Implementing particle image velocimetry (PIV) in particle-laden flows is a challenging task due to the optical effects of the added particles. The addition of particles reduces optical access in the measurement region, degrades image quality and increases the experimental uncertainty. One method for reducing this effect is by using refractive index matching (RIM) which matches the index of refraction of the fluid and added particles rendering the particles optically transparent. Exact matching of the refractive index is difficult to achieve in practice. Even slight mismatches in the refractive indices lead to degradation of the image quality which will impact the higher-order statistics. Comparisons of statistics between loading levels are difficult under these conditions. In this work, the effect of processing methodology on RIM PIV images is investigated. RIM is achieved through the use of superabsorbent hydrogel materials in water. Two processing methods are investigated. The baseline method is a basic PIV correlation process without any pre- or post-processing. The refined process utilizes image pre-processing, data post-processing, and experimental error compensation. The goal for the refined method was to optimize and implement a more sophisticated PIV processing routine using existing commercial software on a particle-laden flow to reduce progressive error artifacts. Images from a fully turbulent channel flow with particle loading levels of 0–15% are utilized. Past literature indicates that turbulence levels will be attenuated for the particles used. Turbulence levels calculated using the baseline method show a linear increase with increasing loading, which is a non-physical result. The refined processing method shows a slight attenuation of turbulence which is consistent with expectations. The study highlights the need for careful processing and interpretation of results in cases where changes in image quality can be expected between different experimental conditions.

中文翻译：

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两相指数匹配流的 PIV 处理程序的开发和验证

由于添加粒子的光学效应，在载有粒子的流中实施粒子图像测速 (PIV) 是一项具有挑战性的任务。颗粒的加入减少了测量区域的光学通路，降低了图像质量并增加了实验的不确定性。减少这种影响的一种方法是使用折射率匹配 (RIM)，它匹配流体和添加的粒子的折射率，使粒子光学透明。在实践中很难实现折射率的精确匹配。即使是折射率的轻微不匹配也会导致图像质量下降，这将影响高阶统计数据。在这些条件下，很难对负载水平之间的统计数据进行比较。在这项工作中，研究了处理方法对 RIM PIV 图像的影响。RIM 是通过在水中使用超吸收性水凝胶材料来实现的。研究了两种处理方法。基线方法是一个基本的 PIV 相关过程，没有任何预处理或后处理。细化过程利用图像预处理、数据后处理和实验误差补偿。改进方法的目标是使用现有的商业软件在载有粒子的流上优化和实施更复杂的 PIV 处理程序，以减少渐进式错误伪影。使用了颗粒负载水平为 0-15% 的完全湍流通道流的图像。过去的文献表明，所使用的粒子的湍流水平将被减弱。使用基线方法计算的湍流水平随着载荷的增加呈线性增加，这是一个非物理结果。精细的处理方法显示湍流略有衰减，符合预期。该研究强调，在不同实验条件之间可以预期图像质量发生变化的情况下，需要仔细处理和解释结果。