Lagrangian measurement of fluid and particle motion using a field‐deployable Volumetric Particle Imager (VoPI)

We describe the development of a field-deployable Volumetric Particle Imager (VoPI) and the methods that enable it to measure three-dimensional particle tracks in situ. The VoPI has the unique ability to obtain three-component particle velocity records in a volume. The device has a slender, single-camera design ideal for optically accessing study sites in the environment, including hard-to-reach places like the inside of coral reefs and marshes. Unlike other underwater measurement techniques, the VoPI implements a defocused imaging method that relies on ambient light in place of laser light for particle illumination. We describe the construction, calibration and validation of this device. We also demonstrate the VoPI's abilities by measuring velocity statistics of passive tracer particles dispersed in a high-Reynolds-number turbulent flow. By measuring velocity variances and integral timescales from these Lagrangian velocity records, we can compute the turbulent diffusivity directly from the VoPI measurements.