10TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY – PIV13 Delft, The Netherlands, July 2-4, 2013 Simultaneous 3D PTV and infrared tomographic PIV measurement of zooplankton distribution in unsteady flow fields Deepak Adhikari 1 , Michael Hallberg 1 and Ellen Longmire 1 1 Department of Aerospace Engineering and Mechanics, University of Minnesota, Minnesota, USA adhi0018@umn.edu ABSTRACT 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. 1. INTRODUCTION Zooplankton generally drift with large eddies (~kilometers) in the ocean, but they may move independently of smaller eddies (~millimeters to centimeters). For instance, copepods, with swimming speeds substantially higher than small- scale fluctuation velocities, can exhibit motion independent of the surrounding flow [1]. Generally, copepods can detect sudden flow perturbations that trigger them to swim at speeds of up to 0.5 m/s. By contrast, brine shrimp, which are capable of only low swimming speeds (~3 mm/s), effectively move passively with the local fluid velocity [2]. This suggests that distributions of different zooplankton species may vary depending on the flow conditions and scales. Modeling such distributions requires understanding unique locomotion and sensory attributes of the individual species. Copepods use their setae to sense flow disturbances and elicit high speed escape responses [3,4]. This flow disturbance refers mainly to local fluid velocity gradients, since the gradients promote differences in velocity near individual setae. Kiørboe et al (1999) [5] subdivided velocity gradients into vorticity, normal strain, shear strain, and acceleration and subjected copepods to different flows exhibiting these conditions. Based on a simplified model, they determined a threshold for either normal or shear strain of 0.4 s -1 as being the minimum required for copepod escape response. In a later work, particle image velocimetry (PIV) was used in shear dominated flow, and the results showed that copepods could respond to shear rates as low as 0.025s -1 [6]. More recently, high resolution PIV and volumetric PIV have been used to understand the propulsion of copepods [7, 8, 9]. Previous work has shown that interaction between turbulent fluid and zooplankton motions can be highly coupled, complex and three-dimensional. For better insight into their interaction, a measurement system needs to be capable of measuring both flow velocity and zooplankton motion in three-dimensions and at high repetition rate. The purpose of the present work is to demonstrate a novel volumetric velocimetry and tracking system capable of quantifying zooplankton and turbulent fluid motion simultaneously and instantaneously. In this paper, we study the behavior and distribution of the two zooplankton types within the wake of a vertically mounted cylinder.