Hydrodynamics of Vortex Generation during Bell Contraction by the Hydromedusa Eutonina indicans (Romanes, 1876)

Swimming bell kinematics and hydrodynamic wake structures were documented during multiple pulsation cycles of a Eutonina indicans (Romanes, 1876) medusa swimming in a predominantly linear path. Bell contractions produced pairs of vortex rings with opposite rotational sense. Analyses of the momentum flux in these wake structures demonstrated that vortex dynamics related directly to variations in the medusa swimming speed. Furthermore, a bulk of the momentum flux in the wake was concentrated spatially at the interfaces between oppositely rotating vortices rings. Similar thrust-producing wake structures have been described in models of fish swimming, which posit vortex rings as vehicles for energy transport from locations of body bending to regions where interacting pairs of opposite-sign vortex rings accelerate the flow into linear propulsive jets. These findings support efforts toward soft robotic biomimetic propulsion.

Experimental Investigation of Fish Farm Hydrodynamic Wake Properties on 1:15 Scale Model Circular Aquaculture Cages

Hydrodynamic experiments on 1:15 scale model arrays of circular fish cages, typically used in eastern Canada, have been completed in the recirculating flume tank located at the Fisheries and Marine Institute of Memorial University in St. Johns, Newfoundland. Scale model cages were designed with a high amount of detail from 100 m circumference cages used in industry. Two different cage spacings were tested, representing spacing of cages typically found at cage sites. A global force ratio scaling technique was developed and applied to the experiment to ensure geometric similarity between cages of model scale and full scale. Planes of 64 (8×8) wake velocity measurements at both cage spacings were taken behind individual cages within the array and at distances in the wake of the entire array, to observe velocity deficits, wake topology, wake recovery and unsteadiness in the flow field. Results show high velocity deficits behind the cages, causing accelerations in the flow underneath and around the sides of the cages. High amounts of unsteadiness is found to be generated at the bottom of the cages due to the presence of a shear layer in the wake of the cages. Dye release was also used to observe many features of the flow field at one time, and to verify results obtained from wake velocity measurements.

First in situ observations of the deep-sea squid Grimalditeuthis bonplandi reveal unique use of tentacles

The deep-sea squid Grimalditeuthis bonplandi has tentacles unique among known squids. The elastic stalk is extremely thin and fragile, whereas the clubs bear no suckers, hooks or photophores. It is unknown whether and how these tentacles are used in prey capture and handling. We present, to our knowledge, the first in situ observations of this species obtained by remotely operated vehicles (ROVs) in the Atlantic and North Pacific. Unexpectedly, G. bonplandi is unable to rapidly extend and retract the tentacle stalk as do other squids, but instead manoeuvres the tentacles by undulation and flapping of the clubs’ trabecular protective membranes. These tentacle club movements superficially resemble the movements of small marine organisms and suggest the possibility that G. bonplandi uses aggressive mimicry by the tentacle clubs to lure prey, which we find to consist of crustaceans and cephalopods. In the darkness of the meso- and bathypelagic zones the flapping and undulatory movements of the tentacle may: (i) stimulate bioluminescence in the surrounding water, (ii) create low-frequency vibrations and/or (iii) produce a hydrodynamic wake. Potential prey of G. bonplandi may be attracted to one or more of these as signals. This singular use of the tentacle adds to the diverse foraging and feeding strategies known in deep-sea cephalopods.

Hydrodynamic wake resonance as an underlying principle of efficient unsteady propulsion

A linear spatial stability analysis is performed on the velocity profiles measured in the wake of an actively flexible robotic elliptical fin to find the frequency of maximum spatial growth, that is, the hydrodynamic resonant frequency of the time-averaged jet. It is found that: (i) optima in propulsive efficiency occur when the driving frequency of a flapping fin matches the resonant frequency of the jet profile; (ii) there can be multiple wake resonant frequencies and modes corresponding to multiple peaks in efficiency; and (iii) some wake structures transition from one pattern to another when the wake instability mode transitions. A theoretical framework, termed wake resonance theory, is developed and utilized to explain the mechanics and energetics of unsteady self-propulsion. Experimental data are used to validate the theory. The analysis, although one-dimensional, captures the performance exhibited by a three-dimensional propulsor, showing the robustness and broad applicability of the technique.

Biomimetic Flow Sensing Using Artificial Lateral Lines

By mimicking fish lateral lines and fish sensing behaviors, we constructed an artificial lateral line and employed it for biomimetic flow sensing. Thirteen hot-film anemometers were mounted to the surface of a NACA0015 airfoil to constitute the artificial lateral line. The resulting fish-like platform was maneuvered by a 3-degree-of-freedom robotic arm for two-dimensional mobility in aquatic environments. Assisted with specially developed algorithms, two biologically relevant flow-sensing scenarios have been realized. They were tracking a nearby dipole source in still water by detecting the dipole flow field and tracking a distant stationary source in running water by detecting its hydrodynamic wake.