Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F23: Biological Fluid Dynamics: Locomotion Swimming  Foils and Plates 
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Chair: Francisco HueraHuarte, California Institute of Technology Room: Georgia World Congress Center B311 
Monday, November 19, 2018 8:00AM  8:13AM 
F23.00001: Dynamics and locomotion of flexible foils in a frictional environment Silas Alben, Xiaolin Wang Flexible foils have long been used as a generic model for the bodies and appendages of organisms (e.g. fish and snakes) in locomotion. To understand the range of possible dynamicsincluding efficient locomotionin a terrestrial environment or granular medium, we have used analysis and computations to study flexible foils moving under frictional forces. When a flexible foil is oscillated by heaving at one end but is not free to locomote, the dynamics change from periodic to nonperiodic and chaotic as the heaving amplitude increases or the bending rigidity decreases. Resonant peaks and bistable states are observed. When the foil is free to locomote, the horizontal motion smoothes the resonant peaks. Locomotion is steady but slow at moderate frictional coefficients, and faster at larger transverse friction and small tangential friction corresponding to wheeled snake robots. Here traveling wave motions arise spontaneously, and move with horizontal speeds that scale as transverse friction coefficient to the power 1/4 and input power that scales as the transverse friction coefficient to the power 5/12. These scalings correspond to boundary layers near the foil’s leading edge. 
Monday, November 19, 2018 8:13AM  8:26AM 
F23.00002: Connections between resonance and nonlinearity in swimming performance of a flexible heaving plate Andres Goza, Daniel Floryan, Clarence Rowley The use of flexibility in underwater vehicles is of interest as a means to augment propulsive performance. As an intermediate step to this aim, the effect of flexibility on performance is often considered for flow past a heaving flexible flat plate. In this setting, previous work has found flexibility to improve performance for certain parameters. While these benefits have been found for a range of flow regimes, the physical role of flexibility remains unclear. Often, resonance is cited as the source of performance peaks, though in certain settings heaving at nonresonant frequencies has been found to be optimal. Other studies have found relationships between flow structures and performance, though the connection of these observations to resonance is still unclear. We use highfidelity nonlinear simulations and a global linear analysis of the fullycoupled fluidstructure interaction system to study twodimensional viscous flow past a heaving geometrically nonlinear EulerBernoulli beam. The linear analysis is used to unambiguously define resonant behavior in the presence of a viscous fluid. Comparisons between these linear results and nonlinear, finiteamplitudeheaving simulations are made to determine the role of nonlinearity in optimal performance. 
Monday, November 19, 2018 8:26AM  8:39AM 
F23.00003: Thrust enhancement of a flexible foil oscillating near an airwater interface Sung Goon Park, Ming Li, Hyung Jin Sung, Lian Shen Fish swimming near water surfaces is widely observed in nature. In this study, twodimensional (2D) simulation was performed to analyze the hydrodynamic effects of the airwater interface on the propulsive performance of a heaving flexible foil. A sharp immersed boundary method was used to solve the fluidflexiblebody interaction problem, and a coupled levelset and volumeoffluid (CLSVOF) method was adopted to capture the airwater interface. The flexible foil was modeled as a thin shell based on the nonlinear Kirchhoff thinshell theory, and a plunging motion was prescribed on its leading edge. The effects of the submergence depth and the freestream velocity were examined. The thrust produced by the foil was reduced near the airwater interface when the freestream velocity was relatively low. The foil moving in a highvelocity stream, on the other hand, produced more thrust near the airwater interface than that far from the interface. The thrust enhancement or reduction due to the freesurface effect was found to be associated with the propagation of the leadingedgeinduced wave, the phase velocity of which was estimated using the nonlinear dispersion relation. It was found that the thrust enhancement can be amplified for up to 49% due to the freesurface effect. 
Monday, November 19, 2018 8:39AM  8:52AM 
F23.00004: Hydrodynamics of multiple selfpropelled flapping flexible plates XiYun Lu, ZeRui Peng, Haibo Huang Inherent hydrodynamic mechanism for aggregates of active swimmers has attracted significant attention. We here study the collective behavior of multiple selfpropelled flexible plates by means of the numerical simulation of fluidstructure interaction, which are driven by sinusoidally oscillating motions of identical frequency and amplitude at the leadingedges of the plates. Both fast mode with compact configurations and slow mode with sparse configurations were observed. The Lighthill conjecture that orderly configuration may emerge passively from hydrodynamic interactions was verified with multiple plates. The whole group may consist of subgroups and individuals with regular separations. Hydrodynamic forces experienced by the plates near their multiple equilibrium locations are all springlike restoring forces, which stabilize the orderly formation and maintain group cohesion. For the cruising speed of the whole group, the leading subgroup or individual plays the role of 'leading goose'.

Monday, November 19, 2018 8:52AM  9:05AM 
F23.00005: Vortex interaction between selfpropelled two tandem flexible fins in wall effect Young Dal Jeong, Jae Hwa Lee It is known that fish can take hydrodynamic advantages by schooling behaviors as well as by wall effects. Inspired by the fish schooling in wall effects, we perform numerical simulations of two tandem flexible fins under the influence of the wall confinements. The movement of the two flexible fins is driven by harmonic heaving motions at the leading edge. The two tandem fins move freely in the streamwise direction by the fluidflexible body interaction and the motions of the fins are affected by a one sided wall or two sided walls. The vortex interaction between the two fins in wall effects is analyzed as a function of the wall proximity, initial gap distance, bending rigidity, heaving amplitude and phase difference. When the leader and follower have the identical heaving amplitude, the input power of the follower is lower 12%, 19% and 5% near a one sided wall, two sided walls and with no wall respectively than that of the leader. The optimal efficiency of the follower is achieved by changing the heaving amplitude and flapping phase of the follower. 
Monday, November 19, 2018 9:05AM  9:18AM 
F23.00006: Controlling flow separation with backward traveling waves: from a turbulent channel flow to a low Reynolds number airfoil Amir Mahdi Akbarzadeh, Iman Borazjani The flow over many aquatic swimmers’ bodies is attached as they propel themselves via backward traveling waves. Inspired by aquatic animals, large eddy simulations (LES) of a turbulent channel flow which its wall is undergoing an undulatory motion are performed to investigate the effect of the wave on the flow dynamics. Effect of different wave speed and amplitude on flow separation is investigated. The results show the reverse flow in the wavy channel decreases when the wave speed increases. To see if the undulatory oscillation can also improve the aerodynamic characteristics of an airfoil, similar low amplitude traveling wave oscillations which can be generated experimentally are performed over the suction side of the airfoil. The Reynolds number (Re) is 50,000 and the angle of attack is ten degrees in these simulations. The results show that the traveling wave oscillation can increase the lift by 8% by consuming a negligible energy to produce the wave. 
Monday, November 19, 2018 9:18AM  9:31AM 
F23.00007: CFSI Study on Effect of Structural Flexibility of a Pitching Hydrofoil in a Freestream Flow Namshad Thekkethil, Atul Sharma, Amit Agrawal Computational FluidStructure Interaction (CFSI) study is presented for the effect of structural stiffness of a flexible hydrofoil on hydrodynamics characteristics and propulsive performance of the pitching foil in a free stream flow. Using a levelset and immersedboundary based hybrid finite volume element method (LSIBHFVEM), inhouse code based 2D simulations are done for various stiffness and Strouhal number at a constant Reynolds number Re = 5000. Unified CFSI analysis is presented by considering a timewise synchronized variation of flow patterns (pressure and vorticity contours) and engineering parameters (strain energy transfer and hydrodynamic forces). Structural flexibility leads to the storage (release) of strain energy to the foil (surroundingfluid) at certain (remaining) time duration. The storage (release) correspond to the accelerating (decelerating) phase of the pendulumlike motion of the foil – for an optimum structural stiffness which leads to the maximum thrust generation. The optimum as compared to other stiffness values leads to maximum deflection of the tip of the foil. This work gives fundamental insights into the physics of momentum transfer in the flexible foil which can lead to a more efficient propulsion system in the future. 
Monday, November 19, 2018 9:31AM  9:44AM 
F23.00008: Propulsive efficiency of a pitching foil with trailing edge control Francisco J HueraHuarte, Morteza Gharib A series of experiments conducted with a robotic fin, demonstrate how by actuating its trailing edge while performing periodic flapping, enhanced efficiencies and thrusts can be achieved depending on the phase difference imposed between the motions. The robotic fin consists of a rigid 3D printed panel in which the trailing edge is articulated. The fin is controlled by two independent servos that actuate the fin and its trailing edge independently. Measurements include forces and moments acting on the fin whilst flapping, as well as the torque applied to the shaft. The flow dynamics around the fin was also measured using planar Particle Image Velocimetry. Results show how the phase difference between the trailing edge and the fin is a key parameter, and how only when the trailing edge moves behind the fin not only thrust is enhanced but efficiency as well. 
Monday, November 19, 2018 9:44AM  9:57AM 
F23.00009: The evolution of largescale structures in the wake of oscillating teardrop foils Arman Hemmati, Ali Tarokh, Alexander John Smits The wake of a twodimensional teardrop foil in combined heavingpitching motion is examined using Direct Numerical Simulations at a Reynolds number of 6000. The Strouhal number of 0.6, and the amplitude and phase differences between pitching and heaving motions were selected to match the experiments by Van Buren et al. (2018) on the performance of underwater propulsors. These conditions represent the cases of maximum thrust and maximum efficiency. Preliminary analysis suggests that a wider expansion of the wake, which is dominated by the early detachment of structures from the foil, results in higher thrust. However, a higher efficiency locomotion was marked by a smaller wake width that resulted from the elongated detachment of stretched structures. More detailed wake analysis will identify potential differences in coherent structures that result in higher thrust versus higher efficiency. 
Monday, November 19, 2018 9:57AM  10:10AM 
F23.00010: Experimental study on the effects of trailing edge geometry and pitching amplitude on the wake structure of bioinspired pitching panels Justin T King, Melissa A Green Some aquatic swimmers create thrust with a caudal fin or a fluke found at the rear of the animal. These surfaces display many geometries, including those with different trailing edges. In the current work, the effects of systematically varying the trailing edge shape are studied using phase and timeaveraged velocity fields obtained from stereoscopic particle image velocimetry in a water tunnel experiment. Results focus on the threedimensional wakes produced by panels with straight, forked, and pointed trailing edges pitched at multiple amplitudes for Strouhal numbers, St, between 0.22 and 0.53. Results show that geometry and St influence wake behavior and dynamics. Portions of the phaseaveraged wakes are often comprised of linked vortex rings, formed by greater amounts of vorticity as St number is increased. The timeaveraged wakes indicate that increasing St leads to an injection of more momentum into the flow. In general, pointed trailing edges appear to generate more vorticity and momentum than straight and forked trailing edges. 
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