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Pulsatile jet propulsion is a highly energy-efficient swimming mode used by various species of aquatic animals that continues to inspire engineers of underwater vehicles. Here, we present a bio-inspired jet propulsor that combines the flexible hull of a jellyfish with the compression motion of a scallop to create individual vortex rings for thrust generation. Similar to the biological jetters, our propulsor generates a nonlinear time-varying exit velocity profile and has a finite volume capacity. The formation process of the vortices generated by this jet profile is analysed using time-resolved velocity field measurements. The transient development of the vortex properties is characterised based on the evolution of ridges in the finite-time Lyapunov exponent field and on local extrema in the pressure field derived from the velocity data. Special attention is directed toward the vortex merging observed in the trailing shear layer. During vortex merging, the Lagrangian vortex boundaries first contract in the streamwise direction before expanding in the normal direction to keep the non-dimensional energy at its minimum value, in agreement with the Kelvin–Benjamin variational principle. The circulation, diameter and translational velocity of the vortex increase due to merging. The vortex merging takes place because the velocity of the trailing vortex is higher than the velocity of the main vortex ring prior to merging. The comparison of the temporal evolution of the Lagrangian vortex boundaries and the pressure-based vortex delimiters confirms that features in the pressure field serve as accurate and robust observables for the vortex formation process.
In this chapter, I focus on three different topics to illustrate the complicated and often surprising ways in which Aristotle’s investigations of animals and of the heavens are related to one another. After an introductory discussion of how Aristotle differentiates these different scientific investigations of nature from one another, this chapter looks at (1) the dependence of Aristotle’s account of cosmic directionality (in De caelo II.4) on his discussion of directional concepts in his account of animal locomotion (De incessu animalium 1-6); (2) the relationship between his account of gestation periods in De generatione animalium IV.10 and his understanding of the complex relationship between the solar and lunar cycles; and (3) his teleological explanation of why there are distinct male and female contributors to animal generation (and why animals generate at all) at the beginning of De generatione animalium II as it relates to his discussion of the cyclical nature of all generation that closes Generatione et Corruptione II.11.
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