Date

8-1-2019

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Keith W. Moored

Abstract

Numerical simulations are conducted to better understand the underlying physics of intermittent swimming. The swimmer is assumed to be effectively splitted into drag producing (the body) and thrust producing (the propulsor) parts and the body is modeled by a surrogate drag law whereas the propulsor is modeled by a hydrofoil. It is shown that the advantage of intermittent swimming over continuous swimming is kinematics dependent. Pitch dominated large amplitude intermittent swimming is shown to save as much as 60% energy over continuous swimming through inviscid mechanisms. Adding heave to the motion reduces the advantage of intermittent swimming, however, the overall efficiency of the swimmers peak with heave dominated continuous swimming. The trends observed in two-dimensional intermittent swimming problem holds for three-dimensional intermittent swimmers. Furthermore, increasing the aspect ratio increases the energy savings from intermittent swimming. The time averaged velocity field of an intermittent swimmer is not symmetric due to the shedding of unequal strength vortices over one cycle of oscillation. It is observed that leading-edge vortex formation and shedding in a viscous flow significantly alter the wake dynamics from the inviscid flow solutions where only trailing edge shedding is modeled. Despite the flow field differences, both inviscid and viscous simulations show similar trends in the force production and predict similar ranges of energy savings. Finally, to understand the effect of non-uniform flexibility on the intermittent swimming performance, a fluid-structure model is employed where the flexibility is modeled with a torsional spring placed along the chord. The leading edge is actively controlled in a pure pitching motion and the trailing edge follows with passive pitching. Flexibility is observed to increase the efficiency of an intermittent swimmer compared to a rigid counterpart. The time averaged thrust force over the coast phase is positive, thus intermittent swimming increases the overall thrust coefficient by releasing stored elastic energy without inputting power into the leading edge motion.

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