Understanding and predicting transition from laminar to turbulent flow in hypersonic boundary layers is an active and important field of research because transitional and turbulent heating can be four or more times higher than laminar heating on hypersonic vehicles, including the Orion Crew Exploration Vehicle (CEV).
A new measurement technique for obtaining time- and spatially-resolved image sequences in hypersonic flows was developed. Nitric-oxide planar laser-induced fluorescence (NO PLIF) has previously been used to investigate transition from laminar to turbulent flow in hypersonic boundary layers using both planar and volumetric imaging capabilities. Low-flow rates of NO were typically seeded into the flow, minimally perturbing the flow, but marking the boundary layer fluid. The volumetric imaging was performed at a measurement rate of 10 Hz using a thick planar laser sheet that excited NO fluorescence. The fluorescence was captured by a pair of cameras having slightly different views of the flow. Subsequent stereoscopic reconstruction of these images allowed the three-dimensional flow structures to be viewed.
In the current work, this approach has been extended to 50,000 times higher repetition rates. A laser operating at 500 kHz excites the seeded NO molecules, and a camera, synchronized with the laser and fitted with a beam-splitting assembly, acquires two separate images of the flow. The resulting stereoscopic images provide three-dimensional flow visualizations at 500 kHz for the first time. The 200-ns exposure time in each frame is fast enough to freeze the flow, while the 500-kHz repetition rate is fast enough to time-resolve changes in the flow being studied. This method is applied to visualize the evolving hypersonic flow structures that propagate downstream of a discrete protuberance attached to a flat plate.