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 EV: Swimming I |
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Chair: Neelesh Patankar, Northwestern University Room: 205A-D |
Sunday, November 22, 2009 4:15PM - 4:28PM |
EV.00001: Fluid Forces and Vortex Wakes of a Flapping Foil Teis Schnipper, Anders Andersen, Tomas Bohr, Jens Honor\'e Walther We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil with pitching oscillations in a two-dimensional free stream. We vary the frequency and amplitude of the oscillations and observe von K{\'a}rm{\'a}n wake, inverted von K{\'a}rm{\'a}n wake, and wakes in which two vortex pairs form per oscillation period. We find a close correspondence between the numerically determined vortex structures and the thickness variations that visualize the flow in the soap film.\footnote{Schnipper, Andersen, and Bohr, J. Fluid Mech. {\bf 633}, 411--423 (2009).} Numerically we obtain systematic maps with $25 \times 40$ simulations in the frequency and amplitude plane of both wake type and average forces and moments, and we discuss the drag-thrust transition in relation to the changes in wake structure. Finally, we investigate the time evolution of the fluid forces and its link to the vortex formation near the round leading edge and the vortex shedding at the sharp trailing edge. [Preview Abstract] |
Sunday, November 22, 2009 4:28PM - 4:41PM |
EV.00002: The Hydrodynamic Origin of Whale Flukeprints Rachel Levy Whales swimming at a shallow depth leave a signature on the ocean surface known as a whale flukeprint. The print is a large, smooth, oval patch surrounded by a small wake or ridge. Informal observations made by biologists have led to hypotheses that the prints are made either by hydrodynamic structures created by the motion of the fluke, or by surfactants. This study employs experiments with an artificial fluke to determine whether prints can be created by hydrodynamic forces without the presence of surfactant. The effect of swim velocity on the width, length and duration of a flukeprint created by the artificial fluke is discussed. The experimental data indicate that prints can be formed solely by hydrodynamic forces. This conclusion is supported by observations of whales, infrared images of footprints and numerical simulations of vorticity. [Preview Abstract] |
Sunday, November 22, 2009 4:41PM - 4:54PM |
EV.00003: Three-dimensional wake of a biologically-inspired propulsor Melissa Green, Clarence Rowley, Alexander Smits Digital Particle Image Velocimetry (DPIV) was used to investigate the wakes of rigid pitching panels with a trapezoidal planform geometry, chosen to model idealized fish caudal fins. Experiments were performed for Strouhal numbers of 0.17 and 0.23. The three-dimensional unsteady vortex wake downstream of the panel trailing edge was visualized using spatially- and temporally-resolved two-component data. A Lagrangian Coherent Structure (LCS) analysis was employed in addition to Eulerian vortex identification criteria to investigate the generation and evolution of the wake. A reverse von K\'{a}rm\'{a}n vortex street pattern was observed near the mid-span immediately downstream of the panel trailing edge, but the complexity and three-dimensionality of the wake increases away from the mid-span as streamwise vortices interact with the swept edges of the panel. Farther downstream of the trailing edge, the wake was observed to shrink in the spanwise direction at both Strouhal numbers. In addition, a quantitative bifurcation in the LCS coincided with a qualitative transition of the wake structure observed with increasing Strouhal number. [Preview Abstract] |
Sunday, November 22, 2009 4:54PM - 5:07PM |
EV.00004: Multi-directional thrusting using oppositely traveling waves in knifefish swimming Oscar Curet, Malcolm MacIver, Neelesh Patankar \textit{Apteronotus albifrons}, also known as the black ghost knifefish, generate a weak electric field for omnidirectional sensing. This is matched by an extraordinary multi-directional swimming ability that is achieved by undulating a ribbon-like anal fin. Forward or backward motion is generated by a traveling wave on the ribbon fin. We have discovered that, for hovering and vertical swimming, the knifefish use two oppositely traveling waves on the ribbon fin. To understand the hydrodynamic mechanism of hovering and heave we performed fully resolved simulations of self-propulsion of the knifefish. We used kinematic inputs based on experimental observations. We found that the counter propagating waves generate two opposite streamwise jets along the bottom edge of the ribbon fin. These two jets meet approximately at the mid-section along the fin length and are deflected downward. The resultant downward momentum imparted to the fluid creates an upward force on the fish body which can be used for hovering or vertical swimming. There is a vortex ring pair of opposite directions at the middle of the fin that is associated with this fluid flow. Further insight into how the knifefish control heave and hovering was obtained from the measurements of force generated by a robotic ribbon fin for different wave parameters. [Preview Abstract] |
Sunday, November 22, 2009 5:07PM - 5:20PM |
EV.00005: On the ``momentum enhancement'' and hydrodynamic efficiency of gymnotiform and balistiform swimmers Anup Shirgaonkar, Neelesh Patankar, Malcolm MacIver Gymnotiform and balistiform swimmers generate thrust by undulating ribbon fins while keeping the body nearly rigid. The question of whether there is a hydrodynamic basis for this evolutionary adaptation was considered by Lighthill and Blake. They used a two-dimensional inviscid approach and explained this adaptation based on their finding that the thrust produced by an undulatory ribbon fin is much higher when it is attached to a rigid body. This was termed momentum enhancement. We revisited this problem by performing high-resolution numerical simulations to calculate the thrust generated by undulatory ribbon fins in a plate-fin model of a gymnotiform swimmer. We did not find momentum enhancement. This disagreement could be explained by noting that an axial jet along the bottom edge of the ribbon fin is the primary thrust producing mechanism. This flow is not significantly affected by the presence of the body thus leading to no momentum enhancement. Lighthill's theory does not capture this dominant mechanism of thrust production. We find that the observed relative size of the body and the ribbon fins is such that it tends to optimize the cost of transport, as opposed to simply maximizing thrust. We present scaling analysis that supports this finding. [Preview Abstract] |
Sunday, November 22, 2009 5:20PM - 5:33PM |
EV.00006: Labriform swimming of a ray-strengthened pectoral fin Kourosh Shoele, Qiang Zhu Labriform swimming is a common locomotion mode used by fish in low speed swimming, in which thrust generation is achieved through a combination of flapping and rowing motions of pectoral fins. Pectoral fins of bony fishes usually consist of a soft collagen membrane strengthened by embedded flexible rays. Morphologically, each ray is connected to a group of muscles so that the fish can control the rotational motion of each ray individually, enabling multi-degree of freedom control over the fin motion and deformation. We have developed a fluid-structure interaction model to simulate the kinematics and dynamic performance of a structurally idealized fin. This method includes a boundary-element model of the fluid motion and a fully-nonlinear Euler-Bernoulli beam model of the embedded rays. Using this model we studied thrust generation and propulsion efficiency of the fin at different combinations of parameters. Effects of kinematic as well as structural properties are examined. It has been illustrated that the fish's capacity to control the motion of each individual ray, as well as the anisotropic deformability of the fin determined by distribution of the rays, are essential to high propulsion performance. Specifically, it is found that a reinforced ray at the leading edge leads to performance enhancement. [Preview Abstract] |
Sunday, November 22, 2009 5:33PM - 5:46PM |
EV.00007: Hydrodynamic characteristics of sailfish and swordfish Woong Sagong, Woo-Pyung Jeon, Haecheon Choi The sailfish and swordfish are known as fastest sea animals, reaching their maximum speeds of more than 100km/h. Recently, Sagong \emph{et al.} (2008, Phys. Fluids) investigated the role of V- shaped protrusions existing on the sailfish skin in the skin-friction reduction but those protrusions did not make a direct role in reducing drag. On the other hand, the long bill has been regarded as a device of reducing drag by separation delay through turbulence generation. In the present study, we investigate the hydrodynamic characteristics of sailfish and swordfish by installing the stuffed ones in a wind tunnel and measuring the drag on their bodies and boundary-layer velocities above the body surfaces. The drag coefficients of sailfish and swordfish are 0.0075 and 0.009 based on the free-stream velocity and wetted area, respectively. They are comparable to or smaller than those of other kinds of fish such as the dogfish, tuna and trout. Next, the role of bill on the drag is studied. The drag without bill or with an artificial short bill is lower than that with the original long bill, indicating that the bill does not reduce the drag at all. From the velocity measurement near the body surfaces, we found that flow separation does not occur even without bill, and thus the conjecture that the flow separation is delayed through turbulence generation by the bill is not valid. [Preview Abstract] |
Sunday, November 22, 2009 5:46PM - 5:59PM |
EV.00008: Turbulence augmentation to achieve separation control over a bristled shark skin model Leah Mendelson, Amy Lang, Drew Smith The skin of fast-swimming sharks is covered with scale-like denticles capable of bristling to form cavities instead of lying flat against the body. These may be valuable for delaying flow separation and reducing net drag forces. This complex 3D roughness geometry alters the flow through the creation of vortices in the cavities and augments the behavior of the boundary layer. Understanding these flow phenomena is necessary for replicating the shark's passive flow control mechanisms. A model of bristled shortfin mako denticles in turbulent flow was tested in a water tunnel facility using Digital Particle Image Velocimetry (DPIV) to study the impact of the shark skin on the boundary layer. Results of these experiments, including influence on the time-averaged boundary layer profiles and Reynolds stresses over the span of the model, will be discussed. [Preview Abstract] |
Sunday, November 22, 2009 5:59PM - 6:12PM |
EV.00009: Effect of Varying the Angle of Attack of the Scales on a Biomimetic Shark Skin Model on Embedded Vortex Formation Jennifer Wheelus, Amy Lang The skin of fast-swimming sharks is proposed to have mechanisms to reduce drag and delay flow separation. The skin of fast-swimming sharks is covered with small denticles, on the order of 0.2 mm, that if bristled create cavities. It has been shown that for an angle of attack of 90 degrees, vortices form within these cavities and impose a partial slip condition at the surface of the cavity. This experiment focuses on smaller angles of attack for denticle bristling, closer to the range thought to be achieved on real shark skin. A 3-D bristled shark skin model with varying angle of attack, embedded below a boundary layer, was used to study the formation of cavity vortices through fluorescent dye visualization and Digital Particle Image Velocimetry (DPIV). The effect of varying angle of attack on vortex formation will be discussed. [Preview Abstract] |
Sunday, November 22, 2009 6:12PM - 6:25PM |
EV.00010: Energetics and Motion Planning for Hamiltonian Fishlike Locomotion Scott Kelly, Parthesh Pujari The self-propulsion of an undulating body suspended in a fluid hinges on the judicious excitation of the fluid itself, in that forward momentum is developed by the body as equal and opposite momentum is imparted to the fluid. The manner in which this is accomplished is recorded in the structure of the body's wake; the efficiency with which it's accomplished is reflected in the fluid's evolving kinetic energy. Focusing on a model for the self-propulsion of a free hydrofoil with variable camber in an infinite planar fluid, we examine relationships between economy of deformation, wake structure, and wake energetics. The model in question simplifies the underlying physics by restricting vortex shedding to the trailing point of the foil, discretizing shed vorticity, and neglecting dissipation --- allowing the remaining dynamics to be framed in a Hamiltonian setting. We consider the self-propulsion of the foil both in a quiescent fluid and in a variety of vortex flows representing wake structures present in vehicle schools. [Preview Abstract] |
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