Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session RT: Biolocomotion XIII: Macro-Swimming III |
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Chair: John Dabiri, California Institute of Technology Room: Long Beach Convention Center Grand Ballroom B |
Tuesday, November 23, 2010 3:05PM - 3:18PM |
RT.00001: Flexibility and Resonance in Thrust Production of a Mechanical Swimming Lamprey Megan Leftwich, Alexander Smits We use a robotic lamprey as a means of investigating the influence of flexibility on the wake structure and thrust production during anguilliform swimming. A programmable microcomputer actuates 11 servomotors that produce a traveling wave along the length of the lamprey body. The waveform is based on kinematic studies of living lamprey. The shape of the tail is taken from CT scan data of the silver lamprey, and it is constructed of flexible PVC gel. Plastic inserts allow the the degree of flexibility to be changed. PIV measurements in the wake of the robot with three different flexible tails show that a 2P structure dominates the flexible wake. However, the large structure is composed of several small vortices (as opposed to the large coherent vortex seen behind a stiff tailed robot). Furthermore, the wake loses coherence as flexibility is increased. Additionally, momentum balance calculations indicate that increasing the tail flexibility yields less thrust. Finally, we find that changing the cycle frequency to match the resonance frequency of the tail increases the thrust production. The project is supported by NIH CNRS Grant 1R01NS054271. [Preview Abstract] |
Tuesday, November 23, 2010 3:18PM - 3:31PM |
RT.00002: How does muscle forcing lead to translational motion in undulatory swimming? Amneet Bhalla, Neelesh Patankar Swimming organisms show variety of complex deformations during their movement. In this work we enquire whether complex muscle forcing is required to create the observed deformation kinematics that cause movement. We interrogate how the muscle forcing leads to the forward translational momentum of an organism. A set of linearized equations of motion, using a spring-link model, is derived for undulatory swimming. We do not consider observed body deformations to be composed of active and passive components. Instead, swimming is treated as a forced oscillation problem. Forcing can be due to the muscles (active swimming) or due to the surrounding fluid (passive swimming). In either case, the forcing triggers the first few fundamental deformation modes of the body which in turn drive the axial translational motion. We explain the reason for observing only the first few fundamental modes. It is seen that simple forcing patterns can trigger complex looking deformation kinematics that lead to movement. We show that there is range of frequency at which the body responds well (i.e. the swimming speed increases with frequency), but after that range the body does not respond well to higher frequencies. It is found, consistent with prior work, that anisotropy in drag enables swimming. [Preview Abstract] |
Tuesday, November 23, 2010 3:31PM - 3:44PM |
RT.00003: Stability of Passive Locomotion in a Perfect Fluid Fangxu Jing, Eva Kanso We investigate the effect of body elasticity on the stability of locomotion in a perfect fluid. Our motivation is to study fish swimming. Actual fish seem to alternate between actively flapping and passively responding to the surrounding fluid, referred to as {\em Burst and Coast} cycle. We study the stability of the coast (passive) phase. It's well known that the passive motion of a single elongated rigid body along its major axis of symmetry is unstable. The question is: can passive shape changes mediated by body elasticity stabilize the motion? The answer is yes. We consider an articulated body with finite number of rigid links, connected by hinge joints with torsional springs at the joints to emulate the elasticity of fish. The motion of the articulated body with constant velocity along its major axis of symmetry is a relative equilibrium. Upon analyzing the stability of this equilibrium, we discover that passive shape changes do {\em stabilize} the motion for appropriate combination of body geometry and spring elasticity. We plot the region of stability in aspect ratio - spring stiffness parameter space. [Preview Abstract] |
Tuesday, November 23, 2010 3:44PM - 3:57PM |
RT.00004: A self-excited flapper from fluid-structure interaction Oscar M. Curet, Kenneth S. Breuer The flexible nature of lifting and propulsive surfaces is a common characteristic of aquatic and aerial locomotion in animals. These surfaces may not only move actively, but also passively or with a combination of both. What is the nature of this passive movement? What is the role of this passive motion on force generation, efficiency and muscle control? Here, we present results using a simple wing model with two degrees of freedom designed to study passive flapping, and fluid-structure interaction. The wing is composed of a flat plate with a hinged trailing flap. The wing is cantilevered to the main body to enable a flapping motion with a well-defined natural frequency. We test the wing model in a wind tunnel. At low speed the wing is stationary. Above a critical velocity the trailing wing section starts to oscillate, generating an oscillating lift force on the wing. This oscillating lift force results on a self-excited flapping motion of the wing. We measure the kinematics and the forces generated by the wing as a function of flow velocity and stiffness of the cantilever. Comparisons with aeroelasticity theory will be presented as well as details of the fluid-structure interactions. [Preview Abstract] |
Tuesday, November 23, 2010 3:57PM - 4:10PM |
RT.00005: Hydroelastic Tuning in Fish Swimming Benjamin Connell Recent studies have indicated the importance of structural properties to the hydroelastic response of passive flexible bodies in uniform flow. One response regime includes a structural traveling wave of increasing amplitude from leading to trailing edge with alternating vortex shedding in the wake. This modal response exhibits the same characteristics as fish swimming, suggesting the importance of the natural hydroelastic response in fish swimming actuation. We explore the concept of underactuation in fish swimming by examining the ability to achieve swimming kinematics through single point forcing of a flexible body. The phenomenon is first studied through simulation of the Navier-Stokes equations coupled to a nonlinear structural solver. This indicates the relationship between passive and active modal response, and the ability to alter the vortex wake and associated hydrodynamic loading through underactuation. A reduced-fidelity model for the fluid-structural dynamics is employed to optimize the properties of a fish body for the desired underactuated modal response. The optimized design is then tested in a captive-swimming experiment to examine the response modes and swimming performance. [Preview Abstract] |
Tuesday, November 23, 2010 4:10PM - 4:23PM |
RT.00006: Recent Observations on Shortfin Mako Scale Flexibility as a Mechanism for Separation Control Amy Lang, Philip Motta, Maria Habegger, Emily Jones, Robert Hueter Recent results obtained from examining the skin of the shortfin mako (\textit{Isurus oxyrinchus}) suggest that scale flexibility may provide a passive, flow actuated mechanism for controlling flow separation. The shortfin mako is considered to be one of the fastest and most agile marine predators. High contragility, or the ability to change direction while already in a turn, requires minimal form drag and thus control of flow separation on body regions aft of the point of maximum girth. Recent biological observations have found that the shortfin mako has highly flexible scales, or denticles, particularly on the sides of the body downstream of the gills; in these regions scale crowns can be easily manipulated to angles in excess of 60 degrees. Histological data of the skin provides preliminary evidence that this flexibility is achieved due, in part, to a reduction in the size of the base of the scale where it is anchored into the skin. Experimental measurements of maximum angle of denticle bristling observed as a function of body location will be presented and a probable mechanism leading to separation control will be discussed. [Preview Abstract] |
Tuesday, November 23, 2010 4:23PM - 4:36PM |
RT.00007: Learning from jellyfish: Fluid transport in muscular pumps at intermediate Reynolds numbers Janna Nawroth, John Dabiri Biologically inspired hydrodynamic propulsion and maneuvering strategies promise the advancement of medical implants and minimally invasive clinical tools. We have chosen juvenile jellyfish as a model system for investigating fluid dynamics and morphological properties underlying fluid transport by a muscular pump at intermediate Reynolds numbers. Recently we have described how natural variations in viscous forces are balanced by changes in jellyfish body shape (phenotypic plasticity), to the effect of facilitating efficient body-fluid interaction. Complementing these studies in our live model organisms, we are also engaged in engineering an artificial jellyfish, that is, a jellyfish-inspired construct of a flexible plastic sheet actuated by a monolayer of rat cardiomyocytes. The main challenges here are (1) to derive a body shape and deformation suitable for effective fluid transport under physiological conditions, (2) to understand the mechanical properties of the muscular film and derive a design capable of the desired deformation, (3) to master the proper alignment and timely contraction of the muscle component needed to achieve the desired deformation, and (4) to evaluate the performance of the design. [Preview Abstract] |
Tuesday, November 23, 2010 4:36PM - 4:49PM |
RT.00008: Examination of Scuba Fin Designs Using Simultaneous Force and DPIV Measurements Lori Halvorson, Erica Sherman, ChiaMin Leong, Timothy Wei Like many commercial products, there is a wide variety of scuba dive fins on the market, each one of which, the designers argue are the best and most efficient. The foundation for these claims invariably are based on some sort of hydrodynamic argument with the full spectrum of scientific credibility attached. In this study, we examine a number of commercially available scuba fins using both DPIV of the fin motion as well as dynamic force measurements of thrust generated by a swimmer kicking against a stationary force balance. Both techniques have been used and reported in the past for studies of world class swimmers and dolphins. This will be the first time that high quality data has been obtained of both flow and force simultaneously. A number of different fin designs were tested. But the most interesting comparison was between the ``monofin'' and the ``split-fin'' designs. A discussion of the relative merits of the two different designs will be presented along with video footage showing flow and force overlaid on the fin motions. [Preview Abstract] |
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