Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A27: Biofluids: Flapping |
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Chair: Matthew Ringuette, State University of New York at Buffalo Room: 308 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A27.00001: Vortex Loop Topology During the Stroke Reversal of a Flapping Wing Matthew Burge, Clara Wysochanski, Matthew Ringuette The effect of kinematic variations on the instantaneous 3-D flow structures formed during stroke-reversal of a 2-degree-of-freedom flapping wing in hover is investigated. Previous work correlates large force and circulation peaks to unsteady motion kinematics, but information from experiments detailing the instantaneous, 3-D flow-structure evolution is lacking. The objective of this work is to generate the vortex topology of a flapping wing in hover and qualitatively study the flow-structure trajectories with multi-color dye-flow visualization. Pure pitching and fixed angle of attack rotation are first examined to identify the vortices produced by each degree-of-freedom separately. For flapping motions, the stroke-reversal phase during various rotational accelerations for a constant pitching reduced frequency is studied, emphasizing vortex interactions and re-connectivity of time-elapsed vortex loops. The flow features are visualized using a scaled wing model in water with an internal dye-manifold, and captured using 2 orthogonal cameras. Motivation exists for both symmetric and advanced timing of the pitching with respect to stroke-reversal, and both are compared against pitching reduced frequency to characterize the 3-D loop structures responsible for lift generation. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A27.00002: Schooling of two tandem flapping wings: Simulations and theory Fang Fang, Sophie Ramananarivo, Leif Ristroph, Michael Shelley We examine theoretically the hydrodynamic interaction of two tandem flapping wings. The two wings heave vertically with the same prescribed sinusoidal motion and each wing is free to choose its locomotion speed in the horizontal direction. We model the wings as flat plates and apply an improved vortex sheet simulation method to study their interaction through the fluid. Multiple stable schooling states are found from simulations and are consistent with experimental results. By applying an external load on the follower wing, we map out an effective hydrodynamic potential acting on the follower as a function of the ``schooling number", which is defined as the tail-to-head separation distance over the wake wavelength. The hydrodynamic potential and drag-induced dissipation function are also calculated theoretically by applying a linear theory for the motion of the leader, the wake it produces, and for its effect on the follower. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A27.00003: ''Schooling'' of wing pairs in flapping flight Sophie Ramananarivo, Jun Zhang, Leif Ristroph The experimental setup implements two independent flapping wings swimming in tandem. Both are driven with the same prescribed vertical heaving motion, but the horizontal motion is free, which means that the swimmers can take up any relative position and forward speed. Experiments show however clearly coordinated motions, where the pair of wings `crystallize' into specific stable arrangements. The follower wing locks into the path of the leader, adopting its speed, and with a separation distance that takes on one of several discrete values. By systematically varying the kinematics and wing size, we show that the set of stable spacings is dictated by the wavelength of the periodic wake structure. The forces maintaining the pair cohesion are characterized by applying an external force to the follower to perturb it away from the `stable wells'. These results show that hydrodynamics alone is sufficient to induce cohesive and coordinated collective locomotion through a fluid, and we discuss the hypothesis that fish schools and bird flocks also represent stable modes of motion. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A27.00004: Experimental Investigation of the Unsteady Flow Structures of Two Interacting Pitching Wings Melike Kurt, Keith Moored Birds, insects and fish propel themselves with unsteady motions of their wings and fins. Many of these animals are also found to fly or swim in three-dimensional flocks and schools. Numerous studies have explored the three-dimensional steady flow interactions and the two-dimensional unsteady flow interactions in collectives. Yet, the characterization of the three-dimensional unsteady interactions remains relatively unexplored. This study aims to characterize the flow structures and interactions between two sinusoidally pitching finite-span wings. The arrangement of the wings varies from a tandem to a bi-plane configuration. The vortex structures for these various arrangements are quantified by using particle image velocimetry. The vortex-wing interactions are also characterized as the synchrony between the wings is modified. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A27.00005: Investigating the Force Production of Functionally-Graded Flexible Wings in Flapping Wing Flight Durlav Mudbhari, Malcolm Erdogan, Kai He, Daniel Bateman, Rory Lipkis, Keith Moored Birds, insects and bats oscillate their wings to propel themselves over long distances and to maneuver with unprecedented agility. A key element to achieve their impressive aerodynamic performance is the flexibility of their wings. Numerous studies have shown that homogeneously flexible wings can enhance force production, propulsive efficiency and lift efficiency. Yet, animal wings are not homogenously flexible, but instead have varying material properties. The aim of this study is to characterize the force production and energetics of functionally-graded flexible wings. A partially-flexible wing composed of a rigid section and a flexible section is used as a first-order model of functionally-graded materials. The flexion occurs in the spanwise direction and it is affected by the spanwise flexion ratio, that is, the ratio of the length of the rigid section compared to the total span length. By varying the flexion ratio as well as the material properties of the flexible section, the study aims to examine the force production and energetics of flapping flight with functionally-graded flexible wings. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A27.00006: Torsional spring is the optimal flexibility arrangement of a flapping wing Nick Moore While it is understood that flexibility can improve the propulsive performance of flapping wings and fins, the flexibility distribution leading to optimal performance has not been explored. Using 2D small-amplitude theory and a fast Chebyshev method, we examine how thrust depends on the chord-wise distribution of wing stiffness. Through numerical optimization, we find that focusing flexibility at the wing's front, e.g.~through a torsional spring, maximizes thrust. A wing with an optimally chosen spring constant typically generates 36\% more thrust than a wing of optimal uniform stiffness. These results may relate to material distributions found in nature, such as insect wings, and may apply to the design of biomimetic swimmers and flyers, such as ornithopters. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A27.00007: Flapping locomotion of a flexible wing with heaving motion Sunghyuk Im, Hyung Jin Sung The flapping locomotion of a freely heaving flexible wing was experimentally explored in a merry-go-round equipment. Two rectangular wings were attached at the both ends of a horizontal support bar submerged in a dodecagonal water tank. The center of the support bar was connected to the vertically flapping axis which is freely rotating. This experimental apparatus generated a pure heaving motion in the vertical direction to the flapping wings in the frequency range of 0 to 5 Hz. The propulsion due to the heaving wing was expressed by a horizontally rotating speed of the support bar. The heaving motion and the rotating speed were retained with a laser displacement sensor and a rotary encoder. The rotating speed according to the heaving frequency was measured with different experimental parameters. Compared to a rigid wing, the flexible wing in the heaving motion showed a better propulsive performance in some conditions. The effects of the flexibility, the aspect ratio, and the thickness of the heaving wing on the propulsive performance were examined. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A27.00008: The role of tip deflection on the thrust produced by rigid flapping fins Francisco Huera-Huarte, Morteza Gharib It is well known that flexibility plays an important role in the propulsion performance and efficiency of oscillating fin based propulsion systems. Compliance is one of the aspects that has received more attention, as it seems to be a common feature in nature's flyers and swimmers. Active control strategies are also common in nature. We will show how by deflecting only the last 10{\%} of length of a rigid fin, at the tip, the thrust can be changed dramatically. This can be thought as an alternative to passive flexibility for controlling very efficiently the momentum transfer in the wake and therefore the thrust generation when flapping. A series of experiments have been carried with a robotic fin that allowed the control of its flapping kinematics as well as the control of the motions of its tip independently. We will be showing situations in which the tip was kept at a certain fixed position during a power stroke, and others in which it moved either in-phase or out-of-phase with the fin. The observed thrust and wake dynamics will be discussed for all these situations. The authors would like to acknowledge the financial support provided by the Gordon and Betty Moore Foundation and by the Spanish Ministerio de Economia y competitividad (MINECO) through grant DPI2012-37904. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A27.00009: Fanning the Optimal Breeze with an Abanico Grace Goon, Joel Marthelot, Pedro Reis Flexible hand-held fans, or abanicos, are universally employed as cooling devices that are both portable and sustainable. Their to and fro axial motion about one's hand generates an airflow that increases the evaporation rate near the skin and refreshes. We study this problem in the context of fluid-structure interaction, through precision model experiments. We first characterize the elastic properties of a semi-circular thin plates with various thickness and evaluate their aerodynamic performance in a custom built apparatus. The air velocity profile that results from the flapping motion of the fan is characterized for different driving conditions. A systematic variation of the geometric and elastic parameters, along with an exploration of the parameter space of the periodic driving motion (amplitude and frequency), allows us to establish optimal design and operational conditions for maximal output of the generated airflow, while minimizing the input power. [Preview Abstract] |
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