Stabilizing Effect of Surface Dimples on a NACA0012 airfoil at Low-Reynolds-Numbers

Understanding and controlling unsteady aerodynamic forces and wake dynamics is essential for improving the efficiency, stability, and noise characteristics of low-Reynolds-number airfoils, which are widely used in small-scale unmanned aerial vehicles, bio-inspired flyers, and wind energy systems. This study investigates how passive surface modifications in the form of shallow dimples affect the unsteady aerodynamics of a NACA0012 airfoil at chord-based Reynolds numbers of Re_c = 5300 and 10,000, at a fixed angle of attack α = 5°, using direct numerical simulations. At Re_c = 5300, the dimpled airfoil exhibited a 26.5% reduction in lift fluctuations and a 33.3% reduction in drag fluctuations, without significantly altering the mean aerodynamic forces. In contrast, at Re_c = 10,000, the presence of dimples amplified force fluctuations by nearly a factor of six without affecting the mean forces, indicating a fundamental change in the unsteady wake behavior. While the smooth airfoil showed irregular, aperiodic force variations at Re_c = 10,000, the dimpled case revealed highly periodic force signatures, suggesting the emergence of a more coherent wake. Three-dimensional flow visualizations using the Q-criterion revealed that dimples generate streamwise vortices within the boundary layer that presumably delay vortex breakdown and stabilize the wake. Spectral Proper Orthogonal Decomposition (SPOD) further showed that dimples redistribute energy among the dominant modes, modifying the underlying flow dynamics. Collectively, these results demonstrate that surface dimples can passively modulate unsteady aerodynamic forces and wake organization in a Reynolds-number-dependent manner, offering a potential pathway for passive flow control in low-Reynolds-number applications.