## Dissertations - ALL

Summer 8-27-2021

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Mechanical and Aerospace Engineering

#### Advisor(s)

Green, Melissa A.

#### Keywords

Bio-Inspired Propulsion, Experimental Fluid Mechanics, Fish Swimming, Vortex Dynamics, Wake Dynamics, Wakes

#### Subject Categories

Aerodynamics and Fluid Mechanics | Aerospace Engineering | Engineering

#### Abstract

Recently, some engineers have turned to the study of swimming and flying animals to provide insight into the design of vehicles that move through air or water. For certain animals swimming in water, thrust is generated primarily by either a caudal fin or fluke located at the posterior end of the swimmer. It has been argued extensively in the existing research literature that biological propulsors of high aspect ration that also display forked, or lunate, planforms are representative of optimized propulsor geometries with respect to propulsive performance. However, recent experimental and computational work on purely pitching panels has demonstrated that such planform shapes are not representative of propulsive optimality. Prior to the results of this dissertation, these counter-intuitive findings have primarily been observed for cases of rigid rectangular panels with varying trailing edge shape oscillating in pure pitch.

In the current work, bio-inspired propulsion is examined through an investigation of multiple rigid pitching panels of trapezoidal planforms with trailing edge shapes that were varied in a systematic manner at moderate Reynolds number. The wakes behaviors and wake dynamics of these panels are explored in a time-varying and time-averaged sense using results obtained from multiple planes of stereoscopic particle image velocimetry. The time-averaged and time-varying performance characteristics of all the bio-inspired panels are examined through time-resolved force and torque measurements. Experiments were designed to serve as rudimentary approximations of animal swimmers. Dimensional planform shape and non-dimensional aspect ratio were chosen to mimic the observed features of different animals, including those considered high and low performance swimmers. The Strouhal number range used in this dissertation, $0.09 \leq St \leq 0.66$, overlaps with and extends beyond the Strouhal number range used by many swimming and flying animals.

Performance results show that purely pitching trapezoidal panels with modifications in their trailing edge shape perform in a manner that could be considered counter-intuitive in light of observations of nature. Trapezoidal panels with pointed trailing edges, which are of relatively low aspect ratio and resemble the shape of propulsors used by certain low performance animal swimmers, tend to pitch with larger propulsive efficiencies and coefficient of thrust than planform shapes associated with high performance swimming. Furthermore, more pointed panels generate greater lateral forces, which can be useful for overall maneuverability.

Wake results indicate that pointed panels develop relatively high propulsive performance, despite their increased production of streamwise vorticity, which is not generally considered useful for propulsion. Wake results also reveal that the wakes of these finite aspect ratio propulsors are complex and highly three-dimensional. Wakes are typically comprised of a series of connected, alternating vortex rings that are comprised of spanwise and streamwise vortices. The influence of these vortex rings are their induced velocity fields are highly dependent on Strouhal number and panel geometry, with increases in Strouhal number and trailing edge convexity resulting in stronger vortices and greater interactions between neighboring vortex structures.

Open Access

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