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

TL;DR

This paper explores how surface dimples on an airfoil influence unsteady aerodynamics and wake stability at low Reynolds numbers, showing potential for passive flow control to reduce force fluctuations or induce wake stabilization.

Contribution

It demonstrates that surface dimples can passively modulate unsteady forces and wake dynamics, with effects varying depending on Reynolds number, through detailed numerical simulations and flow visualization.

Findings

Dimples reduce force fluctuations at Re_c=5300 by over 25%.

At Re_c=10,000, dimples stabilize wake and increase periodicity.

Flow visualizations show dimples generate vortices that stabilize wake structures.

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 $Re_c = 5300$ 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 $\alpha = 5^\circ$. At $Re_c = 5300$, 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 $Re_c = 10{,}000$, 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 $Re_c$, 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 $Re_c = 5300$, 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 $Re_c$, they reduce broadband content and suppress unsteadiness, while at high $Re_c$, 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.