On the challenges of precise velocity measurements in transparent pipes using robotic volumetric LPT with Shake-the-Box

In this study, a comparison between measurements in a transparent pipe with and without arbitrary optical distortions using robotic volumetric Lagrangian Particle Tracking (LPT) is presented. Here, the coaxial volumetric velocimeter (CVV) in combination with a robotic arm enables the measurement from the outside (air and acrylic wall) and the inside (only air) of the tube at the end of the pipe section. Experimental results from the pipe flow using helium-filled soap bubbles (HFSB) as tracers and Shake-the-Box (StB) algorithm for the three-dimensional LPT data analyses are presented. A calibration plate with a high spatial resolution was used at every position of the independent views (sub-volumes) for the measurements with arbitrary optical distortions. The evaluations shows that a complete calibration set, consisting of a geometrical calibration and a volume self-calibration, for every sub-volume is mandatory. For comparison, the total number of recorded double frames is varied in order to study the effect on the velocity fields. In this case, 1000 double frames are a good compromise between accuracy and storage space. Moreover, only half of the particle tracks are present in the case with optical distortions (acrylic wall).