Molecular weight and specific heat ratio effects on convective Mach numbers, entrainment, and mixing in jets in supersonic crossflow

The influence of gas molecular weight and specific heat ratio on entrainment and mixing is investigated for sonic jets in a supersonic crossflow. Five pure gases (argon, ethylene, carbon dioxide, helium, and nitrogen) spanning molecular weights of 4–44 g/mol and specific heat ratios of 1.24–1.66 are injected into a Mach 1.71 crossflow at momentum flux ratios from 1 to 6. Planar Mie scattering and particle image velocimetry provides measurements of velocity fields on the flow's plane of symmetry. The velocity fields provide information about mean and fluctuating velocities, as well as the jet bow shock geometry. Convective Mach number profiles are computed directly from the velocity data. Results show that compressibility effects grow faster for lower molecular weight gases, with peak convective Mach numbers reaching approximately unity for helium compared to higher values for heavier gases. Injectants with higher molecular weight and lower specific heat ratios demonstrate faster entrainment and mixing, as evidenced by decreased velocity fluctuations in the near field of the jet. The convective Mach number estimates explain these trends through compressible suppression of hydrodynamic instabilities. The velocity data and spatially resolved convective Mach number profiles for a large range of injectant properties provide unprecedented quantitative guidance for the design of systems dependent on supersonic injection and mixing.