Unraveling Non-linear Wing Aerodynamics at Low Reynolds Numbers - An Experimental and Computational Approach

A combined experimental/computational study was carried out to understand airfoil characteristics at a very low Reynolds number (Re) of 50,000. In the case of experiments, loads and velocity field measurements were conducted. Specifically, microscopic particle image velocimetry (PIV) technique was applied to capture the fine details present inside the thin boundary layer (a unique characteristic of low Re flows). Boundary layer flow was resolved to a wall-normal coordinate of Y+ = 2. For CFD simulations, state-of-the-art procedures were followed in solving the RANS equations including the application of the γ − Re_θ transition model. The predictions were validated against measurements wherever possible. Both measurements and simulations showed a slope greater than 2π in the C_L − α curve. The validated CFD simulation revealed the presence of a separation bubble on not only the suction side but also the pressure side of the airfoil. Historically, the non-linear slope is often attributed to the presence of the LSB on the upper airfoil surface. Further simulations made by assuming the LSB as a geometric feature suggested that the LSB on the lower surface is the primary source of the augmented lift.