The aerodynamic performance of a 2-D Clark-Y airfoil at low angles of attack (when the baseline flow is fully attached) is significantly improved by fluidic modification of the apparent aerodynamic shape of the surface. The (pressure) drag reduction is accomplished by the deliberate formation of a small stationary flow interaction domain adjacent to the surface that displaces the local streamlines sufficiently to modify the local pressure distribution. The interaction domain is formed on the surface of the airfoil by combining the activation of a synthetic jet actuator placed downstream of a surface-mounted passive obstruction having a characteristic cross-stream dimension of 0.01c. The effects of the actuation frequency and momentum coefficient on the evolution and structure of the interaction domain are investigated using pressure measurements and particle image velocimetry (PIV). The streamwise length of this domain increases with the actuation wavelength and while high (global) actuation frequencies result in reduced (pressure) drag, low actuation frequencies result in increased (pressure) drag and reduction in lift.