Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

David W. Murphy, Ph.D.

Committee Member

Rasim Guldiken, Ph.D.

Committee Member

Andres Tejada-Martinez, Ph.D.

Committee Member

Bradford J. Gemmell, Ph.D.

Committee Member

Amy E. Maas, Ph.D.

Keywords

Clap-and-Fling, Flapping Wing, Heteropod, Pteropod, Soft Robot

Abstract

Pteropods (also known as sea butterflies or sea angels), are holoplanktonic marine snails which swim by flapping a pair of extremely flexible wings. The wings are modified from the molluscan foot and the wing motions are supported by the fluid pressure without any rigid support. Sea angels (gymnosome pteropods) are completely naked; in contrast, sea butterflies (thecosome pteropods) have negatively buoyant aragonite shells which vary in geometry and size among different species. Pteropods are seasonally abundant in the ocean, and an important food source for the other zooplanktons, fishes, and whales. Though studies have been conducted regarding their biology, ecology, and geography, only very limited studies have been conducted on their swimming behavior. Nevertheless, swimming is a critical behavior to sustain their holoplanktonic lifestyle in the water column both for prey capture and to escape from predators. In addition, pteropods migrate close to water surface at night to prey on the phytoplankton and sink to deeper region to avoid visual predators. Thus, pteropods must use their energy efficiently to swim since they migrate a long distance of tens of thousands of body lengths for this diel vertical migration.

First, we focused on the 3D swimming kinematics and trajectories of a diverse group of subtropical marine snails. While different large scale swimming patterns were observed, all species exhibited small scale sawtooth swimming trajectories caused by reciprocal appendage flapping. The coiled shell species had the highest normalized swimming and sinking speeds, reaching swimming speeds of up to 45 body lengths s-1. The sinking trajectories of the coiled and elongated shell pteropods were nearly vertical, but globular shell pteropods use their hydrofoil-like shell to glide downwards at approximately 20° from the vertical, thus retarding their sinking rate. The swimming Reynolds number (Re) increased from the coiled shell species (Re ~ O(10)) to the elongated shell species (Re ~ O(100)) and again for the globular shell species (Re ~ O(1000)), suggesting that more recent lineages increased in size and altered shell morphology to access greater lift-to-drag ratios available at higher Re.

In the second part, a novel cylindrical overlap-and-fling mechanism used by the elongated shelled pteropods was described quantitatively and qualitatively by using high-speed stereophotogrammetry and micro-particle image velocimetry systems. The clap-and-fling mechanism is a well-studied, unsteady lift generation mechanism widely used by flying insects and is considered obligatory for tiny insects flying at low to intermediate Re. However, some aquatic zooplankters including some pteropod and heteropod species swimming at low to intermediate Re also use the clap-and-fling mechanism or its variants. These marine snails have extremely flexible, actively deformed, muscular wings which they flap reciprocally to create propulsive force, and these wings may enable novel lift generation mechanisms not available to insects, which have less flexible, passively deformed wings. In the overlap-and-fling maneuver, the pteropod’s wingtips overlap at the end of each half-stroke to sequentially form a downward-opening cone, a cylinder, and an upward-opening cone. The transition from downward-opening cone to cylinder produces a downward-directed jet at the trailing edges. Similarly, the transition from cylinder to upward-opening cone produces downward flow into the gap between the wings, a leading edge vortex ring, and a corresponding sharp increase in swimming speed. The ability of this pteropod species to perform the cylindrical overlap-and-fling maneuver twice during each stroke is enabled by its slender body and highly flexible wings. The cylindrical overlap-and-fling mechanism observed here may inspire the design of new soft robotic aquatic vehicles incorporating highly flexible propulsors to take advantage of this novel lift generation technique.

Finally, the swimming of an atlantiid heteropod with a shell and single swimming fin was studied and we show that the heteropod actively flaps both the swimming fin and shell in a highly coordinated wing-like manner in order to swim in the intermediate Reynolds number regime (Re=10-100). The fin and shell kinematics indicate that atlantiid heteropods use unsteady hydrodynamic mechanisms such as the clap and fling and delayed stall. Unique features of atlantid heteropod swimming include the coordinated pairing of dissimilar appendages, use of the clap and fling mechanism twice during each stroke cycle, and the fin’s extremely large stroke amplitude which exceeds 180º.

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