Three-dimensional imaging of hypersonic flow at MHz-rate

This paper reports the development of simultaneous MHz-rate multi-dimensional imaging and three-dimensional rendering of Mach 8 transitional flow using a pulse-burst laser and high-rate CCD framing camera system. Experiments are conducted on elliptic cone models and show the evolution of the boundary layer structure near the centerline of the model where the transition occurs. Images are taken using Rayleigh scattering from a CO₂ fog of particulates which condense in the cold core portions of the flow but not in the boundary layer. Planform and spanwise images are simultaneously recorded so the full three- dimensional evolution of the outer portion of the boundary layer can be monitored. The slow transitional structure evolution rate allows the frozen flow approximation to be used so the sequential spanwise images can be assembled into a full three-dimensional rendering of the outer boundary layer surface.