Session AA: Biofluid Dynamics I: Swimming-1

Snail-Inspired Biomimetic Devices

Robosnail 1 is a machine we have designed that uses a waving foot to propel itself over a viscous fluid in the lubrication limit. We present new theoretical and numerical work, including full 3D modeling of finite-width snails, and a design for snails which can move faster than their own waving velocity. We also have experimentally validated the 3D theory using several different versions of Robosnail 1. Robosnail 2 is a machine we have designed that uses in-plane waves of compression to propel itself on a thin layer of finite yield-stress fluid. The latest iteration is able to climb walls and move upside-down on a layer of Carbopol, a gel-like water-based polymer solution. We present basic theory and experimental results.   

Crawling underneath a free surface

The ability of land snails to traverse extreme terrains has been investigated theoretically and experimentally in recent years. However, little is known about how water snails travel inverted underneath a free surface. Based on our experimental observations, we present a lubrication model of water snail locomotion.   

Optimal Stroke Patterns for Purcell's Three-Link Swimmer

This study focuses on the optimal swimming of Purcell's three link swimmer. We model the swimmer as a jointed chain of three links moving in a inertialess flow using slenderbody theory. The kinematics and the geometry of the swimmer is optimized for efficiency and speed. The parametrization of the stroke is developped in Fourier series and the optimal stroke is found via a gradient type search on a finite set of the Fourier coefficients using a modified Newton algorithm. We were able to attain swimmer designs significantly more efficient than those previously suggested by authors who only consider geometric design rather than kinematic criteria (Becker, Koehler and Stone 2003). The influence of slenderness on optimality is considered as well.   

Flow around a Jellyfish

We think to apply the swimming motion of a jellyfish to a micro robot made from soft material. The purpose of this study is to understand the way to propel the jellyfish's. We observed the swimming motion of the jellyfish by using a motion-capture camera, and measured the vector field of flow around the jellyfish by using a PIV system. The jellyfish is principally propelled by a vortex ring ejected at contracting phase of the jellyfish motion. Whereas, we do not have explanation for keeping constant speed and acceleration in the expanding phase of the jellyfish motion. We found that a vortex ring with the opposite vorticity to shed vortex ring was inside the jellyfish body in the expanding phase of the jellyfish motion. We discussed a cause of an increase in thrust force and keeping constant speed in the expanding phase of the jellyfish motion by considering the change in momentum.   

Aerodynamic effect of hind-wing tails on a gliding swallowtail butterfly

In butterfly flight, the relationship between wing morphology and gliding performance have been one of the major concerns. The hind-wing tails, observed in most swallowtail butterfly, have been also conjectured to promote the gliding ability of butterfly. In this study, the aerodynamic role of hind-wing tails in gliding swallowtail butterfly is experimentally investigated using butterfly models with and without tails. The butterfly models are copied from a dried specimen of real swallowtail butterfly, $Papilio~Ulysses$. Varying the attack angle, we measure the lift, drag and pitching moment for both models, and visualize the flow fields using an array of smoke wires at the attack angle of $20^{\circ}$. With the tails, the lift and drag increase by about $10 \sim 20\%$ and $5\%$, respectively, at the attack angles higher than $15^{\circ}$, which results in the increase of lift-to-drag ratio in a wide range of attack angles. Also, with the tails, the nose-down pitching moment increases more rapidly with increasing attack angle, indicating the enhanced longitudinal static stability by the tails. From visualization, it is found that strong vortical structures are drawn closer to the upper wing surface by the tails.   

Time-Resolved Two-Component Force Analysis of a Swimmer’s Kick

A two-dimensional dynamic force balance was constructed to study and improve kicking motions of elite swimmers. Rrevious methods used elastic tethers which altered the swimmer’s motion. Apparently no time-resolved measurements have ever been made either. The balance used was a simple truss in which static dynamics could be applied in order to break the kick's force up into it's x and y components, giving the ability to see in which direction the majority of the force was being applied. An underwater video camera was also implemented so that the forces produced could be directly related to the motion of the swimmer. This relation was then used to determine how a swimmer could change his/her kick to be a more effective swimmer. Measurements made with Megan Jendrick (2000 Olympic gold medalist) and Ariana Kukors (3x US National Champion) will be presented.   

A Lagrangian approach to vortex identification in swimming and flying animal wakes.

The fluid wakes of swimming and flying animals are generally time-dependent. The Eulerian velocity field, which can be measured by existing DPIV measurement techniques, does not directly indicate the flow geometry in this type of unsteady flows. In this study, a Lagrangian approach is developed to determine the Lagrangian Coherent Structures, which are physical boundaries separating flow regions with distinct dynamics, including vortices. The determination of morphology and kinematics of vortices is necessary in estimating time-dependent locomotive forces (Dabiri, J. Exp. Bio., 2006). It also provides information in studying fluid transport in animal swimming and flying. The application of the method is demonstrated by studying the wake of a bluegill sunfish pectoral fin and that of a free-swimming jellyfish.   

Towards direct numerical simulation of freely swimming fish.

Swimming mechanisms employed by fish are currently inspiring unique underwater vehicles and robotic devices as well as basic science research into the neural control of movement. Key engineering issues include propulsion efficiency, precise motion control and maneuverability. A numerical scheme that simulates the motion of freely swimming fish will be a valuable design and research tool. We are working towards this goal. In particular we are interested in simulating the motion of a gymnotiform fish that swims by producing undulations of a ventral ribbon fin while keeping its body rigid. We model the fish as a rigid body with an attached undulating membrane. In our numerical scheme the key idea is to assume that the entire fluid-fish domain is a fluid. Then we impose two constraints: the first requires that the fluid in the region occupied by the fish body moves rigidly (a fictitious domain approach), and the second requires that the fluid at the location of the fin has the traveling wave velocity of the fin (an immersed boundary approach). Given the traveling wave form of the fin, the objective is for the numerical scheme to give the swimming velocity of the fish by solving the coupled fluid-fish problem. We will present results for the forces generated by a fin attached to a fixed body and preliminary results for freely swimming fish.   

A Study of a Mechanical Swimming Lamprey

To develop a comprehensive model of lamprey swimming, the wake structure generated by a swimming mechanical model is investigated using dye flow visualization. The eel is activated by 13 programmable servomotors and a traveling wave is generated along the length of the body.~ The waveform is based on the motion of an American eel (Anguilla rostrata) of Tytell and Lauder (2004).~ A laser scanning system is used to visualize the three-dimensional unsteady wake structure.   

A vorticity-free approach to wake-based swimming/flying force estimation

Traditional wake-based analyses of animal swimming and flying depend largely on knowledge of the vorticity field, which can be difficult or impossible to incorporate in the context of unsteady fluid-structure interactions. This talk will describe the development and application of a technique for estimating swimming/flying forces that does not require measurement of the vorticity field. The method is based on the identification of Lagrangian Coherent Structures in the wake, whose dynamics are governed by the theory for deformable bodies in potential flow (Peng and Dabiri, J. Exp. Biol. 2007). This paradigm for the analysis of unsteady fluid-structure interactions is integrated with existing DPIV measurement techniques to analyze medusan (jellyfish) swimming and the dynamics of the bluegill sunfish pectoral fin.   

Dynamics of tethered versus free-swimming animals: A wake structure comparison in jellyfish

Previous research has shown that jellyfish utilize the formation and shedding of vortices to help feed and move the animal. Laboratory experiments often require restricting the motion of an animal by tethering/fluming to allow for repeatable results. However, past research has not addressed the differences that arise when the motion of an animal is restricted/confined. This presentation will attend to this issue by comparing the wake structure of a tethered and free-swimming \textit{Aurelia aurita}. Digital Particle Image Velocimetry is used to collect measurements of the velocity field surrounding an animal that is either tethered or swimming freely. Dynamical systems methods are used to compute Lagrangian coherent structures (LCS), which is used to identify the geometries of structures in the wake of the animal. Using LCS, a comparison between the wake of a tethered and free-swimming animal can be made. This research provides a quantitative measure of the differences between a tethered and freely moving jellyfish.   

Hydrodynamics of ribbon fin-based swimming with application to highly maneuverable underwater vehicles.

Gymnotiform fish swim by producing undulations of a long, ventral midline fin, also referred to as a ribbon fin. South American weakly electric gymnotiform fish are remarkably maneuverable fish -- MacIver and co-workers have shown they achieve omnidirectional movement on short timescales. We investigated this system's mechanics with a view to gaining insight into sensory processing and neural control of movement, by using the immersed boundary method. The results suggest that this mode of propulsion may be an ideal underwater vehicle propulsion system for low speed applications where precise control of motion is needed. We show that in addition to the surge direction (forward/backward) traditionally associated with the ribbon fin, the ribbon fin can also achieve positive heave (up), pitch, and roll. Utilizing one traveling wave, we show how to control the thrust magnitude and direction over a limited range by varying amplitude and wave speed. By utilizing two traveling waves, our control space significantly expands to include pure heave motions, which have been informally observed both in gymnotiforms and in ammiform fish, where the midline fin is dorsally positioned. The non-dimensional thrust generated by the ribbon fin appears to be insensitive to Reynolds number over the relevant velocities.