Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session BA: Bio-Fluid Dynamics: Flight |
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Chair: John Dabiri, California Institute of Technology Room: Hilton Chicago International Ballroom South |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BA.00001: WITHDRAWN: Vortical structures generated by wings performing single flap motions Laura Guglielmini, Paolo Blondeaux, Michael Triantafyllou Experiments on swimming fish reveal that they often make simple flap of their tail, or two flaps in quick succession, to change direction or rapidly accelerate, with the so called fast- starting and rapid maneuvers. These have the effect of producing individual vortices with `optimal' characteristics, that is vortices which have maximum efficiency, in the sense of maximum thrust for a given expenditure of energy. It has been suggested that in this like in other biological systems the same principles of optimal vortex ring formation discovered in laboratory experiments occur. In the present work we study numerically the flow fields generated by short aspect ratio foils performing biologically inspired kinematics, like aperiodic, single or double stoke motions. We concentrate on the comprehension of the complex dynamics of the vortical structures shed in the flow and on the evaluation of the forces acting on the foil. We also consider different foil geometries, rectangular, elliptical and lunate cross-sections, to get some insight into the apparent evolutionary convergence of the fish tail towards a lunate form and/or suggest different forms able to give optimal efficiency. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BA.00002: Quasi-Steady Simulation of Insect-Like Flapping Wing Brett G. Compton, J.M. McDonough The goal of this study is to computationally compare turbulent and laminar quasi-steady, 3-D models of insect flight using a simplified wing planform with roughly the same dimensions and stroke kinematics as the average bumblebee wing. To simulate flapping motion of the wing we use a velocity distribution at the inlet that varies linearly with distance along the span of the wing. Angle of attack is treated by changing the angle of the input velocity vector while keeping the wing stationary, thus simplifying grid generation efforts. A laminar simulation is run on an unstructured grid of $\sim 2.63\times 10^5$ mesh volumes using the commercial CFD code, {\it Fluent}; the turbulent simulation is run on a structured grid of similar size and resolution using a research LES code developed by the second author. In both cases we are seeking to reproduce the leading edge vortex (LEV) stabilized with span-wise flow as seen from previous experiments, to compare the time series of coefficients of lift and drag from the laminar and turbulent simulations over one half-stroke, and to analyze validity (or lack thereof) of the quasi-steady approximation. [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BA.00003: Wing Kinematics and Wake Velocity Characteristics of Bat Flight Sharon Swartz, Ricardo Galvao, Jose Iriarte, Emily Israeli, Kevin Middleton, Abigail Roemer, Allyce Sullivan, Xiaodong Tian, Kenneth Breuer Bats demonstrate unequalled flight characteristics and are capable of highly efficient flight as well as extreme maneuverability at high speeds. They have morphological properties that are unique in the animal world including jointed wings skeletons, elastic wing membranes and very complex wing motions. We report on a series of experiments on bats flying in a flight cage along both a straight path and through a 90-degree turn. Measurements of their kinematic wing motion (using high speed photography) and wake velocity structures (using stereo PIV) are reported. The live animal measurements are also interpreted with the help of a series of companion wind tunnel experiments using model structures that mimic some key features of bat flight mechanics. The results reveal a complex vortex wake structure which is compared and contrasted to that found in bird and insect flight. [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BA.00004: A Boundary Element Simulation of Flapping Foils with Leading-Edge Separations Qiang Zhu, Xiaoxia Dong, Michael S. Triantafyllou, Dick K.P. Yue We develop a three-dimensional numerical model based on potential-flow theory and boundary-integral formulations to investigate the dynamics of a flapping foil with vortex generation at both the trailing and the leading edges. The shedding at the trailing edge is denoted by a single shear layer originated from the sharp edge itself, while the vorticity generation near the leading edge is modeled as a group of shear layers, each of them starting from a prescribed separation line. With a boundary-element algorithm the problem is solved numerically. We find that without taking into account the effect of leading-edge separations, the predictions of the hydrodynamic forces and propulsion efficiency of a heaving-pitching foil match the experimental measurements only in the regime of small Strouhal number or small angle of attack, while the agreement between numerical results and experiments is significantly improved over a large range of kinematic parameters by including the leading-edge separation model. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BA.00005: Vortex Ring Formation in the Wake of Biologically Inspired Flapping Foils M.B. Read, M.J. Krueger, A.H. Techet The design of biologically inspired propulsion mechanisms for underwater vehicles continues to generate significant interest in the hydrodynamics of fish swimming. Flapping foils, mimicking fish fins, have been shown to produce significant thrust and have been implemented on prototype underwater vehicles. Here, the three-dimensional vortical structures in the wake of a finite aspect ratio flapping foil are investigated in order to model the three dimensional propulsive signature of swimming fish and flapping foils. The vortical patterns in the wake of a flapping foil are visualized using qualitative fluorescent dye methods, imaged in three views: planform, wing-tip and isometric. Reynolds number based on foil chord length is 165. The foil is forced to heave and pitch with a prescribed motion mimicking that of a swimming fish tail. The visualizations reveal the formation of a pair of coherent, curved, and interconnected ring-like vortices for each full flapping cycle. The wake evolution shows a dependence on Strouhal number and foil motion kinematics; Strouhal number was varied between 0.1 and 0.4. Experimental visualization results compare well with recent numerical simulations using the same parameters. An analogy the model of the wake of a swimming fish is also explored. [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BA.00006: Kinematics, Power, and Optimization in Hovering Insect Flight Gordon Berman, Z. Jane Wang Insects are graceful and varied locomotors -- flying, darting, and hovering with remarkable ease. But are they efficient? By determining the forces acting on a wing for a prescribed motion via a quasi-steady model of fluid forces on a thin plate, we run optimization algorithms to find the optimal wing motion that an insect can make. A common belief is that animals move in a way that minimizes their metabolic cost. We test this hypothesis by comparing the results of our optimization with empirically measured data. [Preview Abstract] |
Sunday, November 20, 2005 12:14PM - 12:27PM |
BA.00007: Optimal Flight of a Symmetric Flapping Wing Lionel Rosellini, Jun Zhang We study the unidirectional locomotion that results from symmetry-breaking of fluid response to a wing that is flapped vertically. We seek the optimal parameters of such locomotion. In particular, we investigate (1) at what flapping amplitude the forward flight speed is highest (minimum Strouhal number), and (2) at what amplitude the flapping wing has the lowest threshold to forward flight. We discuss other factors affecting the forward flight performance, such as the chord of the wing and its flexibility. [Preview Abstract] |
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