Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session M9: Swimming VBio Fluids: External
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Chair: Anand Oza, Courant Institute Room: 502 |
Tuesday, November 21, 2017 8:00AM - 8:13AM |
M9.00001: Hydrodynamics of a freely movable flexible fin near the ground Young Dal Jeong, Jae Hwa Lee In the present study, a freely movable flexible fin is numerically modelled to investigate the flapping dynamics of the fin near the ground in a Poiseuille flow. A leading edge of the fin is fixed in the streamwise direction, whereas the lateral motion is spontaneously determined by hydrodynamic interaction between the fin and surrounding fluid. When the fin is initially positioned at $y_{o}$, the fin passively migrates toward another wall-normal position for an equilibrium state by the interaction between passively flapping flexible body and ground. At the equilibrium position, the drag coefficient of the fin ($C_{D})$ significantly decreases due to decaying of the flapping and low flow velocity and the fin can swim consistently without the time-averaged lateral force. Two distinctive behavior at the transient state (flapping and non-flapping migration modes) and three distinctive behaviors at the equilibrium state (deflected-straight, large- and small-amplitude flapping modes) are observed depending on the bending rigidity ($\gamma )$ and mass ratio ($\mu )$ of the fin. The equilibrium position of the fin is investigated as a function of initial position ($y_{o})$, bending rigidity ($\gamma )$, mass ratio ($\mu )$ and the Reynolds number (\textit{Re}). [Preview Abstract] |
Tuesday, November 21, 2017 8:13AM - 8:26AM |
M9.00002: Free Swimming in Ground Effect Jackson Cochran-Carney, Nathan Wagenhoffer, Samane Zeyghami, Keith Moored A free-swimming potential flow analysis of unsteady ground effect is conducted for two-dimensional airfoils via a method of images. The foils undergo a pure pitching motion about their leading edge, and the positions of the body in the streamwise and cross-stream directions are determined by the equations of motion of the body. It is shown that the unconstrained swimmer is attracted to a time-averaged position that is mediated by the flow interaction with the ground. The robustness of this fluid-mediated equilibrium position is probed by varying the non-dimensional mass, initial conditions and kinematic parameters of motion. Comparisons to the foil’s fixed-motion counterpart are also made to pinpoint the effect that free swimming near the ground has on wake structures and the fluid-mediated forces over time. Optimal swimming regimes for near-boundary swimming are determined by examining asymmetric motions. [Preview Abstract] |
Tuesday, November 21, 2017 8:26AM - 8:39AM |
M9.00003: Hydrodynamics of a flexible plate between pitching rigid plates Junyoung Kim, Daegyoum Kim The dynamics of a flexible plate have been studied as a model problem in swimming and flying of animals and fluid-structure interaction of plants and flags. Motivated by fish schooling and an array of sea grasses, we investigate the dynamics of a flexible plate closely placed between two pitching rigid plates. In most studies on passive deformation of the flexible plate, the plate is immersed in a uniform flow or a wavy flow. However, in this study, the flexible plate experiences periodic deformation by the oscillatory flow generated by the prescribed pitching motion of the rigid plates. In our model, the pitching axes of the rigid plates and the clamping position of the flexible plate are aligned on the same line. The flexible plate shows various responses depending on length and pitching frequency of rigid plates, thickness of a flexible plate, and free-stream velocity. To find the effect of each variable on the response of the flexible plate, amplitude of a trailing edge and modal contribution of a flapping motion are compared, and flow structure around the flexible plate is examined. [Preview Abstract] |
Tuesday, November 21, 2017 8:39AM - 8:52AM |
M9.00004: Fluid-mediated stability and speed-increase for heaving hydrofoils swimming side-by-side Joel Newbolt, Jun Zhang, Leif Ristroph As an example of collective motion in active swimmers we study the fluid-mediated interaction between two heaving hydrofoils that swim with a fixed transverse separation (between the heaving mid-heights) but are free to independently choose their forward swimming speeds and positions. Experiments reveal that out-of-phase foils are attracted to a side-by-side configuration which also increases the swimming speed of the pair (up to 59{\%} faster for our parameters), while in-phase foils are repelled from this configuration. Because this type of swimming is qualitatively similar to that of fish and birds this interaction could be important to schooling and flocking. [Preview Abstract] |
Tuesday, November 21, 2017 8:52AM - 9:05AM |
M9.00005: Efficiency of a flapping propulsion system based on two side-by-side pitching foils Francisco Huera-Huarte We explore the propulsive performance of two foils flapping side-by-side in a wide variety of configurations, for different foil separations, pitching amplitudes and frequencies and phase differences. Direct force and torque measurements will be shown in each situation, after a thorough parametric study, that led to the identification of highly efficient modes of propulsion. The especially designed experimental rig allowed the computation of efficiencies globally and at each shaft in the system. Planar and volumetric Particle Image Velocimetry (PIV) allowed a detailed description of the wake generated by the system, for each different kinematics investigated. The investigation is part of an ambitious project with the aim of producing a high efficient and highly manoeuvrable flapping propulsion system for underwater vehicles. [Preview Abstract] |
Tuesday, November 21, 2017 9:05AM - 9:18AM |
M9.00006: Improved Swimming Performance in Hydrodynamically- coupled Airfoils Sina Heydari, Michael J. Shelley, Eva Kanso Collective motion is a widespread phenomenon in the~animal kingdom from fish schools to bird flocks. Half of~the known fish species are thought to exhibit schooling behavior during some phase of their life cycle.~ Schooling likely occurs to serve multiple purposes, including foraging for resources and protection from predators. Growing experimental and theoretical evidence supports the hypothesis that fish can benefit from the hydrodynamic interactions with their neighbors, but it is unclear~whether this requires particular configurations or regulations.~Here, we propose a physics-based~approach that account for hydrodynamic interactions among swimmers~based on the vortex sheet model.~The benefit of this model is that it is scalable to a large number of swimmers.~We start by examining the case of two swimmers,~heaving plates,~moving~in parallel and in tandem. We find that for the same heaving amplitude and frequency, the coupled-swimmers move faster and more efficiently. This increase in velocity depends strongly on~the configuration~and separation distance between the swimmers. Our results are consistent with recent experimental findings on heaving airfoils and underline the role of fluid dynamic interactions in the collective behavior of swimmers. [Preview Abstract] |
Tuesday, November 21, 2017 9:18AM - 9:31AM |
M9.00007: Unsteady Flow Interactions Between Pitching Wings In Schooling Arrangements Melike Kurt, Keith Moored In nature, many fish aggregate into large groups or schools for protection against predators, for social interactions and to save energy during migrations. Regardless of their prime motivation, fish experience three-dimensional flow interactions amongst themselves that can improve or hamper swimming performance and give rise to fluid-mediated forces between individuals. To date, the unsteady, three-dimensional flow interactions among schooling fish remains relatively unexplored. In order to study these interactions, the caudal fins of two interacting fish are idealized as two finite span pitching wings arranged in mixtures of canonical in-line and side-by-side arrangements. The forces and moments acting on the wings in the streamwise and cross-stream directions are quantified as the arrangement and the phase delay between the wings is altered. Particle image velocimetry is employed to characterize the flow physics during high efficiency locomotion. Finally, the forces and flowfields of two-dimensional pitching wings are compared with three-dimensional wings to distinguish how three-dimensionality alters the flow interactions in schools of fish. [Preview Abstract] |
Tuesday, November 21, 2017 9:31AM - 9:44AM |
M9.00008: Cause-and-effect relationships in tandem swimmer models using transfer entropy Sean Peterson, Maxwell Rosen, Antonios Gementzopoulos, Peng Zhang, Maurizio Porfiri Swimming in a group affords a number of advantages to fish, including an enhanced ability to escape from predators, search for food, and mate. To study coordinated movements of fish, principled approaches are needed to unravel cause-and-effect relationships from raw-time series of multiple bodies moving in an encompassing fluid. In this work, we aim at demonstrating the validity of transfer entropy to elucidate cause-and-effect relationships in a fluid-structure interaction problem. Specifically, we consider two tandem airfoils in a uniform flow, wherein the pitching angle of one airfoil is actively controlled while the other is allowed to passively rotate. The active control alternates the pitch angle based upon an underlying two-state ergodic Markov process. We monitor the pitch angle of both the active and passive airfoils in time and demonstrate that transfer entropy can detect causality independent of which airfoil is actuated. The influence estimated by transfer entropy is found to be modulated by the distance between the two airfoils. The proposed data-driven technique offers a model-free perspective on fluid-structure interactions that can help illuminate the mechanisms of swimming in coordination. [Preview Abstract] |
Tuesday, November 21, 2017 9:44AM - 9:57AM |
M9.00009: Traveling waves in a continuum model of 1D schools Anand Oza, Eva Kanso, Michael Shelley We construct and analyze a continuum model of a 1D school of flapping swimmers. Our starting point is a delay differential equation that models the interaction between a swimmer and its upstream neighbors’ wakes, which is motivated by recent experiments in the Applied Math Lab at NYU. We coarse-grain the evolution equations and derive PDEs for the swimmer density and variables describing the upstream wake. We study the equations both analytically and numerically, and find that a uniform density of swimmers destabilizes into a traveling wave. Our model makes a number of predictions about the properties of such traveling waves, and sheds light on the role of hydrodynamics in mediating the structure of swimming schools. [Preview Abstract] |
Tuesday, November 21, 2017 9:57AM - 10:10AM |
M9.00010: Propulsive performance of pitching foils with variable chordwise flexibility. Samane Zeyghami, Keith Moored Many swimming and flying animals propel themselves efficiently through water by oscillating flexible fins. These fins are not homogeneously flexible, but instead their flexural stiffness varies along their chord and span. Here we seek to evaluate the effect stiffness profile on the propulsive performance of pitching foils. Stiffness profile characterizes the variation in the local fin stiffness along the chord. To this aim, we developed a low order model of a functionally-graded material where the chordwise flexibility is modeled by two torsional springs along the chordline and the stiffness and location of the springs can be varied arbitrarily. The torsional spring structural model is then strongly coupled to a boundary element fluid model to simulate the fluid-structure interactions. Keeping the leading edge kinematics unchanged, we alter the stiffness profile of the foil and allow it to swim freely in response to the resulting hydrodynamic forces. We then detail the dependency of the hydrodynamic performance and the wake structure to the variations in the local structural properties of the foil.. [Preview Abstract] |
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