The ability of fish to deform their bodies in steady swimming action is gaining from robotic designers. While bound by the same physical laws, fish have evolved to move in ways that often outperform artificial systems in critical measures such as efficiency, agility, and stealth through thousands of years of natural selection. As we expand our presence in the ocean with deep-sea exploration or offshore drilling for petroleum and natural gas, the demand for prolonging underwater operations is growing significantly. Therefore, it is critical for robotic designers to understand the physics of pisciform (fish-like) locomotion and learn how to effectively implement the propulsive mechanisms into their designs to create the next generation of aquatic robots. Aiming to assist this process, this thesis presents an experimental apparatus called Towed Biolocomotion Emulator (TBE), which is capable of imitating the undulating action of different fish species in steady swimming and can be quickly adapted to different configurations with modular modules. Using the TBE device, an experiment is performed to test its hydrodynamic performance and evaluate the effectiveness of the bio-inspired locomotion implemented on such a mechanical system. The analysis of hydrodynamic data collected from the experiment shows that there exists a small range of kinematic parameters where the undulating motion of the device produces the optimal performance. This result confirms the benefits of utilizing pisciform locomotion for small-scale underwater vehicles. In addition, this thesis also proposes a reduced-order flow model using the unsteady vortex lattice method (UVLM) to predict the hydrodynamic performance of such a system. The proposed model is then validated with the experimental data collected earlier. The tool developed can be employed to quickly explore the possible design space early in the conceptual design stage for such a bio-mimetic vehicle. / Master of Science / It is no surprise that through thousands of years of natural evolution, marine species possess incredible ability to navigate through water. As we expand our presence in the sea, more and more tasks require underwater operations such as ocean exploration, oil-rig maintenance, etc. Yet, most of the underwater robotic vehicles still utilize propellers as the primary propulsive mechanism. In many cases, the bio-inspired propulsion system that mimics the swimming action of fish offers many advantages in agility, maneuverability, and stealth. With the rising interest in the field, the works presented in this thesis aim to expand our understanding of how to implement the bio-inspired propulsive mechanism to robotic design. To achieve this, a mechanical device is designed to mimic the swimming action of different fish species. Then, an experiment is performed to subject the device to different fish-like motions and test their effectiveness. In addition, a reduced-ordered model is also introduced as an alternative method to predict the hydrodynamic performance of this propulsive mechanism. The works presented in this thesis help to expand the toolbox available for the engineer to design the next generation of the underwater robotic vehicle.

The impressive kinematic capabilities and structural adaptations presented by bio-locomotion continue to inspire some of the advancements in today's small-scaled flying and swimming vehicles. These vehicles operate in a low Reynolds number flow regime where viscous effects dominate flow interactions, which makes it challenging to generate lift and thrust. Overcoming these challenges means utilizing non-conventional lifting and flow control mechanisms generated by unsteady flapping body motion. Understanding and characterizing the aerodynamic phenomena associated with the unsteady motion is vital to predict the unsteady fluid loads generated, to implement control methodologies, and to assess the dynamic stability and control authority of airborne and underwater vehicles. This dissertation presents experimental results for forced oscillations on multi-element airfoils and hydrofoils for Reynolds numbers between Re=104 and Re=106. The document divides the work into four main sections: The first topic presents wind tunnel measurements of lift forces generated by an oscillating trailing edge flap on a NACA-0012 airfoil to illustrate the effects that frequency and pitching amplitude have on lift enhancement. The results suggest that this dynamic trailing edge flap enhances the mean lift by up to 20% in the stalled flow regime. Using frequency response approach, it is determined that the maximum enhancement in circulatory lift amplitude occurs at stalled angles of attack for lower pitching amplitudes. The second topic presents wind tunnel measurements for lift and drag generated by a sinusoidal and non-sinusoidal oscillations of a NACA-0012 airfoil. The results show that 'trapezoidal' pitching enhances the mean lift and the RMS lift by up to 50% and 35% in the pre-stall flow regime, respectively, whereas the 'reverse sawtooth' and sinusoidal pitching generate the most substantial increase of the lift-to-drag ratio in stall and post-stall flow regimes, respectively. The third topic involves a study on the role of fish-tail flexibility on thrust and propulsive efficiency. Flexible tails enhance thrust production in comparison to a rigid ones of the same size and under the same operating conditions. Further analysis indicates that varying the tail's aspect ratio has a more significant effect on propulsive efficiency and the thrust-to-power ratio at zero freestream flow. On the other hand, changing the material's property has the strongest impact on propulsive efficiency at non-zero freestream flow. The results also show that the maximum thrust peaks correspond to the maximum passive tail amplitudes only for the most flexible case. The final topic aims to assess the unsteady hydrodynamic forces and moments generated by a three-link swimming prototype performing different swimming gaits, swimming speeds, and oscillatory frequencies. We conclude that the active actuation of the tail's first mode bending produces the most significant thrust force in the presence of freestream flow. In contrast, the second mode bending kinematics provides the most significant thrust force in a zero-freestream flow. / Doctor of Philosophy / It is by no surprise that animal locomotion continues to inspire the design of flying and swimming vehicles. Although nature produces complex kinematics and highly unsteady flow characteristics, simplified approximations to model bio-inspired locomotion in fluid flows are experimentally achievable using low degrees of freedom motion, such as pitching airfoils and trailing edge flaps. The contributions of this dissertation are divided into four primary foci: (a) wind tunnel force measurements on a flapped NACA-0012 airfoil undergoing forced pitching, (b) wind tunnel measurements of aerodynamic forces generated by sinusoidal and non-sinusoidal pitching of a NACA-0012 airfoil, (c) towing tank measurements of thrust forces and torques generated by a one-link swimming prototype with varying tail flexibilities, and (d) towing tank measurements of hydrodynamic forces and moments generated by active tail actuation of a multi-link swimming prototype. From our wind tunnel measurements, we determine that lift enhancement by a trailing edge flap is achieved under certain flow regimes and oscillating conditions. Additionally, we assess the aerodynamic forces for a sinusoidal and non-sinusoidal pitching of an airfoil and show that 'trapezoidal' pitching produces the largest lift coefficient amplitude whereas the sinusoidal and 'reverse sawtooth' pitching achieve the best lift to drag ratios. From our towing tank experiments, we note that the role of tail flexibility enhances thrust generation on a swimming device. Finally, we conclude that different kinematics on an articulating body strongly affect the hydrodynamic forces and moments. The results of the towing tank measurements are accessible from an online public database to encourage research and contribution in underwater vehicle design through physics-based low-order models that can accommodate hydrodynamic principles and geometric control concepts.