Browsing by Subject "Copepod"

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    Copepod response behavior in turbulence
    (2014-09) Krizan, Daniel
    The objective of this thesis is to determine copepod response to turbulence generated by obstacles in cross flow. Mainly, flow and copepod response downstream a square fractal grid is examined but experiments downstream a cylinder provides comparison. This is done by simultaneously measuring the copepods position and velocity using 3D-PTV in a measurement volume and measuring the two dimensional three component velocity vectors of the flow using stereo PIV. These measurements are done in a way that does not elicit copepod response. Tomographic PIV is done downstream the square fractal grid without copepods to gain volumetric velocity knowledge of the flow in the measurement volume. Copepods are known to execute sudden high speed jumps (or escapes) in response to sensed hydrodynamic signals. The fractal grid was shown to elicit copepod escape, specifically directly downstream with escape frequency decreasing further downstream where turbulence levels were much lower. It was found that at a slower freestream speed copepods exhibited jumps not in reaction to flow disturbances but to reorient themselves (cruise swimming). There was almost no copepod response in the wake of a cylinder, but copepods again exhibited cruise swimming behavior at a slower freestream speed. In regions with high maximum principal strain rate (MPSR) downstream of the fractal grid, copepods were observed to exhibit multiple escapes. Moreover, copepods were observed to jump towards regions of lower turbulence and against the freestream direction. From stereo PIV, instantaneous 2D MPSR values of less than 3s^(-1) were shown to create escape in 60% of copepod escapes analyzed. Finally, it was found that on average larger MPSR resulted in larger jumps from copepods.
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    Volumetric velocity measurement of aquatic predator-prey interactions
    (2013-12) Adhikari, Deepak
    The aim of this study is to develop a novel multi-scale volumetric measurement system and flow facility, and apply it to improve understanding of aquatic predator-prey interactions. A combined infrared tomographic PIV + 3D PTV system was developed and demonstrated. Tomographic PIV was used to obtain the volumetric velocity field of the flow, while the 3D PTV was used to track the prey, and the eye of the predator, in the same volume. A visual hull technique was implemented to mask out the objects (such as fish) appearing within the reconstructed tomographic PIV volumes, ensuring that velocity vectors near the object/fish were not contaminated during PIV cross-correlation. Copepods, which make up the majority of the oceanic zooplanktons, are known to sense flow disturbed by approaching predators and can execute sudden high-speed swim (or jump) to escape predation. Although their response to local flow disturbances has been studied, their sensing and swimming response to live predators (fish) is not well understood. Three series of experiments were implemented - (1): Copepod interactions with a wall-mounted cylinder in cross-flow; (2): Predator-prey interactions in still water; (3): Predator-prey interactions in unsteady/turbulent flow. From the experiments, copepods appeared to respond (jump) to a large and sudden increase in local maximum principal strain rate (MPSR) of the fluid, instead of a fixed threshold quantity. For fish predation in still water, zebrafish were first observed to approach slowly, followed by sudden acceleration (ram feeding) to feed on copepods. Using a potential flow model, it was found that this strategy might not be sufficient to capture copepods successfully. Thus, zebrafish were found to execute suction feeding simultaneously to increase chances of predation success. In uniform cross flow, a coral reef fish (blenny) used a similar slow approach - ram feeding strategy to feed on copepods, but it rarely captured them. However, turbulent cross flow increased the chances of predation by allowing the fish to approach closer to a copepod, and preventing the copepod from detecting hydrodynamic signals from the approaching predator.