Experiments were performed to determine the effects of impinging oscillating shocks of different frequencies on a 15° downstream angled, underexpanded, sonic helium jet injected into a supersonic airflow. Information on mixing, penetration, total pressure loss and turbulence structure from these experiments was used to estimate mixing control achieved by adding an oscillating shock to the helium injection flow field. Tests were conducted at Mach 3.0, with a total pressure of 6.5 atm, a total temperature of 290 K and a Reynolds number of 51.0 x 10⁶ per meter. Oscillating shocks of three different frequencies were studied. The frequencies selected were designed to allow tuning of the shock frequency to the estimated frequency, about 100 - 150 kHz, of the largest eddies in the approach boundary layer. Visualization using nanoshadowgraph photography showed large turbulent structures in all cases. In addition, there were clear changes in eddy size with changing shock frequency visible on the nanoshadowgraphs. The primary measurement made for the mixing studies was the molar concentration of helium. Concentration data, as well as mean flow data, was collected at nine lateral positions at each of three axial stations downstream of the helium injector. The resulting data produced contours of helium concentration, total pressure, Mach number, velocity, mass flux and static flow properties. Additional tests were conducted to determine the shock oscillation frequency, the correlation between the oscillating shock and the turbulence in the shear layer and the angle of large-scale structures in the flow. Mixing and penetration rates were determined from the helium concentration data. The major result of this study was that impingement of an oscillating shock on a high-speed shear layer can be used to control the rate of mixing. Depending on the shock oscillation frequency, mixing enhancement or inhibition can be produced. It was found that increasing shock oscillation frequency resulted in more rapid injectant concentration decay and increased freestream air entrainment leading to a stoichiometric H₂-air mixture ratio while also reducing penetration of the helium injectant. A strong correlation was found between the highest frequency shock and changes in the mixing flow field. The maximum oscillation frequency was approximately 140 kHz, which was consistent with numerical estimates for the frequency necessary for mixing augmentation under these test conditions. It was concluded that oscillating shock impingement has promise as a means of controlling gaseous mixing in a high-speed cross-flow. / Ph. D.