Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R6: Biofluids: Predicting Effective Locomotion |
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Chair: Geoffrey Spedding, University of Southern California Room: 3010 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R6.00001: Schooling of flapping wings: Experiments Leif Ristroph, Alexander Becker, Hassan Masoud, Joel Newbolt, Michael Shelley The role of fluid dynamics in mediating schooling and flocking remains unclear because of the complex interactions between locomotors and their flow fields. We study such interactions for flapping wings that swim within the wakes of others in an array and discover ``schooling modes'' characterized by preferred spatial phase relationships. These modes are associated with surprising effects including the doubling of swimming speed for small changes in flapping kinematics and the coexistence of two possible speeds for identical kinematics. Flow visualization shows how these dynamics arise from repeated constructive or destructive interactions of a wing with the wave-like flow into which it swims. By establishing how coherent collective behavior emerges naturally for flapping locomotion, these results provide a physical basis to interpret the structure and dynamics of animal groups. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R6.00002: Schooling of flapping wings: Simulations Hassan Masoud, Alexander Becker, Leif Ristroph, Michael Shelley We examine the locomotion of an infinite array of wings that heave vertically with a prescribed sinusoidal motion and are free to translate in the horizontal direction. To do this, we simulate the motion of a freely translating flapping airfoil in a domain with periodic horizontal boundary conditions. These simulations indicate that the wings can ``take advantage'' of their collectively generated wake flows. In agreement with our experiments in a rotational geometry, we find ranges of flapping frequency over which there are multiple stable states of locomotion, with one of these swimming states having both higher speeds and efficiencies than an isolated flapping and locomoting wing. A simple mathematical model, which emphasizes the importance of history dependence in vortical flows, explains this multi-stability. These results may be important to understanding the role of hydrodynamic interactions in fish schooling and bird flocking. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R6.00003: Performance characteristics of pitching flexible foil propulsors Cody Brownell, Brendan Egan, Mark Murray Performance characteristics of flexible foil propulsors are studied experimentally. The project investigates the dependence of thrust and efficiency on foil elasticity, Strouhal number, and flow velocity. The experiments took place in a large recirculating water channel, using full span flexible propulsor models to approximate a 2D geometry. The propulsor pitched about a fixed axis at its quarter chord, with a six-axis load cell measuring the forces and torques on the shaft. Propulsive efficiency is found to peak at an optimum Strouhal number for each foil tested. Varying elasticity did not produce a similar local maximum over the sampled parameter space. The ensemble data will facilitate the engineering of fish-like propulsion systems for future application of this technology. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R6.00004: Survival of the fastest: Evolving wings for flapping flight Sophie Ramananarivo, Thomas Mitchel, Leif Ristroph To optimize flapping flight with regard to wing shape, we use an evolutionary or genetic algorithm to improve the forward speed of 3d-printed wings or hydrofoils that heave up-and-down and self-propel within water. In this scheme, ``genes'' are mathematical parameters specifying wing shape, and ``breeding'' involves the merging and mutation of genes from two parent wings to form a child. A wing's swimming speed is its ``fitness'', which dictates the likelihood of breeding and thus passing on its genes to the next generation. We find that this iterative process leads to marked improvements in relatively few generations, and several distinct shape features are shared among the fastest wings. We also investigate the favorable flow structures produced by these elite swimmers and compare their shape and performance to biologically evolved wings, fins, tails, and flippers. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R6.00005: PIV-based pressure, force, and torque measurements of a robotic model swimmer John Dabiri, Kelsey Lucas, Patrick Thornycroft, George Lauder We apply a recently developed technique for non-invasive pressure measurement to study the dynamics of anguilliform swimming by a robotic flapping foil. The method is based on spatial integration of time-resolved particle image velocimetry measurements. The pressure gradient computed from the Navier-Stokes equations is integrated along multiple paths in the domain, and the local pressure is determined by the median value of the integration results. In addition, the pressure field is integrated on the surface of the foil to compute the instantaneous forces and torque exerted by the foil on the fluid. Direct force and torque measurements from a load cell are used to confirm the accuracy of the PIV-based measurements. Results for flapping foils of varying flexibility are compared to infer the role of the pressure field in the dynamics and energetic efficiency of locomotion. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R6.00006: Underlying principle of efficient propulsion in flexible plunging foil Xing Zhang, Xiaojue Zhu, Guowei He Recently, it has been reported that passive flexibility in flapping foils can result in the enhancement of propulsive performance. In this study, we investigate the relations among propulsive efficiency, structural resonance and hydrodynamic wake resonance in a flexible plunging foil. We conduct fluid-structure-interaction simulations on flows over flexible plunging foils by using the immersed boundary method. The wake resonant frequency is computed by performing a linear stability analysis on the averaged velocity profile. The results indicates that: (i) optimal efficiency is not necessarily achieved at the structural resonance point; and (ii) optimal efficiency always occurs when the driving frequency matches the wake resonant frequency. By dissecting the wake structures, we found that whether the optimal efficiency is achieved at the structural resonance point depends on the strength of the leading edge vortex (LEV) relative to that of the trailing edge vortex (TEV). In addition, the validity of the aforesaid principle under the condition of free-swimming (self-propulsion) is also discussed. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R6.00007: Swimming Efficiently: An Analytical Study of Optimal Swimming in Fish A. Josh Wiens, Anette Hosoi The Strouhal Number $(St)$, is widely considered to be the defining parameter for efficient undulatory swimming. Biological studies have shown that fish species across a broad range of shapes and sizes adhere to a narrow $St$ range ($0.2 < St < 0.4$). Despite its significance, $St$ alone provides an incomplete description of the kinematics and geometry of a swimming fish. The dimensionless speed and amplitude of the body wave, along with the size and shape of the body can also play a significant role in swimming performance. We apply Lighthill's elongated body theory to construct a simple but powerful reduction of the steady-swimming problem. Through this reduction, the energetic efficiency of a swimming fish can be directly expressed as an analytical function of body geometry and kinematics. In this reduced form, the interplay between the parameters of the system, and their collective role in determining the performance of the swimmer can be readily observed and understood. In particular, the reduced model is applied to understand how wave amplitude, wave speed, and $St$ must relate for optimal swimming efficiency. Following this, we then explore how these relationships are altered by geometric factors such as tail size and compliance. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R6.00008: The hydrodynamic principle for the caudal fin shape of small aquatic animals Jeongsu Lee, Yong-Jai Park, Kyu-Jin Cho, Ho-Young Kim The shape of caudal fins of small aquatic animals is completely different from that of large cruising animals like dolphin and tuna which have high aspect-ratio lunate tail. To unveil the physical principle behind natural selection of caudal fins of small aquatic animals, here we investigate the hydrodynamics of an angularly reciprocating plate as a model for the caudal fin oscillation. We find that the thrust production of a reciprocating plate at high Strouhal numbers is dominated by generation of two distinct vortical structures associated with the acceleration and deceleration of the plate regardless of their shape. Based on our observations, we construct a scaling law to predict the thrust of the flapping plate, which agrees well with the experimental data. We then seek the optimal aspect ratio to maximize thrust and efficiency of a flapping plate for fixed flapping frequency and amplitude. Thrust is maximized for the aspect ratio of approximately 0.7. We also theoretically explain the power law behaviors of the thrust and efficiency as a function of the aspect ratio. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R6.00009: Maximizing the efficiency of a flexible propulsor using experimental optimization Daniel Quinn, George Lauder, Alexander Smits Experimental gradient-based optimization is used to maximize the propulsive efficiency of a heaving and pitching flexible panel. Optimum and near-optimum conditions are studied via direct force measurements and Particle Image Velocimetry (PIV). The net thrust and power are found to scale predictably with the frequency and amplitude of the leading edge, but the efficiency shows a complex multimodal response. Optimum pitch and heave motions are found to produce nearly twice the efficiencies of optimum heave-only motions. Efficiency is globally optimized when (1) the Strouhal number is within an optimal range that varies weakly with amplitude and boundary conditions; (2) the panel is actuated at a resonant frequency of the fluid-propulsor system; (3) heave amplitude is tuned such that trailing edge amplitude is maximized while flow along the body remains attached; and (4) the maximum pitch angle and phase lag are chosen so that the effective angle of attack is minimized. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R6.00010: New drag laws for flapping flight Natalie Agre, Jun Zhang, Leif Ristroph Classical aerodynamic theory predicts that a steadily-moving wing experiences fluid forces proportional to the square of its speed. For bird and insect flight, however, there is currently no model for how drag is affected by flapping motions of the wings. By considering simple wings driven to oscillate while progressing through the air, we discover that flapping significantly changes the magnitude of drag and fundamentally alters its scaling with speed. These measurements motivate a new aerodynamic force law that could help to understand the free-flight dynamics, control, and stability of insects and flapping-wing robots. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R6.00011: Scaling macroscopic aquatic locomotion Mattia Gazzola, Mederic Argentina, Lakshminarayanan Mahadevan Inertial aquatic swimmers that use undulatory gaits range in length $L$ from a few millimeters to 30 meters, across a wide array of biological taxa. Using elementary hydrodynamic arguments, we uncover a unifying mechanistic principle characterizing their locomotion by deriving a scaling relation that links swimming speed $U$ to body kinematics (tail beat amplitude $A$ and frequency $\omega$) and fluid properties (kinematic viscosity $\nu$). This principle can be simply couched as the power law $Re \sim Sw^{\alpha}$, where $Re = UL/\nu \gg 1$ and $Sw = \omega A L/\nu$, with $\alpha = 4/3$ for laminar flows, and $\alpha =1$ for turbulent flows. Existing data from over 1000 measurements on fish, amphibians, larvae, reptiles, mammals and birds, as well as direct numerical simulations are consistent with our scaling. We interpret our results as the consequence of the convergence of aquatic gaits to the performance limits imposed by hydrodynamics. [Preview Abstract] |
Tuesday, November 25, 2014 3:28PM - 3:41PM |
R6.00012: Modulation of a Flow Field by Dragonfly Nymph Valve Kinematics Chris Roh, Morteza Gharib Previously, we visualized a respiratory jet and a propulsive jet of a dragonfly nymph using laser induced fluorescence. A more quantitative measurement of the dragonfly nymph's underwater breathing was investigated using digital particle image velocimetry. Simultaneously, dragonfly's anal valve kinematics were recorded using high-speed videography. The result shows an active usage of the valve during exhalation and inhalation to modulate the flow field. Calculating a Lagrangian particle path by time integration of the velocity field showed that the exhaled fluid is not inhaled back. This result suggests that the anal valve modulation of the flow field prevents the rebreathing of the exhaled jet. [Preview Abstract] |
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