Stereoscopic time-resolved visualization of three-dimensional structures in a hypersonic flow was performed for the first time in NASA Langley Research Center’s 31-inch Mach 10 Air Tunnel. Nitric oxide (NO) was seeded into hypersonic boundary layer flows that were designed to transition from laminar to turbulent. A laser excitation and multiple-camera imaging scheme was used to obtain raw images containing three-dimensional spatial information. The images were processed in a computer visualization environment to provide stereoscopic image pairs that could be viewed several ways, including using the cross-eyed viewing method, with the aid of a stereoscope, as animated image pairs (i.e., wiggle stereoscopy), or as anaglyph images through conventional red/blue 3D glasses.
The images captured three-dimensional information that would be lost if conventional planar laser-induced fluorescence imaging had been used. Two models were studied in various configurations. One model was a 10° half-angle wedge containing a small protuberance to force the flow to transition. The other model was a 1/3-scale, truncated, Hyper-X forebody that included a series of spanwise holes normal to (and flush with) the surface, through which blowing was used to force the boundary layer flow to transition to turbulence. The resulting three-dimensional visualizations had an effective time resolution of about 500 ns, which is fast enough to freeze these hypersonic flows. The 512 × 512 size of the resulting images provided better spatial resolution than that of earlier high-speed laser-sheet scanning systems with similar time response, which typically measure approximately 20 planes in the direction perpendicular to the imaging plane.
One of the most important advantages to this innovation is that the sub-microsecond time response is short enough to freeze the rapidly changing hypersonic flow structures, yielding three-dimensional time-resolved stereoscopic images of detailed turbulent flow features. A laser-sheet scanning mechanism is not required, thereby decreasing the cost and complexity of the experimental setup relative to laser-sheet-sweeping 3D imaging systems. The setup is straightforward to implement if already performing conventional PLIF (planar laser-induced fluorescence) imaging, requiring a slight modification to the sheet-forming optics and the addition of a second imaging camera (or stereoscopic viewing adapter for a single camera). The main drawbacks of this single-camera, thick-laser-sheet technique are that extracting quantitative three-dimensional spatial information from the images — while possible — is difficult, and that seeding the flow with nitric oxide (a toxic gas) is required in facilities where it is not already naturally present.