Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R28: Flapping Flight |
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Chair: James Buchholz, University of Iowa Room: Ballroom II |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R28.00001: Time-resolved measurements of the velocity field over the wing of bats during flight Cosima Schunk, Sharon Swartz, Kenneth Breuer Particle Image Velocimetry (PIV) has become a well-established tool to study flows associated with flying animals. The wake shed by flying bats as it is seen in the Trefftz plane is by now well described for several bat species. However, to complete the understanding of the three-dimensional wake structures, additional views are necessary. To meet this need, bats were trained to fly at a stationary position in the wind tunnel at wind speeds between 2 m/s and 6 m/s. Aligning the laser light sheet parallel to the free stream, we measured, using time-resolved PIV (200 Hz), the air flow in the region of the left wing of the animals. Three high-speed cameras (400 Hz) were used to capture the position and movement of the bat and to reconstruct the wing kinematics. Characteristic flow structures were observed consistently at different spanwise positions, including the starting and stopping vortices at the beginning and end of the downstroke, as well as other vortex and wake features. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R28.00002: Inertial and Fluid Forces during Bat Flight Maneuvers Attila Bergou, Jennifer Franck, Gabriel Taubin, Sharon Swartz, Kenneth Breuer Flying animals generate forces to move through the air with the coordinated movement of their wings. Bats have evolved a particularly impressive capacity in flight control. With more than 24 wing joints bats are able to manipulate wing area, angle of attack, and camber to control their flight through altering aerodynamic forces on their wings. Here we use a model-based tracking framework to reconstruct the highly articulated wing and body kinematics of maneuvering bats from high-speed video. Using this data, we extract a simplified wing geometry and kinematics during various flight maneuvers. The time-dependent fluid flow and resultant forces on the wing are computed with CFD using a direct numerical simulation, and then used in a low-order model of the bat dynamics. This reconstruction identifies the relative importance of both inertial and aerodynamic forces during flight maneuvers. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R28.00003: Large-Eddy Simulations of Flapping-Induced Lift Enhancement Jennifer Franck, Sharon Swartz, Kenneth Breuer This work isolates the heaving motion of flapping flight in order to numerically investigate the fluid-structure interaction at Reynolds numbers relevant to birds and bats. Although there has been much focus on insect flight, larger vertebrates fly at a higher Reynolds number, which leads to different dynamics in terms of flow separation, reattachment, and high-lift mechanisms. In this work, an incompressible large-eddy simulation is used to simulate the periodic heaving of a flat plate at various angles of attack. It is found that the heaving motion can increase the average lift when compared with the steady flow, more so than is expected from the relative angle of attack. The additional lift is attributed to the vortex dynamics at the leading edge. The lift enhancement and flow features are compared with experimental results. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R28.00004: Lift force enhancement and fluid-structure interactions on a self-excited flapping wing model Oscar Curet, Sharon Swartz, Kenneth Breuer We present data from a mechanical model that we have used to explore a physical mechanism that may have aided transition from gliding to flapping flight over fifty million years ago. The model is composed of a cantilevered flat plate with a hinged trailing flap and is tested in a low-speed wind tunnel. For slow wind speeds the model is stationary, but above a critical wind speed the wing starts to oscillate due to an aeroelastic instability. A positive angle of attack on the wing results in a positive lift force. However, this lift force is significantly enhanced once the wing starts to oscillate. We used particle image velocimetry (PIV) to understand the unsteady aerodynamics of the self-excited flapping wing, and to identify and characterize the mechanisms that generate the enhanced lift force. We also discuss the implications of our results on the evolution of powered biological flight. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R28.00005: Fluid-structure interactions on compliant membrane wings Rye Waldman, Sharon Swartz, Kenneth Breuer Membrane wings are characteristic of flying animals such as bats, as well as low Reynolds number Micro Air Vehicles. These wings exhibit interesting features such as self-cambering, soft stall, and good performance at large angles of incidence. The interaction between the membrane and the vortical structures over the wing play an important role in the wing's performance. Vortices shed from the leading edge and tip interact with the membrane to select vibration modes, which depend on the details of the wing loading. However, the vibration modes are sensitive to the boundary conditions of the membrane. Here, we present results using force and membrane displacement measurements as well as Particle Image Velocimetry (PIV) from wind tunnel experiments on rectangular membrane wings with identical planform, but with different kinds of perimeter support. The different boundary conditions on the membrane affect the wing shape, support different vibration modes at different frequencies, and affect aerodynamic performance. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R28.00006: A bi-directional leading-edge vortex in slow-flying bats Shizhao Wang, Xin Zhang, Guowei He A leading-edge vortex (LEV) is crucial to bat afloat, since a LEV could generate high lift which could not be predicted by the conventional aerodynamics theories. The LEV usually exhibits an intensive spiral vortex of a unidirectional axial flow on the top surface of wing. In this study, we numerically simulate a slowing-flying bat using immersed boundary method. The morphology and kinematics of bat are taken from experimental measurements. It is observed from our simulation that the stretching and collapse motions of wing could induce a bi-directional axial flow. The bi-directional axial flows stabilize the LEV and enhance its intensity. The observation is further investigated by using a simple model: the flows around a spanwise oscillating plate. The spanwise oscillation could enhance the LEV and make its more stable. This result implies a link of bat kinematics with its unusual aerodynamic performances. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R28.00007: Force estimation and turbulence in the wake of a freely flying European Starling Hadar Ben-Gida, Adam Kirchhefer, Gregory Kopp, Roi Gurka Flapping wings are one of the most complex yet widespread propulsion method found in nature. Although aeronautical technology has advanced rapidly over the past 100 years, natural flyers, which have evolved over millions of years, still feature higher efficiency and represent one of nature's finest locomotion methods. One of the key questions is the role of the unsteady motion in the flow due to the wing flapping and its contribution to the forces acting on a bird during downstroke and upstroke. The wake of a freely flying European Starling is investigated as a case study of unsteady wing aerodynamics. Measurements of the near wake have been taken using long duration high-speed PIV in the wake behind a freely flying bird in a specially designed avian wind tunnel. The wake has been characterized by means of velocity and vorticity fields. The measured flow field is decomposed based on the wing position phases. Drag and lift have been estimated using the mean velocity deficit and the circulation at the wake region. In addition, kinematic analysis of the wing motion and the body has been performed using additional high-speed cameras that recorded the bird movement simultaneously with the PIV. Correlations between the wing kinematics and the flow field characteristics are presented as well as the time evolution of the velocity, vorticity and additional turbulence parameters. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R28.00008: Characterization and Scaling of Vortex Shedding from a Plunging Plate Azar Eslam Panah, James Buchholz Leading-edge and trailing-edge vortices (LEV and TEV) are investigated for a plunging flat plate airfoil at a chord Reynolds number of 10,000 while varying plunge amplitude and Strouhal number. Digital Particle Image Velocimetry is used to examine the strength and dynamics of shed vortices. Vortex strength, timing, pinch-off and trajectory are examined. By tracking the development of both the LEV and TEV in phase-locked measurements throughout the cycle and extracting the respective vortex circulation, the dimensionless circulation of both the LEV and TEV at each phase in the cycle could be determined. Guided by theoretical considerations for vorticity generation and aerodynamic theory, we will discuss the role of kinematic parameters on vortex shedding and the applicability of a scaling factor for the circulation of the shed vortex structures. Whereas a scaling parameter based on plate kinematics effectively collapses the circulation values of the shed leading-edge vortices with variation in Strouhal number, plunge amplitude, and angle of attack, it is found that the strength of the trailing-edge structures vary little with variation in plunge amplitude and angle of attack. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R28.00009: Optimization of Flapping Based Locomotion Shawn Walker, Michael Shelley Locomotion at the macro-scale is important in biology and industrial applications, such as for understanding the fundamentals of flight to enable design of artificial locomotors. We present results on optimal actuation profiles for locomotion of a rigid, flapping body at intermediate Reynolds number. The actuation consists of a vertical velocity control attached to a pivot point of an ellongated rigid body, which is allowed to rotate and is affected by a torsional spring; the spring acts as an elastic recoil. No a priori assumption is made on the form of the vertical actuation, except for smoothness. Thus, we pose an infinite dimensional time-varying, PDE-constrained optimization problem (with additional constraints on the vertical control) and solve it by variational methods. We explore the effects of parameter variations on the optimal locomotion profile, such as the torsional spring constant, relative mass density of body to fluid, and discuss the effects on locomotion strategies. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R28.00010: Spontaneous motion of flapping wings driven by hydrodynamic instability Olivier Marquet, Juan Guzman Inigo Recent experimental [1] and numerical [2] studies have examined the dynamics of rigid symmetric wings flapping vertically in a quiescent fluid and free to move in the horizontal direction. It has been observed that above a critical flapping frequency the flow loses its symmetry while the wing starts to move horizontally and eventually reaches a quasi constant horizontal speed. The present work reconsiders this problem from an hydrodynamics instability point of view. The basic flow is periodic and symmetric, the periodicity being imposed by the vertical forcing frequency while the symmetry of the velocity field ensures no motion in the horizontal direction. The linear stability is examined using the Floquet theory, with the assumption of asymmetric perturbations to explain the onset of horizontal forces. Numerical results of the stability problem will be shown. The cases of wings fixed or free to move will be analyzed and compared.\\[4pt] [1] Vandenberghe N., Zhang J. {\&} Childress S., ``A symmetry-breaking leads to forward flapping'', Journal of Fluid Mechanics, 506, 147 (2004)\\[0pt] [2] Alben S. {\&} Shelley M. ``Coherent locomotion as an attracting state for a free flapping body``, Proc. Natl. Acad. Sci., U.S.A., 102, 11163 (2005) [Preview Abstract] |
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