Wake stabilization and force modulation via surface dimples on an airfoil at low Reynolds numbers

This study investigates the effect of surface dimples on the unsteady aerodynamics of a National Advisory Committee for Aeronautics airfoil (NACA0012) at chord-based Reynolds numbers of Rec=5300 and 10 000 using direct numerical simulations. Dimples were placed on the suction side at non-dimensional chordwise locations of lD/c = 0.035 and 0.35, and the flow response was studied at a fixed angle of attack α=5°. At Rec=5300, dimples placed at lD/c = 0.35 reduced lift and drag fluctuations by 26.5% and 33.3%, respectively, with minimal change in mean forces. At Rec=10 000, the same configuration led to a sevenfold increase in force fluctuations, while the mean remained unchanged. The smooth airfoil exhibited irregular, aperiodic force signals at this Rec, whereas the dimpled case showed highly periodic behavior, indicating wake stabilization. Flow visualizations revealed that dimples generate streamwise vortices within the boundary layer. These vortices are found to have a stabilizing effect on wake dynamics at Rec=5300, reducing vortex breakdown and enhancing the coherence of wake structures. Spectral proper orthogonal decomposition (SPOD) showed that dimples redistribute modal energy depending on Reynolds numbers: at low Rec, they reduce broadband content and suppress unsteadiness, while at high Rec, they amplify dominant shedding modes and broaden the spectral energy distribution. Leveraging the SPOD energy modes, it is found that dimpled foil exhibits 26% lower spectral entropy compared to the smooth case at St≈2 and Rec=10 000, suggesting a more concentrated energy at one or few dominant modes. These results demonstrate that dimples can passively modulate unsteady forces and wake dynamics for a flow over a streamlined body, either suppressing or enhancing flow instabilities depending on the regime.