Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session GV: Swimming II |
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Chair: Moteza Gharib, California Institute of Technology Room: 205A-D |
Monday, November 23, 2009 8:00AM - 8:13AM |
GV.00001: Repeatability of arm pull patterns in front crawl swimming Lester K. Su, John C. Kegelman The arm pull in human swimming has seen extensive study, particularly involving the front crawl stroke. This work has primarily been aimed either at clarifying the mechanisms of thrust generation by the arm and hand, or at comparing the relative performance of different canonical pulling patterns. In this work we investigate the degree to which swimmers adjust their arm and hand trajectories in response to instantaneous ambient conditions. Video imaging data from competitive swimmers indicates that there may be wide stroke-to-stroke variations in pull trajectories. This suggests that optimal stroking form may be less about a swimmer's ability to repeat idealized pull patterns, than about the swimmer's ability to respond to local flow conditions, or what is referred to in the swimming vernacular as the ``feel'' for the water. [Preview Abstract] |
Monday, November 23, 2009 8:13AM - 8:26AM |
GV.00002: Dynamics of wake structure in clapping propulsion Daegyoum Kim, Morteza Gharib Some animals such as insects and frogs use a pair of symmetric flaps for locomotion. In some cases, these flappers operate in close proximity or even touch each other. In order to understand the underlying physics of these kinds of motion, we have studied the wake structures induced by clapping and their associated thrust performance. A simple mechanical model with two acrylic plates was used to simulate the power stroke of the clapping motion and three-dimensional flow fields were obtained using defocusing digital particle image velocimetry. Our studies show that the process of vortex connection plays a critical role in forming a downstream closed vortex loop. Under some kinematic conditions, this vortex loop changes its shape dynamically, which is analogous to the process of an elliptical vortex ring switching its minor and major axis. As the length of the plate along the rotating shaft decreases to change an aspect ratio, the downstream motion of the vortex is retarded due to the outward motion of side edge vortices and less propulsive force is generated per the surface area of the plate. The impact of compliance and stroke angle of the plate on wake structures and thrust magnitudes are also presented. [Preview Abstract] |
Monday, November 23, 2009 8:26AM - 8:39AM |
GV.00003: Can drag and thrust be separated in undulatory swimming? Neelesh Patankar, Anup Shirgaonkar, Malcolm MacIver Aquatic organisms are motivating new biomimetic underwater vehicles. To that end it is essential to obtain the swimming velocity and efficiency of organisms using reduced order models. The swimming velocity is often determined by equating the drag and thrust on swimming bodies. This has led to many conflicting results in the past. It has been proposed that one of the root causes of the disagreements is that, in general, drag and thrust on swimming bodies can not be separated from each other. This is considered to be true when movement is generated by undulations as in anguilliform, gymnotiform, and balistiform modes of swimming, among others. We did high-resolution numerical simulations to study the forces acting on the undulatory ribbon fin of a gymnotiform swimmer -- the black ghost knifefish. In spite of the above expectations, we have surprisingly found a new way to approximately decompose the net force into drag and thrust producing mechanisms in undulatory swimming modes. Such decomposition is unexpected for non-linear finite Reynolds number problems. This result appears conceptually analogous to how a linearization of the Navier--Stokes equations, or Carrier's equation, captures the drag-determining features of the flow around objects. [Preview Abstract] |
Monday, November 23, 2009 8:39AM - 8:52AM |
GV.00004: The effects of fluid viscosity on undulating swimmers Josue Sznitman, Xiaoning Shen, Paulo Arratia The swimming behavior of the nematode \emph{C. elegans} ($L\approx$~1 mm) as a function of the surrounding fluid viscosity $\mu$ is investigated using both particle- and nematode-tracking methods. Nematode tracking data show that \emph{C. elegans} move in a highly periodic fashion characterized by traveling waves. The nematode swimming speed $U$ decays nonlinearly with increasing fluid viscosity such that $U\sim \mu^{-0.2}$. Velocimetry data shows flow re- circulation regions along the nematode's body. The velocity profiles measured in the direction normal to the swimming nematode show a decay that is similar for fluid viscosities ranging from from 1 cP to 20 cP. The normalized velocity decays follow a single mater curve with $d/L$ as the independent variable, where $d$ is the normal distance from the swimming nematode. This result suggests that \emph{C. elegans} may be a good canditate to investigate low Re locomotion. [Preview Abstract] |
Monday, November 23, 2009 8:52AM - 9:05AM |
GV.00005: High speed x-ray observation of a sand swimming lizard Daniel Goldman, Ryan Maladen, Yang Ding We use high-speed x-ray imaging to reveal how a small (10~cm) desert dwelling lizard, the sandfish ({\em Scincus scincus}), swims within a granular medium, and how its locomotion is affected by the volume fraction $\phi$ of the media~\footnote{Maladen et. al, Science, {\bf 325}, 314, 2009}. We use an air fluidized bed to prepare 0.3~mm glass beads (similar in size to desert sand) into naturally occurring loose ($\phi=0.58$) and close ($\phi=0.62$) packed states. On the surface, the lizard uses a standard diagonal gait, but once below the surface, the organism no longer uses limbs for propulsion. Instead it propagates a large amplitude single period sinusoidal traveling wave down its body and tail to propel itself at speeds up to $\approx 1$ body-length/sec. For fixed $\phi$ the animal increases forward swimming speed $v_f$ by increasing temporal frequency $f$. For fixed $f$, $v_f$ is independent of $\phi$, despite resistance forces that nearly double from loose to close packed states. Surprisingly, the greatest sandfish velocity (and $f$) occur in the close packed state. [Preview Abstract] |
Monday, November 23, 2009 9:05AM - 9:18AM |
GV.00006: Resistive force theory for sand swimming Yang Ding, Ryan Maladen, Chen Li, Daniel Goldman We discuss a resistive force theory~\footnote{Maladen et. al, Science, \textbf{325}, 314, 2009} that predicts the ratio of forward speed to wave speed (wave efficiency, $\eta$) of the sandfish lizard as it swims in granular media of varying volume fraction $\phi$ using a sinusoidal traveling wave body motion. In experiment $\eta\approx0.5$ independent of $\phi$ and is intermediate between $\eta \approx 0.2$ for low $Re$ Newtonian fluid undulatory swimmers like nematodes and $\eta \approx 0.9$ for undulatory locomotion on a deformable surface. To predict $\eta$ in granular media, we developed a resistive force model which balances thrust and drag force over the animal profile. We approximate the drag forces by measuring the force on a cylinder (a ``segment'' of the sandfish) oriented at different angles relative to the displacement direction. The model correctly predicts that $\eta$ is independent of $\phi$ because the ratio of thrust to drag is independent of $\phi$. The thrust component of the drag force is relatively larger in granular media than in low $Re$ fluids, which explains why $\eta$ in frictional granular media is greater than in viscous fluids. [Preview Abstract] |
Monday, November 23, 2009 9:18AM - 9:31AM |
GV.00007: Sandfish numerical model reveals optimal swimming in sand Ryan Maladen, Yang Ding, Adam Kamor, Andrew Slatton, Daniel Goldman Motivated by experiment and theory examining the undulatory swimming of the sandfish lizard within granular media~\footnote{Maladen et. al, Science, \textbf{325}, 314, 2009}, we study a numerical model of the sandfish as it swims within a validated soft sphere Molecular Dynamics granular media simulation. We hypothesize that features of its morphology and undulatory kinematics, and the granular media contribute to effective sand swimming. Our results agree with a resistive force model of the sandfish and show that speed and transport cost are optimized at a ratio of wave amplitude to wavelength of $\approx0.2$, irrespective of media properties and preparation. At this ratio, the entry of the animal into the media is fastest at an angle of $\approx20^{\circ}$, close to the angle of repose. We also find that the sandfish cross-sectional body shape reduces motion induced buoyancy within the granular media and that wave efficiency is sensitive to body-particle friction but independent of particle-particle friction. [Preview Abstract] |
Monday, November 23, 2009 9:31AM - 9:44AM |
GV.00008: No slip locomotion of hatchling sea turtles on granular media Nicole Mazouchova, Chen Li, Nick Gravish, Andrei Savu, Daniel Goldman Sea turtle locomotion occurs predominantly in aquatic environments. However after hatching from a nest on a beach, the juvenile turtles (hatchlings), must run across several hundred meters of granular media to reach the water. To discover how these organisms use aquatically adapted limbs for effective locomotion on sand, we use high speed infrared video to record hatchling Loggerhead sea turtles (\textit{Caretta caretta}) kinematics in a field site on Jekyll Island, GA, USA. A portable fluidized bed trackway allows variation of the properties of the granular bed including volume fraction and angle up to the angle of repose. Despite being adapted for life in water, on all treatments the turtles use strategies similar to terrestrial organisms when moving on sand. Speeds up to 3 BL/sec are generated not by paddling in sand, but by limb movement that minimizes slip of the flippers, thus maintaining force below the yield stress of the medium. We predict turtle speed using a model which incorporates the yield stress of the granular medium as a function of surface angle. [Preview Abstract] |
Monday, November 23, 2009 9:44AM - 9:57AM |
GV.00009: Tree-inspired Piezoelectric Energy Harvesting William Hobbs, David Hu We design and build a tabletop wind energy harvester inspired by the swaying of trees. The device consists of cantilevered cylinders (``tree trunks'') arranged linearly downwind. The bases of the cylinders contain piezoelectric transducers that capture energy from vibration of the cylinder transverse to the flow. For a particular Reynolds number, and ratio of vortex shedding frequency to cylinder natural frequency, we experimentally measure the power generated ($\sim $ 1 micro-watt) as a function of cylinder arrangement. We report optimal spacings for generating peak power. We also report the distribution of power down the array. We qualitatively account for these trends using flow visualizations of vortex shedding using a flowing soap film dynamically matched with our piezoelectric system. [Preview Abstract] |
Monday, November 23, 2009 9:57AM - 10:10AM |
GV.00010: Flow energy harvesting -- another application of the biomimetic flapping foils Qiang Zhu, Zhangli Peng Imitating fish fins and insect wings, flapping foils are usually used for biomimetic propulsion. Theoretical studies and experiments have demonstrated that through specific combinations of heaving and pitching motions, these foils can also extract energy from incoming wind or current. Compared with conventional flow energy harvesting devices based upon rotating turbines, this novel design promises mitigated impact upon the environment. To achieve the required motions, existing studies focus on hydrodynamic mode coupling, in which a periodic pitching motion is activated and a heaving motion is then generated by the oscillating lifting force. Energy extraction is achieved through a damper in the heaving direction (representing the generator). This design involves a complicated control and activation system. In addition, there is always the possibility that the energy required to activate the system exceeds the energy recovered by the generator. We have discovered that a much simpler device without activation, a 2DOF foil mounted on a rotational spring and a damper undergoing flow-induced motions can achieve stable flow energy harvesting. Using Navier-Stokes simulations we predicted different behaviors of the system during flow-induced vibrations and identified the specific requirements to achieve controllable periodic motions essential for stable energy harvesting. The energy harvesting capacity and efficiency were also determined. [Preview Abstract] |
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