Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G15: Biofluids: Large Swimmers III |
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Chair: David Hu, Georgia Institute of Technology Room: 28A |
Monday, November 19, 2012 8:00AM - 8:13AM |
G15.00001: Thrust and power measurements of Olympic swimmers Timothy Wei, Vicki Wu, Sean Hutchison, Russell Mark Elite level swimming is an extremely precise and even choreographed activity. Swimmers not only know the exact number of strokes necessary to take them across the pool, they also plan to be a precise distance from the wall at the end of their last stroke. Too far away and they lose time by drifting into the wall. Too close and their competitor may slide in before their hand comes forward to touch the wall. In this context, it is important to know, in detail, where and how a swimmer propels her/himself through the water. Over the past decade, state-of-the-art flow and thrust measurement diagnostics have been brought to competitive swimming. But the ability to correlate stroke mechanics to thrust production without somehow constraining the swimmer has here-to-fore not been possible. Using high speed video, a simple approach to mapping the swimmer's speed, thrust and net power output in a time resolved manner has been developed. This methodology has been applied to Megan Jendrick, gold medalist in the 100 individual breast stroke and 4 x 100 medley relay events in 2000 and Ariana Kukors, 2009 world champion and continuing world record holder in the 200 individual medley. Implications for training future elite swimmers will be discussed. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G15.00002: Effects of flexibility on bio-inspired aquatic propulsion Peter Dewey, Birgitt Boschitsch, Alexander Smits We present the results of an experimental investigation aimed at understanding the role that flexibility plays in bio-inspired aquatic propulsion. A rectangular pitching panel apparatus, where both the flexibility and aspect ratio can be systematically varied, is utilized as a simplified model for bio-inspired propulsion in water at a Reynolds number of 7200. It is found that, when optimized, flexibility can double the thrust produced and propulsive efficiency achieved in comparison to a rigid panel. There is a notable thrust enhancement when the flexible panels are operating near resonance; however, it is found that resonance is not the primary mechanism governing efficient propulsion. Peaks in propulsive efficiency are found below, at, and above the resonant frequency depending on the flexibility of the panel. Finally, a scaling law is derived that is shown to collapse the thrust production, power consumption, and propulsive efficiency data across all panels examined. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G15.00003: Thrust production of free-to-pivot plates at low Reynolds number Kenneth Granlund, Michael Ol, Luis Bernal As an abstraction of flapping-wing aerodynamics, rigid flat plates free-to-pivot at the leading edge between incidence angle limits of $\pm $45\r{ } are considered in rectilinear as well as waving motion in a quiescent fluid. Thrust (lift) and resistive-force are measured, forming a hover Figure-of-Merit (FoM). The evolution of spatial retention of a leading edge vortex is tracked throughout the motion cycle, showing vortex formation shortly after the plate completes its rotation, and in some cases shedding of subsequent vortices after the initial leading edge vortex is ejected. Vortex evolution in rectilinear- vs. rotating and steady vs. accelerating motion is visualized with fluorescent dye illuminated by a laser light sheet at several spanwise stations along the leading edge. Experiments in water reveal a Reynolds number indifference in thrust and FoM for 8,000$<$Re$<$31,000 based on the maximum velocity of the leading edge. A study on aspect ratio from 3.4 to a nominally 2D-plate also shows an indifference in vortex shedding, force coefficients and FoM. The main operative parameter for aerodynamic coefficients is the stroke-to-chord ratio of the leading edge, with decay in both thrust production and FoM as the ratio approaches unity. Prescribed kinematics of varying phase lead/lag of the pitch- vs. stroke history from free-to-pivot cases shows the effect on attachment of the leading edge vortex and related thrust production and FoM. The effect on flow and force for several orders of magnitude lower Reynolds number are investigated by performing the experiment in varying mixtures of glycerin and water. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G15.00004: Swimming Near the Wall Daniel Quinn, Keith Moored, Peter Dewey, George Lauder, Alexander Smits The aerodynamic loads on rectangular panels undergoing heave and pitch oscillations near a solid wall were measured using a 6-axis ATI sensor. Over a range of Strouhal numbers, reduced frequencies and flexibilities, swimming near the wall was found to increase thrust and therefore the self-propelled swimming speed. Experimental particle image velocimetry revealed an asymmetric wake structure with a momentum jet angled away from the wall. Both the thrust amplification and the asymmetric wake structure were verified and investigated further using an in-house inviscid panel method code. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G15.00005: On the efficiency of fish like swimming Michel Bergmann, Angelo Iollo The aim of this talk is to present a parametric study of underwater locomotion via numerical simulations. The Navier-Stokes equations are discretized onto a cartesian mesh and the interface between the fluid and the fish is computed using an immersed boundary method. The lagrangian motion of the swimmer is computed from the Newton's laws. We present results showing how the swimming efficiency is influenced by the reynolds number and the swimming law. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G15.00006: The Effects of Limb Coordination on the Swimming Efficiency of Crayfish Robert Guy, Jiawei Zhang, Qinghai Zhang, Timothy Lewis Limbs of crayfish, called swimmerets, move rhythmically in a metachronal wave that progresses from back to front during forward swimming. Neighboring swimmerets maintain phase-lags of about 25\% over a wide range of frequencies. This ``phase constancy'' suggests that there may be mechanical advantages to this stroke pattern. We use the immersed-boundary method to simulate the coupled mechanics of the swimmerets and the surrounding fluid in order to explore how stroke patterns affect swimming efficiency. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G15.00007: Model of skin friction enhancement in undulatory swimming Uwe Ehrenstein, Christophe Eloy To estimate the energetic cost of undulatory swimming, it is crucial to evaluate the drag forces originating from skin friction. This topic has been controversial for decades, some claiming that animals use ingenious mechanisms to reduce the drag and others hypothesizing that the undulatory motion induces a drag increase because of the compression of the boundary layers. In this paper, we examine this latter hypothesis, known as the ``Bone--Lighthill boundary-layer thinning hypothesis''.\footnote{M.J. Lighthill, {\emph{Proc. R. Soc. Lond. B}} {\bf{179}}, 125 (1971).} Considering a plate of section $s$ moving perpendicular to itself at velocity $U_\perp$ and applying the boundary-layer approximation for the incoming flow, the drag force per unit surface is shown to scale as $\sqrt{U_\perp/s}$. An analogous two-dimensional Navier-Stokes problem by artificially accelerating the flow in a channel of finite height is solved numerically, showing the robustness of the analytical results. Solving the problem for an undulatory plate motion similar to fish swimming, we find a drag enhancement which can be estimated to be of the order of 20 to 100\%, depending on the geometry and the motion. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G15.00008: Optimal number of waves for ribbon fin propulsion Rahul Bale, Amneet P.S. Bhalla, Malcolm A. MacIver, Neelesh A. Patankar Many species of fish, such as \textit{rajiform, diodontiform, amiiform, gymnotiform} and \textit{balistiform }swimmers, use a ribbon fin as their primary mode of propulsion. It has been observed that each fish species, depending on its size, uses a specific number of waves on its ribbon fin. For example, the black ghost knifefish (10$\times $1cm fin) typically uses 2-2.5 waves while a giant oarfish (3$\times $0.1m fin) uses 6-8 waves. In this work we investigate whether this leads to optimal axial thrust. The axial thrust generated depends on the efficiency with which fin waves transport the fluid backward. We find that there are two competing mechanisms. On the one hand, an increase in wavelength (at fixed amplitude and frequency of the wave), and therefore the wave velocity, leads to an increased ability to transport the fluid backward. This leads to more thrust. On the other hand, longer wavelength leads to shallower waves. This reduces the efficacy to transport the fluid backward and reduces the thrust. The optimal wavelength, and therefore the optimal number of waves, is a result of a balance between the two competing mechanisms. We do our analysis in terms of specific wavelength, which is the wavelength non-dimensionalized by the wave amplitude. We find that the value of the specific wavelength at which the axial thrust is maximized is universal for ribbon fins and it is in agreement with biological data. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G15.00009: Shark Skin Bristling: A Passive Flow-Actuated Separation Control Mechanism Amy Lang, Jonathon Smith, Michael Bradshaw, Jennifer Wheelus, Philip Motta, Maria Habegger, Jessica Davis, Robert Hueter A collaborative experimental effort between biologists and engineers has proven the separation control capability of shark skin, with a specific focus on the shortfin mako (\textit{Isurus oxyrinchus}) known for its high speed and agility. Biological measurements of the denticles, or scales, as a function of body location (DOI:10.1002/jmor.20047) will be presented together with data on bristling angle of scales and the morphological implications. Results show key regions of high bristling capability to correspond with those most prone to flow separation; these include the tail, flank regions aft of the gills, and on pectoral fins with scale flexibility increasing towards the trailing edge. Fresh shark skin samples were also tested in a water tunnel facility using DPIV and evidence of flow separation control was observed under laminar and tripped boundary layer conditions. It was concluded that the experiments conducted in the Re $\sim $ 10$^{5}$ range resulted in sufficiently strong backflow induced close to the surface such that the shear threshold to induce bristling on the real skin sample was achieved since flow control at lower Re was not as evident. It is hypothesized that backflow initiated close to the wall in a region of adverse pressure gradient induces localized scale bristling thereby interrupting the subsequent flow development that leads to global flow separation from the surface and increased drag. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G15.00010: Underlying principles of flexible bio-inspired propulsion: Hydrodynamic wake resonance analysis Keith Moored, Peter Dewey, Alexander Smits, Hossein Haj-Hariri Experiments on flexible pitching panels have demonstrated that flexibility can be utilized to double both thrust production and propulsive efficiency. Yet, the exact mechanisms that lead to efficient locomotion with flexible propulsors are not understood. Using experimental particle image velocimetry data from flexible pitching panels, a linear stability analysis is performed and compared with efficiency data. It is shown that when the driving frequency of motion matches a wake resonant frequency a peak in efficiency occurs not just for rigid panels but also for flexible panels, and that the panel flexibility that leads to global optimally efficient locomotion is the one where a structural resonant frequency is tuned to a wake resonant frequency. The primary mechanism to achieve efficient locomotion is to tune the frequency of motion to a wake resonant frequency, while the secondary mechanism is to tune a structural resonant frequency to a wake resonant frequency. The untuned flexible panels exhibit at most a 46\% increase in efficiency over the rigid panel, while the tuned flexible panels attain global optimally efficient locomotion with a 100 to 108\% increase in efficiency. [Preview Abstract] |
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