A high-speed tomographic PIV system for measuring zooplanktonic flow

Quantification of the hydrodynamic disturbance of free-swimming zooplankton provides insight into propulsion as well as organism sensory interactions. Current flow measurement techniques, such as planar particle image velocimetry (PIV), are limited by their two-dimensional nature. These techniques also are challenged by the small spatial scale of zooplankton, by the high speeds achieved by many zooplankton species, and by zooplankton photosensitivity to a broad range of wavelengths. We present a high-speed tomographic PIV system using near-infrared laser illumination that is capable of measuring three-dimensional velocity vectors in a volume surrounding a plankter. This technique is assessed by recording and analyzing the time-resolved flow field created by a high-speed escape of a copepod (Calanus finmarchicus). Persistent body and wake vortices are created by the impulsive momentum transfer to the fluid surrounding the animal. It is shown that some aspects of this flow can be analytically modeled as an impulsive stresslet. Azimuthal asymmetry of the strength and position of the wake vortex is analyzed and attributed to the strong ventral flows created by the metachronally beating swimming legs. In addition, the energy required by a copepod escape jump is estimated by calculating the viscous energy dissipation rate using the spatial gradients of the measured three-dimensional velocity field. Finally, the challenges and benefits of the tomographic PIV technique are discussed.