Contribution of lower surface laminar separation bubble to nonlinear airfoil lift characteristics at low Reynolds number

A combined experimental/computational study was carried out to understand the 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, the microscopic particle image velocimetry technique was applied to capture the fine details present inside the thin boundary layer (a unique characteristic of low Re flows). The boundary layer flow was resolved to a wall-normal coordinate of upper Y Superscript plus Baseline equals 2 Y + = 2 $Y^+ = 2$ . For computational fluid dynamics (CFD) simulations, state-of-the-art procedures were followed in solving the Reynolds-averaged Navier Stokes equations including the application of the gamma minus italic Re Subscript theta γ − Re θ $\gamma -\textit{Re}_\theta$ transition model, where gamma γ $\gamma$ is the intermittency, and upper R e Subscript theta R e θ $Re_\theta$ is the Critical momentum thickness Reynolds number. The predictions were validated against measurements wherever possible. Both measurements and simulations showed a slope greater than 2 pi 2 π $2\pi$ in the upper C Subscript upper L Baseline minus alpha C L − α $C_L-\alpha$ curve, where upper C Subscript upper L C L $C_L$ is the coefficient of lift, alpha