A SHADOW PTV TECHNIQUE FOR PARTICLE TRACKING IN AN INHOMOGENEOUS TURBULENT FLOW

International audienceWe use a Shadow Particle Tracking Velocimetry technique (S-PTV) with collimated light to investigate the dynamics of a turbulent von Kármán (VK) flow of water. Such a PTV technique, easy to calibrate as apparent particle position on the images do not depend on the camera-particle distance, allows to track small objects (100 µm particles, fibers, ...) with low power LEDs as light sources. Investigating the dynamics of Lagrangian tracers, we observe the flow produced in a square tank is bistable, the two states having a life time of several minutes so that the high speed recordings appear as snapshots of one of the two states that can be reconstructed from the particle trajectories. We show the two states topologies strongly differ from the one of the mean flow, and how the bistability affects the dispersion of tracers