Wall mounted cavity has established itself as a plausible candidate for air-fuel mixing in supersonic flows. A lot of research has been carried out by changing the geometrical properties of the cavity. In spite of some advantages, the cavity suffers the cons of combustor wall heating and asymmetrical mixing. These cons have led to a new design of open strut cavity. Open strut cavity is the cavity between two struts placed in tandem, along with the flow. The current work focuses on the numerical study of flow inside these types of cavities when placed in supersonic flow at different Mach numbers. Also the effect of aspect ratio on the oscillation field is being studied. Comparing open strut cavity with the corresponding wall cavity, an attempt is being made to propose a superior design for supersonic air-fuel mixing.