Study applied optical flow to measure 3D density in underexpanded jets, revealing background effects on noise levels.
Background-Oriented Schlieren (BOS) is a quantitative visualization method that measures background displacement, usually with cross-correlation. Optical Flow (OF) has recently emerged as a promising alternative, yet its application to free-background BOS - removing restrictions on background patterns - remains limited. This study applied OF to free-background BOS and reconstructed the 3D density field of an underexpanded jet using brick and concrete block backgrounds. Both backgrounds captured the jet’s characteristic density structures, but high-frequency noise appeared depending on the OF averaging scale. The noise was markedly stronger with the brick background.
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Sakuma et al. (2025) studied this question.
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