Surface Roughness Effects on Unsteady Transition Property over a Pitching Airfoil

The characteristics of unsteady boundary-layer transition on a small-amplitude pitching NACA0012 airfoil are investigated using the γ-Reθt¯-Ar transition model, which consists of the γ-Reθt¯ transition model and the roughness amplification factor (Ar) transport equation. The present transition model is validated by three roughness surface cases, including a zero pressure gradient flat plate, the NACA0012 airfoil, and NREL-S814 airfoil with different roughness surface. The numerical results for these cases are in agreement with experimental data. Three pitching NACA0012 airfoils with smooth surface, fully distributed roughness surface, and roughness leading edge are simulated by the present model. The impact of reduced frequencies ranging from 0.019 to 0.301 is also taken into consideration. The results show that as the reduced frequency increases, the transition location moves downstream in the upstroke and upstream in the downstroke. With the influence of distributed roughness, the transition moves upstream and the time delay of the transition location is decreased. Moreover, the roughness leading edge enhances the asymmetry of the propagation speed and makes it more sensitive to the reduced frequency.