MHz rate imaging of boundary layer transition on elliptic cones at Mach 8

A series of flow visualizations has been performed on both 4:1 and 2:1 sharp-nosede lliptic cones at Mach 8. A new high-speed imaging system consisting of a pulse-burst Nd:YAG laser and a 1 MHz rate CCD framing camera was used to produce 30-frame motion pictures and volumetric images of the flow. Boundary layers ranging from fully laminar to late-transitional in character were imaged using Filtered Rayleigh Scattering with streamwise, spanwise, and planform laser sheet orientations. The pressure gradient and associated crossflow from the major axis to the minor axis of the cone cause increased growth and subsequent early transition of the centerline boundary layer. Volumetric image sets of the centerline region reveal hairpin structures characteristic of the early stages of subsonic turbulent spot formation. At low Reynolds number, the unstable region on the cone is confined to the bulge of the centerline boundary layer and the lateral evolution of these hairpin structures is minimal. Images of the off-axis regions reveal the presence of elongated streaks, which are believed to be the footprints of stable traveling crossflow vortices. At higher Reynolds numbers the breakdown of these streaks involve the formation of a series of chain-like structures. The behavior appears qualitatively similar to visualizations of the breakdown of crossflow vortices at subsonic speeds.