The complex wake modifications produced by a Gurney flap on symmetric NACA airfoils at low Reynolds number are investigated. Two-dimensional incompressible flows over NACA 0000 (flat plate), 0006, 0012, and 0018 airfoils at a Reynolds number of Re = 1.0 × 103 are analyzed numerically to examine the flow modifications generated by the flaps for achieving lift enhancement. Although high lift can be attained by the Gurney flap on airfoils at high angles of attack, the highly unsteady nature of the aerodynamic forces is also observed. An analysis of the wake structures along with the lift spectra reveals four characteristic wake modes (steady, 2S, P, and 2P), influencing the aerodynamic performance. The effects of the flap over a wide range of angles of attack and flap heights are considered to identify the occurrence of these wake modes, and are encapsulated in a wake-classification diagram. Companion three-dimensional simulations are also performed to examine the influence of three-dimensionality on the wake regimes. The spanwise instabilities that appear for higher angles of attack are found to suppress the emergence of the 2P mode. The use of the wake-classification diagram as a guidance for Gurney-flap selection at different operating conditions to achieve the required aerodynamic performance is discussed.
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