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Effects of a gurney flap were numerically investigated on the supercritical NASA airfoil by solving the two-dimensional Reynolds-averaged Navier-Stokes equations for a range of transonic Mach numbers and angles of attack, using turbulence compressible KW SST model. The height of the gurney flap was selected to be 1.65 percent chord length. A high-resolution mesh was applied to accurately predict the flow field specifically in the vicinity of the airfoil. Below the drag divergence Mach number, the gurney flap has a remarkable influence on the aerodynamic coefficients especially at -1 and 0 degrees angle of attack resulting in 50 percent increase in L over D ratio. At high Mach numbers and angles of attack, Gurney flap loses its effects and the clean airfoil has better aerodynamic performance since it significantly boosts both the pressure and shear drag. It was observed that the gurney flap mitigates the transonic lambda shock on both surfaces of the airfoil. Moreover, it alters the Kutta condition by changing the separation point location at the trailing edge which provides the airfoil more bound circulation and lift force., Comment: Submitted to Journal of Aircraft, under revision