Abstract Particle Image Velocimetry (PIV) measurement accuracy is lower along the phase boundaries of two-phase-flows, because the interrogation windows contain information from both phases. Different seeding density, background intensity, velocity magnitude and flow direction conditions often exist across the boundary, and the cross-correlation-based PIV algorithm selects only the highest correlation peak. The highest correlation peak is either influenced by the wrong phase (across the boundary), or the correctly calculated displacement is erroneously detected as an outlier at a later stage and is subsequently replaced. Phase-separated PIV measurements minimize this problem, and increase accuracy along the boundary by treating each phase separately. This type of measurement requires for each time step; (i) the accurate detection of the phase boundary in consecutive frames, (ii) generation of dynamic phase masks, (iii) an accurate PIV evaluation of each phase and (iv) recombination of the flow fields. In this article, we focus on the first step and test a hybrid phase boundary detection (PBD) technique in three different two-phase-flow configurations which manifest different challenges: The first configuration is the mixing of two liquids in a magnetic micromixer, the second is a combustion experiment where a turbulent, pre-mixed, low-swirl, lifted flame is investigated, and the third is a bubble column reactor where air bubbles are rising in a water tank. The PBD implementation uses a three-step procedure: approximate global thresholding, local Otsu thresholding, and discrimination of image gradients. Comparison of results with and without the use of PBD and phase separation indicate that there are significant measurement accuracy improvements along the boundary.

中文翻译：

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一种用于两相流 PIV 测量的混合相边界检测技术

摘要 粒子图像测速 (PIV) 测量精度沿两相流的相边界较低，因为询问窗口包含来自两相的信息。跨越边界经常存在不同的播种密度、背景强度、速度幅度和流向条件，基于互相关的PIV算法只选择最高的相关峰。最高相关峰值要么受到错误相位（跨边界）的影响，要么在后期将正确计算的位移错误地检测为异常值并随后被替换。相分离 PIV 测量最大限度地减少了这个问题，并通过分别处理每个相来提高边界的准确性。这种类型的测量需要每个时间步；(i) 连续帧中相位边界的准确检测，(ii) 动态相位掩模的生成，(iii) 每个相位的准确 PIV 评估和 (iv) 流场的重组。在本文中，我们专注于第一步，并在三种不同的两相流配置中测试混合相边界检测 (PBD) 技术，这些配置表现出不同的挑战：第一种配置是在磁性微混合器中混合两种液体，第二个是燃烧实验，其中研究了湍流、预混合、低涡流、离火的火焰，第三个是气泡塔反应器，其中气泡在水箱中上升。PBD 实现使用三步程序：近似全局阈值、局部 Otsu 阈值和图像梯度判别。