An experimental investigation, aimed at delaying flow separation as a result of the occurrence of a shockwave/boundary-layer interaction, is reported. The experiment was performed using a NACA 0015 airfoil at high-Reynolds-number incompressible and compressible flow conditions. The effects of Mach and Reynolds numbers were identified, using the capabilities of the cryogenic-pressurized facility to maintain one parameter fixed and change the other. The main objectives of the experiment were to study the effects of periodic excitation on airfoil drag divergence and to alleviate the severe unsteadiness associated with shock-induced separation (known as buffeting). Zero-mass-flux oscillatory blowing was introduced through a downstream directed slot located at 10% chord on the upper surface of the NACA 0015 airfoil. The effective frequencies generated 2-4 vortices over the separated region, regardless of the Mach number. Even though the excitation was introduced upstream of the shock wave, it had pronounced effects downstream of it. Wake deficit (associated with drag) and unsteadiness (associated with buffeting) were reduced. The spectral content of the wake pressure fluctuations indicates steadier flow throughout the frequency range when excitation is applied. This is especially evident at low frequencies which are more likely to interact with the airframe.