Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D17: Biofluids: Locomotion II - Swimming |
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Chair: Azar Eslampanah, University of Iowa Room: 305 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D17.00001: Hovering of a jellyfish-like flying machine Leif Ristroph, Stephen Childress Ornithopters, or flapping-wing aircraft, offer an alternative to helicopters in achieving maneuverability at small scales, although stabilizing such aerial vehicles remains a key challenge. Here, we present a hovering machine that achieves self-righting flight using flapping wings alone, without relying on additional aerodynamic surfaces and without feedback control. We design, construct, and test-fly a prototype that opens and closes four wings, resembling the motions of swimming jellyfish more so than any insect or bird. Lift measurements and high-speed video of free-flight are used to inform an aerodynamic model that explains the stabilization mechanism. These results show the promise of flapping-flight strategies beyond those that directly mimic the wing motions of flying animals. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D17.00002: Do resonating bells increase jellyfish swimming performance? Alexander Hoover, Laura Miller A current question in swimming and flight is whether or not driving flexible appendages at their resonant frequency results in faster or more efficient locomotion. It has been suggested that jellyfish swim faster and/or more efficiently when the bell is driven at its resonant frequency. Previous work has modeled the jellyfish bell as a damped harmonic oscillator, and this simplified model suggests that work done by the bell is maximized when force is applied at the resonant frequency of the bell. We extend the idea of resonance phenomena of the jellyfish bell to a fluid structure interaction framework using the immersed boundary method. We first examine the effects of the bending stiffness of the bell on its resonant frequency. We then further our model with the inclusion of a ``muscular'' spring that connects the two sides of a 2D bell and drives it near its resonant frequency. We use this muscular spring to force the bell at varying frequencies and examine the work done by these springs and the resulting swimming speed. We finally augment our model with a flexible, passive bell margin to examine its role in propulsive efficiency. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D17.00003: A Simple Computational Model of a jellyfish-like flying machine Fang Fang, Leif Ristroph, Michael Shelley We explore theoretically the aerodynamics of a jellyfish-like flying machine recently fabricated at NYU. This experimental device achieves flight and hovering by opening and closing a set of flapping wings. It displays orientational flight stability without additional control surfaces or feedback control. Our model machine consists of two symmetric massless flapping wings connected to a body with mass and moment of inertia. A vortex sheet shedding and wake model is used for the flow simulation. Use of the Fast Multipole Method (FMM), and adaptive addition/deletion of vortices, allows us to simulate for long times and resolve complex wakes. We use our model to explore the physical parameters that maintain body hovering, its ascent and descent, and investigate the stability of these states. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D17.00004: Ultra-fast Escape of a Octopus-inspired Rocket Gabriel Weymouth, Michael Triantafyllou The octopus, squid, and other cephalopods inflate with water and then release a jet to accelerate in the opposite direction. This escape mechanism is particularly interesting in the octopus because they become initially quite bluff, yet this does not hinder them in achieving impressive bursts of speed. We examine this somewhat paradoxical maneuver using a simple deflating spheroid model in both potential and viscous flow. We demonstrate that the dynamic reduction of the width of the body completely changes the flow and forces acting on the escaping rocket in three ways. First, a body which reduces in size can generate an added mass thrust which counteracts the added mass inertia. Second, the motion of the shrinking wall acts similar to suction on a static wall, reducing separation and drag forces in a viscous fluid, but that this effects depends on the rate of size change. Third, using a combination of these two features it is possible to initially load the fluid with kinetic energy when heavy and bluff and then recover that energy when streamlined and light, enabling ultra-fast accelerations. As a notable example, these mechanisms allow a shrinking spheroid rocket in a heavy inviscid fluid to achieve speeds greater than an identical rocket in the vacuum of space. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D17.00005: Hydrodynamics of a Digitized Adult Humpback Whale Flipper Wesley N. Fassmann, Samuel J. McDonald, Scott L. Thomson, Frank E. Fish During feeding, humpback whales turn with a turn radius of up to $1/6^{th}$ of their length towards schools of fish enclosed by bubble nets. This high maneuverability requirement is facilitated by high aspect ratio flippers with leading edge tubercles that delay stall. Previous experimental and computational studies have used idealized models, such as airfoils with scalloped leading edges, to explore the influence of leading edge tubercles on boundary layer separation, vortex generation, and airfoil lift and drag characteristics. Owing to the substantial size of the flipper, no studies have been performed on a digitized adult humpback flipper with real geometry. In this study the hydrodynamics of a realistic humpback flipper model were explored. The model was developed by digitizing a sequence of 18 images circumscribing the suspended flipper of a beached humpback whale. A physical prototype was constructed based on the resulting 3D model, along with a complementary model with the tubercles removed. Experimentally-obtained measurements of lift and drag were used to study the influence of the tubercles. In the presentation, digitization and flow measurement methods are described, and the flow data and results are presented and discussed. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D17.00006: Effects of leading edge tubercles on the flow over a humpback whale flipper Heesu Kim, Jooha Kim, Haecheon Choi In the present study, we conduct a laboratory experiment for the effect of tubercles on the hydrodynamic performance of a humpback whale flipper. The shape of the flipper used is the same as that of Miklosovic et al. (2004, 2007), and the Reynolds number considered is 100,000 based on the free-stream velocity and mean chord length. The lift and drag forces on the flipper with and without tubercles are measured by varying the angle of attack, and PIV measurements are conducted in several cross-flow planes at a few different angles of attack. As observed in previous studies, the stall angle is delayed and the maximum lift coefficient is increased. Without tubercles, the cross flow above the flipper does not show large-scale vortical motions except tip vortex. With tubercles, however, strong streamwise vortices having negative streamwise vorticity are observed along the tubercles, but the vortices with positive streamwise vorticity are either relatively weak or unobserved. This result is very different from those found in a two-dimensional wing with tubercles with which strong counter-rotating streamwise vortex pair were observed. Those vortical motions reattach the flow on the flipper and delay the separation. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D17.00007: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D17.00008: Efficiency is designed into free swimming Mehdi Saadat, Hossein Haj-Hariri In free swimming the swim speed and Strouhal number (St) are outputs. St alone is insufficient to decide optimal motion because many inefficient combinations of amplitude and frequency lead to the same St. This is manifested by the coincidence of the iso-lines for speed, St, and thrust. For a given combination of propulsor and body, St of motion is essentially independent of amplitude, frequency, and speed, and is only a function of shape. Some motions are efficient, and some are not. But they all have the same St. For a simple swimmer, there is a sweet spot in the dimensionless amplitude vs. frequency plane (for a fixed U) where the power efficiency is maximized. That is the place where the swimmer lives. And as long as the swimmer modulates its speed by keeping its amplitude fixed, and modulating the frequency, then the animal will always swim efficiently. So nature is efficient not because the animals are monitoring their motion in real time, but because the design of the animal is such that it cannot be inefficient. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D17.00009: Hydrodynamic flows can induce selective advantages among species Francesca Tesser, Roberto Benzi, Herman J.H. Clercx, David R. Nelson, Prasad Perlekar, Federico Toschi Evolutionary forces such as genetic drift, selection, mutation and spatial diffusion act to change the genetic composition of populations. Such problems can be modeled as a system of binary reactions between competing individuals, involving births and deaths, and progressing at specific rates. An inhomogeneous or time-dependent spatial structure has the effect of modulating the interaction between individuals. To explore this problem further, we consider the dynamics and evolution of genetically diverse populations in a fluid environment where a flow field transports individuals in combination with birth and death processes [1], thus driving genetic inhomogeneities. An individual-based model in continuous space with spatial diffusion implements stochastic demographic rules for a fluctuating population size and introduces the advection of simple realistic flow fields. The system is analyzed in terms of fixation probabilities and fixation times as well as the behavior of spatial correlations. Provided organismic reproduction times are faster than the characteristic time scales of the flow, fluid ecosystems can by themselves induce spatially non-homogeneous selective advantages. \\[4pt] [1] Pigolotti et al. Theoretical Population Biology 84, 72 (2013) [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D17.00010: Mechanism of maximum thrust generation by oscillating compliant caudal-fin model in a quiescent fluid Hyungmin Park, Yong-Jai Park, Kyu-Jin Cho, Haecheon Choi A certain level of flexibility of moving appendage like a fin enhances its hydrodynamic performance (e.g., thrust generation). However, little efforts have been spent to characterize the condition in which the beneficial impact of compliance is maximized. Recent report (Park \emph{et al}. 2012, TRO) has shown that a sinusoidally oscillating caudal-fin model generates the maximum thrust when its compliance creates a phase difference ($\xi$) of $\pi/2$ between the oscillating and fin-bending angles, irrespective of its planform shape. To establish the underlying mechanism, we have analyzed the time-averaged and instantaneous flow fields around nine (9) oscillating caudal-fin models with varying their compliance. A series of particle image velocimetry measurements were performed in a quiescent water tank. When $\xi < \pi/2$, a strong interaction between the separated trailing-edge vortex (TEV) and the TEV forming at next stroke directs the flow in a transverse way, thereby enhancing the decay of thrust-generating jet velocity. At $\xi = \pi/2$, this interaction is weak such that the fast jet velocity is retained along the streamwise direction. On the other hand, when $\xi > \pi/2$, the trailing-edge is moving opposite to the oscillation reducing the rotational circulation of TEV. [Preview Abstract] |
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