Visualization of the trapping of inertial particles in a laminar mixing tank

We report on new methods and results for the trajectory visualization and tracking of large inertial particles in a laminar (chaotic) mixing tank. Using specially designed light-emitting capsules, particle orbits were visualized using streak photography to reveal pathlines of capsules moving in the laminar fluid flow. We present visualization results on unexpected trapping of particles whereby finite-sized particles/capsules can spontaneously localize into the vortex tubes in the laminar stirred system. Once into tubes, large inertial particles move in helical orbits as do passive particles. Mixing tanks have been in use over a number of centuries but this trapping phenomenon has been consistently ignored. We also measure the three-dimensional position of large inertial particles over time using an externally positioned camera and planar mirror in a calibrated setup. As the light capsules used in this study give very high contrast even when the fluid is murky, the work provides a sound basis for further research development of measurement techniques for large (industrial) scale flows.