Numerical Investigation of Leading-Edge Modifications of a NACA Airfoil

A parametric investigation was carried out to understand the flow characteristics of tubercle airfoils and to determine the best approach and parameters for designing a tubercle airfoil. For this purpose, a straight edge base airfoil (NACA 4414) and several tubercle airfoils, by modifying the leading edge of the base airfoil, were created in SolidWorks and tested with Computational Fluid Dynamics (CFD) application software Star CCM+. Alternative tubercle airfoil with elliptical bumps demonstrated superior post-stall performance when compared to their straight edge counterparts; their post-stall lift did not decrease drastically. However, their pre-stall lift coefficients were always lower than the base NACA 4414 airfoil. Alternative tubercle airfoil with spherical bumps at the leading edge showed good agreement with the base NACA 4414 lift curve while providing slightly higher lift coefficients for all tested angles. However, the drag coefficient was also higher for this model which resulted in a poor lift to drag ratio. Tubercle models with varying amplitude suffered drastically at high angles of attack while also stalling earlier. Early flow separation took place at tubercles with high maximum amplitudes. Gradual increase of lift and stall angle were achieved by lowering the maximum amplitude of tubercles. The varying amplitude model 4414_sin_0.015t_0.4_100 with a maximum amplitude of 1.5% of chord length provided a higher lift to drag ratio than the base airfoil at low angles of attack between 0° and 4°. Conventional sinusoidal models were created with various magnitudes of amplitude and wavelength. It was found that low amplitude and long wavelength contribute to the best aerodynamic performance. An additional study found that surface waviness contributes to the enhancement of post stall lift coefficient. Following these parametric studies, an optimal tubercle airfoil (4414_sin_0.6_0.2_100) configuration was identified with a uniform amplitude of 0.6% of chord length and wavelength of 31.4% (0.2 factor) of chord length. Finally, the effect of Reynolds number on the optimal tubercle airfoil was studied by testing the airfoil at three Reynolds numbers: 1x106, 5x106, 10x106. A trend of increasing lift and a 4° increase of stall angle was observed with the increase of Reynolds number