Effects of the acoustic disturbance on the 2D mixing layer in Mach 1.78 air flow were investigated. Subsonic gas was injected parallel to the main air flow. Acoustic wave generated by a cavity impinged on the initial mixing layer. When air was injected, it caused large scale structures in the mixing layer, which resulted in the mixing zone expansion. In this case, antisymmetry of the pressure distribution in the mixing layer was observed. According to the linear stability theory, the antisymmetric mode was much more unstable than the symmetric mode. The effects could not be observed when helium, whose density was lower than air, was injected.
