Large-scale particle shadow tracking and orientation measurement with collimated light

Lagrangian particle tracking experiments are a key tool to understanding particle transport in fluid flows. However, tracking particles over long distances is expensive and limited by both the intensity of light and number of cameras. In order to increase the length of measured particle trajectories in a large fluid volume with minimal cost, we developed a large-scale particle-shadow-tracking method. This technique is able to accurately track milimeter-scale particles and their orientations in meter-scale laboratory fluid flows. By tracking the particles' shadows cast by a wide beam of collimated light from a high-power LED, 2D particle position and velocity can be obtained, as well as their 3D orientation. Compared with traditional volumetric particle tracking techniques, this method is able to measure particle kinematics over a larger area using much simpler imaging and tracking techniques. We demonstrate the method on sphere, disk, and rod particles in a wavy wind-driven flow, where we successfully track particles and reconstruct their orientations.