Assessment of three-dimensional turbulent density measurements from tomographic background-oriented schlieren (BOS)

The accuracy of three tomographic BOS reconstruction methods are assessed by generating synthetic BOS displacements from the instantaneous density field of a heated jet at Reynolds number Re = 5000 and inlet temperature 75K above ambient computed via direct numerical simulation. Synthetic displacements are produced by ray tracing through the jet’s refractive index field, obtained using the Gladstone-Dale relation. The displacements form the basis of tomographic reconstruction of the refractive index gradient fields. Reconstruction is performed using filtered back-projection (FBP), iterative algebraic reconstruction technique (ART) and a sequential FBP-ART method. Eighteen views around the flow are used, which mimics a typical experimental setup. FBP features strong reconstruction artefacts even with 18 simultaneous independent views. The reconstruction performs well in the laminar jet core, but accuracy decreases as the flow transitions to turbulence. The sequential FBP-ART uses FBP as the first guess to ART iterations. Unlike previous investigations, improvements were found to marginal, and could degrade overall accuracy in the turbulent region where FBP gives strong artefacts. Using a pre-filter for FBP before ART accounting for the jet spreading downstream is expected to produce a more favourable comparison.