What question did this study set out to answer?

This research aims to explore the effectiveness of using optical flow in free-background BOS for 3D density field measurement in underexpanded jets.

April 27, 2026

Application of Free Backgrounds to 3D Density Field Measurement of Under-expanded Jets using Optical Flow based BOS method

Key Points

This research aims to explore the effectiveness of using optical flow in free-background BOS for 3D density field measurement in underexpanded jets.
Applied optical flow to free-background BOS for 3D density field reconstruction.
Utilized brick and concrete block backgrounds to analyze the underexpanded jet.
Compared noise levels affecting the density measurement based on background choice.
Both brick and concrete block backgrounds effectively captured the jet's density structures.
High-frequency noise was observed, with increased severity using the brick background.
Noise levels varied depending on optical flow averaging scale.

Abstract

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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Cite This Study

Sakuma et al. (Wed,) studied this question.

synapsesocial.com/papers/69eefc6dfede9185760d37fc https://doi.org/https://doi.org/10.1299/jsmeyamanashi.2025.d13

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