Treatment of light reflections in 3D PIV systems

Abstract

Measuring the velocity field around a complex object by volumetric PIV is hindered by shadow formation (illumination) as well as camera occlusion (imaging). These have been recently dealt with by multiplying illumination and imaging directions (redundancy) and by the integration of ray-tracing techniques to include the effect of visual blockage caused by the object (object-aware particle reconstruction, Wieneke & Rockstroh, 2024). The problem of light reflections blinding regions of the images has not been afforded yet. The latter pertains to interactions between illumination and imaging through the object surface and it poses additional challenges to ghost particle formation, particle detection and tracking in general. This study proposes a method to effectively detect such regions, and measures to modify the particle triangulation algorithm.
The viability of this novel reflection-aware Lagragian particle tracking (RA-LPT) approach is examined by application to two experiments of varying complexity. The first case is the flow around a stationary wall-mounted cube as imaged with a redundant number of cameras. The second experiment tackles an elite runner sprinting across the measurement region obtained with the Ring-of-Fire technique. A considerable reduction of ghost particles (false positives) is attained, while the formation of voids (false negatives) is also minimized. The overall result of the method maximizes the measurement region around and in proximity of the object of interest.