Simultaneous 3D PTV and infrared tomographic PIV measurement of zooplankton distribution in unsteady flow fields

Copepods and brine shrimp, which represent different species of zooplankton with similar inertial time constant, can behave very differently when subjected to velocity gradients in unsteady or turbulent flows. Upon sensing hydrodynamic disturbances, copepods can respond with rapid acceleration and high speed. Brine shrimp, on the other hand are more passive and capable of only small relative swimming speeds. A goal of the current study is to understand copepod response thresholds and their effect on copepod number distribution in complex flows. We describe a simultaneous 3D PTV and infrared tomographic PIV measurement system as a means to obtain volumetric flow fields and three-dimensional tracks of zooplankton within a volume. A tomographic PIV measurement volume was illuminated with an Oxford Firefly infrared laser (wavelength: 808 nm) and viewed by four high-speed cameras (1280 × 800 pixels) fitted with infrared pass optical filters. The infrared pass filters are needed to allow only the infrared illumination scattered from the seed particles to pass through the lens. The 3D PTV system included two white LED lamps placed in a dark-field illumination configuration, and two additional high-speed cameras (1280 × 800 pixels) fitted with infrared-blocking optical filters. The LED lamps and infrared-blocking filters were effective at illuminating the zooplankton but not the tracer particles. Experiments were carried out in a re-circulating seawater channel driven by a paddlewheel. The paddlewheel ensures that zooplanktons in the seawater are not damaged. We present results of brine shrimp and copepod distributions ahead of and behind a cylinder in crossflow with Re = 930. The number distributions are compared with distributions of principal strain rate of the flow at the position of each zooplankton, and the results are assessed. While brine shrimp exhibit uniform distributions across the cylinder wake, the distribution of copepods shows a deficit directly behind the cylinder. This difference in distribution is attributed not to local strain in the wake, but instead to escape responses of copepods upstream of the cylinder due to sudden decelerations.