Applications of PIV and Holography to Characterize Flow and Particle Distributions in the Ocean

This presentation discusses in-situ applications of particle image velocimetry (PIV) and digital holography to characterize the flow and particle distributions in oceanic flows. The PIV measurements focused on the mean flow and turbulence in the inner part of the coastal ocean bottom boundary layer. The stationary submersible system recorded 2D velocity distributions in two 30x30 cm planes, the first aligned with the tidal current, and the second with the dominant wave direction. The orientation of the illuminated planes were controlled remotely and re-adjusted to maintain alignment with the current and waves. Data were transmitted via optical fibers to a ship-board acquisition system. In recent field tests, the system was deployed off New Jersey at a depth of 25 m. Data acquired at 6 Hz (12 frames/s) covered varying velocity, bottom roughness, as well as relative wave-current orientations and magnitude. Sets spanning entire tidal cycles were recorded in multiple 35-60 min subsets, each providing 12,600-21600 velocity distributions. Co-deployed ADV facilitated calculations of Reynolds stresses by filtering out the contribution of wave-induced motion, and a pencil-beam sonar was used to characterize the seabed topography. Sample profiles of mean flow and turbulence parameters will focus on the interaction of currents and waves with the bottom roughness, and will show, e.g., instabilities at the interface between wave and current boundary layers, signature of bottom roughness in the turbulent energy spectra, and variations in velocity profiles across bottom ripples. The particle distributions were measured using a free-drifting, submersible, digital holography system, which maintained a low relative velocity (<1 cm/s) with the local fluid. Consequently, images of the same particles could be observed in multiple frames, enabling us to examine their behavior, as well as directly measure the local profiles of shear strain and dissipation rates form the velocity gradients. Digital holograms were recorded simultaneously at 14 Hz, at two different magnifications, and data were transmitted via an optical fiber to a disk array located on the research vessel. Sample results from several deployments demonstrating the resolution of digital holography, and the advantage in using free drifting systems will be presented. For one of these deployments, statistical analysis of tens of thousands of holograms, recorded while the system was ascending slowly through the water column, provided size, orientation and spatial distributions of particles, characterized based on their shape and size. The results demonstrated formation of multiple layers with high concentration of small particles in regions of low shear and dissipation rates. The presentation will conclude with sample laboratory applications of digital holographic microscopy, some recorded at high speed, to characterize interactions among marine organisms located in three-dimensional dense suspensions. Samples will include e.g. studies of swimming behavior and predator-prey interactions involving toxic and non-toxic marine dinofalgellates, their cryptophye prey as well as copepods. Sponsored by NSF and ONR. Numerous collaborators contributed to this research.