Masanori Ota

Background-oriented schlieren (BOS) is an optical method for measuring refractive dis- 13 placement without inserting a probe into the flow. Fourier-based methods such as fast 14 checker demodulation (FCD) can be affected by background occlusions and may produce 15 large displacement artifacts around obstructed regions or sharp displacement boundaries. 16 Correlation-based methods are generally more robust to local image disturbances, but 17 their spatial resolution is limited by the interrogation-window size. Here, we propose 18 stripe-decomposition BOS (SD-BOS), which extends GPU-accelerated Simplified BOS (S- 19 BOS) to two-component displacement measurement using a monochrome checkerboard. 20 SD-BOS separates the image into two orthogonal stripe carriers and calculates their phase 21 differences independently. Edge-adaptive blending and confidence-based occlusion han- 22 dling are used to reduce artifacts near sharp gradients and background obstructions. In 23 synthetic tests, SD-BOS reduced vector RMSE by 29.8% for a smooth Gaussian field and 24 21.9% for a shock-like field relative to FCD. For an opaque circular obstruction, the 95th- 25 percentile displacement error outside the detected mask was 86.4% lower than for FCD; 26 vectors within the mask were excluded. In wind-tunnel images, SD-BOS showed lower 27 image-space high-pass fluctuations than FCD, but no independent reference displacement 28 field was available to assess absolute accuracy. The complete SD-BOS pipeline processed 29 512 × 512-pixel images at 115.3 frames per second (fps) on an NVIDIA GeForce RTX 4070 30 SUPER GPU, approximately 23 times the CPU rate measured on an AMD Ryzen 7 5700X. 31 These results indicate improved robustness in the evaluated synthetic occlusion case and 32 highlight the need for independent validation of wind-tunnel measurements.

Three-dimensional density measurement around a Hayabusa re-entry capsule model using the background-oriented schlieren (BOS) technique is carried out in the ballistic range at the Institute of Fluid Science, Tohoku University. A Hayabusa re-entry capsule model with a diameter of 15 mm is used for a projectile, and the Mach number was 1.20. A method of model Position and Pose detection from BOS images was proposed for a capsule model, and unsteady flow field measurement around a free-flight test model was conducted. From the results, we succeeded in measuring the three-dimensional density distribution around a capsule model and visualizing the detailed structure in the wake region.