Improved Flow Visualization Methods Based on the Background Oriented Schlieren Technique

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This paper propose two new flow visualization methods based on the Background-Oriented Schlieren (BOS) technique for compressible flow. One is “Simplified background-oriented schlieren (S-BOS) ”. This technique does not require a cross-correlation algorithm, which is typically used in BOS. The data processing is much simpler than that of the original BOS, since image displacement associated with density gradient is algebraically calculated from intensities of the images with the periodic background pattern. Moreover, it easily allows us to automate the data processing, because it is neither necessary to remove incorrect vectors nor to optimize parameters such as the interrogation window and search window. The other is referred to “Wavelet-based background-oriented schlieren (W-BOS)”. This technique provides a schlieren image using continuous wavelet transformation for the periodic background pattern. By transforming the periodic intensity pattern into the phase, a schlieren image can be obtained easily. Since the optical setup in both techniques is simpler than that of a conventional schlieren imaging, they could be used in various situations including field tests. A wind tunnel test was conducted in a 1 m × 1 m supersonic wind tunnel. Their usefulness was demonstrated by comparing with the conventional schlieren images.