Visualization of supersonic turbulent vortical flows with filtered Rayleigh scattering

The interactions of multiple streamwise vortices have the potential to enhance the fuel/air mixing process in the supersonic flowfields intrinsic of scramjet combustors. As part of an ongoing experimental study on the mixing enhancement capabilities of imposed streamwise vortex interactions, high quality visualizations of the resulting turbulent vortical flowfields have been obtained with the filtered Rayleigh scattering technique. By visualizing the structure and organization of the studied flowfields, which are generated by a ramped strut injector in a Mach 2.5 flow, insight into the relevant flow physics governing the vortex interactions is obtained. Filtered Rayleigh scattering is a non-intrusive laser-based diagnostic technique that images the light scattered from the constituent gas molecules of a flow, thus providing a distinct advantage over other flow visualization techniques, in that it is not required to seed the flow with gaseous or particulate flow tracers. However, because the Rayleigh scattered signal is relatively weak, it is easily obscured by light scattered from other sources, necessitating the implementation of a molecular notch filter. This fact, combined with the complex supersonic turbulent vortical flows that are of interest, makes the application of the filtered Rayleigh scattering technique challenging and non-trivial. This work will highlight the experimental considerations necessary to achieve successful visualization of the flows of interest by presenting the methods used to image a single downstream station of a non-merging vortex interaction mode. The resulting flow visualizations illustrate the highly turbulent nature of the probed flowfield while confirming the behavior observed in previous experiments.