0° to 24°), and different Reynolds numbers (Re = 2.5 × 104, 5 × 104 and 7.5 × 104). The controlled case gave benefits of up to 2 times in lift coefficient at lower angles of attack (α = 0° − 10°), at the same time drag coefficient decreased. Moreover, the partially flexible airfoil presented high power efficiencies at the pre-stall angles of attack (α = 0° − 10°). Using the flexibility caused the shear layer to approach, laminar separation bubble to suppress, and the wake region to shrunk, indicating the high lift force and less drag force production. The membrane vibrated and deformed due to the flow over the airfoil. Then, the membrane vibration caused the flow to trigger, which caused fluid–structure interaction to perform and the laminar separation bubble to shrink. The transition to turbulence formed earlier and these two flow phenomena moved towards upstream, resulting in having fewer flow fluctuations and increasing the flow stability. This ensures important advantages such as enhancement of aerodynamic performance, power efficiency, and decreasing vibration and noise for wind turbine blades.