[en] Air flow fields have nonuniform velocity distributions because of the compressibility of air. Current optical flow-based particle image velocimetry (PIV) methods cannot thoroughly solve these distributions because of the global optical flow formulation. In this study, we propose a novel field-segmentation-based variational optical flow (FS-VOF) method for preserving the spatially discontinuous characteristics of nonuniform flow fields. The proposed method is designed based on the level-set segmentation method, in which the particle image is segmented according to the velocity distribution of the fluid flow. Subsequently, a segmented smoothness constraint term built from the assumption that velocity varies continuously in each segmented particle image region and the discontinuous structures of the velocity field are preserved at the boundaries of the segmented regions. In addition, the data term of the proposed method is based on the segmented regions and derived from the physics-based optical flow equation. The proposed method is evaluated over synthetic and experimental images, and the comparison with advanced variational optical flow algorithms demonstrates the effectiveness of our method in preserving the spatially discontinuous characteristics of nonuniform flow fields. Graphic abstract:

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