Plenoptic Self-Aligned Focusing Schlieren at the Probe Calibration Tunnel

A self-aligned focusing schlieren (SAFS) system utilizing a plenoptic light-field camera was used to capture images at NASA Langley Research Center's Probe Calibration Tunnel to qualitatively evaluate the behavior of the nozzle shear layer at various tunnel stagnation pressures. Utilizing a plenoptic camera in the SAFS system allowed for a single image of the flow to be acquired, which was then computationally refocused to different planes in the flow during post-processing. This single image thus contained three-dimensional spatial information which could be used to assess the flow state of the entire nozzle and model at a single instant in time. These images revealed the transitioning state of the Mach 3.45 nozzle, which remained laminar up to a stagnation pressure of 20 psia, showed strong Gortler vortices at a stagnation pressure of 25 psia, and comprised varying levels of turbulence up to a fully turbulent nozzle shear layer between stagnation pressures of 30 psia and 70 psia. Comparisons of the plenoptic SAFS system with both conventional path-averaged schlieren and non-plenoptic SAFS systems for the same flow conditions demonstrated some of the advantages and compromises to consider when determining which flow imaging technique to use.