Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H19: Bio: Flapping and Swimming II |
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Chair: Leif Ristroph, The Courant Institute, New York University Room: D136 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H19.00001: Dynamic Schooling of a Tandem Pair of Heaving Hydrofoils Joel Newbolt, Leif Ristroph, Jun Zhang The reverse von K\'{a}rm\'{a}n wake generated by a heaving hydrofoil has recently been shown to provide stable positions to a second hydrofoil heaving in the wake (Ramananarivo et al. at NYU). Because a similar wake structure is seen for many swimming and flying animals this fluid-mediated interaction is suspected to play a role in schooling and flocking. A newly designed experimental apparatus allows us to study this interaction in the case where the two foils are powered independently so that each foil may take on a different flapping amplitude, phase and frequency. Measurements show that the stable positions of the following foil can be shifted to any arbitrary downstream position by varying only the relative flapping phase between the foils. At different relative frequencies and amplitudes the following foil exhibits several distinct trajectories. When the following foil has a lower frequency and higher amplitude than the leader the spacing between the foils can undergo a periodic trajectory. When driven at a common frequency the follower occupies stable positions in the wake of the leader. When the follower has a higher frequency the spacing between the foils is unstable, either increasing or decreasing in time, depending on the relative amplitudes and initial conditions. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H19.00002: Schooling behavior of heaving flexible airfoils Sunghyuk Im, Hyung Jin Sung The schooling behavior of rigid and flexible NACA0017 airfoils in the heaving motion is experimentally explored in a merry-go-round equipment. The airfoil was attached to the end of a horizontal support bar whose other end was connected to the freely rotating vertical axis. The axis was forced to undergo a sinusoidal motion in the vertical direction to make a pure heaving motion of the airfoils in the frequency range of 0.5 to 5 Hz. The propulsion due to the heaving airfoils is expressed by a horizontally rotating speed of the support bar. This experimental setup is simulating infinite schooling situations of airfoils in an in-phase heaving motion with the streamwise distance $d$. The ratio of the distance to the chord length $d$/$c$ was determined by the number of airfoils $(1\le n\le 8)$. The rotational frequency $F$ according to the heaving frequency $f$ was measured with different experimental parameters. The schooling number $S=f$/(\textit{nF}), representing the number of heaving oscillations between each airfoil, was introduced to explain the schooling behavior of the airfoils. The effects of the flexibility, $d$/$c$ and $f$ on the propulsive performance were examined with the schooling behavior of the airfoils. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H19.00003: Leveraging fluid-structure interaction for passive control of flapping locomotion Chan-ye Ohh, Yangyang Huang, Ziteng Wen, Eva Kanso, Mitul Luhar While many living organisms employ active feedback control during flapping locomotion, there is increasing evidence to suggest that passive fluid-structure interactions also play a central role in dictating stability and efficiency (Liu et al. \textit{Phys. Rev. Lett.} 108:06081-3, 2012). The current project seeks to experimentally evaluate and numerically verify the rotational stability and dynamics of a rigid $\Lambda$-flyer oscillating up-and-down in a rest fluid. We explore the dynamic behavior of the flyer in terms of three dimensionless parameters: opening angle, oscillation amplitude, and acceleration of the flyer. Within the parameter ranges tested, we identify four types of behavior: periodic rotation, chaotic dynamics, stable behavior (concave down position), and bistability (concave up and down position). The emergence of periodic and chaotic rotation depends primarily on the oscillation amplitude of the flyer, whereas transition from stability to bistability is dependent on both the amplitude and acceleration. The transition to bistability occurs at a constant ratio of drag to gravity, indicating that the stabilizing effect is hydrodynamic. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H19.00004: Cyber-physical experiments on the efficiency of swimming protocols Nathaniel Wei, Daniel Floryan, Tyler Van Buren, Alexander Smits We present results from experiments on a biologically inspired cyber-physical system, composed of a two-dimensional heaving and pitching rigid airfoil attached to a six component load cell, mounted to a traverse that can move along a water channel. A feedback controller, influenced by the apparatus of Mackowski and Williamson (J. Fluid Struct., 2011), introduces the effects of a fictional drag force specified by a virtual body profile and drives the traverse accordingly. Free-swimming protocols using the force-feedback system are compared with similar motions on a motionless traverse. The propulsive efficiency of burst-and-coast kinematics is also considered. Of particular interest are (1) the implementation of the cyber-physical control system with respect to the accessible experimental parameter space, (2) the impact of force-based streamwise actuation on experimental data, and (3) the effects of burst-and-coast motions on propulsive efficiency. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H19.00005: Airflow Actuation of Shortfin Mako Shark Denticles Sean Devey, Paul Hubner, Amy Lang The shortfin mako shark is covered in microscopic scales called denticles, which may act as a mechanism for passive flow control. Recent research has investigated the theory that reversing flow could passively bristle these denticles, which could delay flow separation. Water tunnel studies have supported this theory, yet a wind tunnel study at a greater dynamic pressure found no significant differences between an airfoil covered with mako skin and a smooth airfoil. A likely cause is that surface tension between denticles, which must be wet to retain flexibility, prevented bristling. This would not be an issue in water. To determine what reverse airflow characteristics cause denticle bristling in air, a benchtop study was conducted in which a jet of air was impinged upon a sample of wet mako skin in the reverse flow direction. A microscope and camera captured video of the denticles under the air jet, and image analysis techniques were used to detect bristling. Analysis shows sporadic bristling around 16 m/s (q $=$ 150 Pa) but full bristling does not occur until above 35 m/s (q $=$ 740 Pa). The free stream velocities required to achieve such reversal speeds are much higher. For this reason, mechanical analogues will be used rather than real skin in future studies of this mechanism. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H19.00006: Bistable flapping of flexible flyers in oscillatory flow Yangyang Huang, Eva Kanso Biological and bio-inspired flyers move by shape actuation. The direct control of shape variables for locomotory purposes is well studied. Less is known about indirect shape actuation via the fluid medium. Here, we consider a flexible $\rm \Lambda$-flyer in oscillatory flow that is free to flap and rotate around its fixed apex. We study its motion in the context of the inviscid vortex sheet model. We first analyze symmetric flapping about the vertical axis of gravity. We find that there is a finite value of the flexibility that maximizes both the flapping amplitude and elastic energy storage. Our results show that rather than resonance, the flyer relies on fluidic effects to optimize these two quantities. We then perturb the flyer away from the vertical and analyze its stability. Four distinct types of rolling behavior are identified: mono-stable, bistable, bistable oscillatory rotations and chaotic dynamics. We categorize these types of behavior in terms of the flyer’s and flow parameters. In particular, the transition from mono-stable to bistable behavior occurs at a constant value of the product of the flow amplitude and acceleration. This product can be interpreted as the ratio of fluidic drag to gravity, confirming the fluid role in this transition. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H19.00007: Intermittent Swimming with a Flexible Propulsor Emre Akoz, Samane Zeyghami, Keith Moored Some animals propel themselves by using an intermittent swimming gait known as a burst-and-glide or a burst-and-coast motion. These swimmers tend to have a more pronounced pitching of their caudal fins than heaving leading to low non-dimensional heave-to-pitch ratios. Recent work has shown that when this ratio is sufficiently low the efficiency of an intermittently heaving/pitching airfoil can be significantly improved over a continuously oscillating airfoil. However, fish that swim with an intermittent gait, such as cod and saithe, do not have rigid fins, but instead have highly flexible fins. To examine the performance and flow structures of an intermittent swimmer with a flexible propulsor, a fast boundary element method solver strongly coupled with a torsional-spring structural model was developed. A self-propelled virtual body combined with a flexible-hinged pitching airfoil is used to model a free-swimming animal and its flexible caudal fin. The duty cycle of the active to the coasting phase of motion, the torsional spring flexibility and the forcing frequency are all varied. The cost-of-transport and the swimming speed are measured and connected to the observed wake patterns. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H19.00008: Unsteady propulsion in ground effects Sung Goon Park, Boyoung Kim, Hyung Jin Sung Many animals in nature experience hydrodynamic benefits by swimming or flying near the ground, and this phenomenon is commonly called `ground effect'. A flexible fin flapping near the ground was modelled, inspired by animals swimming. A transverse heaving motion was prescribed at the leading edge, and the posterior parts of the fin were passively fluttering by the fin-fluid interaction. The fin moved freely horizontally in a quiescent flow, by which the swimming speed was dynamically determined. The fin-fluid interaction was considered by using the penalty immersed boundary method. The kinematics of the flexible fin was altered by flapping near the ground, and the vortex structures generated in the wake were deflected upward, which was qualitatively analyzed by using the vortex dipole model. The swimming speed and the thrust force of the fin increased by the ground effects. The hydrodynamic changes from flapping near the ground affected the required power input in two opposite ways; the increased and decreased hydrodynamic pressures beneath the fin hindered the flapping motion, increasing the power input, while the transversely reduced flapping motion induced the decreased power input. The Froude propulsive efficiency was increased by swimming in the ground effects [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H19.00009: Internally-actuated flexible fins swim faster and more efficiently with a passive attachment Peter Yeh, Alexander Alexeev Using three dimensional computer simulations, we probe biomimetic free swimming of an internally-actuated flexible plate in the regime near the first natural frequency. The plate is driven by an oscillating internal moment approximating the actuation mechanism of a piezoelectric MFC bimorph. We show in our simulations that the addition of a passive attachment increases both swimming velocity and efficiency. Specifically, if the active and passive sections are of similar size, the overall performance is the best. We determine that this optimum is a result of two competing factors. If the passive section is too large, then the actuated portion is unable to generate substantial deflection to create sufficient thrust. On the other hand, a large actuated section leads to a bending pattern that is inefficient at generating thrust especially at higher frequencies. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H19.00010: Enhancing propulsive efficiency through proper design of bending patterns of a flexible pitching foil samane zeyghami, Emre Akoz, Keith Moored Many aquatic animals propel themselves efficiently through water by oscillating flexible fins. These fins are, however, not homogeneously flexible, but instead their flexural rigidity varies along their chord and span. To detail the flow structures and propulsive performance of these functionally-graded propulsors a simple model of an unsteady pitching airfoil with a flexible hinge of varying location is examined. This acts as a first-order model of a functionally-graded fin by varying both the flexibility and bending pattern of the propulsor. Recent experiments have shown that adding a flexible `tail' with the proper stiffness to a rigid pitching foil can effectively delay/suppress the formation of a deflected wake thereby enhancing the cycle-averaged wake momentum in the swimming direction. To extend these observations, we investigate the dependency of the wake pattern of a hinged pitching airfoil to the location and flexibility of the hinge by employing a fast boundary element method solver that is strongly coupled with a torsional spring structural model. The observed wake patterns are further connected to the thrust production and propulsive efficiency with the goal of determining the proper combinations of parameters that yields the maximum gain in efficiency. [Preview Abstract] |
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