Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session AM: Bio-Fluids: Flight I |
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Chair: Jeff Eldredge, University of California, Los Angeles Room: 103B |
Sunday, November 23, 2008 8:00AM - 8:13AM |
AM.00001: Why Twist? Promode R. Bandyopadhyay, David N. Beal Free swimming and flying animals twist their wings. But why? We have carried out force and efficiency measurements with twistable finite fins in water. Twist increases the hydrodynamic efficiency of a rolling and pitching fin, but only up to 5{\%}. Animals tend to operate in narrow frequency ranges of flapping oscillation and amplitude. In such kinematic constraint, twist can increase thrust forces by 20{\%}--a large range, while Strouhal number is held constant (frequency, tow speed and roll angle are held constant) and maximum efficiency is retained. Less than 5{\%} of the roll power is spent in twist to produce this variation in thrust force. Therefore, while our biorobotic underwater vehicles have so far used the square of frequency for thrust control, animals that have resonant design could use twist for control of both cruise and maneuvering. The angle of attack along the span becomes more uniform with twist, becoming the most uniform at 20 degrees. We propose that twist is a method for controlling the direction of the induced flow jetting out of the closed stall vortex that is shed from the fin. [Preview Abstract] |
Sunday, November 23, 2008 8:13AM - 8:26AM |
AM.00002: ABSTRACT HAS BEEN MOVED TO ML.00009 |
Sunday, November 23, 2008 8:26AM - 8:39AM |
AM.00003: Spider capture thread: form and function Sunghwan Jung, Christophe Clanet, John Bush We present the results of a combined theoretical and experimental investigation of spider capture thread. While the radial threads in a spider web are simply cylindrical, the circumferential threads are pre-wound helices immersed in a viscous fluid. These so-called capture threads are subject to an instability reminiscent of Rayleigh-Plateau that results in the formation of a series of droplets along the thread, each filled with a series of coils. We demonstrate that this instability is a natural example of capillary origami that will arise when the surface tension exceeds the tension of the spring. Moreover, we demonstrate its efficacy in prey capture through augmenting damping during prey impact. [Preview Abstract] |
Sunday, November 23, 2008 8:39AM - 8:52AM |
AM.00004: Sideways flight of insects by phased wing flips Leif Ristroph, Gordon Berman, Attila Bergou, Z. Jane Wang, Itai Cohen Insects are enviable flyers and are capable of unusual maneuvers, such as sideways flight. We show that fruit flies generate sideways forces in flight, and we propose an aerodynamic mechanism that takes advantage of the unique features of flapping flight. Specifically, flies induce asymmetries between the right and left wing angles of attack just as the wings rapidly flip over, and this leads to unbalanced drag forces that contribute to the lateral force. Remarkably, these delicate asymmetries can be simply induced by flipping each wing at slightly different times. We measure that fruit flies use wing rotation timing differences of around 1 millisecond while undergoing a half $g$ lateral acceleration. [Preview Abstract] |
Sunday, November 23, 2008 8:52AM - 9:05AM |
AM.00005: Rotational timing and the alteration of aerodynamic forces in insect flight Y. Sudhakar, S. Vengadesan Delayed pronation and advanced supination enable the insects using inclined stroke plane motion to generate forces substantially higher than those predicted using quasi-steady aerodynamic principles. Insects, by controlling the timing of wing rotation, subtly modulate the magnitude and direction of aerodynamics forces generated by them, and can perform complex aerial maneuvers by adjusting the timing of rotation in each wing independently. The goal of the present study is to investigate the fluid dynamic changes and the corresponding alterations in forces generated by their flapping wings. The Immersed Boundary Method is used to simulate the flow field over a 2D flat plate of thickness ratio 0.02 undergoing prescribed wing kinematics along the 45$^{0}$ inclined stroke plane at Re=100. The pronation and supination timings are varied independently and the influence of these changes on the force production is investigated. In all the simulations, the downstroke and upstroke angle of attack are held constant. [Preview Abstract] |
Sunday, November 23, 2008 9:05AM - 9:18AM |
AM.00006: Leaping of a flexible loop on water Ho-Young Kim, Eun Jin Yang, Min-Hee Lee, Bongsu Shin Small aquatic arthropods, such as water striders and fishing spiders, are able to leap on water to a height several times their body length. We study a simple model using a floating flexible loop to provide fundamental understanding and mimicking principle of the leaping on water. Motion of a loop, initially bent into an ellipse from equilibrium circular shape using a thin thread, is visualized with a high speed camera upon cutting the thread with a laser. We find that the loop may merely oscillate while afloat, penetrate into the water, or soar into air depending on the hydrophobicity, the bending stiffness, the weight and the degree of initial deflection of the loop. We also construct a scaling law for the leaping height by balancing the initial elastic bending energy with the loop's translational and vibrational energy and a loss imparted to the water in the forms of interfacial, kinetic and viscous energy. [Preview Abstract] |
Sunday, November 23, 2008 9:18AM - 9:31AM |
AM.00007: Three-dimensional vortical structures around the fore- and hind-wings of dragonfly in hovering motion Jihoon Kweon, Haecheon Choi In the present study, we investigate three-dimensional vortical structures around the fore- and hind-wings of dragonfly in hovering motion. The three-dimensional wing shape is based on that of {\it Aeschna juncea} (Noberg, JCP 1972) and numerically realized using an immersed boundary method (Kim et al., JCP 2001). The wing motion is modelled using sinusoidal functions and the mid-stroke angles of attack are 60$^\circ$, 20$^\circ$ with the stroke plane angle 60$^\circ$. The Reynolds numbers considered are 150 and 1000 based on the maximum translational velocity and mean chord length. During the downstroke, the strong wing- tip vortex produces the vortex ring and the downwash, and at the supination this vortex influences the force generation in a similar way to the normal hovering of {\it Drosophila}. During the stroke reversal, dipole vortices are observed all over the spanwise direction, but the time sequence of their development is different at different spanwise location. Near the wing tip, two vortex pairs are generated at the leading and trailing edges, respectively. To further understand the interaction between the wing and vortices, the wing-sectional force and torque are examined. The results will be discussed in the presentation. [Preview Abstract] |
Sunday, November 23, 2008 9:31AM - 9:44AM |
AM.00008: High-Fidelity Computational Models of Insects in Flight: Wing Deformation and Flight Maneuvers Rajat Mittal, Lingxiao Zheng, Tyson Hedrick, Varun Gupta Highly accurate kinematical and geometrical models are used to examine the aerodynamics of insect flight. The simulations employ a sharp-interface immersed boundary method for the flow simulations, and the non-dissipative numerical method used, allows us to capture the vortex dynamics of the wake. One focus of the current study is on understanding the role of wing deformation in insect flight and for this, we examine the flight of a moth in hover with deformable and rigid wings. The second focus of the current work in on flight maneuvers in insects and the presentation will also show preliminary results from simulations of flight maneuvers in butterflies. [Preview Abstract] |
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