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 G5: Biofluids: Flapping Wings |
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Chair: David Rival, Queen’s University, Kingston Room: 3008 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G5.00001: The Hovering Hat Yangyang Huang, Monika Nitsche, Eva Kanso Birds and insects often flap their wings to hover and fly. Interestingly, recent experiments have shown that non-flapping rigid objects are also capable of stably hovering in an oscillatory background flow, given they possess particular geometric asymmetry in the vertical direction. The up-down asymmetry creates downwash vortex shedding and thus generates lift against gravity. Here, we use a two-dimensional vortex sheet model to study the motion of an inverted V-shaped object, moving passively in an oscillatory flow. We found that, depending on the fluid oscillation frequency, the hat descends, hovers or ascends. We also found an optimal opening angle of the hat for hovering that requires minimal energy from the background flow. We conclude by showing the passive stability of the hat and the role of the hydrodynamic forces and moments in preventing it from tipping over. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G5.00002: Thrust generated by a flapping flexible plate Florine Paraz, Christophe Eloy, Lionel Schouveiler In order to a gain better insight into the physics of swimming with a flexible caudal fin, we have performed experiments with a rectangular elastic plate immersed in a water flow. The plate leading edge is forced into harmonic motion, while its trailing edge responds passively to this actuation. A resonance has been evidenced experimentally, pointing out a strong coupling between the natural frequencies of the structure and the forcing frequencies. In this experiment, the forcing amplitude plays a non-trivial role, emphasizing the role of non-linearities in this problem. To better understand the origin of these non-linearities, a weakly non-linear model has been developed. We assumed a quasi two-dimensional plate of zero thickness immersed in a potential flow and subject to a resistive drag-like force. The plate deflection has then been decomposed into a forcing heaving mode and natural flexural modes. This modeling approach allowed us to predict the response to the heave forcing as a function of its amplitude and frequency. The frequencies of the resonances, as well as the deflection enveloppes, are well captured by this model. The performance of the system, measured through the generated thrust, is also well predicted by this model. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G5.00003: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 8:39AM - 8:52AM |
G5.00004: Model-Based Optimization for Flapping Foil Actuation Jacob Izraelevitz, Michael Triantafyllou Flapping foil actuation in nature, such as wings and flippers, often consist of highly complex joint kinematics which present an impossibly large parameter space for designing bioinspired mechanisms. Designers therefore often build a simplified model to limit the parameter space so an optimum motion trajectory can be experimentally found, or attempt to replicate exactly the joint geometry and kinematics of a suitable organism whose behavior is assumed to be optimal. We present a compromise: using a simple local fluids model to guide the design of optimized trajectories through a succession of experimental trials, even when the parameter space is too large to effectively search. As an example, we illustrate an optimization routine capable of designing asymmetric flapping trajectories for a large aspect-ratio pitching and heaving foil, with the added degree of freedom of allowing the foil to move parallel to flow. We then present PIV flow visualizations of the optimized trajectories. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G5.00005: Flapping propulsion with tip pitch control Francisco Huera-Huarte, Morteza Gharib The effect of flexibility in the propulsion performance and efficiency of oscillating pitching foils has received a large amount of attention in the past years. Scientists have used simplified robotic models that mimic the kinematics of flying and swimming animals, in order to get inspiration to build more efficient engineering 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 or passive control elements are also common in nature. We will show how thrust generation in a pitching fin, can be greatly affected by controlling the tip pitch motion dynamically and independently of the fin itself. This is in fact a controlled local change of curvature of the end of the fin. A robotic system has been designed in a way that not only flapping amplitudes and frequencies can be controlled, but also the amplitudes and frequencies of the tip and the phase difference between the tip and the fin. We measured thrust forces and the vortex dynamics in the near wake of the system, by using planar DPIV (Digital Particle Image Velocimetry) in a wide variety of flapping situations with tip control. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G5.00006: Critical Point Matching and Distance Metrics of Unsteady Flow Separation from a Pitching Plate Faegheh Hooman, Paul Krueger Unsteady flow separation is of interest for force and moment generation by flapping airfoils, but it is often difficult to determine how small differences in the motion lead to differences in the flow field and resulting forces. To better understand the flow evolution during unsteady separation in pitching maneuvers, analysis was performed of two numerical data sets for the pitch-up of a two-dimensional flat plate in a free stream flow with Re$=$1000 (data provided by Prof. J.D. Eldredge at UCLA). Flow fields were compared by finding the best match of first order critical points according to weighted physical location and topological characteristics. Weighting and smoothing helped eliminate outliers, especially after adding noise, and made the method robust. A total distant metric for matched critical points was defined to provide a global metric for identifying similarities and differences between flow fields. Comparisons of the flow evolution for the two data sets using the distance metric will be presented. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G5.00007: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 9:31AM - 9:44AM |
G5.00008: Experimental study of the fluid and structure interaction for gravity driven falling plates Ruijun Tian, Fangjun Shu Falling motion of thin plates and the induced flow field were investigated in this study. Time-resolved 2-D PIV measurements were conducted to investigate the dynamic development of the flow field induced by falling plates submerged in water. Two types of falling motions were observed, fluttering (sliding from side to side while descending) and tumbling (continuously rotating while falling downward and sliding to one direction), depending on the plate material and the physical dimensions, which forms a few governing non-dimensional parameters. The time-resolved PIV images, which also contain the plate location information, were further processed to extract the location and orientation of the plate. The data were then numerically differentiated to acquire the plate's translational and angular speeds and accelerations. Thus, the instantaneous hydrodynamic force/moment on the plates and the surrounding flow field were correlated to perform empirical analysis on this classical unsteady fluid and structure interaction (FSI) problem. It is discovered that the leading edge vortex plays an important role since its development is drastically related to the dynamic features of falling plates, though it is still unclear if the vortex shedding causes or results from the plates' movement. A theoretical model is proposed to simulate the dynamic features of the falling plates, which will be compared with the experimental data. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G5.00009: Combined turning and propulsion of a flexible plate in viscous fluid Alexander Alexeev, Peter Yeh We use three dimensional computer simulations to study the flow and structural deformation of an oscillating elastic rectangular plate submerged in a viscous fluid. The elastic plate is actuated at the root near the first natural frequency and undergoes a combined sinusoidal plunging and twisting motion. This complex motion results in not only a forward propulsive force, but also a force perpendicular to the swimming direction. The latter force leads to turning. We find that the strength of the turning force depends on oscillation amplitudes as well as the phase difference between the plunging and twisting oscillations. Our simulations reveal an optimal phase difference and twisting amplitude that leads to maximum turning potential. These results can be used to design a basic mechanism for changing direction in a micro underwater autonomous vehicle actuated using flexible fins. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G5.00010: The influence of circulation distribution on LEV development for an impulsively-started, spanwise-flexible profile Jaime Wong, David Rival As a spanwise-flexible profile is accelerated from rest, the profile bends thereby causing a component of the free-stream flow to align with the spanwise direction. For the rapid accelerations typical in biological swimming and flying, accelerating a profile from rest will simultaneously result in the formation of a leading-edge vortex (LEV). The spanwise flow resulting from profile bending rearranges the distribution of circulation in the LEV along the span of the wing via vorticity convection, which does not occur in an otherwise equivalent rigid case. The effect of this circulation redistribution on LEV detachment and force history is difficult to separate from other flexibility effects, such as the varying shear-layer feeding rate and local acceleration. Therefore, the current study utilizes cyber-physical fluid dynamics (CPFD) to simulate an impulsively-started spanwise-flexible profile in the absence of spanwise flow. Nominally two-dimensional CPFD results are combined in a blade-element scheme that replicates the distributed load on a flexible profile. In this way, the effect of spanwise flow on LEV development and detachment and the resulting force histories can be isolated from other flexibility effects. [Preview Abstract] |
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