Hydrodynamic Performance of Aquatic Flapping: Efficiency of Underwater Flight in the Manta

The manta is the largest marine organism to swim by dorsoventral oscillation (flapping) of the pectoral fins. The manta has been considered to swim with a high efficiency stroke, but this assertion has not been previously examined. The oscillatory swimming strokes of the manta were examined by detailing the kinematics of the pectoral fin movements swimming over a range of speeds and by analyzing simulations based on computational fluid dynamic potential flow and viscous models. These analyses showed that the fin movements are asymmetrical up- and downstrokes with both spanwise and chordwise waves interposed into the flapping motions. These motions produce complex three-dimensional flow patterns. The net thrust for propulsion was produced from the distal half of the fins. The vortex flow pattern and high propulsive efficiency of 89% were associated with Strouhal numbers within the optimal range (0.2–0.4) for rays swimming at routine and high speeds. Analysis of the swimming pattern of the manta provided a baseline for creation of a bio-inspired underwater vehicle, MantaBot

Fluid-structure interaction modeling on a 3D ray-strengthened caudal fin

In this paper, we present a numerical model capable of solving the fluid-structure interaction problems involved in the dynamics of skeleton-reinforced fish fins. In this model, the fluid dynamics is simulated by solving the Navier-Stokes equations using a finite-volume method based on an overset, multi-block structured grid system. The bony rays embedded in the fin are modeled as nonlinear Euler-Bernoulli beams. To demonstrate the capability of this model, we numerically investigate the effect of various ray stiffness distributions on the deformation and propulsion performance of a 3D caudal fin. Our numerical results show that with specific ray stiffness distributions, certain caudal fin deformation patterns observed in real fish (e.g. the cupping deformation) can be reproduced through passive structural deformations. Among the four different stiffness distributions (uniform, cupping, W-shape and heterocercal) considered here, we find that the cupping distribution requires the least power expenditure. The uniform distribution, on the other hand, performs the best in terms of thrust generation and efficiency. The uniform stiffness distribution, per se, also leads to 'cupping' deformation patterns with relatively smaller phase differences between various rays. The present model paves the way for future work on dynamics of skeleton-reinforced membranes

The development of a biologically inspired propulsor for unmanned underwater vehicles

IEEE Journal of Oceanic Engineering, 32(3): pp. 533-550Fish are remarkable in their ability to maneuver and to control their body position. This ability is the result of the coordinated movement of fins which extend from the body and form control surfaces that can create and vector forces in 3-D. We have embarked on a research program designed to develop a maneuvering propulsor for unmanned undersea vehicles (UUVs) that is based on the pectoral fin of the bluegill sunfish. For this, the anatomy, kinematics, and hydrodynamics of the sunfish pectoral fin were investigated experimentally and through the use of computational fluid dynamics (CFD) simulations. These studies identified that the kinematics of the sunfish pectoral fin are very complex and are not easily described by traditional “rowing”- and “flapping”-type kinematics. A consequence of the complex motion is that the pectoral fin can produce forward thrust during both its outstroke (abduction) and instroke (adduction), and while doing so generates only small lateral and lift forces. The results of the biological studies were used to guide the design of robotic pectoral fins which were built as experimental devices and used to investigate the mechanisms of thrust production and control. Because of a design that was based heavily on the anatomy of the sunfish fin, the robotic pectoral fins had the level of control and degrees of freedom necessary to reproduce many of the complex fin motions used by the sunfish during steady swimming. These robotic fins are excellent experimental tools, and are an important first step towards developing propulsive devices that will give the next generation of UUVs the ability to produce and control thrust like highly maneuverable fish

Review of Experimental Work in Biomimetic Foils

Significant progress has been made in understanding some of the basic mechanisms of force production and flow manipulation in oscillating foils for underwater use. Biomimetic observations, however, show that there is a lot more to be learned, since many of the functions and details of fish fins remain unexplored. This review focuses primarily on experimental studies on some of the, at least partially understood, mechanisms, which include 1) the formation of streets of vortices around and behind two- and three-dimensional propulsive oscillating foils; 2) the formation of vortical structures around and behind two- and three-dimensional foils used for maneuvering, hovering, or fast-starting; 3) the formation of leading-edge vortices in flapping foils, under steady flapping or transient conditions; 4) the interaction of foils with oncoming, externally generated vorticity; multiple foils, or foils operating near a body or wall

On the role of tip curvature on flapping plates

During the flapping motion of a fish's tail, the caudal fin exhibits antero-posterior bending and dorso-ventral bending, the latter of which is referred to as chord-wise bending herein. The impact of chord-wise tip curvature on the hydrodynamic forces for flapping plates is investigated to explore potential mechanisms to improve the maneuverability or the performance of autonomous underwater vehicles. First, actuated chord-wise tip curvature is explored. Comparison of rigid curved geometries to a rigid flat plate as a baseline suggests that an increased curvature decreases the generated forces. An actuated plate with a dynamic tip curvature is created to illustrate a modulation of this decrease in forces. Second, the impact of curvature is isolated using curved plates with an identical planform area. Comparison of rigid curved geometries as a baseline corroborates the result that an increased curvature decreases the generated forces, with the exception that presenting a concave geometry into the flow increases the thrust and the efficiency. A passively-actuated plate is designed to capitalize on this effect by presenting a concave geometry into the flow throughout the cycle. The dynamically and passively actuated plates show potential to improve the maneuverability and the efficiency of autonomous underwater vehicles, respectively

Undulatory Locomotion in Freshwater Stingray Potamotrygon Orbignyi: Kinematics, Pectoral Fin Morphology, and Ground Effects on Rajiform Swimming

Fishes are the most speciose group of living vertebrates, making up more than half of extant vertebrate diversity. They have evolved a wide array of swimming modes and body forms, including the batoid elasmobranchs, the dorsoventrally flattened skates and rays, which swim via oscillations or undulations of a broad pectoral fin disc. In this work I offer insights into locomotion by an undulatory batoid, freshwater stingray Potamotrygon orbignyi (Castelnau, 1855), combining studies of live animals, physical models, and preserved specimens. In Chapter 1, I quantify the three-dimensional kinematics of the P. orbignyi pectoral fin during undulatory locomotion, analyzing high-speed video to reconstruct three-dimensional pectoral fin motions. A relatively small portion (~25%) of the pectoral fin undulates with significant amplitude during swimming. To swim faster, stingrays increase the frequency, not the amplitude of propulsive motions, similar to the majority of studied fish species. Intermittently during swimming, a sharp, concave-down lateral curvature occurred at the fin margin; as the fin was cupped against the pressure of fluid flow this curvature is likely to be actively controlled. Chapter 2 employs a simple physical model of an undulating fin to examine the ground effects that stingrays may experience when swimming near a substrate. Previous research considering static air- and hydrofoils indicated that near-substrate locomotion offers a benefit to propulsion. Depending on small variations in swimming kinematics, undulating fins can swim faster near a solid boundary, but can also experience significant increases (~25%) in cost-of-transport. In Chapter 3, I determine how pectoral and pelvic fin locomotion are combined in P. orbignyi during augmented punting, a hybrid of pectoral and pelvic fin locomotion sometimes employed as stingrays move across a substrate. The timing of pectoral and pelvic fin motions is linked, indicating coordination of thrust production. Chapter 4 discusses pectoral fin structure and morphological variations within the fin, correlating morphology with the swimming kinematics observed in Chapter 1. Passive and active mechanisms may stiffen the anterior fin to create the stable leading edge seen during swimming; stingrays have converged on several structural features (fin ray segmentation and branching) shared by actinopterygian fishes