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 A19: Biological Fluid Dynamics: Flying and Gliding 
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Chair: Jake Socha, Virginia Polytechnic Institute Room: Georgia World Congress Center B306 
Sunday, November 18, 2018 8:00AM  8:13AM 
A19.00001: Flying Spiders: Effects of the length of a dragline and the spider mass in ballooning Tessa Stevens, Jodi C Turk, Longhua Zhao, Wei Zhang Most spiders use a type of aerial dispersal "ballooning" to move from one location to another. By ballooning, spiders can reach distances as far as 3200 km and heights of up to 5 km. Though a large number of observations of spider ballooning have been reported, it remains a mysterious phenomenon. What dominate the three stages of spider takeoff, flight, and settling? There are many factors to consider, including a spider's mass, morphology, posture, the silken dragline properties, and local meteorological conditions. A thorough understanding of the roles of these critical parameters is not only of ecological significance but also critical to improving advanced technologies for bioinspired innovations of airborne robotic devices. This preliminary test is to determine how the silk dragline length and spider mass affect the interaction in the freefall at Reynolds numbers of several thousand (based on the spider size and the relative wind speed), using recordings by a highspeed camera in a laboratory setting. The vertical velocities of the dragline and the induced flow structures are compared against numerical models of coupled fluidstructure interaction. Such results are expected to shed lights on the intriguing flow physics of spider ballooning and help to validate new models.

Sunday, November 18, 2018 8:13AM  8:26AM 
A19.00002: Optimal design of autorotating wings Lionel Vincent, Eva Kanso Autorotation is a passive flight mode in which lift is primarily created by the revolution of the flyer around itself. The aerodynamics of autorotating flyers is not well understood, and design rules derived in the context of fixed wings may not apply. Using carefullycontrolled experiments, we study the aerodynamic performance of thin, quasirectangular autorotating wings. In this framework, we systematically look for the optimal dynamics by varying several parameters such as mass distribution, flexibility, shape, and winglet size. We use highspeed photography to extract flight characteristics such as flight duration, descent angle, and flight range, and develop reducedorder models to predict the behavior change. We find that a heterogenous redistribution of the wing mass, leading to spanwise tip flexibility and chordwise reduction in the wing’s moment of inertia, can result in improvement of aerodynamic performance. The design rules extracted from our investigation may contribute to the understanding of the settling dynamics of heterogeneous objects, shed light on the mechanism of seed dispersal, or help with the design of microair vehicles. 
Sunday, November 18, 2018 8:26AM  8:39AM 
A19.00003: On the stabilization of the leading edge vortex of an autorotating winged seed Gonzalo Arranz, Alejandro Gonzalo, Manuel Moriche, Oscar Flores, Manuel GarcíaVillalba, Markus Uhlmann Direct numerical simulations of the autorotation of a wingedseed model are performed at Reynolds number between 80 and 240. The simulations are performed by coupling the NavierStokes equations with the NewtonEuler equations of the seed. Due to this coupling, the flow around the seed and the kinematics of the seed change with Re. In particular, as the Reynolds number increases, the coning angle decreases and the angular velocity of the seed increases. In terms of the flow characterization, it is observed that a stable leading edge vortex (LEV) develops on the upper surface of the wing of the autorotating seed, in accordance to existing literature. Using a reference frame attached to the seed, the effect of the Reynolds number in the relative velocity, relative vorticity and pressure is analyzed. Three possible stabilization mechanisms for the LEV are evaluated (i.e., Coriolis/centrifugal accelerations, vorticity transport along the LEV and viscous effects). The results suggest that, for the present geometry and range of Reynolds number, only the effect of Coriolis/centrifugal accelerations is relevant in the stabilization of the LEV. 
Sunday, November 18, 2018 8:39AM  8:52AM 
A19.00004: How the dandelion stabilises a dragenhancing vortex ring Cathal Cummins, Madeleine Seale, Enrico Mastropaolo, Naomi Nakayama, Ignazio Maria Viola We previously reported the extraordinary vortex produced by dandelion seeds, showing that the dandelion’s parachute design stabilizes the vortex as the seed flies. In this talk, we consider the slow viscous flow (100 < Re < 600) past dandelion seeds and thin porous disks approximating dandelion seeds using flow visualisation techniques and particle image velocimetry. We show that the flow around porous disks exhibits striking similarities to that around impervious disks, even at very high porosities. We show that the vorticity is produced at the edges of the porous disks, even though there is virtually no material there. The critical Re for the onset of vortex shedding behind the disks changes as a function of porosity, as reported previously; here, we show that the Strouhal number is similar for porous disks as it is for solid ones. 
Sunday, November 18, 2018 8:52AM  9:05AM 
A19.00005: Vorticity dynamics and transient force generation during the leadingedge vortex formation on a revolving wing at low Reynolds number Long Chen, Jianghao Wu, Bo Cheng Given previous efforts on explaining the stability of leadingedge vortex (LEV) when it reaches the steady state, its formation process and its contribution to the transient force generation remain largely underexplored. Here, we examine the vorticity dynamics and transient force generation during the LEV formation on a revolving wing with AR=3 and Re=1500, operating in a mineraloil tank. The wing starts with a constant acceleration and then rotates at a constant velocity. The accelerating distance is based on the chord (c) length of travel and is varied from 0.25c to 2c, and the AoA is varied from 15 to 60 degs. The 'ShakeTheBox' Lagrangian Particle Tracking Velocimetry system together with a volumetric patching process are employed to reconstruct the entire flow generated by the wing. Results show that the LEV reaches the steady state after approximately 4c of travel regardless of the accelerating distance or the AoA. However, the circulatory lift peaks around the end of acceleration, resulted from a combined effect of the LEV growth and the expansion of the region outlined by the LEV, starting vortex and tip vortex. Our findings indicate an indispensable role of transient LEV dynamics in understanding insect flight. 
Sunday, November 18, 2018 9:05AM  9:18AM 
A19.00006: Undulation enhances stability, enabling gliding in flying snakes John J. Socha, Isaac J. Yeaton, Shane D. Ross Aerial undulation is a unique form of undulation used by flying snakes as theyglide through the air. Ostensibly, the role of aerial undulation is the same as terrestrial and aquatic undulation: to generate propulsive thrust. However, flying snakes must contend with nonplanar aerodynamic forces and rotational motion to glide successfully, which may not require undulation. Is aerial undulation a nonfunctional behavioral vestige, or is it a novel aerial control strategy? We used highspeed motion capture to fully quantify the aerial undulation waveform. The body motion consists of two waves of bending and a net outofplane motion of the body. The vertical wave has twice the spatial and temporal frequencies as the horizontal wave and is phaseshifted by 90°. Using these results, we tested the effects of undulation by developing an anatomically accurate mathematical model, which indicates that undulation is not a strict requirement for the simulated snake to glide. However, the addition of undulation, even in the absence of feedback control, drastically increases the glide performance by stabilizing the rotational motion. This work shows that aerial undulation serves a different purpose than other known uses of undulation in animals. 
Sunday, November 18, 2018 9:18AM  9:31AM 
A19.00007: Estimate strength and trajectory of leadingedge vortex: a universal analytical model Di Chen, Dmitry Kolomenskiy, Hao Liu For flying animals, leadingedge vortex (LEV) in flapping flight of insects, bats and birds, is created by dynamic stall, persisting on the the upper surface of the wing at a large angle of attack (AoA) and maintaining highlift. Current study presents a novel reducedorder analytical model that provides fast estimations in closedform expressions for the strength and position of LEV on rotating wings at arbitrarily large AoA. Downwashmodified vorticity production at the leading edge and its subsequent outboarddirected transport are included as two competing effects in the theory of LEV development based on the case of AoA = 90^{o} in our previous study (Chen et al. 2017 J. Fluid Mech.). The analytical model is well validated with the corresponding experimental data and the CFD solutions in a wide range of the Reynolds number, as well as bioinspired wing shapes of fruit fly, bumblebee and hawkmoth. Moreover, an important parameter of spanwise vorticity transport K_{sp} which depends only on Reynolds number at constant AoA, is found to determine the LEV trajectory in the competition of vorticity production and transport. 
Sunday, November 18, 2018 9:31AM  9:44AM 
A19.00008: Efficiency Optimization and Three Dimensional Flow Visualization of Flapping Flight Yagiz E Bayiz, Long Chen, Yano Chavrin ShadeAlexander, Aaron Nicholas Aguiles, Jianghao Wu, Bo Cheng Finding optimal locomotion strategies for small fliers or swimmers poses a serious challenge due to the complex fluid behaviors in conjunction with the vast kinematic parameter space available. Recent advancement of machine learning techniques, especially the policy search methods, can be potentially used to solve such problems. In this work, an evolutionarystrategybased policy search algorithm is applied to a robotic wing to optimize flappingwing trajectories that maximize the average lift generated per power consumption. The robotic wing have two degrees of freedom, i.e. stroke and pitch rotation which are transcribed with periodic triangular and trapezoidal functions, respectively. The learning experiments are repeated for four different prescribed stroke amplitudes while the Reynolds Number (Re) is maintained at 1000. The efficiency is observed to increase with the stroke amplitude and the lift is mainly generated through delayed stall. Moreover, advanced wing rotation is the preferred strategy for lower stroke amplitude. We also perform Particle Tracking Velocimetry (PTV) measurement of the flow created by the optimized trajectories to evaluate the underlying physics of efficient lift generation. 
Sunday, November 18, 2018 9:44AM  9:57AM 
A19.00009: Flowinduced bending of disks settling in water Lisa Maillard, Lionel Vincent, Eric Clément, Eva Kanso Interactions between flexible bodies and the surrounding fluid medium are ubiquitous in nature. Examples range from the great trash island of plastic bags in the Pacific Ocean to the swimming of jellyfish. To study such interactions, a widely used model system is the settling of a body under the influence of gravity. Here, we investigate the bending of a thin, freelyfalling elastic disk in water as a function of its relative flexibility. The parameters of the disk are fixed to produce a zigzag, fluttering motion. Nonmonotonous changes in the fluttering amplitude are observed as a function of increasing flexibility. We show that, as the disk flutters, it experiences outofplane bending that dynamically switches between the leading edge and the sides of the disk. These observations differ from those of rectangular fluttering wings in which only the leading edge deflects. These findings suggest that flexibility alters the nature of the fluiddisk interactions in a nontrivial manner, that could be potentially exploited for various applications in engineering and biorelated problems. 
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