A comparative study of planar Mie and Rayleigh scattering for supersonic flowfield diagnostics

Planar Mie and Rayleigh scattering has been applied in a supersonic ramjet model combustion chamber (Scramjet chamber) which allows for the direct visualization of the fluid shear/mixing layer zone along the central axis of the combustion chamber. Hydrogen was injected parallel to the center line of the combustion chamber through holes at the base of an injector plate into a Mach 2.0 air flow. The evolution of shear/mixing layer downstream of the injector part has been visualized by laser light sheet scattering directly from the gas molecules (Rayleigh scattering) or from TiO₂ seed particles (Mie scattering). The laser-based optical techniques reveal details of the turbulent structure of the supersonic mixing process by single shot measurements and allow for the direct determination of the absolute visual shear/mixing layer thickness by averaged image processing. Compared to former experimental investigations applying the optical schlieren method significant smaller values of the absolute shear/mixing layer thickness have been measured by planar Rayleigh scattering and even smaller ones by planar Mie scattering. This difference is partly due to the different measurement techniques used and the different definitions of the shear/mixing layer thickness. Nevertheless the measured shear/mixing layer growth rates agree with previous results by other experimenters, confirming the compressibility effect of decreasing shear/mixing layer growth rates with increasing compressibility. Furthermore a slight but significant shear/mixing layer growth rate enhancement has been measured by shock/shear layer interactions