Wake Stabilization and Force Modulation via Surface Dimples on an Airfoil at Low-Reynolds-Numbers

Abstract

This study investigates the effect of surface dimples on the unsteady aerodynamics of a National Advisory Committee for Aeronautics airfoil (NACA0012) at a chord-based Reynolds numbers of $R{e}_c=5300$ and $10,000$ using direct numerical simulations. Dimples were placed on the suction side at non-dimensional chordwise locations of $l_D/c=0.035$ and $0.35$, and the flow response was studied at a fixed angle of attack $\hat{I}\pm =5^{\circ }$. At $R{e}_c=5300$, dimples placed at $l_D/c=0.35$ reduced lift and drag fluctuations by $26.5\%$ and $33.3\%$, respectively, with minimal change in mean forces. At $R{e}_c=10,000$, the same configuration led to a seven-fold increase in force fluctuations, while the mean remained unchanged. The smooth airfoil exhibited irregular, aperiodic force signals at this $R{e}_c$, 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 $R{e}_c=5300$, reducing vortex breakdown and enhancing the coherence of wake structures. Spectral Proper Orthogonal Decomposition (SPOD) showed that dimples redistribute modal energy depending on Reynolds number: at low $R{e}_c$, they reduce broadband content and suppress unsteadiness, while at high $R{e}_c$, they amplify dominant shedding modes and broaden the spectral energy distribution. 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.