Lagrangian particle tracking in three dimensions via single-camera in-line digital holography

Abstract

Lagrangian particle trajectories are measured in three spatial dimensions with a single camera using the method of digital in-line holography. Lagrangian trajectories of 60–120 μm diameter droplets in turbulent air obtained with data from one camera compare favorably with tracks obtained from a simultaneous dual-camera data set, the latter having high spatial resolution in all three dimensions. Using the single-camera system, particle motion along the optical axis is successfully tracked, allowing for long, continuous 3D tracks, but the depth resolution based on standard reconstruction methods is not sufficient to obtain accurate acceleration measurements for that component. Lagrangian velocity distributions for all three spatial components agree within reasonable sampling uncertainties and Lagrangian acceleration distributions agree for the two lateral components. An equivalent single-camera, imaging-based 2D tracking system would be challenged by the particle densities tested, but the holographic configuration allows for 3D tracking in the dilute limit. The method also allows particle size, shape and orientation to be measured along the trajectory. Lagrangian measurements of particle size provide a direct measure of particle size uncertainty under realistic conditions sampled from the entire measurement volume.