Turbulent structures in an airfoil wake at ultra-low to low Reynolds numbers

The flow around a NACA0012 airfoil was investigated for the Reynolds number regime of 1.0 x 104 < Rec < 3.0 x 104. The effect of chord-based Reynolds number (Rec) on the time-averaged flow characteristics and turbulent structures was examined at two different angles of attack (alpha = 0 degrees at alpha = 5 degrees). Particle Image Velocimetry (PIV) measurements were performed in a closed-loop water channel. The results were elaborated employing velocity profiles, root-mean-square fluctuations, two-point correlations, and energy spectra. Substantial changes in flow structures were observed for both attack angles by varying Rec. The formation and evolution of the vortices in the airfoil wake depicted that the increasing Rec decreases the vortex formation length and alters the fluctuations at alpha = 0 degrees. On the other side, large separation from the leading edge and consequent fluctuations downstream were attenuated for alpha = 5 degrees with increasing Rec. According to the two-point correlations, some instability in the turbulent structures was observed for Rec = 2.0 x 104 and Rec = 2.5 x 104 at alpha = 0o, which gave rise to subharmonics in the vortex shedding frequency. Separated and attached flow conditions were interpreted for Rec <= 1.5 x 104 and Rec >= 2.0 x 104, respectively, at alpha = 5 degrees. The outputs of the evaluated parameter range were linked and discussed with significant deviations in the drag polar of the studied NACA0012 airfoil. It is believed that the revealed results will contribute to the understanding of this specific flow regime which has many implementations in engineering.